A text-book of veterinary anatomy (part 4 of 5)

A text-book of veterinary anatomy (part 4 of 5)

=Structure.=—The hilus is about in the middle of the inner border. The pelvis is funnel-shaped, and divides into two calyces majores, which pass in a curve forward and backward respectively, and give off some eight to twelve short calyces minores; each of the latter contains a papilla. Some papillæ are narrow and conical, and correspond to a single pyramid; others are wide and flattened, and result from the fusion of two or more pyramids; some project directly through the wall of the pelvis without the formation of a calyx. The renal pyramids are distinct, but it is apparent that some are compound, i. e., formed by fusion of primitively separate pyramids. The renal artery enters the ventral part of the hilus, and the ureter leaves it dorsally.

The only special feature in regard to the =ureter= is that it is at first relatively wide and gradually diminishes in caliber.

The =bladder= is relatively very large; when full, it lies chiefly in the abdominal cavity. It is almost completely covered with peritoneum.

URINARY ORGANS OF THE DOG

The =kidneys= are relatively large, forming about ¹⁄₁₅₀ to ¹⁄₂₀₀ of the body-weight; the weight of the kidney of a medium-sized dog is about two ounces (ca. 50 to 60 grams). They are both bean-shaped, thick dorso-ventrally, with a rounded ventral surface and a less convex dorsal surface; the surfaces are smooth.

The =right kidney= (Fig. 343) is not subject to much variation in position; it is situated usually under the last rib and the first three lumbar transverse processes. Its anterior part lies in the deep renal impression of the liver; its posterior part is related to the sublumbar muscles dorsally, and the pancreas and duodenum ventrally.

The =left kidney= (Fig. 342) is subject to some variation in position; this is due to the fact that it is loosely attached by the peritoneum, and is affected by the degree of fullness of the stomach. When the stomach is nearly empty, the kidney usually lies under the transverse processes of the second, third, and fourth lumbar vertebræ, so that its anterior pole is opposite to the hilus of the right kidney. As the stomach fills it pushes the left kidney backward, so that the anterior pole of the latter may be opposite the posterior pole of the right kidney. The dorsal surface is related to the sublumbar muscles. The ventral surface is in contact with the terminal part of the colon (Colon descendens). The external border is related to the spleen and the flank. The anterior extremity touches the stomach and the left extremity of the pancreas.

The external border of the left kidney has considerable contact with the flank, and hence it may be palpated more or less distinctly in the living animal about half-way between the last rib and the crest of the ilium.

=Structure.=—The hilus is in the middle of the inner border and is, relatively wide. Cortex, limiting zone, and medulla are clearly defined. On frontal sections it is seen that the medullary substance forms a horizontal renal crest like that of the horse, but with the important difference that curved ridges proceed dorsally and ventrally from the crest somewhat like buttresses. Sections above or below the renal crest often cut these ridges in such a manner as to give the appearance of conical papillæ, and thus tend strongly to produce a false impression. The pelvis is adapted to this arrangement of the medullary substance. It incloses a central cavity into which the renal crest projects, and is prolonged outward between the ridges, forming cavities for the latter, thus simulating the appearance of calyces which do not exist.

The =ureters= present no special features.

The =bladder= is abdominal in position, the neck lying at the anterior border of the pubic bones. It is relatively large, and when full, the vertex may reach to the umbilicus. It has a practically complete peritoneal coat.

THE ADRENALS

The =right adrenal= lies between the anterior part of the inner border of the kidney and the posterior vena cava. It is somewhat prismatic, and is pointed at either end.

The =left adrenal= lies along the posterior aorta, from the renal vein forward, but is not in contact with the kidney. It is elongated and flattened dorso-ventrally. (There is a furrow on the ventral surface for the vein which crosses it; the part in front of this furrow is discoid, and may be taken for the entire organ in a fat subject.)

The cortex is pale yellow in color, the medulla dark brown.

THE MALE GENITAL ORGANS

The male =genital organs= (Organa genitalia masculina) are: (1) The two =testicles=, the essential reproductive glands, with their coverings and appendages; (2) the =vasa deferentia=, the ducts of the testicles; (3) the =vesiculæ seminales=; (4) the =prostate=, a musculo-glandular organ; (5) the two =bulbo-urethral= (or Cowper’s) =glands=; (6) the male =urethra=, a canal which transmits the generative and urinary secretions; (7) the =penis=, the male copulatory organ. The vesiculæ seminales, the prostate, and the bulbo-urethral glands discharge their secretions into the urethra, where they mix with the seminal fluid secreted by the testicles; hence they are often termed the accessory sexual glands.

MALE GENITAL ORGANS OF THE HORSE

THE TESTICLES

The =testicles= (Testes) are situated in the inguinal region, inclosed in a diverticulum of the abdomen termed the scrotum. Their long axes are nearly longitudinal. They are ovoid in form, but considerably compressed from side to side. Each presents two surfaces, two borders, and two extremities. The =internal= and =external surfaces= (Facies medialis, lateralis) are convex and smooth. The =free= or ventral =border= (Margo liber) is convex in both directions. The =attached= or dorsal =border= (Margo epididymidis) is nearly straight, and is the one by which the gland is suspended in the scrotum by the spermatic cord; the epididymis is attached to this border and overlies it externally. The =anterior= and =posterior extremities= (Extremitas capitata, caudalis) are rounded.

At the anterior extremity there is often a sessile or pedunculated sac which contains a clear fluid; this is the =appendix testis= or hydatid of Morgagni, from which a thread-like process extends backward toward the vas deferens. It is regarded as a remnant of the Müllerian duct of the embryo.

A testicle of average size of an adult stallion is about four or five inches (ca. 10 to 12 cm.) long, two and a half inches (ca. 6 to 7 cm.) high, and one and a half to two inches (ca. 4 to 5 cm.) wide; it weighs about ten and one-half ounces (ca. 300 grams). They vary much in size in different subjects, and are commonly of unequal size, the left one being more often the larger.

The =epididymis= is adherent to the attached border of the testicle, and overlaps somewhat the external surface. Its anterior enlarged end is termed the =head= or globus major (Caput epididymidis), and its posterior, slightly enlarged end is the tail or globus minor (Cauda epididymidis); while the intermediate narrow part is the =body= (Corpus epididymidis). The head is closely connected with the testicle by the efferent ducts of the latter, by connective tissue, and by the serous membrane. The body is less closely attached by the serous covering, which forms externally a pocket beneath the epididymis termed the =digital fossa= (Sinus epididymidis). The tail is firmly attached to the posterior extremity of the testicle by a short ligament (Lig. epididymidis), formed by a thick fold of the tunica vaginalis, which contains smooth muscle-fibers; it is continued by the vas deferens.

=Structure of the Testicle and Epididymis.= The greater part of the surface of the testicle is covered by a serous membrane, the =tunica vaginalis propria=, which is the visceral layer of the serous envelop of the cord and testicle; this is reflected from the attached border of the gland, leaving an uncovered area at which the vessels and nerves in the spermatic cord reach the testicle. Beneath this serous covering is the =tunica albuginea=, a strong capsule composed of dense white inelastic fibrous tissue and unstriped muscle-fibers. From the deep surface of this tunic delicate membranous =septa= (Septula testis) pass into the gland, dividing it into pyramidal spaces, the bases of which are peripheral. At the anterior part of the attached border of the testicle connective tissue and smooth muscle-fibers pass into the substance of the gland, forming the =mediastinum testis=; from this the septa radiate. The mediastinum contains a network of fine canals, the =rete testis=; it is traversed by blood-vessels, which radiate on the septa to the deep face of the tunica albuginea, forming a delicate network—the so-called tunica vasculosa. The spaces imperfectly marked off by the septa contain the =parenchyma testis=, which consists of =seminiferous tubules= (Tubuli seminiferi), supported by loose intertubular connective tissue. The tubules are collected into small masses or =lobules= (Lobuli testis). They are at first very tortuous (Tubuli contorti); toward the apex of the lobule they unite with other tubules, forming larger straight tubules (Tubuli recti); these pass into the mediastinum testis, where they form the =rete testis=. From the latter a number (ca. 15 to 20) of larger =efferent tubules= (Vasa efferentia) pierce the albuginea and enter the head of the epididymis.

The =epididymis= is covered by the tunica vaginalis propria and a thin albuginea. Its head consists of =lobules= (Lobuli epididymidis), each of which consists of a coiled tubule or =conus vasculosus=, which succeeds a vas efferens. By the union of these tubules is formed a single tube, the =duct of the epididymis= (Ductus epididymidis), which, by its complex coils, forms the body and tail of the epididymis and terminates in the vas deferens. The tubules and the coils of the duct of the epididymis are held together by connective tissue and unstriped muscle-fibers. The tubules and duct are lined with ciliated epithelium, and the duct has a muscular coat which consists of longitudinal and circular fibers.

=Vessels and Nerves.=—The testicle is richly supplied with blood by the =spermatic artery=, a branch of the posterior aorta. The artery descends in the anterior part of the spermatic cord, and is very tortuous near the testicle; on reaching the attached border of the gland it passes backward in a flexuous manner, giving branches to the testicle and epididymis, turns around the posterior extremity, and runs forward on the free border to the anterior extremity. It is partially embedded in the tunica albuginea, and detaches lateral branches which ascend in a tortuous fashion on each surface of the testicle; these give off small branches which enter the gland on the septa.

The =spermatic veins= on leaving the testicle, form the =pampiniform plexus= around the artery in the spermatic cord. The vein which issues from this plexus usually joins the posterior vena cava on the right side, the left renal vein on the left side.

The =nerves=, derived from the renal and posterior mesenteric plexuses, form the =spermatic plexus= around the vessels.

THE SCROTUM

The =scrotum=, in which the testicles and the lower parts of the spermatic cords are situated, is somewhat globular in form, but is commonly asymmetrical, since one testicle—more often the left—is the larger and more dependent. It varies in form and appearance in the same subject, according to the condition of its subcutaneous muscular tissue. The latter contracts on exposure to cold, so that the scrotum is drawn up and becomes thicker and wrinkled; when relaxed under the influence of heat or fatigue, or from debility, it becomes smooth and pendulous, with a constriction or neck superiorly. It consists of layers which correspond with those of the abdominal wall; considered from without inward these are:

(1) The =skin=, which is thin, elastic, usually black in color, and smooth and oily to the touch. It presents scattered short fine hairs, and is abundantly supplied with very large sebaceous and sweat glands. It is marked centrally by a longitudinal furrow, in which is a line, the =raphé scroti=; this is continued forward on the prepuce and behind on the perineum.

(2) The =dartos= (Tunica dartos) is reddish in color and is closely adherent to the skin except superiorly. It consists of fibro-elastic tissue and unstriped muscle. Along the raphé it forms a median partition, the =septum scroti=, which divides the scrotum into two pouches. Dorsally the septum divides into two layers which diverge on either side of the penis to join the abdominal tunic. At the bottom of the scrotum fibers connect the dartos closely with the tunica vaginalis and the tail of the epididymis, constituting the =scrotal ligament=.[143] Elsewhere the dartos is loosely connected with the underlying tunic by areolar tissue which contains no fat.

(3) The scrotal =fascia=, which is apparently derived from the oblique abdominal muscles.

It has been customary to describe three layers of fascia, in conformity with the accounts given in text-books of human anatomy. These are: (1) the intercolumnar or spermatic fascia, derived from the margin of the external inguinal ring; (2) the cremasteric fascia, derived from the internal oblique muscle; (3) the infundibuliform fascia, derived from the fascia transversalis. The first two cannot be distinguished by dissection and the third is (in the scrotum) fused with the parietal peritoneum of the tunica vaginalis.

(4) =The parietal layer of the tunica vaginalis.=—This is a fibro-serous sac which is continuous with the parietal peritoneum of the abdomen at the internal inguinal ring. It is thin above, but is thick in its scrotal part, where it is strengthened by fibrous tissue (Lamina fibrosa) derived from the transversalis fascia. It will be described further under the caption tunica vaginalis.[144]

THE VAS DEFERENS

This tube (Ductus deferens) extends from the tail of the epididymis to the ejaculatory duct. It passes upward in the inguinal canal, inclosed in a fold detached from the inner surface of the mesorchium, near the posterior (attached) border of the latter. At the vaginal ring it separates from the other constituents of the spermatic cord, and turns backward and inward into the pelvic cavity (Fig. 272). For some distance it lies in the free edge of the urogenital fold, by which it is attached to the lower part of the lateral wall of the pelvis. In its further course (over the dorsal surface of the bladder) it leaves the edge of the fold and inclines inward between its layers, coming in contact with the inner face of the vesicula seminalis. Over the neck of the bladder the two vasa lie very close together, flanked laterally by the necks of the vesiculæ seminales, and having the uterus masculinus between them. They then disappear under the isthmus of the prostate, and are continued through the wall of the urethra by the =ejaculatory ducts=. The latter are formed in the urethral wall by the union of the vas deferens with the duct (or neck) of the corresponding vesicula seminalis; they are thin-walled tubes, about 2 to 3 mm. long and 6 to 7 mm. wide, which open into the urethra on either side of the colliculus seminalis. In about 15 per cent. of subjects the vas deferens and duct of the seminal vesicle do not unite, but open side by side into the urethra. From its origin until it reaches the dorsal surface of the bladder the vas deferens has a uniform diameter of about a quarter of an inch (ca. 6 mm.). It then forms a fusiform dilatation, the =ampulla ductus deferentis= (Fig. 394); this part is about six to eight inches (ca. 15 to 20 cm.) long, and in its largest part nearly an inch (ca. 2 cm.) in diameter in the stallion; in geldings the dilatation is usually not very pronounced. Beyond the ampulla the duct abruptly contracts.

=Structure.=—The wall of the vas deferens is thick and the lumen relatively small, so that the tube has a firm and cord-like character. It is covered with peritoneum, except in the last few inches of its course. The loose =adventitia= contains numerous vessels and nerves. The thick =muscular coat= consists of longitudinal and circular layers. The =mucous membrane= has an epithelium of short columnar cells. In the posterior part of the tube, and especially in the ampulla, there are numerous tubulo-alveolar glands.

=Blood-supply.=—Spermatic and umbilical arteries.

=Nerve-supply.=—Spermatic and pelvic plexuses.

THE SPERMATIC CORD

The =spermatic cord= (Funiculus spermaticus) consists of the structures carried down by the testicle in its migration through the inguinal canal from the abdominal cavity to the scrotum. It begins at the internal inguinal ring, where its constituent parts come together, extends obliquely downward through the inguinal canal, passes over the side of the penis, and ends at the attached border of the testicle. It consists of the following structures:

(1) The =spermatic artery=.

(2) The =spermatic veins=, which form the pampiniform plexus around the artery.

(3) The =lymphatics=, which accompany the veins.

(4) Sympathetic =nerves=, which run with the artery.

(5) The =vas deferens=.

(6) The =internal cremaster muscle=, which consists of bundles of unstriped muscular tissue.

(7) The =visceral layer= of the =tunica vaginalis= or =mesorchium=.

The first four of these constituents are gathered into a rounded mass which forms the anterior part of the cord; they are united by connective tissue, interspersed with which are bundles of the cremaster internus. The vas deferens is situated posteriorly, inclosed in a special fold detached from the inner surface of the mesorchium; hence it is not visible externally.

The term spermatic cord is to a certain extent misleading as applied to most animals, while in man the structure is distinctly cord-like. In the horse, when the tunica vaginalis is slit open and the “cord” stretched out, the latter is seen to have the form of a wide sheet, the mesorchium, which has a thick, rounded anterior edge, the so-called “vascular part” of the cord. The posterior edge of the mesorchium is continuous with the parietal layer of the tunic; its inner surface presents posteriorly the deferential fold (Plica ductus deferentis). Between the two layers of the mesorchium are bundles of unstriped muscle (cremaster internus) and small vessels.

THE TUNICA VAGINALIS

The =tunica vaginalis= is a somewhat pyriform serous sac which extends from the internal inguinal ring through the inguinal canal to the bottom of the scrotum. Like the abdominal peritoneum, of which it is an evagination, it consists of two layers—parietal and visceral. The =parietal layer=, or =tunica vaginalis communis= (s. reflexa), lines the scrotum below; its narrow, tubular part lies in the inguinal canal and is directly continuous with the parietal peritoneum of the abdomen at the internal inguinal ring. The =cavity= of the tunica vaginalis (Cavum vaginale) is a diverticulum of the general peritoneal cavity, with which it communicates through the =vaginal ring= (Annulus vaginalis). It contains normally a small quantity of serous fluid. The parietal layer is reflected from the posterior wall of the inguinal canal around the structures of the cord, forming the =mesorchium=, a fold analogous to the mesentery of the intestine. The =visceral layer=, or =tunica vaginalis propria=, covers the spermatic cord, testicle, and epididymis.

The =external cremaster muscle= (M. cremaster externus) lies on the outer and posterior part of the tunic, to the scrotal part of which it is attached.

Confusion has arisen from the use of the term internal abdominal or inguinal ring in two senses. The term is used to designate the upper or abdominal opening of the inguinal canal, but it is also applied to the upper opening of the cavity of the tunica vaginalis. It should only be applied to the abdominal opening of the canal, the =subperitoneal ring=. The =peritoneal ring= at which the cavity of the tunica vaginalis opens into the general peritoneal sac is distinguished by the name =vaginal ring=. It is placed about four or five inches (ca. 10 to 12 cm.) from the linea alba, and about two or three inches (ca. 6 to 8 cm.) in front of the ilio-pectineal eminence. In stallions it will usually admit the end of the finger readily, but it may be abnormally large and allow a loop of bowel to enter the cavity of the tunica vaginalis. In the gelding it is smaller and sometimes partially occluded. In man the cavity is almost always obliterated early, except in its scrotal portion, thus abolishing the vaginal ring.

DESCENT OF THE TESTICLES

During early fœtal life the testicle is situated against the dorsal wall of the abdominal cavity, in contact with the ventral surface of the corresponding kidney. As growth proceeds it gradually migrates from this primitive position, and finally passes down the inguinal canal into the scrotum. Previous to its descent through the abdominal wall the testicle is suspended by a fold of peritoneum, termed the =mesorchium=. This fold contains the vessels and nerves of the testicle in its anterior border. In its posterior edge is the elongated tail of the epididymis, and two cords of fibrous tissue and unstriped muscle. One of these cords is short and connects the tail of the epididymis with the testicle; later it becomes shorter, and is termed the ligament of the epididymis. The other cord, the =gubernaculum testis=, extends from the tail of the epididymis to the subperitoneal tissue in the vicinity of the future vaginal ring. The =deferential fold= (Plica ductus deferentis) is given off from the inner face of the mesorchium, and joins the urogenital fold posteriorly. The body of the epididymis at this time lies in the edge of an oblique fold formed by the outer layer of the mesorchium. After the middle of fœtal life, a pouch or diverticulum of the peritoneum, the =processus vaginalis=, grows downward through the inguinal canal, carrying with it cremaster fibers derived from the internal oblique muscle and a layer from the transversalis fascia. It is accompanied by an inguinal extension of the gubernaculum testis. The latter blends below with the subcutaneous tissue which later becomes the dartos. The tail of the epididymis first enters the processus vaginalis, followed by the testicle with its mesorchium, which descends within this diverticulum of the peritoneum until it reaches the scrotum. The vas deferens and its fold descend synchronously with the epididymis and testicle.

The mechanical factors concerned in the migration of the testicle are matters on which much uncertainty still exists. That the gubernaculum exerts sufficient traction to guide the epididymis and testicle to the inguinal canal seems plausible. The internal inguinal ring may constitute a locus minoris resistentiæ in the abdominal wall, especially after the descent of the processus vaginalis. Progressive shortening of the gubernaculum was formerly considered to be the chief cause of the descent through the abdominal wall. Increase in the intra-abdominal pressure is probably an important factor.

In the foal the descent of the testicles is often complete at birth, but it frequently happens that one testicle or both may be retained in the inguinal canal or in the abdomen for some months. In other cases the testicle may return into the canal or abdomen, since in the young foal the vaginal ring is large and the testicle small and soft, and not yet closely anchored by the scrotal ligament. In rare cases the descent may be completed as late as the fourth year (personal observation).

Indefinite retention of one testicle or both in the abdominal cavity or inguinal canal is not rare in horses; this condition is termed cryptorchism. Abdominal retention is the more usual form of cryptorchism in adult horses, inguinal retention being usually temporary. The retained testicle is usually, but not always, small, thin, soft, and flabby, and is non-spermiogenic. The processus vaginalis and the inguinal part of the gubernaculum are usually present, but may be rudimentary. The ligament of the epididymis and the corresponding part of the mesorechium are often so long that the tail of the epididymis may be several inches distant from the testicle. The abdominal part of the gubernaculum may be 8 to 10 inches (ca. 20 to 25 cm.) long, and the ligament of the epididymis much elongated (10 to 15 cm. in length according to Vennerholm); thus the testicle may have a wide range. The vaginal ring is sometimes closed.

In many mammals the testicles normally remain in the abdominal cavity; such animals are termed testiconda, and include the elephant, some insectivora, hyrax, sloths, ant-eaters, armadillos, and cetacea. In others the testicles descend periodically during the period of œstrum, and then return into the abdomen, or they may be extruded and retracted voluntarily; this is true of most rodents, many insectivora (moles, shrews, hedgehog), and bats.

THE VESICULAE SEMINALES

The =vesiculæ seminales= (Fig. 394) are two elongated and somewhat pyriform sacs, which lie on either side of the posterior part of the dorsal surface of the bladder. They are partly inclosed in the urogenital fold, and are related to the rectum dorsally. Their long axes are parallel with the vas deferentia and converge posteriorly. Each consists of a rounded blind end, the =fundus=, a middle, slightly narrower part, the =body=, and a posterior constricted part, the =neck= or =duct=.

In the stallion they are about six to eight inches (ca. 15 to 20 cm.) long, and their greatest diameter is about two inches (ca. 5 cm.); in the gelding they are usually much smaller.[145]

The vesiculæ are, for the most part, retroperitoneal, but the fundus extends forward into the urogenital fold and hence has a serous covering. The =neck= or =duct= (Ductus excretorius) dips under the prostate, and usually unites with the end of the vas deferens to form the =ejaculatory duct=.

=Structure.=—The wall, exclusive of the partial serous coat, consists of a fibrous =adventitia=, a middle =muscular coat=, and a =mucous lining=. The muscular coat is thickest at the fundus, and consists of two planes of longitudinal fibers with a circular layer between them. The mucous membrane is thin, and is arranged in numerous folds which form a network; the spaces so inclosed present the openings of tubulo-alveolar glands. The epithelium is columnar. The =blood-supply= is derived from the =internal pudic artery=.

THE PROSTATE

The =prostate= (Prostata) is a musculo-glandular organ which lies on the neck of the bladder and the beginning of the urethra, beneath the rectum. It consists of two lateral lobes and a connecting isthmus.

The =lateral lobes=, right and left (Lobus dexter et sinister), are somewhat prismatic in form, and are directed forward, outward, and somewhat upward. The antero-internal surface of each lobe is concave and partly embraces the corresponding vesicula seminalis. The dorsal surface is concave and is in relation with the rectum. The ventral surface is convex and lies on the obturator internus muscle and fat. The apex is pointed and lies near the posterior end of the superior ischiatic spine.

The =isthmus= is a thin, transverse band, about four-fifths of an inch (ca. 2 cm.) wide. It lies over the junction of the bladder with the urethra, the uterus masculinus, the terminal parts of the vasa deferentia, and the necks of the vesiculæ seminales. Dorsally it is partly covered by transverse fibers of the urethral (Wilson’s) muscle.

=Structure.=—The prostate is inclosed in a fibro-muscular =capsule=, which sends thick, somewhat radially arranged =trabeculæ= into the gland substance, dividing it into spheroidal or ovoid =lobules=. The lobules contain a central space in which the prostatic secretion (Succus prostaticus) collects. The walls of these spaces are pouched out to form tubular diverticula, and the latter present saccular dilatations. These cavities are all lined by cubical epithelium. There are 15 to 20 =prostatic ducts= (Ductus prostatici) on either side, which perforate the urethra and open lateral to the colliculus seminalis. The =blood-supply= is derived from the =internal pudic artery=.

The surface of the prostate is commonly tuberculate in old subjects, and amyloid bodies and calcareous concretions may be found in it.

THE UTERUS MASCULINUS

The =uterus masculinus= or =prostatic utricle= is a rudimentary structure of variable size and form, situated centrally on the posterior part of the dorsal surface of the bladder. When well developed, it consists of a median flattened tube, some three or four inches (ca. 7.5 to 10 cm.) long, and about half an inch (ca. 1 to 1.5 cm.) wide, the anterior part of which lies in the urogenital fold and gives off two slender processes or cornua; the latter curve forward and outward in the fold a variable distance, being sometimes traceable as far as the anterior end of the ampulla of the vas deferens. The posterior extremity of the tube passes under the isthmus of the prostate, and opens into the urethra on the summit of the colliculus or joins an ejaculatory duct. It has a muscular coat and a mucous lining. In many cases it consists merely of a very small central tubule with a blind anterior end, or a band, not at all sharply marked off from the adjacent tissue; in other cases it cannot be recognized. It is interesting chiefly as being a remnant of the ducts of Müller and the homologue of the uterus and vagina.

THE BULBO-URETHRAL GLANDS

The =bulbo-urethral= (or Cowper’s) =glands= (Glandulæ bulbourethrales) are two in number, and are situated on either side of the pelvic part of the urethra close to the ischial arch (Fig. 452). They are covered by the urethral (Wilson’s) muscle. They are ovoid in form, somewhat depressed dorso-ventrally, and their long axes are directed obliquely forward and outward. In the stallion they may measure about two inches (ca. 5 cm.) in length, and an inch or more (ca. 3 cm.) in width. In the gelding they are about the size of an average hazel nut.

=Structure.=—They resemble the prostate in general structure, but the interstitial tissue is much less abundant, especially in regard to its muscular elements. In the peripheral part there are striped muscle-fibers. Each gland has six to eight =excretory ducts= (Ductus excretorii) which open into the urethra on a series of small papillæ behind the prostatic ducts and close to the median plane. The =blood-supply= comes from the =internal pudic artery=.

THE PENIS

The =penis=, the male organ of copulation, is composed essentially of erectile tissue, and incloses the extrapelvic part of the urethra. It extends from the ischial arch forward between the thighs on to the umbilical region of the abdominal wall. It is supported by the fascia penis and the skin, and its prescrotal portion is situated in a cutaneous pouch, the prepuce or sheath. It is cylindrical in form, but much compressed laterally in the greater part of its extent.

In the quiescent state it is about 20 inches (ca. 50 cm.) long; of this, about 6 to 8 inches (ca. 15 to 20 cm.) is free in the prepuce. In erection it increases 50 per cent. or more in length.

It may be divided into a root, a body, and a terminal enlargement, the glans.

The =root= (Radix penis) is attached to the lateral parts of the ischial arch by two =crura=, which converge and unite below the arch (Fig. 452). The urethra passes over the ischial arch between the crura, and curves forward to become incorporated with the penis.

The =body= (Corpus penis) begins at the junction of the crura and constitutes the bulk of the organ. At its origin it is attached to the symphysis ischii by two strong flat bands, the =suspensory ligaments= of the penis (Ligamenta suspensoria penis), which blend with the tendon of origin of the graciles muscles (Figs. 451, 456). This part of the penis is flattened laterally for the most part, but becomes rounded and smaller anteriorly. It presents four surfaces. The =dorsal surface= or =dorsum penis=, is narrow and rounded; on it are the dorsal arteries and nerves of the penis and a rich venous plexus. The =ventral= or =urethral surface= (Facies urethralis) is rounded, and along it runs the urethra, embedded in the deep =urethral groove= of the corpus cavernosum. The =lateral surfaces= are high and flattened, except anteriorly, where they are lower and rounded.

The =glans penis= is the enlarged free end of the organ. It is marked off from the body by a faintly marked =neck= (Collum glandis). In front of this is a prominent circular ridge, the =corona glandis=, which is notched below. The base of the glans is rounded and extends further backward dorsally than ventrally; it is marked in its lower part by a deep depression, the =fossa glandis=, in which the urethra protrudes for about an inch (ca. 2.5 cm.) as a free tube, the =urethral process= (Processus urethræ), covered by a thin integument. The urethra is thus surrounded by a circular fossa, which opens superiorly into the =urethral sinus=, a bilocular diverticulum lined by thin black skin. This diverticulum is filled sometimes with a caseous mass of sebaceous matter and epithelial débris.

=Structure.=—The penis consists essentially of two erectile bodies, the corpus cavernosum penis and the corpus spongiosum.

The =corpus cavernosum penis= forms the greater part of the bulk of the penis except at its free extremity. It arises from each side of the ischial arch by a =crus=, which is embedded in the ischio-cavernosus muscle. Below the ischial arch the crura fuse completely to form the laterally compressed body of the corpus cavernosum; this presents ventrally the =urethral groove= (Sulcus urethralis), which contains the urethra and corpus spongiosum. Anteriorly, the corpus cavernosum divides into three processes, a long central one, which is capped by the glans penis, and two short blunt lateral ones. The corpus cavernosum is inclosed by the =tunica albuginea=, a thick capsule of fibrous tissue which contains many elastic fibers and some unstriped muscle. Externally the fibers are chiefly longitudinal; internally they are mainly circular and are looser in arrangement. Numerous =trabeculæ= pass inward from the tunic and form a sponge-like framework in the interior of the corpus cavernosum, which is thus divided into numerous spaces (Cavernæ). These spaces may be regarded as greatly enlarged capillaries; they contain blood, are lined with flat endothelial cells, and are directly continuous with the veins of the penis. Their walls are composed very largely of unstriped muscle. Erection is produced by distention of these spaces with blood; at other times the spaces are mere slits.

In man there are two distinct corpora cavernosa, separated by a complete septum penis, except in the middle part of the organ, where the septum is composed of vertical trabeculæ, between which are slit-like intervals; through the latter the blood-spaces of the two corpora cavernosa communicate. In the horse no distinct septum exists except near the root, but in the proximal and distal parts of the corpus cavernosum there are vertical trabeculæ.

The =corpus spongiosum= or =corpus cavernosum urethræ= forms a tube around the urethra, and is much enlarged at its free end to form the bulk of the glans penis; throughout the remainder of its course it is practically uniform in size, and does not form any very distinct bulb (Bulbus urethræ) at its origin, as in man and many animals. The glans has been described in part; it should be noted that the corpus spongiosum extends backward dorsally a distance of about four inches (ca. 10 cm.) on the middle process of the corpus cavernosum penis, forming the processus dorsalis glandis. The urethral process is covered by a thin layer of corpus spongiosum.

In structure the corpus spongiosum has a general resemblance to the corpus cavernosum, but the trabeculæ are finer and more elastic. In the glans the trabeculæ are highly elastic, and the spaces are large and very extensible; the latter are continuous at the extremity of the processus dorsalis with large veins on the dorsum penis. There is a partial septum glandis. The skin covering the glans is thin, destitute of glands, and richly supplied with nerves and special nerve-endings.

=Vessels and Nerves.=—The penis is supplied with blood by three =arteries=, viz., the =internal pudic=, =obturator=, and =external pudic=. The =veins= form a rich plexus on the dorsum and sides of the penis, which is drained by the =external pudic= and =obturator veins=; from the root the blood is carried by the =internal pudic veins=. The =lymphatics= run with the veins and go to the inguinal and lumbar glands. The =nerves= are derived chiefly from the =pudic nerves= and the =pelvic plexus= of the sympathetic.

MUSCLES OF THE PENIS (Figs. 183, 451, 452, 456)

1. The =ischio-cavernosus= (or erector penis) is a short but strong paired muscle, which arises from the tuber ischii and the adjacent part of the sacro-sciatic ligament, and is inserted on the crus and adjacent part of the body of the penis. It is somewhat fusiform, incloses the crus as in a sheath, and is situated in a deep depression in the semimembranosus muscle. It pulls the penis against the pelvis, and assists in producing and maintaining erection by compressing the dorsal veins of the penis. Its blood-supply is derived from the obturator artery, and the nerve-supply from the pudic nerve.

2. The =retractor penis= is an unstriped muscle which arises on the ventral surface of the first two or three coccygeal vertebræ. It divides into two flat bands, half an inch or more (ca. 1.5 cm.) in width, which pass downward over the sides of the rectum to meet below the anus. Here there is a decussation of fibers, thus forming a sort of suspensory apparatus for the posterior part of the rectum and the anus. From the decussation the muscle passes for a short distance between superficial and deep layers of the bulbo-cavernosus, and then along the ventral surface of the penis, to which it is loosely attached. Near the glans penis it splits up into bundles which pass through the bulbo-cavernosus and are attached to the tunica albuginea. Below the anus the muscle is attached to the sphincter ani externus. On the penis the two muscles are intimately united to each other. Their action is to withdraw the penis into the sheath after erection or protrusion. The proximal part would draw the anus upward and forward and support it during defecation.

THE PREPUCE

The =prepuce= (Præputium), popularly called the “sheath,” is a double invagination of the skin which contains and covers the free or prescrotal portion of the penis when not erect. It consists of two parts, external and internal. The external part or sheath extends from the scrotum forward close to the umbilicus, where the external layer is reflected ventrally and laterally, forming the thick margin of the =preputial orifice= (Ostium præputiale); dorsally it is directly continuous with the integument of the abdominal wall. It is marked by a median =raphé præputii=, a continuation of the scrotal raphé. At the lower margin of the preputial orifice there are often in the stallion two papillæ, which are rudimentary teats or nipples. The internal layer =passes= backward from the preputial orifice a distance of about six to eight inches (ca. 15 to 20 cm.), lining the cavity of the external prepuce, and is then reflected forward until it approaches the orifice, where it is again reflected backward. It thus forms within the cavity of the sheath a secondary tubular invagination, the prepuce proper, in which the anterior part of the penis lies. This tubular cavity is closed behind by the reflection of the internal layer on to the penis to form the penile layer of the prepuce. Its orifice is surrounded by a thick margin, the =preputial ring= (Annulus præputialis), which is connected ventrally with the outer part of the sheath by the =preputial frenum= (Frenulum præputii).

The arrangement differs from that found in man in the fact that the inner part of the prepuce as described above is equivalent to the entire human prepuce. This part, the prepuce proper, is well seen on sagittal sections, and can be demonstrated by pulling the penis, inclosed in this prepuce, out of the cavity of the sheath; the arrangement of the free part of the penis and prepuce is then like that in man. (In paraphimosis the penis is strangulated by the preputial ring.)

=Structure.=—The =external skin= of the prepuce resembles that of the scrotum. The =inner layers= of skin are hairless, variable in color, and often irregularly pigmented; they form irregular folds, and are supplied with numerous large sebaceous glands and coil glands, which reach their greatest size at the preputial ring. Beyond this the glands are absent, and the skin resembles a non-glandular mucous membrane. The secretion of the =preputial glands= (Glandulæ præputiales), together with desquamated epithelial cells, forms the fatty =smegma præputii=, which has a strong unpleasant odor, and often accumulates in considerable amount. Beneath the skin is a large amount of loose connective tissue, except over the glans penis, where the skin is closely attached to the tunic of the corpus spongiosum. The external part of the prepuce is strengthened by a layer of elastic tissue, derived from the abdominal tunic, and termed the suspensory ligament.

=Vessels and Nerves.=—The =arteries= are branches of the =external pudic artery=, and the =veins= go chiefly to the =external pudic vein=. The =lymphatics= go to the superficial inguinal and lumbar lymph glands. The =nerves= are derived from the =pudic=, =ilio-hypogastric=, and =ilio-inguinal= nerves.

THE MALE URETHRA

The =urethra= of the male (Urethra masculina) is the long mucous tube which extends from the bladder to the glans penis. It passes backward on the floor of the pelvis, turns around the ischial arch, forming an acute angle, and passes forward inclosed in the corpus spongiosum. It may, therefore, be divided into two segments, pelvic and extrapelvic.

The =pelvic part= (Pars pelvina) is four or five inches (ca. 10 to 12 cm.) long. At its origin it is not distinguishable from the neck of the bladder in size or structure; in fact no line of demarcation exists between the two. Behind the prostate the tube dilates to a potential width of two inches or more (ca. 5 to 6 cm.). Near the ischial arch, between the bulbo-urethral (Cowper’s) glands, it contracts again, forming the =isthmus urethræ=. It is related dorsally to the rectum and the prostate, ventrally to the internal obturator muscles, and laterally to the bulbo-urethral glands. It is inclosed, except at its origin, by the urethral (Wilson’s) muscle. Beneath this is a rich venous plexus, forming a sort of erectile tissue.[146]

The =extrapelvic part= (Pars cavernosa) passes between the two crura of the penis and runs along the groove on the ventral surface of the corpus cavernosum penis, inclosed by the corpus spongiosum and the bulbo-cavernosus muscle. It passes through the glans penis and projects forward about an inch in the fossa glandis as a free tube, the =processus urethræ=; this part is covered by a delicate integument, under which there is a thin layer of erectile tissue.

The lumen of the tube is largely obliterated in the inactive condition of the parts. When moderately distended its dimensions in a horse of medium size are as follows: At its origin the diameter is about half an inch (1 to 1.5 cm.). The pelvic dilatation at its widest part measures one and a half to two inches (ca. 3.5 to 5 cm.) transversely, and about an inch (2 to 3 cm.) vertically; it is elliptical in cross-section when fully distended. The isthmus at the ischial arch is a little smaller than the initial part. Beyond this the lumen is about one-half to three-fourths of an inch (ca. 1.5 cm.) in diameter, and is fairly uniform to the glans penis. Here there is a slight fusiform dilatation (Fossa navicularis), beyond which the tube contracts.

The opening from the bladder into the urethra is termed the =internal urethral orifice= (Orificium urethræ internum); it is closed except during urination. The terminal opening is the =external urethral orifice= (Orificium urethræ externum) or meatus urinarius. The two orifices of the =ejaculatory ducts= are situated dorsally on either side of the colliculus seminalis, about two inches (ca. 5 cm.) behind the internal urethral orifice; they are slit-like and close together. The small orifice of the uterus masculinus is placed centrally on the colliculus; it is inconstant. The orifices of the =prostatic ducts= are on two groups of small papillæ, placed lateral to the openings of the ejaculatory ducts. The =ducts= of the =bulbo-urethral= (Cowper’s) =glands= open on two lateral series of small papillæ, about an inch (ca. 2.5 cm.) further back and close to the median line. The small orifices of the =urethral glands= are situated laterally in the wide pelvic portion.

=Structure.=—The =mucous membrane= contains an unusually large amount of fine elastic fibers, and in its pelvic part there are alveolar =glands= (Glandulæ urethrales). The epithelium is at first like that of the bladder, then becomes cylindrical, and in the terminal part is stratified squamous. In the dorsal wall the membrane forms a median ridge, the =urethral crest= (Crista urethralis); this terminates about two inches from the internal urethral orifice in a rounded elevation, the =colliculus seminalis=, on the sides of which the ejaculatory ducts open.

At the origin of the urethra there is a layer of circular =unstriped muscle-fibers= outside of the mucous coat. Beyond this the latter is inclosed by a layer of =erectile tissue=, composed of plexuses of veins supported by trabeculæ of elastic and smooth muscular tissue. This is continued by the corpus spongiosum. Outside of the erectile tissue there is a continuation of the intrinsic unstriped muscular coat, consisting of external and internal longitudinal strata, with a layer of circular fibers between them.

Except at its origin and termination the urethra is provided with a continuous layer of striped muscle, placed outside of the erectile tissue. This is described as consisting of two parts or muscles.

(1) The =urethral= (or Wilson’s) =muscle= (M. urethralis) consists of dorsal and ventral layers of transverse fibers on the wide pelvic part of the urethra, and covers the bulbo-urethral (Cowper’s) glands. It is a compressor of the pelvic part of the urethra and the bulbo-urethral glands. By its forcible contraction it plays an important rôle in the ejaculation of the seminal fluid, and also in evacuating the last of the urine in micturition.[147]

(2) The =bulbo-cavernosus muscle= (or accelerator urinæ) is the continuation of the urethral muscle on the extrapelvic part of the urethra; it extends from the ischial arch to the glans penis. At the root of the penis it is the thickest, and forms a complete layer of circular fibers which inclose the corpus spongiosum and the urethra. Beyond this it diminishes very gradually in thickness, and consists of fibers which arise on a median ventral raphé and curve around the corpus spongiosum to end on the tunica albuginea of the corpus cavernosum. Its action is to empty the extrapelvic part of the urethra.

The =ischio-urethral muscles= (Fig. 452) are small bands which arise on the ischial arch and crura of the penis and pass forward to become lost on the ventral layer of the urethral muscle. They may assist in erection of the penis by exerting pressure on the dorsal veins.

MALE GENITAL ORGANS OF THE OX

The =scrotum= is situated somewhat further forward than in the horse. It is long, pendulous, and has a well-marked neck. The skin here is usually flesh-colored, but in some breeds is more or less pigmented; it is sparsely covered with short hairs. Just in front of it are four (sometimes only two) rudimentary teats.

The =testicles= are relatively larger than those of the horse, and have an elongated, oval outline. The long axis is vertical. The inner surface is somewhat flattened.

A testicle of an adult bull measures on the average about four or five inches (10 to 12 cm.) in length, exclusive of the epididymis; with the latter the length is about six inches (15 cm.). The width is about two and a half to three inches (ca. 6 to 8 cm.), and the antero-posterior diameter about the same. The weight is about nine or ten ounces (ca. 250 to 300 grams).

The tunica albuginea is thin. The parenchyma is yellowish in color. The mediastinum testis descends from the anterior part of the upper extremity deeply into the gland as an axial strand about 5 mm. in thickness. The vasa efferentia leave at the upper end of the mediastinum.

The =epididymis= is very closely attached to the testicle. The head is long; it curves over the upper extremity and nearly half-way down the anterior border of the testicle. The body is very narrow, and lies along the outer part of the posterior border of the testicle, to which it is attached by a narrow peritoneal fold. The tail is large and is closely attached to the lower extremity of the testicle.

The =spermatic cord= and the =tunica vaginalis= are much longer than in the horse; the extra-inguinal part of the cord is about eight to ten inches (20 to 25 cm.) in length. The =cremaster externus= is well developed, and almost completely incloses the tunic to the nook of the scrotum; it does not extend so far down as in the horse, but is inserted about at the level of the upper pole of the testicle. The =vaginal ring= is relatively very small, and is about four inches (ca. 10 cm.) from the linea alba. The cremaster internus is feeble.

The spermatic cords emerge through the outer angles of the external inguinal rings, and curve downward and inward to the neck of the scrotum. The external rings are about four inches (ca. 10 cm.) long, and are two and a half to three inches (6 to 8 cm.) apart. In animals in good condition there is a large mass of fat above the neck of the scrotum, between the spermatic cords. The mesorchium extends to the bottom of the tunica vaginalis forming a narrow fold (ca. 1 cm. wide) which attaches the epididymis to the posterior part of the tunic.

The inguinal canal presents several special features in the bull. The internal ring is very long. Its anterior border, formed by the edge of the internal oblique, is decidedly concave and is tendinous in its inner part. The long axis of the external ring is directed outward, forward, and downward. The spermatic cord lies in its outer part, the external pudic vessels in its middle. A muscular band about an inch wide, detached from the internal oblique muscle, crosses the outer side of the vaginal ring.

The =vas deferens= is small in caliber. The urogenital fold is narrow, so that the vasa are closer together in it than in the horse. On reaching the posterior part of the bladder they are in apposition for a distance of about four inches (10 cm.), flanked and overlapped by the vesiculæ seminales. They form ampullæ about four or five inches (10 to 12 cm.) long and half an inch (1.2 to 1.5 cm.) wide, the mucous lining of which is plicated. They then pass under the body of the prostate and unite with the ducts of the vesiculæ seminales to form the ejaculatory ducts. The latter have slit-like openings on either side of the colliculus seminalis.

The =vesiculæ seminales= are not bladder-like sacs, as in the horse, but are compact glandular organs with a lobulated surface. In the adult they measure about four or five inches (ca. 10 to 12 cm.) in length, two inches (5 cm.) in width in their largest part, and an inch or more (ca. 3 cm.) in thickness. The dorsal surface of each faces upward and inward and is partially covered with peritoneum. The ventral surface faces in the opposite direction and is non-peritoneal. Each may be regarded as consisting of a very thick-walled, sacculated tube, bent on itself in a tortuous manner. This tube, if straightened out, would be about ten inches (25 cm.) in length. They are commonly unsymmetrical in size and shape. Short branches are often given off from the chief tube. The excretory duct or neck joins the outer side of the vas deferens at an acute angle to form the ejaculatory duct.

The structure of the vesicula is masked by a thick capsule of fibrous tissue and unstriped muscle, which maintains it in its bent condition and also sends trabeculæ between the alveoli. There is a central canal into which the secretion formed in the alveoli passes. The cavities (central canal and alveoli) are lined with columnar epithelium. On account of this structure the term glandulæ vesiculares is preferred by some anatomists.

The =prostate= is pale yellow in color, and consists of two parts. The =body= (Corpus prostatæ) is a mass which stretches across the dorsal surface of the neck of the bladder and the origin of the urethra. It measures about an inch and a half (ca. 3.5 to 4 cm.) transversely, and about half an inch (ca. 1 to 1.5 cm.) in width and thickness. The =pars disseminata= surrounds the pelvic part of the urethra; dorsally it forms a layer about half an inch (ca. 10 to 12 mm.) thick, but ventrally it is quite thin. It is concealed by the urethral (Wilson’s) muscle; hence it often escapes notice, but is very evident on cross-section (fig. 402). The =prostatic ducts= open into the urethra in rows between two folds of the mucous membrane which proceed backward from the colliculus seminalis.

The =uterus masculinus= appears to be absent frequently. Ellenberger states that it opens between the ejaculatory ducts, while Martin says that it usually has two orifices at the colliculus, but that it often unites with the vasa deferentia.

The =bulbo-urethral= (Cowper’s) =glands= are somewhat smaller than in the stallion. They are liable to escape notice, since they are covered by a thick layer of dense fibrous tissue and also partially by the bulbo-cavernosus muscle. Each has a single duct which opens into the urethra under cover of a fold of the mucous membrane.

The =penis= is cylindrical, and is longer and of very much smaller diameter than in the horse. Just behind the scrotum it forms an =S=-shaped curve, the =sigmoid flexure=; thus about one foot is folded up when the penis is fully retracted. The flexure is effaced during erection. The =glans penis= is about three inches (ca. 8 cm.) in length. It is flattened dorso-ventrally, and its extremity is pointed and twisted. The =external urethral orifice= is situated at the end of a groove formed by this twist; it is only large enough to admit a probe of medium size. Even in the non-erect state the penis is remarkably dense and firm. The =tunica albuginea= is very thick, and incloses the urethra. In the first part, as far as the first curve, there is a thick median =septum penis=. Beyond this there is a central axial rod of dense fibrous tissue from which numerous strong trabeculæ radiate. The erectile tissue is small in amount, so that the organ undergoes very little enlargement in erection.

The length of the penis in the adult is about three feet (ca. 90 cm.). The crura are flattened laterally. The suspensory ligaments are attached to the ventral ridge on the symphysis. The body is somewhat flattened dorso-ventrally beyond the first curve. The extremity of the glans is unsymmetrical, the urethral orifice being situated ventro-laterally. The glans contains little erectile tissue, and hence undergoes little enlargement in erection.

The =bulbo-cavernosus muscle= presents several remarkable features. It is for the greater part an inch or more (ca. 3 cm.) in thickness, but its length is only six to eight inches (15 to 20 cm.). It is covered by a strong aponeurosis, and is divided by a median raphé into two lateral halves except at its origin. It diminishes in size from behind forward, and its anterior extremity is pointed.

The =ischio-cavernosus muscle= resembles in general that of the horse, but is flattened laterally. It is covered by a close aponeurosis.

The =retractor penis= muscle resembles that of the horse in origin. Its two parts are about an inch (ca. 2 to 5 cm.) apart on the root of the penis, where they lie in a groove on either side of the bulbo-cavernosus. They then come close together and pass on either side of the ventral curve of the sigmoid flexure. Further forward they lie along the ventral aspect of the penis, and end about five or six inches (ca. 12 to 15 cm.) behind the glans.

The =prepuce= is very long and narrow. Its orifice is about two inches (ca. 5 cm.) behind the umbilicus; it is only large enough to admit a finger readily, and is surrounded by long hairs. The cavity of the sheath is about fifteen inches (35 to 40 cm.) long, and a little over an inch (ca. 3 cm.) in diameter. The lining membrane forms longitudinal folds; it is covered with squamous stratified epithelium, and has coiled tubular glands. The penile layer is glandless, and is reddish in color. It presents lymph nodules in its posterior part.

There are two pairs of =preputial muscles= which are derivatives of the panniculus carnosus. The anterior pair, the =protractors= of the prepuce (Mm. præputiales craniales), are two flat bands, two inches or more (5 to 6 cm.) in width, which arise close together in the xiphoid region, about eight inches (20 cm.) in front of the preputial orifice. Traced backward they diverge, leaving the umbilicus and an area about one and a half inches wide free, and then unite behind the preputial orifice. They draw the prepuce forward. The posterior pair, the =retractors= of the prepuce (Mm. præputiales caudales), arise in the inguinal region and converge on the anterior part of the prepuce. They draw the prepuce backward.

These muscles are subject to a good deal of variation. The retractor may be absent. Many fibers come in from the panniculus on either side, dip under the protractor, and are inserted into the skin just behind the preputial opening. The homologue of the protractor is present in the cow.

The =urethra= in its pelvic part is about five inches (12 cm.) long, and is of relatively small and uniform caliber. The =urethral= (Wilson’s) =muscle= incloses the tube ventrally and laterally; it is very thick, crescentic in cross-section, and is covered by an aponeurosis. Dorsally, the aponeurosis is thick and the muscle absent. Inside of these the pars disseminata of the prostate surrounds the urethra as far back as the bulbo-urethral glands. The =colliculus seminalis= is short, but prominent; from it the urethral crest extends forward to the trigonum vesicæ. The extrapelvic part has an enlargement, the =bulb=, at its origin; it then gradually diminishes in diameter, and is relatively very small at its termination. Other features have been mentioned in preceding paragraphs.

In the ram the urethra lies in a groove on the ventral surface of the corpus cavernosum. Its terminal part projects commonly 3 to 4 cm. beyond the glans penis, forming a twisted processus urethræ.

MALE GENITAL ORGANS OF THE PIG

The =scrotum= is situated a short distance from the anus, and is not so sharply defined from the surrounding parts as in the other animals.

The =testicles= are very large and are soft in texture. Their long axis is almost vertical, the free border being posterior and the tail of the epididymis dorsal. The mediastinum testis is well developed. The epididymis is closely attached to the testicle and the tunica vaginalis; the tail is specially large.

The =spermatic cord= is necessarily very long (20 to 25 cm. in a boar of medium size). The =vas deferens= in its testicular part is flexuous, and is closely attached to the tunica vaginalis; it forms no ampullæ. The =cremaster externus= is well developed, and extends to about the middle of the scrotal part of the tunic.

The =vesiculæ seminales= are exceedingly large, and extend into the abdominal cavity. They are three-sided pyramidal masses, are in apposition with each other medially, and cover the posterior part of the bladder and the ureters, the vasa deferentia, the body of the prostate, and the anterior part of the urethra. They are pale pink in color, distinctly lobulated and glandular in structure, and contain large alveoli, but no central cavity. There are two excretory ducts which open into the urethra close to the vasa deferentia; the two ducts frequently unite.

In the adult boar the vesiculæ are about five to six inches (ca. 12 to 15 cm.) long, two to three inches (ca. 5 to 8 cm.) wide, and one and a half to two inches (ca. 4 to 5 cm.) thick; they weigh about 6 to 8 ounces each.

The =prostate= consists of two parts, as in the ox. The =body= is about an inch (2.5 cm.) wide, and embraces the neck of the bladder and the urethra at their junction dorsally and laterally. It is concealed by the vesiculæ seminales. The =pars disseminata= forms a layer which surrounds the pelvic part of the urethra, and is covered by the urethral muscle.

The uterus masculinus is small.

The =bulbo-urethral= (Cowper’s) =glands= are very large and dense. They are somewhat cylindrical, and lie on either side of and upon the posterior two-thirds of the pelvic urethra. In a large boar they are about five inches (ca. 12 cm.) in length and an inch or more (ca. 2.5 to 3 cm.) in diameter. They are partially covered with a layer of striated muscle (M. ischio-glandularis), and have a lobulated surface. Each gland has a large excretory duct which leaves at the posterior extremity, perforates the dorsal wall of the urethra at the ischial arch, and opens in a cul-de-sac covered by a fold of mucous membrane.

It is to be noted that these accessory glands are very small in animals which have been castrated early.

The =penis= resembles in general that of the ox. The sigmoid flexure is, however, prescrotal. The anterior portion has no glans, and is spirally twisted, especially in erection. The =urethral orifice= is slit-like and is situated ventro-laterally, close to the pointed extremity. The penis in the adult boar measures about 18 to 20 inches (ca. 45 to 50 cm.) in length. Its muscles resemble those of the bull; the bulbo-cavernosus is very strong but short.

The =prepuce= has a narrow orifice, around which there are a number of stiff hairs. The cavity is partially divided by a circular fold into a posterior narrow portion and a much wider anterior portion. The lining membrane of the posterior part is papillated, and is in close contact with the penis. In the dorsal wall of the wide part there is a circular opening which leads into a cul-de-sac, the =preputial diverticulum=. This pouch is ovoid in form (when distended), and varies greatly in size in different subjects. It extends for the most part backward over the narrow part of the prepuce. Its cavity is partially divided by a narrow median septum. It contains usually decomposing urine and macerated epithelium, which have a characteristic and very unpleasant odor. Concrements have been found in it.

Oehmke found that a cast of the pouch in a Yorkshire boar weighing about 500 pounds measured 9 cm. in length, 12½ cm. in breadth, and 6 cm. in height. The opening into the prepuce will admit two fingers in the adult, but is ordinarily closed by folds of the lining membrane. The sac is much smaller in animals which were castrated young, and the opening is vertical and further back; in them it is often empty or contains only a little clear urine. The pouch is covered by a layer of striped muscle which is mainly derived from the homologue of the protractor of the prepuce of ruminants.

The =urethra= has a very long pelvic portion (ca. 15 to 18 cm. long in the adult); it is inclosed (with the pars disseminata of the prostate) by a thick =urethral muscle=. There is a distinct =bulb= at the root of the penis. The penile part is of small caliber, and is surrounded by erectile tissue which, however, does not extend to the extremity of the penis.

MALE GENITAL ORGANS OF THE DOG

The =scrotum= is situated about half-way between the inguinal region and the anus. The skin covering it is pigmented and is covered sparsely with fine hairs. The raphé is not very distinct.

The =testicles= are relatively small, and have a round-oval form. The long axis is oblique, and is directed upward and backward. The mediastinum testis is central and is well developed. The epididymis is large, and is closely attached along the outer part of the dorsal border of the testicle.

The =spermatic cord= and the =tunica vaginalis= are long; they cross the side of the penis very obliquely. The upper end of the tunic is sometimes closed, so that there is then no vaginal ring. The vasa deferentia have very narrow ampullæ.

The =vesiculæ seminales= are absent; hence there are no ejaculatory ducts.

The =prostate= is relatively large; it is yellowish in color, dense in structure, and lies at the anterior border of the pubis. It is globular, and surrounds the neck of the bladder and the urethra at their junction. A faint median furrow indicates a division into two lateral lobes. The capsule and stroma contain a large amount of unstriped muscle. The ducts are numerous. Lobules of prostatic tissue (pars disseminata) are also found on the urethra further back. The gland is subject to much variation in size, and is often enlarged, especially in old subjects.

The =uterus masculinus= is a very small saccule (Utriculus prostaticus), often difficult to find.

The =bulbo-urethral= (Cowper’s) =glands= are absent.

The =penis= presents several special features. In its posterior part there are two distinct corpora cavernosa, separated by a median septum penis. In its anterior part there is a bone, the =os penis=, which in large dogs reaches a length of four inches (ca. 10 cm.) or more. It is regarded as a continuation forward of the corpus cavernosum. Ventrally it is grooved for the urethra; dorsally it is convex, and anteriorly it becomes smaller and has a curved fibrous prolongation. The =glans penis= is very long, extending over the entire length of the os penis; posteriorly it forms a rounded enlargement, the =bulbus glandis=; anteriorly it is cylindrical, smaller, and pointed at the extremity. Just in front of the bulbus glandis there is a thick circular layer of glands, forming the =annulus glandularis= (Schmaltz). The two dorsal veins arise from the bulbus glandis, pass backward on the dorsum penis, and unite at the ischial arch. A small muscle (M. compressor venæ dorsalis penis) arises from the tuber ischii on either side; the two converge on the dorsum penis near the bulbus glandis. They compress the dorsal veins, and may also tend to elevate the penis and thus assist in copulation. The other muscles offer no features worthy of special description.

The =prepuce= forms a complete sheath around the anterior part of the penis. The outer layer is ordinary integument. The inner layer has the character of a glandular mucous membrane; it is thin and reddish in color. The penile layer is glandless. In the fundus of the præputial cavity there are numerous lymph follicles. The protractor muscles arise in the xiphoid region and decussate posteriorly around the extremity of the penis.

The pelvic part of the =urethra= is relatively long. Its first part is inclosed in the prostate.[148] At the ischial arch the urethra has a well-developed bilobed =bulb=, formed by an enlargement here of the corpus spongiosum. It is covered by the strong but short bulbo-cavernosus muscle. The other erectile bodies have been described.

The =urethral= (Wilson’s) =muscle= is very strong; it encircles the urethra and has a median raphé dorsally.

THE FEMALE GENITAL ORGANS

The =female genital organs= (Organa genitalia feminina) are: (1) The two =ovaries=, the essential reproductive glands, in which the ova are produced; (2) the =uterine= or =Fallopian tubes=, which convey the ova to the uterus; (3) the =uterus=, in which the ovum develops; (4) the =vagina=, a dilatable passage through which the fœtus is expelled from the uterus; (5) the =vulva=, the terminal segment of the genital tract, which serves also for the expulsion of the urine; (6) the =mammary glands=, which are glands of the skin, but are so closely associated functionally with the generative organs proper as to be described with them.

GENITAL ORGANS OF THE MARE

THE OVARIES

The =ovaries= (Ovaria) of the mare are bean-shaped, and are much smaller than the testicles. Their size varies much in different subjects, and they are normally larger in young than in old animals; one ovary is often larger than the other. They are about three inches (ca. 7 to 8 cm.) long and an inch to an inch and a half (ca. 3 to 4 cm.) thick. The weight is about two and a half to three ounces (ca. 70 to 80 grams).

Each presents for description two surfaces, two borders, and two extremities or poles. The =surfaces= are termed internal and external (Facies medialis, lateralis); they are both smooth and rounded.[149] The =attached= or =mesovarial border= (Margo mesovaricus) is convex. It is inclosed in a part of the broad ligament termed the mesovarium; the vessels and nerves reach the gland at this border. The =free border= (Margo liber) is marked by a notch which leads into a depression, the =ovulation fossa=. The =tubal= or =anterior extremity= (Extremitas tubaria) is rounded, and is related to the fimbriated end of the uterine or Fallopian tube. The =uterine= or =posterior extremity= (Extremitas uterina) is also round, and is connected with the horn of the uterus by the ovarian ligament.

The ovaries are situated in the sublumbar region of the abdominal cavity, about a handbreadth behind the corresponding kidney. They are in contact with the lumbar wall of the abdomen, and are commonly about two or three inches (ca. 5 to 7.5 cm.) from the median plane. The distance from the ovaries to the vulvar orifice is about twenty inches (ca. 50 cm.) in a mare of medium size. The attached (mesovarial) border is commonly directed toward the median plane, the free border laterally and somewhat ventrally.

The position of the ovaries is not constant, but is subject to less variation than might be expected from their mode of attachment. The normal variations in the non-pregnant state are chiefly in a transverse direction, and do not usually exceed two or three inches (ca. 5 to 7 cm.). It is not at all rare to find the ovary deflected medially, so that the external surface faces ventrally. In the longitudinal direction little variation appears to occur. The right ovary is usually about four or five inches (ca. 10 to 12 cm.), and the left one about three or four inches (ca. 8 to 10 cm.), behind the corresponding kidney. Thus they lie about in a transverse plane tangent to the external angles of the ilia, i. e., through the fifth lumbar vertebra. They may be about two inches (ca. 5 cm.) from the extremity of the corresponding cornu of the uterus or in contact with it. The right ovary commonly lies a little (ca. 2.5 cm.) further forward than the left one. Except during pregnancy, they are normally in contact with the lumbar abdominal wall, and do not hang down among the adjacent viscera.

The ovary is attached to the sublumbar region by the anterior part of the broad ligament; this part of the ligament, the =mesovarium=, is about three or four inches (ca. 8 to 10 cm.) wide, measured from the ovary directly to the parietal attachment. The uterine or posterior extremity of the ovary, is connected with the extremity of the cornu of the uterus by the =ovarian ligament= (Ligamentum ovarii proprium); this is a band of unstriped muscle inclosed between the layers of the broad ligament.

=Structure.=—The greater part of the surface of the ovary has a covering of peritoneum. The peritoneal investment is absent at the attached border where the vessels and nerves enter; this area is termed the =hilus=. The ovulation fossa is covered by a layer of columnar cells, a remnant of the primitive =germinal epithelium=. The =stroma= of the ovary (Stroma ovarii) is a network of connective tissue and unstriped muscular fibers. In the deeper or medullary portion of the stroma (Zona vasculosa) the meshes are largely occupied by the numerous blood-vessels. In the peripheral or cortical portion (Zona parenchymatosa) are numerous =ova= (Ovula) in various stages of development. The immature ova are surrounded by follicle cells; those more advanced in development are inclosed also by a condensation of the stroma termed the =theca folliculi=; within the theca is a quantity of fluid, the =liquor folliculi=. These constitute the =Graafian follicles= (Folliculi oöphori), which enlarge as they mature, becoming visible to the naked eye as vesicles with a diameter of a centimeter or more. When fully developed, the follicles are superficially situated, and often project slightly from the surface of the ovary. At intervals follicles rupture and their contents escape. This process, which sets free the ovum, is termed =ovulation=; it takes place in the mare only at the ovulation fossa, and occurs during the periods of œstrum.

After rupture of a follicle its cavity is partly occupied by a blood-clot, constituting what may be termed a =corpus rubrum=. By proliferation and enlargement and fatty changes, the follicle cells are transformed into lutein cells, forming a yellow mass known as a =corpus luteum=. If impregnation takes place, the accompanying increase in vascularity of the organs may cause the corpus luteum to reach a large size; if impregnation does not occur, it is much smaller, and is sooner replaced by scar tissue, the =corpus albicans=.

In the young foal the ovaries are large and oval in form. The free border is convex and is covered by germinal epithelium, which extends over part of the surfaces also. This area is distinguishable by its dull gray appearance from the peritoneal surface, which has the usual smooth glistening character. As growth proceeds the ovary gradually becomes bent until it assumes its definitive curved shape. The germinal epithelium is then limited to the ovulation fossa. The ovary migrates somewhat during development from its primitive position, which is the same as that of the testicle.

In old animals the ovaries commonly consist largely of fibrous tissue, in which there are often cysts of various sizes. The ova, present in enormous number at birth, have then been extruded, or destroyed by phagocytic action or degeneration.

=Vessels and Nerves.=—The =arteries= of the ovary are derived from the =ovarian artery=. The artery is relatively large and is flexuous; it reaches the attached border of the ovary by passing between the layers of the mesovarium. The veins are large and numerous. They form a plexus somewhat like that of the spermatic cord. The =lymph vessels= pass to the lumbar glands. The =nerves= are derived from the sympathetic system through the renal and aortic plexuses. They accompany the arterial branches.

THE UTERINE OR FALLOPIAN TUBES

The =uterine= or =Fallopian tubes= (Tubæ uterinæ) act as excretory ducts of the ovaries, since they convey the ova from the reproductive glands to the uterus. They are not, however, in direct continuity with the glands, but rather partly in contiguity with, and partly attached to, them.[150] They are two flexuous tubes, about ten to twelve inches (ca. 25 to 30 cm.) long, which extend from the extremities of the uterine cornua to the ovaries. Each is inclosed in a peritoneal fold, derived from the external layer of the broad ligament, and termed the =mesosalpinx=. This largely covers the outer aspect of the ovary, and forms with it and the broad ligament a pouch called the =bursa ovarica=. The =uterine extremity= of the tube communicates with the cavity of the cornu by a minute orifice, the =ostium uterinum=. The =ovarian= or =fimbriated extremity= is expanded into a funnel-shaped =infundibulum=. The margin of the latter is slit into irregular processes or =fimbriæ=, some of which, the =fimbriæ ovaricæ=, are attached in the ovulation fossa. In the center of the fimbriated extremity is a small opening, the =ostium abdominale=, by which the tube communicates with the peritoneal cavity. The tube appears normally to be applied to the ovary, so that the extruded ova pass into it and are conveyed to the uterus.[151]

Pedunculated cysts, the hydatids of =Morgagni=, are often found on one or more of the fimbriæ. In the mesosalpinx are blind flexuous tubules, which constitute the =paroöphoron=, a remnant of the Wolffian body. They are most evident in the young adult, and tend to disappear with increasing age. Not uncommonly they give rise to cysts.

=Structure.=—The tube is covered externally by a =serous coat= formed by the mesosalpinx. The serous membrane is continued on the fimbriæ, and meets the mucous lining on them. The =fibrous coat= or adventitia is continuous with the fibrous lamina of the broad ligament. The =muscular coat= consists chiefly of circular fibers, although longitudinal fibers occur externally. The =mucous coat= is thin and is much plicated. The folds (Plicæ tubariæ) are chiefly longitudinal, but in the wide part of the tube (Ampullæ tubæ) they are very complex, so that on cross-sections the spaces between the folds may be mistaken for branched tubular glands. The folds are continued on to the fimbriæ. The =epithelium= is a single layer of columnar ciliated cells, the cilia producing a current directed toward the uterus. At the fimbriated extremity this epithelium passes gradually into the squamous type of the serous coat.

=Vessels and Nerves.=—The =arteries= are derived from the =utero-ovarian artery=. The =veins= are satellites of the arteries. The =lymphatics= pass with the ovarian vessels to the lumbar glands. The nerves have a similar origin to those of the ovary.

THE UTERUS

The =uterus= is a hollow muscular organ, which receives the Fallopian tubes anteriorly and opens into the vagina posteriorly. It is situated chiefly in the abdominal cavity, but extends a short distance into the pelvic cavity. It is attached to the lumbar wall of the abdomen and the lateral walls of the pelvic cavity by two folds of peritoneum termed the broad ligaments. It consists of two cornua or horns, the body, and the neck.

The =cornua= or =horns= of the uterus (Cornua uteri) are situated entirely in the abdomen. They appear to vary considerably in position; commonly they are pressed up against the sublumbar muscles by the intestine (left portions of large colon, small colon, and small intestine). They are cylindrical, and are about ten inches (ca. 25 cm.) in length. The anterior extremity of each forms a blunt point which receives the Fallopian tube. Posteriorly they increase somewhat in caliber, converge, and unite with the body. The =dorsal= (or attached) =border= is slightly concave and is attached to the broad ligament. The =ventral border= is convex and free.

When a soft uterus is distended, the cornua are strongly curved, but this does not represent the natural form. When fixed in situ the horns are either slightly curved or nearly straight. The two horns are commonly unsymmetrical in length and diameter in mares which have borne young.

The =body= of the uterus (Corpus uteri) is situated partly in the abdomen, partly in the pelvic cavity. It is cylindrical, but considerably flattened dorso-ventrally, so that in cross-section it is elliptical. Its average length is about six inches (15 cm.), and its diameter, when moderately distended, about four inches (10 cm.). Its =dorsal surface= (Facies rectalis) is related to the small colon and rectum. Its =ventral surface= (Facies vesicalis) is in contact with the bladder, and has inconstant relations with various parts of the intestine. The term =fundus uteri= is applied to the anterior wide part from which the cornua diverge.

The position of the body of the uterus is variable, especially in regard to its anterior part. It is often pressed up against the rectum, and may be deflected to either side—most frequently to the left—by the pelvic flexure of the colon or coils of the small colon.

The =cervix= or =neck= of the uterus (Cervix uteri) is the constricted posterior part which joins the vagina. Part of it (Portio vaginalis uteri) projects into the cavity of the vagina; it is therefore not visible externally, but may be felt through the vaginal wall. It is about two inches (5 cm.) in length, and an inch and a half (3.5 to 4 cm.) in diameter.

=Attachments.=—The body and cornua are attached to the abdominal and pelvic walls by two extensive peritoneal folds, the =broad ligaments= (Ligamenta lata). These extend on either side from the sublumbar region and the lateral pelvic walls to the dorsal border of the cornua and the lateral margins of the body of the uterus. They contain the vessels and nerves of the uterus and ovaries, connective tissue, and unstriped muscular fibers. The ureters are situated along their parietal margins. The outer layer of each forms a fold, the =round ligament= of the uterus (Ligamentum teres uteri), which blends with the parietal peritoneum over the internal inguinal ring; its anterior extremity is situated above the extremity of the cornu and is rounded and free. It contains muscular tissue, vessels, and nerves, and is the homologue of the gubernaculum testis. The anterior part of the neck is continuous with the vagina, and thus has a more fixed position than the rest of the organ.

=The cavity= of the uterus (Cavum uteri) is largely obliterated in the non-pregnant state by the contraction of the wall and by folds of the mucous lining. At the extremity of each cornu it communicates by a minute opening with the Fallopian tube. The cavity of the neck is termed the =cervical canal= (Canalis cervicis); it is closed ordinarily by mucous folds and a plug of mucus. It opens into the vagina by the =os uteri externum=, and into the body by the =os uteri internum=.

=Structure.=—The wall of the uterus consists of three coats—an external serous, a middle muscular, and an internal mucous. The =serous coat= (Perimetrium) is, for the most part, closely adherent to the muscular coat. It is continuous with the broad ligaments. The =muscular coat= consists essentially of two layers, a thin external stratum of longitudinal fibers, and a thick layer of circular fibers. Between these is a very vascular layer (Stratum vasculare) of connective tissue with circular and oblique muscular fibers in it. The circular coat is very thick in the neck, where it forms a =sphincter= half an inch or more (ca. 1.5 cm.) in thickness. The =mucous membrane= rests directly on the muscular coat. It is thick, and is covered by columnar epithelium. It contains numerous long tubular =uterine glands= (Glandulæ uterinæ). It is brownish-red in color, except in the neck, where it is pale.

=Vessels and Nerves.=—The chief =arteries= are the =uterine= and the =uterine branch= of the =utero-ovarian=, which have a flexuous course in the broad ligaments; there is also a branch from the =internal pudic= artery. The =veins= form pampiniform plexuses and accompany the arteries. The =lymphatics= are numerous and go to the internal iliac and lumbar glands. The =nerves= are derived from the sympathetic through the uterine and pelvic plexuses.

The foregoing statements refer to the non-gravid uterus. In the pregnant state it undergoes important changes in size, position, and structure. The increase in size affects chiefly the gravid cornu (except in the case of twins) and the body. The cornu attains a length of about two and a half to three feet (ca. 80 to 90 cm.), and a corresponding diameter; in this process it extends much beyond the ovary, and the broad ligament. The gravid uterus is entirely abdominal in position, and extends along the ventral wall, chiefly to the left of the median plane. It weighs about nine pounds (ca. 4 kg.), according to Ellenberger. The broad ligaments increase greatly in size and contain more muscular tissue. The vessels are greatly enlarged and form new branches. The muscular coat, in spite of the increase in size and number of the fibers, is somewhat thinner, except in the neck. The mucous membrane is thicker and more vascular; the epithelium becomes non-ciliated and stratified.

THE VAGINA

The =vagina= is the passage which extends horizontally through the pelvic cavity from the neck of the uterus to the vulva. It is tubular, is about six to eight inches (ca. 15 to 20 cm.) in length, and, when slightly distended, about four or five inches (ca. 10 to 12 cm.) in diameter. Its dilatability appears to be limited only by the pelvic wall. There is no external line of demarcation between the vagina and the uterus or the vulva.

It is related dorsally to the rectum, ventrally to the bladder and urethra, and laterally to the pelvic wall. The recto-genital pouch of the peritoneum extends between the vagina and rectum for a distance of about two inches (5 cm.), and ventrally the vesico-genital pouch passes backward somewhat less between the vagina and bladder. Thus most of the vagina is retroperitoneal and is surrounded by a quantity of loose connective tissue, a venous plexus, and a variable amount of fat.

=Structure.=—With the exception of the short peritoneal part, as indicated above, the proper wall of the vagina is composed of muscular and mucous coats. The =muscular coat= is composed of a thin layer of longitudinal fibers, and a thicker layer of circular fibers; it contains also many elastic fibers. The =mucous coat= is highly elastic, and is covered with a stratified epithelium. It has no glands.

Under usual conditions the cavity is practically obliterated by apposition of the walls, so that the lumen is a transverse slit; this condition is pronounced when the ampulla of the rectum is full. The anterior end of the vagina is largely occupied by the intravaginal part of the neck of the uterus, so that the cavity is here reduced to an annular recess termed the =fornix vaginæ=. The posterior part is directly continuous with the vulva without any line of demarcation except the valvular fold, which covers the external urethral orifice; in very young subjects this fold is continued on either side, forming the =hymen=, which narrows the entrance to the vagina (Introitus vaginæ).[152]

=Vessels and Nerves.=—The blood-supply is derived from the branches of the =internal pudic arteries=. The =veins= form a plexus which is drained by the =internal pudic veins=. The =lymphatics= pass to the internal iliac lymph glands. The =nerves= are derived from the sympathetic through the pelvic plexus.

THE VULVA

The =vulva= or =urogenital sinus= is the terminal part of the genital tract.[153] It is continuous in front with the vagina, and opens externally at the vulvar cleft below the anus. There is no external line of demarcation between the vagina and vulva. The tube is about five inches (ca. 12 cm.) in length, measured from the external urethral orifice to the inferior commissure; dorsally it is considerably shorter. It is related dorsally to the rectum and anus, ventrally to the pelvic floor, and laterally to the sacro-sciatic ligament, the semimembranosus muscle, and the internal pudic artery. The external orifice or =vulvar cleft= (Rima vulvæ) has the form of a vertical slit, about four to six inches (ca. 10 to 15 cm.) high, and is margined by two prominent rounded folds, the labia. The =labia= of the vulva (Labia vulvæ) meet above at an acute angle, forming the =superior commissure= (Commissura dorsalis), which is about two inches (ca. 5 cm.) below the anus. Below they unite to form the thick rounded =inferior commissure= (Commissura ventralis), which lies about two inches (ca. 5 cm.) behind and below the ischial arch. When the labia are drawn apart, a rounded body, about an inch (ca. 2.5 cm.) wide, is seen in the inferior commissure; this is the =glans clitoridis=, the homologue of the glans penis. Overlying it is a thin fold, the =prepuce= of the clitoris (Præputium clitoridis). At the anterior extremity of the ventral wall of the vulva, i. e., about four or five inches (ca. 10 to 12 cm.) from the inferior commissure, is the =external urethral orifice= or meatus urinarius (Orificium urethræ externum). It readily admits the finger and is very dilatable. It is covered by a fold of mucous membrane, the free edge of which is directed backward.

=Structure.=—The =labia= are covered by thin, pigmented, smooth skin, which is richly supplied with sebaceous and sweat glands. This is continuous at a distance of about half an inch (ca. 1 to 1.5 cm.) from the free edge with a thin glandless mucous membrane. Between these is a layer of striped muscle, the =constrictor vulvæ=; this fuses above with the sphincter ani, and embraces the clitoris below, spreading out laterally at the inferior commissure. It constricts the vulvar orifice and elevates the clitoris. The =constrictor vestibuli= muscle embraces the vulva in front of the preceding; it is joined on either side by a band of unstriped muscle which arises from the first and second coccygeal vertebræ, and is homologous with the proximal part of the retractor penis. It constricts the vulva. Under this muscle in the lateral wall is a flattened, oval body, the =bulbus vestibuli= (Fig. 453); this is an erectile structure, homologous with the corpus spongiosum urethræ of the male. It is about two and a half to three inches (ca. 6 to 8 cm.) long, and an inch or more (ca. 3 cm.) wide. It consists of a venous network inclosed in a fibrous capsule, and is supplied with blood by a large branch of the internal pudic artery. The =mucous membrane= of the vulva is reddish in color, and forms longitudinal and transverse folds. It presents ventrally two linear series of small papillæ which converge toward the inferior commissure; these mark the orifices of the ducts of the =glandulæ vestibulares minores=. On either side of the dorsal wall is a group of eight to ten larger prominences on which the ducts of the =glandulæ vestibulares majores= (of Bartholin) open.

Quite exceptionally there may be found on either side of the urethral orifice the opening of the canals of Gartner (Ductus epoöphori longitudinales).

The =clitoris= is the homologue of the penis. It arises from the ischial arch by two crura, which unite to form a =body= as large as one’s little finger. This projects backward into the inferior commissure of the vulva, capped by a small rounded =glans=. The organ is composed of erectile tissue similar to the corpus cavernosum penis. The =ischio-cavernosus= (or erector clitoridis) is a very feeble muscle which depresses the clitoris. The veins of the clitoris communicate by an intermediate plexus on either side with the bulbus vestibuli.

THE FEMALE URETHRA

The female =urethra= (Urethra feminina) represents only that part of the canal of the male which lies between the internal urethral orifice and the openings of the ejaculatory ducts. Its length is about two inches (5 cm.), and its lumen is sufficient to admit of the introduction of the finger; it is, however, capable of remarkable dilatation if sufficient care and patience are exercised in the process. It lies centrally on the pelvic floor, and is related and attached dorsally to the vagina.

=Structure.=—The intrinsic =muscular coat= consists of longitudinal and circular layers of unstriped fibers. Between these is a layer of areolar tissue which contains a rich =venous plexus=. The =mucous membrane= is thrown into longitudinal folds when the canal is closed; it is highly elastic, and is covered with stratified epithelium.

The =urethral muscle= (M. compressor urethræ) embraces the urethra and is continued backward for some distance under the constrictor vestibuli. It is covered by a fibro-elastic membrane.

THE MAMMARY GLANDS

The =mammary glands= (Glandulæ lactiferæ) are modified cutaneous glands which are so closely associated functionally with the genital organs as to be considered accessory to them.

In the mare they are two in number, and are placed on either side of the median plane in the inguinal region. Each gland has the form of a short cone, much compressed transversely, and having a flat inner surface. It consists of the glandular mass or body of the gland (Corpus mammæ) and the =teat= or =nipple= (Papilla mammæ). The base is related to the abdominal wall, to which it is attached by areolar tissue, which contains a venous plexus, the superficial inguinal lymph glands, and a variable amount of fat. The apex is constituted by the teat, which is also flattened transversely and varies in length from one to two inches (ca. 2.5 to 5 cm.). Between the bases of the teats is the intermammary groove. On the apex of each teat two or three small orifices are placed close together; these are the openings of the lactiferous ducts.

=Structure.=—The =skin= over the glands is thin, pigmented, chiefly hairless, and supplied with numerous large sebaceous and sweat glands. Under this are two layers of =fascia= except on the teats. The superficial fascia presents no special features. The deep fascia consists of two laminæ detached from the abdominal tunic which descend on either side of the median plane, forming a =septum= between the two glands, and constituting their ligamentum suspensorium.

These laminæ are separated by a layer of areolar tissue, so that it is possible to remove one gland if diseased by careful dissection between the layers of the septum.

The =gland substance= or =parenchyma= is pinkish gray in color, and of firmer consistence than the fat which is found around and within the gland. It is inclosed by a fibro-elastic capsule which sends inward numerous trabeculæ; these form the =interstitial tissue=, and divide the gland into =lobes= and =lobules=. In the latter are the secretory =tubules= and =alveoli=, which unite to form the larger =ducts=. Each lobe has a duct, which opens at the base of the teat into a space called the =lactiferous sinus= (Sinus lactiferus); this is lined with mucous membrane, and from it two (or three) =lactiferous ducts= (Ductus lactiferi) pass through the extremity of the nipple. These ducts are lined with a non-glandular mucous membrane, which is covered with stratified squamous epithelium. They are surrounded by unstriped muscular tissue, the bulk of the fibers being arranged in a circular manner to form a =sphincter=.

The size and form of the mammary glands are subject to much variation. In the young subject, before pregnancy, they are small and contain little gland tissue. During the latter part of gestation, and especially during lactation, they increase greatly in size, and the gland tissue is highly developed. After lactation the secretory structures undergo marked involution, and the gland is much reduced in size. The relative amounts of gland substance and interstitial tissue vary greatly; in some cases a gland of considerable size contains little secretory tissue and is consequently functionally deficient.

=Vessels and Nerves.=—The =arteries= are derived from the =external pudic artery=, which enters the gland at the posterior part of its base. The =veins= form a plexus on either side of the base of the gland, which is drained by the =external pudic vein= chiefly. The =lymphatics= are numerous and pass chiefly to the superficial inguinal (or supramammary) lymph glands. The =nerves= are derived from the =inguinal nerves= and the =posterior mesenteric plexus= of the sympathetic system.

GENITAL ORGANS OF THE COW

The =ovaries= of the cow are much smaller than those of the mare, measuring usually about one to one and a half inches (ca. 3 to 4 cm.) in length, and a little less than an inch (ca. 2 cm.) in thickness in their largest part. They are oval in form, pointed at the uterine end, and have no ovulation fossa. They are situated usually a little above the middle of the lateral margin of the pelvic inlet in the non-pregnant subject, but may be found a little further forward. They are thus about 16 to 18 inches (ca. 40 to 45 cm.) from the vulva in a cow of medium size. The greater part of the surface of the gland is covered with germinal epithelium, the peritoneal epithelium being limited to a narrow zone along the attached border. Follicles of various sizes are often seen projecting from the surface, as well as corpora lutea; a corpus luteum verum has a pronounced yellow color, and may reach a width of half an inch or more (ca. 1 to 1.5 cm.).

The =Fallopian= or =uterine tubes= are long and less flexuous than in the mare. The junction with the cornu of the uterus is not so abrupt as in the mare, since the extremities of the horns are pointed. The uterine orifice of the tube is rather large and funnel-shaped. The fimbriæ are not so extensive as in the mare.

The =uterus= lies almost entirely within the abdominal cavity in the adult. The =body= is only about one inch (ca. 2 to 3 cm.) in length, although externally it appears to be much longer. This false impression is due to the fact that the posterior parts of the cornua are united by connective and muscular tissue and have a common peritoneal covering. The =cornua= are, therefore, really longer than they appear externally. They taper gradually toward the free end, so that the junction with the Fallopian tubes is not abrupt, as in the mare. The free part of the horn curves at first downward, forward, and outward, and then turns upward, forming a spiral coil. The =cervix= is about four or five inches (ca. 8 to 10 cm.) long; its wall is remarkably dense, and may be nearly an inch (ca. 2 cm.) in thickness. Its lumen, the =cervical canal=, is spiral, and is ordinarily tightly closed and very difficult to dilate. The =muscular coat= of the uterus is thicker than in the mare. It consists of an external longitudinal layer and two circular strata. The inner circular layer is about a fourth of an inch (ca. 6 mm.) thick in the cervix. The other layers are continued in the vagina. The =mucous membrane= of the horns and body presents as a characteristic feature the =uterine cotyledons= (Cotyledones uterinæ). These are oval prominences, about a hundred in number, which are either irregularly scattered over the surface or arranged in rows of about a dozen.

In the non-gravid uterus they average about ½ to ¾ inch (ca. 15 to 18 mm.) in length, ⅓ inch (ca. 8 mm.) in width, and ⅛ inch (ca. 2 to 4 mm.) in thickness. During pregnancy they become greatly enlarged and pedunculated. The larger ones then measure about 4 to 5 inches (10 to 12 cm.) in length, 1 to 1½ inches (3 to 4 cm.) in width, and 1 inch (2 to 2.5 cm.) in thickness. The deep face has a hilus at which the vessels enter. The rest of the surface has a spongy appearance, due to numerous crypts which receive the villi of the chorion.

The =uterine glands= are long and branched. The mucous membrane of the cervix is pale and forms numerous folds. The latter are arranged in several series which obliterate the lumen. At the external orifice (os uteri) the folds (Plicæ palmatæ) form rounded prominences arranged circularly, which project into the cavity of the vagina. There are no glands in the cervix, but a thick mucus is secreted by goblet cells.

The =broad ligaments= are not attached in the sublumbar region as in the mare, but to the upper part of the flanks, about a handbreadth below the level of the external angle of the ilium. They contain a conspicuous amount of unstriped muscle. The =round ligaments= are well developed, and can be traced distinctly to the vicinity of the internal inguinal ring.

The =vagina= is somewhat longer and more roomy than that of the mare; its wall is also thicker. Its average length in the non-pregnant animal is about eight inches (ca. 20 cm.); but in the pregnant cow the length increases to twelve inches (30 cm.) or more. The recto-genital pouch of peritoneum extends backward about five inches (ca. 12 cm.) on the dorsal surface, while ventrally the serous coat only extends backward about two inches (ca. 5 cm.).

This gives ample space dorsally for the incision of the fornix vaginæ in oöphorectomy.

In the ventral wall of the vagina, between the muscular and mucous coats, there are frequently present the two =canals of Gartner= (Ductus epoöphori longitudinales). When well developed they may attain the diameter of a goose quill, and may be traced forward to the anterior part of the vagina or even further. They open posteriorly near the external urethral orifice.

These tubes are remnants of the Wolffian ducts, and, like other fœtal vestiges, are very variable. Röder states that the right canal was absent in over 52 per cent., the left in only 22 per cent., of the cows examined by him. In some cases they may be traced in the broad ligaments for a variable distance toward the ovary. They are of clinical interest in that cysts frequently form along their course.

The =vulva= has thick wrinkled labia, and both commissures are acute; the lower one is pointed, and has on it a number of long hairs; it lies about two inches (5 cm.) behind, and about the same distance below, the level of the ischial arch. The =glans clitoridis= is small; the =crura= are about four inches (ca. 10 cm.) long and are flexuous. The =external urethral orifice= or meatus urinarius is four or five inches (10 to 12 cm.) from the ventral commissure. Beneath it is a blind pouch, the =suburethral diverticulum=, which is more than an inch (ca. 3 to 4 cm.) long, and readily admits the end of a finger.

The form and position of this pouch should be carefully noted on account of the difficulty it causes in catheterizing the bladder. If the catheter is passed along the ventral wall of the vulva (as in the mare), it will always enter the pouch instead of the urethra.

The =glandulæ vestibulares majores= (or glands of Bartholin) are situated on the lateral walls of the vulva, under the constrictor vulvæ. They are little over an inch (ca. 3 cm.) long and about half an inch (ca. 1.5 cm.) in width. Each has a single duct which opens about an inch and a half (ca. 3 to 4 cm.) lateral to and behind the external urethral orifice.

The =urethra= of the cow is about four inches (10 cm.) in length; it is narrower and much less dilatable than that of the mare. It is fused dorsally with the wall of the vagina, while laterally and ventrally it is covered by the constrictor vaginæ muscle.

The =mammary glands=, normally two in number, are popularly termed the udder. They are very much larger than in the mare, and the body of each is somewhat ellipsoidal in form, but flattened transversely. The =base= (or dorsal surface) of each gland is slightly concave and slopes obliquely downward and forward in adaptation to the abdominal wall, to which it is adherent; posteriorly it is in relation to the large supramammary lymph glands and a quantity of fat. The inner surface is flat, and is separated from the other gland by a well-developed double septum. The outer surface is convex. Four well-developed teats are present; they average about three inches (ca. 7 to 8 cm.) in length. It is customary to consider the udder to consist of four “quarters.” There is no septum nor visible division between the two quarters of the same side, but, on the other hand, injections of fluids of different colors into the two teats of the gland demonstrate that the cavities drained by them do not communicate.

The size and form of the glands vary greatly; in the highly specialized dairy breeds (e. g., Holsteins) they often attain enormous dimensions. The size of the udder is not a certain index of its productivity, since some so-called “fleshy” udders contain a very large amount of interstitial tissue and relatively little parenchyma. Small accessory teats often occur posteriorly; commonly these are rudimentary and imperforate (polythelia), but sometimes they drain a small amount of gland substance (polymastia). Sanson described a case in which there were seven teats, all giving milk.

Each teat has a single duct which widens superiorly and opens freely into a roomy =lactiferous sinus= or milk cistern (Sinus lactiferus). The =lactiferous duct= or teat canal is lined by a glandless mucous membrane which is covered with stratified squamous epithelium; the lower part (ca. 1 cm.) of the canal is narrow, and is closed by a =sphincter= of unstriped muscle. The =mucous membrane= of the sinus forms numerous folds which render the cavity multilocular.

=Vessels and Nerves.=—The blood-supply is derived from the =external pudic arteries=. The =veins= form a circle at the base of the udder, from which the blood is drained by three trunks, viz., the very large =subcutaneous abdominal= (“milk vein”), the =external pudic=, and the =perineal vein=. The =lymphatics= are numerous, and pass to the supramammary glands chiefly. The nerves are derived from the =inguinal nerves= and the =posterior mesenteric plexus= of the sympathetic.

GENITAL ORGANS OF THE SOW

The =ovaries= are concealed in the bursa ovarii, owing to the large extent of the mesosalpinx. They are more rounded than in the bitch, and have a distinct hilus. They are situated usually as in the cow, but their position is quite variable in animals which have borne young. The surface commonly presents rounded prominences, so that sometimes the gland has an irregular, mulberry-like aspect; this is due to the fact that there is very little stroma and the larger Graafian follicles project from the surface.

The =Fallopian= or =uterine tubes= are long (ca. 15 to 30 cm.), and less flexuous than in the mare. The fimbriated extremity forms an ampulla and has a large abdominal opening. The uterine end shades insensibly into the small extremity of the cornu of the uterus.

The =uterus= presents several striking features. The =body= is only about two inches (ca. 5 cm.) long. The =horns= are extremely long and flexuous, and are freely movable, on account of the large extent of the broad ligaments. The extremities of the horns taper to about the diameter of the Fallopian tubes.[154] The =neck= is remarkable for its length (ca. 15 to 20 cm.) and the fact that it is directly continued by the vagina without forming any intravaginal projection (os uteri). It can be distinguished by the transverse folds of its mucous membrane and the thickness of its wall.

The changes in form and position of the uterus during pregnancy are similar to those mentioned later in the case of the bitch.

The =vagina= is about four or five inches (ca. 10 to 12 cm.) long in a sow of medium size. It is small in caliber, and has a thick muscular coat which consists mainly of circular fibers. The mucous membrane is plicated, and is intimately united with the muscular coat.

The =vulva= is relatively long. The inferior commissure is pointed and dependent. The mucous lining is plicated. There is a small =suburethral diverticulum=. The vestibular glands are small and variable in number. The vestibular bulbs are little developed. The canals of Gartner may sometimes be traced as far as the ovary, but are often difficult to find.

The =clitoris= is long, flexuous, and pointed; beneath it is a cul-de-sac.

The =urethra= is very long; its vulvar orifice is not concealed by a valvular fold.

The =mammary glands= are usually ten or twelve in number, and are arranged in two rows, as in the bitch. Each teat has commonly two excretory ducts.

GENITAL ORGANS OF THE BITCH

The =ovaries= (Fig. 336) are small, elongated-oval in outline, and flattened. Each ovary is situated close to or in contact with the posterior pole of the corresponding kidney, and thus lies ventral to the third or fourth lumbar vertebræ, or about half-way between the last rib and the crest of the ilium. Each is concealed in a peritoneal pouch, the =bursa ovarii=, which has a slit-like opening ventrally. The two folds which form this pouch contain a quantity of fat and unstriped muscle. They are continued to the cornu of the uterus, constituting the =mesosalpinx= and the =ovarian ligament=. The surface of the ovary presents prominences caused by projecting Graafian follicles. There is no distinct hilus.

The =Fallopian= or =uterine tubes= are small and average two or three inches (ca. 5 to 8 cm.) in length. Each passes at first forward and then turns backward, having a straight or only slightly flexuous course. The fimbriated extremity lies in the bursa ovarii, and has a rather large opening. The uterine orifice is very small.

The =uterus= has a very short body and two long narrow horns. In a bitch of medium size the body is about an inch (ca. 2 to 3 cm.) and the cornua five or six inches (ca. 12 to 15 cm.) long. The horns are of uniform diameter, are nearly straight, and lie entirely within the abdomen. They diverge from the body in the form of a V toward each kidney. Their posterior parts are united by the peritoneum. The neck is very short and is fused dorsally with the vagina. It has a thick muscular coat. The mucous membrane of the uterus has long uterine glands and also short tubular crypts.

The =broad ligaments= contain fat and unstriped muscle. They are much longer in the middle than at either end. The posterior part is attached to the anterior part of the vagina. The =round ligaments= are very long, extending through the inguinal canals, and traceable almost to the vulva.

The horns of the gravid uterus present dilatations or =ampullæ=, which contain the fœtuses, and are separated by constrictions. The gravid uterus lies on the ventral abdominal wall, and toward the end of gestation extends forward to the stomach and liver.

The =vagina= is relatively long. It is narrow anteriorly, and has no distinct fornix. The muscular coat is thick and consists chiefly of circular fibers. The mucous membrane forms longitudinal folds. The canals of Gartner are usually absent.

The =vulva= has thick labia which form a pointed inferior commissure. The mucous lining is smooth. On either side of the urethral orifice there is a small depression. The glandulæ vestibulares majores are absent, but the smaller glands are present, and their ducts open ventrally on either side of a median ridge. The vestibular bulbs are relatively large and join dorsally. The clitoris has a small pointed glans, beneath which is a diverticulum. The corpus cavernosum is a little more than an inch (ca. 3 to 4 cm.) long in a subject of medium size, and is infiltrated with fat.

The =mammary glands= are usually ten in number, and are arranged in two series extending from the posterior part of the pectoral region to the inguinal region; they are, therefore, designated according to location as =pectoral=, =abdominal=, and =inguinal=. The teats are short, and present on their apices six to twelve small orifices of the excretory ducts.

ANGIOLOGY: THE VASCULAR SYSTEM

=Angiology= is the description of the organs of circulation of the blood and lymph—the heart and vessels. The =heart= is the central hollow muscular organ which functions as a suction and force pump; the differences in pressure caused by its contraction and relaxation determine the circulation of the blood and lymph. It is situated in the middle mediastinal space of the thorax, between the two lungs, and is inclosed in a fibro-serous sac—the pericardium. The vessels are tubular and run through almost all parts of the body. They are designated according to their contents as =blood-= and =lymph vessels=.

The =blood-vascular system= consists of: (1) the =arteries=, which convey blood from the heart to the tissues; (2) the =capillaries=, microscopic tubes in the tissues which permit of the necessary interchange between the blood and the tissues; (3) the =veins=, which convey the blood back to the heart.

The blood-vessels are divided into the =pulmonary= and the =systemic=. The =pulmonary artery= conveys the blood from the right ventricle of the heart to the lungs, where it is arterialized, and is returned by the =pulmonary veins= to the left atrium of the heart, and passes into the left ventricle. The =systemic arteries= convey the blood from the left ventricle all over the body, whence it is returned by the =venæ cavæ= to the right atrium, and passes into the right ventricle.

It should be noted, however, that the lungs receive arterial blood through the systemic bronchial arteries. This blood is returned in part by the bronchial veins to the right atrium (indirectly), in part by the pulmonary veins to the left atrium.

The term =portal system= is often applied to the portal vein and its tributaries which come from the stomach, intestine, pancreas, and spleen. The vein enters the liver, where it branches like an artery, so that the blood in this subsidiary system passes through a second set of capillaries before being conveyed to the heart by the hepatic veins and the posterior vena cava.

The =arteries= (Arteriæ), as a rule, divide at an acute angle, giving off finer and finer branches. In some cases branches come off at a right angle, and others are recurrent, i. e., run in a direction opposite to that of the parent stem. The intercommunication of branches of adjacent arteries is termed =anastomosis=. Most commonly the connections are made by a network of numerous fine branches (Plexus vasculosus). Relatively large communicating branches (Rami communicantes) occur in certain places; they may be transverse or in the form of arches. Wide-meshed networks of vessels are termed =Retia vasculosa=. =Terminal= or =end arteries= are such as form isolated networks, i. e., do not anastomose with adjacent arteries. A =rete mirabile= is a network intercalated in the course of an artery. A =collateral vessel= (Vas collaterale) is one which pursues a course near and similar to that of a larger vessel.

The =veins= (Venæ) are in general arranged like the arteries, but are usually of greater caliber. When a vein accompanies an artery, it is termed a =vena comitans= or =satellite vein=; in many places two veins accompany a single artery. The primitive venous trunks do not run with the arteries, and most of the superficial veins (Venæ cutaneæ) pursue independent courses. They anastomose even more freely than the arteries, and large communicating branches are very common. The veins form very rich plexuses (Plexus venosi) in many places. Some veins which are without independent walls and are inclosed by dense membranes and run usually in bony grooves are termed (venous) =sinuses=; examples of this are the sinuses of the dura mater of the brain.

=Structure of Arteries.=—The wall consists of three coats. The =external coat= (Tunica externa) consists chiefly of fibrous connective tissue. In the deeper part are some elastic fibers, and in some arteries there are also longitudinal unstriped muscle-fibers. The =middle coat= (Tunica media) is composed of unstriped muscle and elastic tissue in medium-sized arteries. In small vessels there is only the muscular tissue, and in the largest trunks only elastic tissue. The =internal coat= or =intima= (Tunica intima) consists of a layer of endothelial cells, resting on an elastic membrane. The =sheath= (Vagina vasis) is a condensation of the surrounding connective tissue, and is attached more or less closely to the external coat.

=Structure of Veins.=—The walls of veins are similar in structure to those of the arteries, but are very much thinner, so that veins collapse more or less completely when empty, while arteries do not. The =middle coat= is very thin and consists to a large extent of ordinary connective tissue. The =intima= is also less elastic than in the arteries. In many veins this coat forms semilunar =valves=, the free edges of which are directed toward the heart. They are most numerous in the veins of the skin and the deep veins of the extremities (except the foot), while in most veins of the body cavities and viscera they are absent or occur only where the veins open into larger ones or where two veins join.

The walls of the vessels are supplied with blood by numerous small arteries, called =vasa vasorum=. These arise from branches of the artery which they supply or from adjacent arteries, ramify in the external coat, and enter the middle coat also.

The =nerves= of the vessels consist of both medullated and non-medullated fibers. They form plexuses around the vessels, from which fibers pass mainly to the muscular tissue of the middle coat.

BLOOD-VASCULAR SYSTEM OF THE HORSE

THE PERICARDIUM[155]

The =pericardium= is the fibro-serous sac which surrounds the heart, and also the great vessels in connection with it to a greater or less extent. Its form is in general similar to that of the heart. The =fibrous layer= is relatively thin, but strong and inelastic. It is attached dorsally to the large vessels at the base of the heart, and is continued in part up to the longus colli muscle. Ventrally it is firmly attached to the middle of the posterior half of the thoracic surface of the sternum. The =serous layer= is a closed sac, surrounded by the fibrous pericardium, and invaginated by the heart. It is smooth and glistening, and contains a small amount of clear serous fluid, the =liquor pericardii=. Like other serous membranes, it may be regarded as consisting of two parts, parietal and visceral. The =parietal part= lines the fibrous layer, to which it is closely attached. The =visceral layer= covers the heart and parts of the great vessels, and is therefore also termed the =epicardium=. The serous pericardium is composed of a connective-tissue membrane, rich in elastic fibers, and covered on its free surface by a layer of flat endothelial cells.

The pericardium is covered by the mediastinal pleura (Pleura pericardica) and is crossed laterally by the phrenic nerves. Its lateral surfaces are related chiefly to the lungs, but the lower part is in partial contact with the chest-wall. On the left side the area of contact is from the third to the fifth intercostal space inclusive. On the right side the contact is smaller and is at the third and fourth ribs. The anterior border of the base is opposite to the second intercostal space, and the posterior border is opposite to the sixth rib or space. The base is related to the great vessels, the trachea and its bifurcation, the bronchial lymph glands, the œsophagus, and the vagus, left recurrent, and cardiac nerves.

The two parts of the serous pericardium are, of course, continuous with each other at the line of reflection on to the great vessels. The latter are covered in varying degree by the visceral layer. The aorta and pulmonary artery are inclosed in a complete common sheath as far as the bifurcation of the latter. A pouch passes inward between the pulmonary artery and the left auricle, and is connected with another pouch which passes backward between the right auricle and the aorta, thus forming the great transverse sinus of the pericardium. The posterior vena cava is covered on the right and below for a distance of an inch or a little more (ca. 3 cm.). The pulmonary veins receive practically no serous covering. The epicardium is closely adherent to the muscular tissue of the heart, but is attached to the vessels by loose tissue and fat, and hence is easily dissected off them.

THE HEART

The =heart= (Cor) occupies the greater part of the middle mediastinal space. Its shape is that of an irregular and somewhat flattened cone. It is attached at its base by the great vessels, but is otherwise entirely free in the pericardium. It presents two surfaces, two borders, a base, and an apex for description.

In =position= the heart is decidedly asymmetrical, about two-fifths being to the right of the median plane and three-fifths to the left. The long axis (from the middle of the base to the apex) is directed downward, backward, and somewhat to the left. The =base= (Basis cordis) is directed dorsally and lies a little above the middle of the dorso-ventral diameter of the thorax. It is opposite to the ribs from the third to the sixth inclusive. The =apex= (Apex cordis) lies above the last segment of the sternum, and is separated by a very short interval from the sternal part of the diaphragm. The =anterior border= (Margo cranialis) is strongly convex and is directed obliquely downward and backward; its lower part is parallel with the sternum. The =posterior border= (Margo caudalis) is much shorter, nearly vertical, and is opposite to the sixth rib or intercostal space. The =surfaces=, =right= and =left= (Facies dextra, sinistra), are convex and are marked by =grooves= which indicate the division of the heart into four chambers, the two atria above and two ventricles below. The left surface (covered by the pericardium) is in contact with the lower third of the chest-wall from the third to the fifth intercostal space. On the right side the cardiac notch of the lung is smaller and further forward, so that the area of contact is at the third and fourth ribs.

The =coronary= or =atrio-ventricular groove= (Sulcus coronarius) indicates the division between the atria and the ventricles. It almost completely encircles the heart, but is interrupted by the origin of the pulmonary artery. The =longitudinal= or =interventricular grooves=, right and left (Sulci longitudinales sinister et dexter), correspond to the septum between the ventricles. The left groove is left-anterior in position. It begins at the coronary groove behind the origin of the pulmonary artery, and passes downward almost parallel to the posterior border, but does not reach the apex. The right groove is right posterior in position. It begins at the coronary groove below the termination of the posterior vena cava and passes downward and a little backward, ending about an inch and a half (ca. 3 to 4 cm.) above the apex. The grooves are occupied by the coronary vessel and a variable quantity of fat.

=Size and Weight.=—In horses of medium size the heart usually weighs about seven or eight pounds (ca. 3.5 kg.), or about 0.7 per cent. of the body-weight. There is, however, great range of variation in apparently normal specimens.

The following measurements were obtained in medium-sized hearts in diastole:

Sagittal diameter of base 25 cm. Greatest width of base 18 to 20 cm. Circumference at coronary groove 65 to 70 cm. Distance between origin of pulmonary artery and apex 25 cm. Distance between termination of posterior vena cava and apex 18 to 20 cm.

THE RIGHT ATRIUM

The =right atrium= (Atrium dextrum) or auricle forms the right-anterior part of the base of the heart, and lies above the right ventricle. It consists of a =sinus venosus=, into which the veins open, and an =auricle= or auricular appendix. The latter is a conical diverticulum which curves around the right and anterior surfaces of the aorta, its blind end appearing on the left side in front of the origin of the pulmonary artery.

There are five chief openings in the right atrium. The =opening of the anterior vena cava= (Ostium venæ cavæ cranialis) is in the upper and fore part. The =opening of the posterior vena cava= (Ostium venæ cavæ caudalis) is at the lower posterior part. Between the two the wall pouches upward somewhat, forming what is sometimes termed the sinus venosus or sac of Lower. In the anterior part of this the =vena azygos= opens. The =coronary sinus= opens immediately below the posterior vena cava; the orifice is provided with a small semilunar valve or valve of Thebesius (Valvula sinus coronarii). The middle coronary vein has a separate opening in some cases close to that of the coronary sinus. The =atrio-ventricular opening= is in the lower part, and leads into the right ventricle. In addition to the foregoing there are several small orifices of the venæ cordis parvæ; these are concealed in the depressions between the musculi pectinati.

The atrium is lined with a glistening membrane, the =endocardium=. Its walls are smooth except on the right and in the auricle (or appendix), where it is crossed in various directions by muscular ridges, the =musculi pectinati=. Small bands extend across some of the spaces inclosed by the musculi pectinati. The latter terminate above on a curved crest, the =crista terminales=, which indicates the junction of the primitive sinus venosus of the embryo with the atrium proper, and corresponds with the sulcus terminalis externally. The openings of the venæ cavæ are without valves. A ridge, the intervenous crest,[156] projects downward and forward from the roof just in front of the opening of the posterior vena cava; it tends to direct the flow of blood from the anterior vena cava to the atrio-ventricular opening. The =fossa ovalis= is an oval depression in the septal wall at the point of entrance of the posterior vena cava, bounded internally by a thick margin (Limbus fossæ ovalis). The fossa is the remains of an opening, the =foramen ovale=, through which the two atria communicate in the fœtus.

THE LEFT ATRIUM

The =left atrium= (Atrium sinistrum) or auricle forms the posterior part of the base of the heart. It lies behind the pulmonary artery and the aorta and above the left ventricle. The auricle (or appendix) extends outward and forward on the left side, and its blind end is behind the origin of the pulmonary artery. The =pulmonary veins=, usually seven or eight in number, open into the atrium behind and on the right side. The cavity of the atrium is smooth, with the exception of the auricle (or appendix), in which the musculi pectinati are present. In some cases there is a depression on the septal wall opposite the fossa ovalis, bounded above by a fold which is the remnant of the valve of the foramen ovale of the fœtus. The =atrio-ventricular opening= is situated below and in front; it usually appears smaller than the right one on account of the contraction of the left ventricle in the dead subject. The apertures of small veins of the heart are found in the spaces inclosed by the musculi pectinati.

The number and the arrangement of the pulmonary veins are variable. They may be five to nine in number. The largest orifice is posterior. Usually three veins of considerable size enter close together on the right above the posterior vena cava, and three or four open close to the ridge which projects from the roof at the base of the auricle (appendix).

THE RIGHT VENTRICLE

The =right ventricle= (Ventriculus dexter) constitutes the right-anterior part of the ventricular mass. It forms almost all of the anterior border of the heart, but does not reach the apex, which is formed entirely by the left ventricle. It extends from the third rib to the fourth intercostal space on the left side, to the fifth rib and space on the right side. It is somewhat triangular in outline, and is semilunar in cross-section. Its base faces upward and a little to the right and is connected largely with the right atrium, with which it communicates through the atrio-ventricular orifice; but its left part projects higher and forms the =conus arteriosus=, from which the pulmonary artery arises. Its apex is two inches or more (ca. 5 to 6 cm.) above the apex of the heart. On opening the cavity it is seen that the two openings are separated by a thick rounded ridge (Crista supraventricularis). The axis of the cavity, taken from this ridge to the apex forms a spiral curve downward and to the right. The septal wall is convex and faces obliquely forward and to the right.

The =right atrio-ventricular orifice= (Ostium venosum dextrum) is oval and is situated opposite to the lower parts of the third and fourth ribs. It is guarded by a =tricuspid valve= (Valvula tricuspidalis); of the three large cusps of this valve, one is between the atrio-ventricular opening and the conus arteriosus, one is septal, and the third is on the right margin. Small intermediate cusps intervene between the large ones. The peripheral edges of the cusps are attached to the fibrous ring at the atrio-ventricular opening. The central edges are irregular and hang down into the ventricle; they give attachment to chordæ tendineæ. The auricular surfaces are smooth. The ventricular surfaces are rough and furnish attachment to interlacing branches of the chordæ tendineæ. The valves are folds of the endocardium, strengthened by fibrous tissue and at the periphery by muscular fibers also. The =chordæ tendineæ= are attached below to the three =musculi papillares=, which project from the ventricular wall; superiorly they divide into branches which are inserted into the ventricular surfaces and the free edges of the valves. Each segment of the valve receives chordæ tendineæ from two papillary muscles. Of the latter, two are on the septum and the third and largest springs from the anterior wall.

The =pulmonary orifice= (Ostium arteriæ pulmonalis) is circular and is situated at the summit of the conus arteriosus, opposite to the lower part of the third intercostal space. It is guarded by the =pulmonary valve=, composed of three semilunar cusps (Valvulæ semilunares arteriæ pulmonalis); of these, one is internal, one external, and the third posterior. The convex peripheral border of each cusp is attached to the fibrous ring at the junction of the pulmonary artery and the conus arteriosus. The central border is free and slightly concave. Each cusp consists of a layer of endocardium on its ventricular surface, a continuation of the inner coat of the artery on its arterial surface, and an intermediate layer of fibrous tissue. The upper edge of the conus arteriosus forms three arches with intermediate projecting angles or horns, to all of which the cusps are attached; and the artery forms opposite each cusp a pouch, the sinus of the pulmonary artery (or of Valsalva).

The walls of the ventricle (except in the conus arteriosus) bear muscular ridges and bands, termed =trabeculæ carneæ=. These are of three kinds, viz., (1) ridges or columns in relief; (2) musculi papillares, somewhat conical flattened projections, continuous at the base with the wall and giving off the chordæ tendineæ to the tricuspid valve; (3) moderator bands (Musculi transversi cordis) which extend from the septum to the opposite wall. The latter are partly muscular, partly tendinous, and vary in different subjects. The strongest one is usually about midway between the base and apex and extends from the septum to the base of the anterior musculus papillaris. It is considered that they tend to prevent overdistention.

THE LEFT VENTRICLE

The =left ventricle= (Ventriculus sinister) forms the left posterior part of the ventricular mass. It is more regularly conical than the right ventricle and its wall is much thicker except at the apex. It forms all of the posterior border of the ventricular part and the apex of the heart. Its base is largely continuous with the left atrium, with which it communicates through the left atrio-ventricular opening, but in front it opens into the aorta. The cavity usually appears smaller than that of the right ventricle in the dead subject, on account of the greater contraction of its wall. It is almost circular in cross-section.

The =left atrio-ventricular orifice= (Ostium venosum sinistrum) is opposite to the fifth rib and intercostal space. It is almost circular and is guarded by the =bicuspid= or =mitral valve= (Valvula bicuspidalis). The two segments of this valve are larger and thicker than those of the right side of the heart. One is placed in front and separates the atrio-ventricular and aortic openings. The other is placed behind and laterally and is usually divided into two or three flaps.

The =aortic orifice= (Ostium aorticum) is directed upward and slightly forward. It is situated opposite to the fourth rib. It is guarded by the =aortic valve=, composed of three semilunar cusps (Valvulæ semilunares aortæ); one cusp is anterior, the others right and left posterior. They are similar to those of the pulmonary valve, but are much stronger. The free edge of each contains a central nodule of fibrous tissue, the corpus Arantii (Nodulus valvulæ semilunaris).

The =chordæ tendineæ= are fewer but larger than those of the right ventricle.

There are two large =musculi papillares=, one on each side; they are usually compound. The moderator bands are variable. Commonly two or three larger ones (which are often branched) extend from the musculi papillares to the septum. Smaller ones may be found in various places, especially at the apex. The other trabeculæ are fewer and less prominent than in the right ventricle.

The =interventricular septum= (Septum ventriculorum) is the musculo-membranous partition which separates the cavities of the two ventricles. It is placed obliquely. One surface is convex, faces forward and to the right, and bulges into the right ventricle. The other surface, which faces into the left ventricle, is concave and looks backward and to the left. The greater part of the septum is thick and muscular (Septum musculare), but the upper part is thin and membranous (Septum membranaceum). The latter intervenes between the left ventricle, on the one hand, and the right ventricle and atrium on the other.

STRUCTURE OF THE HEART

The heart-wall consists mainly of peculiar striped muscle, the myocardium, which is covered externally by the visceral part of the serous pericardium or epicardium, and is lined by the endocardium.

The =epicardium= is in general closely attached to the muscular wall, but is loosely attached over the coronary vessels and the associated subepicardial fat. It consists of a layer of flat polygonal cells, resting on a membrane of white and elastic fibers.

The =myocardium= consists of planes of fibers arranged in a somewhat complicated manner. The muscular tissue of the atria is almost completely separated from that of the ventricles by the fibrous rings around the atrio-ventricular orifices.

In the atria the muscle bands fall naturally into two groups—superficial and deep. The former are common to both atria, the latter special to each. The =superficial= or =common fibers= for the most part begin and end at the atrio-ventricular rings, but some enter the interatrial septum. The =deep= or =special bundles= also form two sets. Looped fibers pass over the atria from ring to ring, while annular or spiral fibers surround the ends of the veins which open into the atria, the auricles, and the fossa ovalis.

The muscular wall of the ventricles is much stronger than that of the atria. That of the left ventricle is in general about three times as thick as that of the right one, but is thin at the apex. The =superficial fibers= are attached above to the atrio-ventricular fibrous rings and pass in a spiral toward the apex. Here they bend upon themselves and pass deeply upward to terminate in a papillary muscle of the ventricle opposite to that in which they arose. The loops so formed at the apex constitute a whorl, the =vortex cordis=. The =deep fibers=, although they appear to be proper to each ventricle, have been shown by MacCallum to be in reality almost all common to both. Their arrangement is scroll-like. They begin on one side, curve around in the wall of that ventricle, then pass in the septum to the opposite side, and curve around the other ventricle. There is a layer of deep fibers which is confined to the basal part of the left ventricle; it is attached to the left atrio-ventricular ring.

Four =fibrous rings= (Annuli fibrosi) surround the orifices at the bases of the ventricles. The atrio-ventricular rings separate the musculature of the atria from that of the ventricles. Those which surround the origins of the pulmonary artery and aorta are festooned in conformity with the attached borders of the valves. The aortic ring contains on the right side a plate of cartilage (Cartilago cordis), which frequently becomes more or less calcified in old animals. Sometimes a smaller plate is present on the left side.

The =endocardium= lines the cavities of the heart and is continuous with the internal coat of the vessels which enter and leave the organ. Its free surface is smooth and glistening and is formed by a layer of endothelial cells. The latter rest on a thin layer of connective tissue, which is connected with the myocardium by a subendocardial elastic tissue containing vessels and nerves.

=Vessels and Nerves.=—The heart receives its blood-supply through the two =coronary arteries= which arise from the aorta opposite to the anterior and left cusps of the aortic valve. Most of the blood is returned by the =coronary veins=, which open into the right atrium by the coronary sinus.[157] A few small veins open directly into the right atrium, and others are said to open into the left atrium and the ventricles. The =lymph vessels= form a subepicardial network which communicates through stomata with the cavity of the pericardium. There is a less distinct subendocardial network. The vessels converge usually to two trunks which accompany the blood-vessels in the atrio-ventricular grooves and enter the glands at the bifurcation of the trachea. The =nerves= are derived from the =vagus= and =sympathetic= through the cardiac plexus.

THE ARTERIES

THE PULMONARY ARTERY

The =pulmonary artery= (A. pulmonalis) springs from the conus arteriosus at the left side of the base of the right ventricle. It curves upward, backward, and inward, and divides behind the arch of the aorta into right and left branches. It is related in front to the right auricle (appendix), behind to the left auricle (appendix), and internally to the aorta. It is enveloped with the latter in a common sheath of the visceral layer of the serous pericardium. Near the bifurcation it is connected with the arch of the aorta by a fibrous band about half an inch (ca. 1.2 cm.) in width; this is the =ligamentum arteriosum=, a remnant of the large =ductus arteriosus=, which conducts most of the blood from the pulmonary artery to the aorta in the fœtus. The artery is bulbous at its origin, and forms three =sinuses= or pouches, which correspond to the cusps of the semilunar valve. Beyond this it gradually diminishes in caliber.

In a horse of medium size the artery is about seven inches (ca. 17 to 18 cm.) long. At the origin it is about two and a half inches (ca. 6 to 6.5 cm.) in width; at the bifurcation its caliber is about one and a half inches (ca. 3.5 to 4 cm.). The wall is relatively thin, especially at the origin.

The =right branch= (Ramus dexter) of the pulmonary artery is a little longer and wider than the left one. It passes over the fore part of the left atrium and below the bifurcation of the trachea to the hilus of the right lung, and enters the latter below the right bronchus. In the lung it passes to the outer and lower side of the stem bronchus and accompanies it to the base of the organ. The branches correspond to the ramification of the bronchi. The =left branch= (Ramus sinister) is very short. It passes backward and enters the lung below the left bronchus. Its branches within the lung are arranged like that of the right one.

THE SYSTEMIC ARTERIES

The =aorta= is the main systemic arterial trunk. It begins at the base of the left ventricle and is practically median at its origin. It passes upward and slightly forward between the pulmonary artery on the left and right atrium on the right. It then curves sharply backward and upward and inclines somewhat to the left, forming the =arch of the aorta= (Arcus aortæ), and reaches the ventral surface of the spine at the eighth or ninth thoracic vertebra. After passing backward along the ventral aspect of the bodies of the vertebræ between the lungs it traverses the hiatus aorticus and enters the abdominal cavity, where it lies below the vertebral bodies and the psoas minor, just to the left of the median plane. It divides under the fifth lumbar vertebra into the two internal iliac or hypogastric arteries.

From the bifurcation a small vessel, the middle sacral artery (A. sacralis media), sometimes passes backward on the pelvic surface of the sacrum. It becomes lost in the periosteum or joins the coccygeal artery, or in exceptional cases is traceable to the sphincter ani externus.

The caliber of the aorta is greatest at its origin, which is termed the =bulbus aortæ=. Here it forms three pouch-like dilatations, the =sinuses of the aorta= (or of Valsalva). These correspond to the cusps of the aortic valve, and the coronary arteries arise from the left posterior and anterior sinuses. At the arch the diameter is about two inches (ca. 5 cm.), and beyond this it diminishes gradually in width.

It is convenient to divide the aorta into thoracic and abdominal parts. The =thoracic aorta= (Aorta thoracica) lies within the pericardium to the point of attachment of the ligamentum arteriosum, and is inclosed with the pulmonary artery in a prolongation of the epicardium. Beyond this it is between the two pleural sacs. It is crossed on the right by the œsophagus and trachea, on the left by the left vagus nerve. The left recurrent nerve winds around the concavity of the arch from left to right, and the vena azygos and thoracic duct lie along the dorsal part of its right face. The trachea causes it to deviate to the left; but beyond this it becomes median. The =abdominal aorta= (Aorta abdominalis) (Fig. 450) is related above to the lumbar vertebræ, the inferior common ligament, and the left psoas minor muscle; in the hiatus aorticus it is related to the cisterna chyli. On its right is the posterior vena cava, and on its left the left kidney and ureter.

BRANCHES OF THE THORACIC AORTA

1. CORONARY ARTERIES

The two coronary arteries, right and left, are distributed almost entirely to the heart, but send some small twigs to the origins of the great vessels.

The =right coronary artery= (A. coronaria dextra) arises from the anterior sinus of the aorta. It passes forward between the conus arteriosus and the right auricle (appendix) to the coronary groove, in which it curves around to the right and backward. It then descends in the right ventricular groove almost to the apex of the heart.

The =left coronary artery= (A. coronaria sinistra) arises from the left posterior sinus of the aorta, passes to the left behind the origin of the pulmonary artery, and divides into two branches. The =descending branch= (Ramus descendens) passes down the left ventricular groove toward the apex. The =circumflex branch= (Ramus circumflexus) runs backward in the coronary groove, in which it winds around to the right side.

2. COMMON BRACHIOCEPHALIC TRUNK OR ANTERIOR AORTA

The =common brachiocephalic trunk= or =anterior aorta= (Truncus brachiocephalicus communis) is a very large vessel which arises from the convexity of the arch of the aorta within the pericardium. It is directed forward and upward. Its length in horses of medium size is usually about two inches (ca. 5 to 6 cm.), but it is sometimes only half an inch or less (ca. 1 cm.). It is crossed on the left by the left vagus and cardiac nerves, and the left recurrent nerve runs between it and the trachea. It divides opposite to the second intercostal space or third rib into the brachiocephalic and left brachial arteries.

The =brachiocephalic artery= (A. brachiocephalica) is directed forward and a little upward in the anterior mediastinum, beneath the trachea. Opposite the first rib it gives off the bicarotid trunk and is continued as the =right brachial artery=. The latter (A. subclavia dextra) turns downward and bends around the anterior border of the first rib and the insertion of the scalenus muscle above the brachial vein. Its course and branches beyond this point will be described with the vessels of the thoracic limb.

The =left brachial artery= (A. subclavia sinistra) is longer than the right one and rises to a higher level. It forms an almost semicircular curve, the concavity being ventral. It is related internally to the œsophagus, trachea, and thoracic duct, and the left vagus, phrenic, and cardiac nerves cross under its origin. It emerges from the thorax like the artery of the right side. There is thus a difference at first between the trunks of opposite sides, but beyond this their course and distribution are similar.

The brachial and brachiocephalic arteries within the thorax give off the dorsal, superior cervical, vertebral, and internal thoracic arteries. At the first rib they give off the external thoracic and inferior cervical arteries.

1. The left =dorsal= or =dorso-intercostal artery= (A. costo-cervicalis) passes dorsally across the left face of the trachea and œsophagus toward the second intercostal space. The right artery arises usually by a common trunk with the superior cervical, crosses the right face of the trachea and has no contact with the œsophagus. Both detach small branches to the trachea, lymph glands, and pleura, and divide on reaching the longus colli into two branches. Of these the =subcostal artery= (A. intercostalis suprema) is the smaller. It passes backward under the costo-vertebral joints with the sympathetic trunk. It gives off the second, third, and fourth intercostal arteries, and ends at the fifth space, where it anastomoses with the first aortic intercostal artery, or constitutes the fifth intercostal artery, or dips into the longissimus muscle. It also gives off spinal branches and twigs to the longus colli and the pleura. The other branch (A. transversa colli) is the direct continuation of the trunk. It emerges through the upper end of the second intercostal space, passes across the transversalis costarum and longissimus toward the withers, and divides into several diverging branches. An anterior branch passes upward and forward between the splenius and complexus and anastomoses with branches of the superior cervical artery; the others are directed upward under the serratus magnus and rhomboideus to the withers, supplying the muscles and skin of this region.

The left dorsal artery sometimes arises with the superior cervical by a common trunk. On the right side there may be a common stem for the dorsal, superior cervical, and vertebral arteries. Occasionally the artery arises from the anterior aorta. Sometimes it emerges through the third space. The subcostal may arise independently behind the dorsal or from the superior cervical artery.

2. The =superior= or =deep cervical artery= (A. cervicalis profunda) arises in front of the dorsal or by a common trunk with it. It crosses the œsophagus (left side), the trachea (right side), and the longus colli, and emerges from the thoracic cavity by passing through the space behind the first costo-transverse articulation. In the thorax it gives off a small branch (A. mediastini cranialis) to the mediastinum and the pericardium; also the first intercostal artery (A. intercostalis prima), a very small vessel which passes down in the first intercostal space. After leaving the thorax the artery passes upward and forward on the spinalis muscle and the lamellar part of the ligamentum nuchæ, covered by the complexus. Its terminal branches anastomose with branches of the occipital and vertebral arteries in the region of the axis. Numerous collateral branches are detached to the extensor muscles of the head and neck, the ligamentum nuchæ, and the skin, and anastomoses occur with the dorsal artery above and the vertebral artery below.

The artery sometimes emerges through the second intercostal space.

3. The =vertebral artery= (A. vertebralis) arises from the brachial (or brachiocephalic) opposite the first intercostal space and passes upward and forward. On the left side it crosses the œsophagus, on the right the trachea. Emerging from the thorax it passes between the longus colli internally and the scalenus externally, under the transverse process of the seventh cervical vertebra, and continues along the neck through the series of foramina transversaria, between which it is covered by the intertransversales colli. Emerging from the foramen of the axis, it crosses the capsule of the atlanto-axial joint, and joins the retrograde branch of the occipital artery under cover of the great oblique muscle of the head. At each intervertebral foramen a =spinal branch= (Ramus spinalis) is given off which enters the vertebral canal and reinforces the ventral spinal artery. It also gives off series of dorsal and ventral =muscular branches= (Rami musculares). The dorsal branches are the larger; they supply the deep extensor muscles of the head and neck, and anastomose with the deep cervical and occipital arteries. The ventral branches supply chiefly the scalenus, longus colli, intertransversales, and rectus capitis anterior major. The artery is accompanied by the vertebral vein and a sympathetic nerve-trunk.

In some cases the last cervical transverse process has a foramen transversarium, through which the artery passes.

4. The =internal thoracic artery= (A. thoracica s. mammaria interna) is a large vessel which arises from the ventral side of the brachial opposite the first rib. It curves downward and backward, being at first on the inner surface of the rib, and then crosses the lower part of the first intercostal space and passes under the transversus thoracis muscle. It runs backward under cover of that muscle over the chondro-sternal joints to the eighth costal cartilage, where it divides into asternal and anterior abdominal branches. At each intercostal space two collateral branches are detached. The =intercostal branches= (Rami intercostales) pass upward in the intercostal spaces and anastomose with homonymous descending arteries. The lower branches detach small twigs to the transversus thoracis, pleura, and pericardium, and pass out between the costal cartilages as =perforating branches= (Rami perforantes) to supply the pectoral muscles and skin, anastomosing with the external thoracic artery. A very small pericardiaco-phrenic artery ascends in the mediastinum on the left side, in the caval fold of pleura on the right side; it supplies fine twigs to the pericardium and pleura and accompanies the phrenic nerve to the diaphragm. In the young subject it gives small branches (Aa. thymicæ) to the thymus gland. The =asternal artery= (A. musculophrenica) passes along the ninth costal cartilage and continues along the costal attachment of the transversus abdominis (Fig. 185). It gives off intercostal branches which anastomose with those descending from the thoracic aorta, and twigs to the diaphragm and transversus abdominis. The =anterior abdominal artery= (A. epigastrica cranialis) is the direct continuation of the internal thoracic. It passes between the ninth costal cartilage and the xiphoid cartilage, runs backward on the abdominal surface of the rectus abdominis and then becomes embedded in it (Fig. 466). It supplies the ventral wall of the abdomen and anastomoses with the posterior abdominal artery.

5. The =external thoracic artery= (A. thoracica externa) is given off from the ventral aspect of the brachial, usually at the inner surface or anterior border of the first rib. It turns around the first rib below or behind the brachial vein (when given off within the thorax) and passes backward under the deep pectoral muscle; it is continued as a small vessel in the panniculus carnosus, where it accompanies the external thoracic (“spur”) vein. It sends branches to the pectoral muscles and the axillary lymph glands.

This artery varies in origin and size. Not rarely it arises from the internal thoracic or from the brachial outside of the thorax. In other cases it arises by a common trunk with the inferior cervical. It may be very small or even absent, in which case the perforating branches of the internal thoracic compensate.

6. The =inferior cervical artery= (Truncus omo-cervicalis) arises usually from the dorsal surface of the brachial opposite the first rib or where that vessel winds around the rib. It is directed downward and a little forward across the external surface of the jugular vein and the deep face of the scalenus among the lymph glands at the thoracic inlet, and divides into ascending and descending branches. The =ascending branch= (A. cervicalis ascendens) passes upward and forward along the external surface of the jugular vein, then turns sharply backward and runs upward along the anterior border of the anterior deep pectoral muscle, between the omo-hyoideus and mastoido-humeralis and in relation to the prescapular lymph glands; it gives branches to these muscles and the prepectoral and prescapular lymph glands. The =descending branch= (A. transversa scapulæ) passes downward and outward across the surface of the anterior deep pectoral and then runs in the groove between that muscle and the mastoido-humeralis in company with the cephalic vein. It supplies branches to these muscles and the skin of the breast.

THE COMMON CAROTID ARTERY

These two vessels (Arteriæ carotides communes) arise from the brachiocephalic artery by a common trunk. This stem, the =truncus bicaroticus= or =cephalic artery=, is detached from the inner face of the brachiocephalic opposite the first rib and passes forward in the median plane beneath the trachea. It is related ventrally to the prepectoral lymph glands, the terminal parts of the jugular veins, and the anterior vena cava, and laterally to the vagus and recurrent nerves. It is commonly two or three inches (ca. 5 to 7 cm.) in length, but it may vary between one and eight inches (ca. 2.5 to 20 cm.).

The =right common carotid artery= passes obliquely from the ventral face of the trachea to its right side. In this position it runs upward and forward and divides at the crico-pharyngeus muscle and under the submaxillary gland into external carotid, internal carotid, and occipital arteries. It is inclosed in a fibrous sheath, and is accompanied dorsally by the vagus and sympathetic nerves, ventrally by the recurrent nerve. At the last two cervical vertebræ it is in contact superficially with the jugular vein, but further forward the omo-hyoideus muscle intervenes between the artery and vein. Near its termination the artery becomes more deeply placed and is related externally to the submaxillary and parotid glands, internally to the œsophagus. In some cases it is in contact ventrally with the thyroid gland, especially when the latter is larger than usual.

The =left common carotid artery= differs from the right one in that it is related deeply to the œsophagus, which separates it from the trachea in the greater part of its course.

The collateral branches of the common carotids are in the main small. They comprise:

1. =Muscular branches= (Rami musculares) of variable size, which go to the ventral muscles of the neck and the skin.

2. =Œsophageal= and =tracheal branches= (Rami œsophagei et tracheales). Small twigs go to the cervical lymph glands also.

3. The =parotid artery=. This comes off near the termination and enters the ventral part of the parotid gland. It also supplies the subparotid lymph glands, and sometimes sends a branch to the submaxillary gland. It is inconstant.

4. The =thyro-laryngeal artery= (A. thyreoidea cranialis). This, the largest collateral branch of the carotid, arises from the latter two or three inches before it divides. It curves over the anterior end of the thyroid gland, into which it sends several branches. It gives off a =laryngeal branch= (A. laryngea), which sends twigs to the external muscles of the larynx and the constrictors of the pharynx, passes between the cricoid and thyroid cartilages, and supplies the internal muscles and the mucous membrane of the larynx. A small =pharyngeal branch= (A. pharyngea ascendens) runs upward and forward to the crico-pharyngeus, and supplies twigs to the posterior part of the pharynx and the origin of the œsophagus. Small innominate twigs are given off to the trachea, the œsophagus, and the sterno-thyro-hyoideus and omo-hyoideus muscles.

In some cases the thyroid and laryngeal arteries arise from the carotid separately or by a short common stem. A laryngeal branch is often detached from the carotid in front of the thyro-laryngeal and enters the larynx with the superior laryngeal nerve. The pharyngeal branch frequently comes directly from the carotid.

5. The =accessory thyroid artery= (A. thyreoidea caudalis) is an inconstant vessel which arises from the carotid at a variable distance behind the thyro-laryngeal—sometimes from the latter or from the parotid artery. It sends branches into the posterior part of the thyroid gland and detaches small tracheal and muscular twigs. In some cases it is distributed chiefly or entirely to the adjacent muscles.

THE OCCIPITAL ARTERY

The =occipital artery= (A. occipitalis) is the second in size of the terminals of the carotid. It arises usually just in front of the internal carotid, but in some cases with that artery by a common trunk of variable length. It pursues a somewhat flexuous course to the fossa atlantis, where it divides into anterior and posterior branches. It is related superficially to the submaxillary gland and the mastoido-humeralis, and deeply to the guttural pouch and the rectus capitis anterior major.[158] The internal carotid artery, the inferior cerebral vein, and the accessory, vagus, and sympathetic nerves cross its deep face. It gives twigs to the submaxillary gland, the anterior straight muscles, the guttural pouch and the adjacent lymph glands, and two named collateral branches. The =condyloid= or =prevertebral artery= (A. condyloidea) is a small vessel which passes upward and forward on the guttural pouch, and divides into muscular and meningeal branches. The latter enter the cranium through the foramen lacerum and hypoglossal foramen and are distributed to the dura mater. This artery is very variable in its origin. The =posterior meningeal= or =mastoid artery= (A. meningea caudalis) is a much larger vessel which runs upward and forward between the small oblique muscle and the paramastoid process, passes through the mastoid foramen into the parieto-temporal canal, enters the cranial cavity, and is distributed to the dura mater.

The =posterior= or =retrograde branch= (Ramus descendens) of the occipital passes up through the foramen transversarium of the atlas and joins the vertebral artery. It gives branches to the great oblique muscle of the head, which covers it.

The =anterior= or =occipital branch= (Ramus occipitalis) passes through the alar (antero-external) foramen of the atlas and supplies the muscles and skin of the poll, anastomosing with the deep cervical artery and its fellow of the opposite side. In the alar furrow it gives off the =cerebrospinal artery=, which enters the spinal canal through the intervertebral (antero-internal) foramen of the atlas, perforates the dura mater, and divides into cerebral and spinal branches. The =cerebral branch= unites with that of the opposite side to form the =basilar artery=, and the =spinal branch= similarly forms by union with its fellow the =middle spinal artery=.

The =basilar artery= (A. basilaris cerebri) passes forward in the median groove on the ventral surface of the medulla and pons and divides into the two posterior cerebral arteries. The collateral branches of the basilar are:

1. Medullary branches (Rami medullares), ten or twelve in number, distributed to the medulla oblongata.

2. Posterior cerebellar arteries (Aa. cerebelli caudales) which pass outward around the medulla behind the pons to the cerebellum, to which they are distributed after giving twigs to the medulla and pons.

3. The small auditory artery (A. auditiva interna) accompanies the eighth nerve to the internal ear. It often arises from the posterior cerebellar.

4. Anterior cerebellar arteries (Aa. cerebelli nasales). These are very variable in number and origin. There are often two or three on either side and they frequently arise from the posterior cerebrals. They pass outward in front of the pons and supply the anterior part of the cerebellum.

The =posterior cerebral arteries= (Aa. communicantes caudales) diverge at an acute angle and join the posterior communicating branches of the internal carotid arteries on the inferior surface of the cerebral peduncles. They are connected by a transverse branch and by a network of fine twigs which form often a rete mirabile.

THE INTERNAL CAROTID ARTERY (Figs. 434, 436, 437)

This artery (A. carotis interna) is somewhat smaller than the occipital. It usually arises just behind that artery, crosses its deep face, and runs upward and forward on the guttural pouch to the foramen lacerum. It is closely related to the vagus nerve and the superior cervical ganglion of the sympathetic nerve, fibers from which accompany it. It is crossed externally by the ninth and twelfth cranial nerves and the pharyngeal branch of the vagus. It passes through the inferior petrosal sinus and enters the cavernous sinus, within which it forms an =S=-shaped curve. It is connected with the opposite artery by a transverse branch (A. intercarotica), which lies behind the pituitary body in the intercavernous sinus. A branch (A. caroticobasilaris) sometimes connects it with the basilar artery. It then perforates the dura mater, gives off the posterior communicating branch, and passes forward and divides at the side of the optic chiasma into anterior and middle cerebral arteries.

The =posterior communicating artery= (A. communicans posterior) turns backward and joins the posterior cerebral. It gives off the =deep cerebral artery= (A. cerebri profunda) which winds around the cerebral peduncle and is distributed chiefly to the mid-brain. A smaller collateral branch is the anterior choroid artery (A. chorioidea nasalis) which passes along the optic tract and is distributed in the choroid plexus of the lateral ventricle.

The =anterior cerebral artery= (A. cerebri anterior) unites with the corresponding branch of the opposite artery above the optic chiasma. From this junction proceeds the =artery of the corpus callosum= (A. corporis callosi), which turns around the genu of the corpus callosum, enters the great longitudinal fissure, divides into two branches, and is distributed to the inner aspect of the cerebral hemispheres. A small =anterior meningeal= branch (A. meningea nasalis) of the anterior cerebral is distributed to the anterior part of the dura, and assists in forming a network in the ethmoidal fossa (Rete ethmoidale), anastomosing with the ethmoidal branch of the ophthalmic artery.

The =middle cerebral artery= (A. cerebri media) passes outward in the lateral fissure (of Sylvius) and divides into branches on the outer surface of the hemisphere.

The =arterial circle of Willis= (Circulus arteriosus) (Fig. 434) is formed at the interpeduncular space of the base of the brain by the union of the anterior cerebral arteries in front, by the diverging posterior cerebral arteries behind, and is completed laterally by the junction of the latter with the posterior communicating arteries and by the internal carotid. It is irregularly polygonal in outline.

The cerebral arteries are very variable in arrangement, and the foregoing account is a brief statement of the more usual disposition of the larger vessels. The internal carotid artery often arises with the occipital by a common trunk of variable length.

THE EXTERNAL CAROTID ARTERY (Figs. 436, 437)

This artery (A. carotis externa) by its size and direction constitutes the continuation of the common carotid. It passes forward on the lateral wall of the pharynx at the lower border of the guttural pouch, covered by the submaxillary gland and the stylo-maxillaris, digastricus, and stylo-hyoideus muscles. It then emerges between the stylo-hyoideus and the great cornu of the hyoid bone, passes upward on the latter parallel with the posterior border of the lower jaw, and terminates about two inches (ca. 5 cm.) below the temporo-maxillary articulation by dividing into superficial temporal and internal maxillary branches. It is crossed deeply near its origin by the superior laryngeal and pharyngeal branches of the vagus nerve. Just before its emergence its superficial face is crossed by the hypoglossal nerve, and the glosso-pharyngeal nerve passes over its inner surface at the ventral border of the great cornu. The chief collateral branches are the masseteric, external maxillary, and posterior auricular. It also furnishes variable branches to the submaxillary and parotid glands, the guttural pouch, and the pharyngeal lymph glands, as well as twigs to some adjacent muscles.

1. The =inferior masseteric= or maxillo-muscular artery (A. masseterica inferior) is given off from the external carotid at its emergence from beneath the stylo-hyoideus. It passes downward and slightly forward under cover of the parotid gland and over the tendon of insertion of the sterno-cephalicus to the posterior border of the mandible and appears on the masseter muscle, into which it plunges after a short course on its surface. It gives branches also to the internal pterygoid and stylo-maxillaris muscles and the parotid gland.

2. The =external maxillary=, =facial=, or =submaxillary artery= (A. maxillaris externa)[159] arises from the external carotid on the inner surface of the posterior belly of the digastricus (Fig. 436). It runs downward and forward on the lateral wall of the pharynx across the deep face of the stylo-hyoideus toward the great cornu of the hyoid bone, accompanied by the glosso-pharyngeal nerve in front and the hypoglossal nerve behind. After giving off the lingual artery at the posterior border of the great cornu, it inclines more ventrally on the inner surface of the internal pterygoid muscle, crosses over the hyo-glossus muscle, the hypoglossal nerve, the submaxillary duct, and the intermediate tendon of the digastricus, and turns forward in the submaxillary space. Here it lies on the lower part of the internal pterygoid muscle, and is related internally to the submaxillary lymph glands, above to the anterior belly of the digastricus, and below to the homonymous vein. At the anterior border of the masseter it turns around the lower border of the jaw and runs upward on the face in front of that muscle.[160] At the turn the artery is in front, the vein in the middle, and the parotid duct posterior. The artery and vein pass upward along the anterior border of the masseter, under cover of the facial panniculus and the zygomaticus, and are crossed superficially by branches of the facial nerve and deeply by the parotid duct. The artery terminates over the levator labii superioris proprius by dividing into the dorsal nasal and the angular artery of the eye. The chief branches of the external maxillary are as follows:

(1) The =pharyngeal artery= (A. palatina ascendens) arises usually behind the stylo-pharyngeus, passes between that muscle and the great cornu, and runs forward on the lateral wall of the pharynx under the elastic pharyngeal fascia. It is distributed to the soft palate, pharynx, and tonsil.

(2) The =lingual artery= (A. lingualis) is a large branch which diverges from the parent trunk at an acute angle, runs along the ventral border of the great cornu of the hyoid bone, and dips under the hyo-glossus muscle. It then passes across the kerato-hyoideus, turns inward under the intercornual joint of the hyoid bone, and runs forward in the tongue between the hyo-glossus and genio-glossus. This part (A. profunda linguæ) is flexuous and is accompanied by branches of the hypoglossal and lingual nerves. It is the chief artery of the tongue, and anastomoses with the opposite artery and the sublingual.

(3) The =sublingual artery= (A. sublingualis) is a smaller vessel which arises at the anterior extremity of the submaxillary gland (Fig. 436). It passes forward on the anterior belly of the digastricus between the ramus of the mandible and the mylo-hyoideus, perforates the latter, runs along the lower border of the sublingual gland, and ramifies in the mucous membrane of the anterior part of the floor of the mouth. It detaches branches to the muscles and skin in the submaxillary space, the submaxillary lymph glands, and the sublingual gland. It also gives off the small =submental artery=, which runs forward superficially toward the lower lip, supplying twigs to the skin and the mylo-hyoideus.

In some cases the sublingual artery arises from the lingual and the submental from the external maxillary. Sometimes the sublingual remains on the external face of the mylo-hyoideus—thus resembling the submental of man—and the sublingual gland is supplied by a special branch of the lingual.

Not uncommonly a considerable branch, given off in the submaxillary space, turns round the lower border of the jaw and enters the middle of the lower part of the masseter muscle. In some cases this artery is of large size and its pulsation can be felt. It is accompanied by a vein.

(4) The =inferior labial artery= (A. labialis inferior) arises from the facial a little before it reaches the depressor labii inferioris (Fig. 435). It passes forward, dips under the depressor muscle, and continues to the lower lip. It supplies branches to the muscles and skin in this region, to the inferior buccal glands, the mucous membrane of the cheek, and the lower lip, anastomosing with the mental artery and the corresponding vessels of the opposite side. It detaches a branch (A. anguli oris) to the angle of the mouth, which anastomoses with the superior labial.

(5) The =superior labial artery= (A. labialis superior) arises from the facial in front of the facial crest (Fig. 435). It passes forward under the dilatator naris lateralis and levator nasolabialis to the upper lip, gives twigs to the upper part of the cheek and the lateral nasal region, and ramifies in the upper lip, anastomosing with the opposite artery and the palato-labial.

(6) The =lateral nasal artery= (A. lateralis nasi) arises usually a little above the preceding one, and runs forward parallel with it and under the levator nasolabialis to the nostril (Fig. 435). It supplies branches to the lateral nasal region and the nostril.

The vessel is often double. It may arise from the superior labial at the point of bifurcation of the facial or with the dorsal nasal from the infraorbital artery (as in the ox). In some cases it gives off a dorsal nasal branch.

(7) The =dorsal nasal artery= (A. dorsalis nasi) arises on the levator labii superioris proprius and passes forward under the levator nasolabialis to the dorsum nasi (Fig. 435).

(8) The =angular artery of the eye= (A. angularis oculi) runs toward the internal canthus of the eye, where it anastomoses with the orbital branch (Ramus malaris) of the infraorbital artery (Fig. 435).

In addition to the preceding, unnamed branches are supplied to the submaxillary salivary gland and the submaxillary lymph glands.

3. The =posterior auricular artery= (A. auricularis posterior) arises at an acute angle from the external carotid just above the origin of the masseteric. It passes upward under cover of the parotid gland, to which it gives branches, and divides into several branches which supply the skin and muscles of the external ear (Fig. 435). The =posterior branch= passes to the posterior part of the base of the ear, where it divides into two branches; of these, one (Ramus intermedius) passes up the convex surface of the external ear to the apex, while the other (Ramus medialis) winds around to the anterior (inner) border, and forms an arch with the intermediate branch. The =external branch= (Ramus lateralis) passes up the posterior (external) border of the ear and forms an arch with the intermediate branch. The =deep branch= (A. auricularis profunda) enters the interval between the external auditory canal and mastoid process, and passes through an opening into the interior of the external ear and ramifies in the skin which lines it. It gives off the =stylo-mastoid artery=, which passes through the stylo-mastoid foramen into the tympanum, forms an arch around the membrana tympani, and supplies the middle ear and its muscles.

THE SUPERFICIAL TEMPORAL ARTERY

This artery (A. temporalis superficialis) is much the smaller of the two terminal branches of the external carotid. It passes upward behind the posterior border of the ramus of the mandible under cover of the parotid gland, and divides below the level of the condyle into the anterior auricular and transverse facial arteries. It is crossed superficially by the facial nerve.

The =anterior auricular artery= (A. auricularis anterior) passes upward behind the temporo-mandibular articulation under cover of the parotid gland, crosses the zygomatic arch, and reaches the temporalis muscle. It is crossed deeply at its origin by the superficial temporal nerve and is accompanied by a satellite vein and the auriculo-palpebral branch of the facial nerve. It is distributed to the skin and the temporal and anterior auricular muscles, and sends a branch through the conchal cartilage to the skin which lines it. Collateral twigs are detached to the parotid gland, and an anterior branch anastomoses with the supraorbital artery. A branch sometimes passes into the parieto-temporal canal and anastomoses with the posterior meningeal (mastoid) artery.

The =transverse facial artery= (A. transversa faciei) is larger than the preceding. It turns around the neck below the condyle of the mandible and emerges from beneath the parotid gland (Fig. 435). It then passes forward a short distance on the masseter about half an inch below the zygomatic arch and plunges into the muscle, in which it commonly divides into two chief branches. It is accompanied by a vein and a branch of the superficial temporal nerve. It supplies the masseter and the skin of this region, and anastomoses with the external maxillary and posterior deep temporal arteries.

The pulse may usually be felt in this artery as it passes below the articulation of the jaw.

THE INTERNAL MAXILLARY ARTERY (Figs. 437, 438, 439)

This artery (A. maxillaris interna) is much the larger of the two terminal branches of the external carotid. It begins at the inner side of the posterior border of the mandible about two inches (ca. 5 cm.) below the articulation of the jaw and ends in the anterior part of the pterygo-palatine fossa. On account of its complex course and the large number of branches given off it is convenient to divide it into three parts.

The =first part= is much the longest, forms an =S=-shaped curve, and is in great part in contact with the guttural pouch. It passes upward and forward on the inner surface of the mandible a distance of about an inch (ca. 2 to 3 cm.) and is related here to the vein, which is ventral. It then turns inward below the external pterygoid muscle and the inferior maxillary nerve, passes between that muscle and the tensor palati, and runs forward to enter the alar (pterygoid) foramen. This part gives off the following branches:

1. The =inferior alveolar= or =dental artery= (A. alveolaris mandibulæ s. inferior) passes downward and forward with the homonymous vein and nerve, being at first between the external and internal pterygoid muscles, then between the latter and the ramus of the mandible. It enters the mandibular foramen, passes downward and forward in the mandibular canal, and terminates at the mental foramen by dividing into mental and incisor branches. The =mental branch= (A. mentalis) emerges through the mental foramen and anastomoses in the lower lip with the opposite artery and the inferior labial. The =incisor branch= continues forward in the bone, supplies twigs to the canine and incisor teeth, and anastomoses with its fellow of the opposite side. Collateral branches are detached to the pterygoid and mylo-hyoid muscles, and within the bone to the teeth, alveolar periosteum, the gums, and the spongy substance of the mandible.

2. The =pterygoid arteries=, two or three in number, are distributed to the pterygoid and tensor and levator palati muscles.

3. The =tympanic artery= is a very small vessel which passes along the Eustachian tube to the petro-tympanic fissure (of Glaser) and enters the middle ear.

4. The =middle meningeal artery= (A. meningea media) arises beneath the buccinator nerve where the internal maxillary turns forward. It passes backward across the temporal wing of the sphenoid to the antero-external part (foramen spinosum) of the foramen lacerum. Entering the cranium, it divides into branches which course in the grooves on the temporal and parietal bones and supply the dura mater. It anastomoses with the posterior meningeal.

The size of this artery is variable, and is in inverse ratio to that of the posterior meningeal.

5. The =posterior deep temporal artery= (A. temporalis profunda aboralis) arises from the internal maxillary just before the latter enters the canal of the pterygoid process. It passes upward and backward in the temporal fossa on the deep face of the temporalis muscle, in which it ramifies. It sends a branch outward to the masseter, and anastomoses with the superficial temporal and middle meningeal arteries.

In some cases the tympanic and middle meningeal arise from this artery.

The =second part= lies in the alar canal and is about an inch (ca. 2 to 3 cm.) in length. It gives off two branches—the anterior deep temporal and the ophthalmic.

1. The =anterior deep temporal artery= (A. temporalis profunda oralis) emerges from the canal through the temporal or small alar foramen, and passes upward in the anterior part of the temporal fossa on the deep face of the temporalis muscle, in which it is chiefly distributed. It gives twigs to the orbital fat and the skin of the frontal region.

2. The =ophthalmic artery= (A. ophthalmica externa) emerges from the anterior opening of the alar canal, and enters the apex of the periorbita or ocular sheath. Within this it forms a semicircular bend under the rectus oculi superior and is continued by the ethmoidal artery. Its branches are as follows:

(a) The =supraorbital= or =frontal artery= (A. supraorbitalis)[161] is a small vessel which often arises from the anterior deep temporal or the internal maxillary. It passes along the inner wall of the orbit in company with the nerve of the same name to the supraorbital foramen, through which it emerges. It is distributed to the orbicularis oculi, the corrugator supercilii, and the skin of the supraorbital region.

(b) The =lacrimal artery= (A. lacrimalis) runs upward and forward within the periorbita along the outer edge of the levator palpebræ superioris to the lacrimal gland, in which it is chiefly distributed. It also sends twigs to the upper eyelid.

(c) =Muscular branches= (Rami musculares) supply the orbital muscles, the periorbita, the third eyelid, and the conjunctiva.

(d) =Ciliary branches= (Aa. ciliares), two sets of very slender arteries, arise from the ophthalmic direct and from the muscular branches. The anterior ciliary arteries (Aa. ciliares anteriores) pierce the sclera in front of the equator and ramify chiefly in the ciliary body and the iris. The posterior ciliary arteries (Aa. ciliares posteriores) pierce the posterior part of the sclera; most of them ramify in the choroid coat as the short ciliary arteries, but two of larger size, the long ciliary arteries, run forward, one on each side, between the sclera and choroid to the periphery of the iris. Here they divide into branches which anastomose and form a circle (Circulus iridis major). From this secondary branches are detached which form a second circle around the pupil (Circulus iridis minor).

(e) The =central artery of the retina= (Arteria centralis retinæ) is a small vessel which arises from the ophthalmic or from a posterior ciliary artery. It pierces the optic nerve a short distance behind the sclera and runs in its center to the lamina cribrosa, where it breaks up in thirty to forty fine branches. These appear in the fundus of the eye at the margin of the optic papilla and radiate in the posterior part of the retina.

(f) The =ethmoidal artery= (A. ethmoidalis) is the continuation of the ophthalmic. It enters the cranial cavity through the ethmoidal foramen, passes inward on the cribriform plate, and divides into meningeal and nasal branches. The former ramify in the anterior part of the dura mater and anastomose with branches of the artery of the corpus callosum. The nasal branch passes through the cribriform plate, gives branches to the mucous membrane of the lateral mass of the ethmoid and the adjacent part of the septum nasi, and runs forward on the superior turbinal.

The =third part= passes forward in the pterygo-palatine fossa, accompanied by branches of the maxillary nerve. On reaching the posterior palatine foramen it is continued by the palatine artery. Its branches are as follows:

(1) The =buccinator artery= (A. buccinatoria) arises from the lower aspect of the internal maxillary shortly after its emergence (Fig. 437). It turns around the maxillary tuberosity, accompanied by the buccinator nerve, and under the masseter muscle, enters the cheek, and runs forward in it. It supplies branches to the cheek, the superior buccal glands, and the masseter and pterygoid muscles. Near its origin it gives off a branch to the orbital fat behind the periorbita.

(2) The =infraorbital= or =superior dental artery= (A. infraorbitalis) arises from the upper aspect of the internal maxillary a little in front of the preceding vessel. It passes upward and forward to the maxillary foramen, runs in the infraorbital canal in company with the nerve of the same name, and is continued forward within the jaw to the incisor teeth. It gives branches to the teeth and gums, and detaches a branch through the infraorbital foramen which anastomoses with the lateral nasal and superior labial. About midway between its origin and the maxillary foramen it gives off the =malar= or =orbital branch= (Ramus malaris), which passes along the floor of the orbit to end in the lower lid and anastomose with the angularis oculi. It gives twigs to the inferior oblique muscle and the lacrimal sac.

The infraorbital artery is usually small at its emergence upon the face, but in some cases it is rather large and may partially replace the superior labial and lateral nasal arteries.

(3) The =staphyline= or =small palatine artery= (A. palatina minor) is a small vessel which passes forward in the groove at the inner side of the maxillary tuberosity to the soft palate. In the groove it is accompanied by the nerve of the same name and the palatine vein.

(4) The =sphenopalatine artery= (A. sphenopalatina) arises in the extreme anterior part of the pterygo-palatine fossa and passes into the nasal cavity, where it divides into internal and external branches. The internal branch is distributed to the mucous membrane of the septum nasi; the external one goes to the inferior turbinal, the inferior meatus, the posterior nares, and the maxillary and frontal sinuses. It may arise from the infraorbital.

(5) The =palatine= or =palato-labial artery= (A. palatina major) is the direct continuation of the internal maxillary. It passes through the palatine canal to the roof of the mouth, accompanied by the palatine nerve, and runs forward in the palatine groove, where it is joined by the vein. A little behind the plane of the corner incisor teeth it curves inward over a bar of cartilage to the foramen incisivum, where it unites with its fellow of the opposite side. The single artery thus formed passes up through the foramen and divides under the transverse dilator of the nostril into two branches. These ramify in the upper lip and anastomose with the lateral nasal and superior labial arteries. In its course in the roof of the mouth the palatine artery gives off branches to the hard and soft palate and the gums, and others which pass through the accessory palatine foramina to be distributed in the mucous membrane of the lower part of the nasal cavity. Commonly two branches, right and left, are detached from the convexity of the arch formed by the union of the two arteries; these run forward in the anterior part of the hard palate.

ARTERIES OF THE THORACIC LIMB

THE BRACHIAL ARTERY

The =brachial artery=, after turning around the lower border of the scalenus at the first rib, passes backward and a little downward across the origin of the coraco-brachialis and the insertion of the subscapularis muscle at the inner side of the shoulder joint. At the posterior border of the subscapularis it gives off the subscapular artery, and turns downward on the inner surface of the arm. In its course in the arm it inclines a little forward, crosses the humerus very obliquely, and divides at the antero-internal aspect of the distal third of the shaft of the bone into the anterior and posterior radial arteries.[162]

It is related internally to the serratus magnus and the posterior deep pectoral muscles, and the cubital lymph glands. Externally it lies on the subscapularis, the brachialis, the insertion of the teres major and latissimus dorsi, and the inner head of the triceps. The coraco-brachialis muscle lies along the front of the artery, which, however, overlaps the muscle distally. The vein crosses the inner face of the artery and runs down behind it, accompanied part way by the ulnar and radial nerves. The musculo-cutaneous and median nerves form a loop around the artery close to its origin, and the latter nerve then passes down in front of the artery. The chief branches are as follows:

1. The =suprascapular artery= (A. thoracico-acromialis) is a small and somewhat flexuous vessel, which arises near the anterior border of the subscapularis, runs upward, and dips in between that muscle and the supraspinatus. It gives branches to these muscles, the anterior deep pectoral, and the mastoido-humeralis. A branch passes in front of the coraco-brachialis to the shoulder joint and the proximal end of the humerus.

This vessel may be double, or be represented by one or more branches of the inferior cervical artery.

2. The =subscapular artery= (A. subscapularis) is a very large vessel which arises at the posterior border of the subscapularis muscle. It passes upward in the interstice between that muscle and the teres major on the inner surface of the long head of the triceps, turns around the posterior border of the scapula below the posterior angle, and ends in the infraspinatus and deltoid. Besides collaterals (Rami musculares) to the subscapularis, teres major, triceps, and tensor fasciæ antibrachii it gives off the following named branches:

(a) The =thoracico-dorsal= (A. thoracicodorsalis) is a long artery which is usually given off about an inch from the origin of the subscapular, crosses the inner face of the teres major, and runs upward and backward on the latissimus dorsi. It gives branches to these muscles, the abdominal panniculus and the axillary lymph glands.

(b) The =posterior circumflex artery= (A. circumflexa humeri posterior) arises a little above the preceding vessel and passes outward behind the shoulder joint between the long and external heads of the triceps with the axillary (or circumflex) nerve. It gives branches to these muscles, the joint capsule, and the muscles and skin of the outer side of the shoulder, anastomosing with the anterior circumflex artery.

(c) The =circumflex artery of the scapula= (A. circumflexa scapulæ) arises about two or three inches (ca. 5 to 7.5 cm.) above the shoulder joint, passes forward to the posterior border of the scapula, and divides into two branches. The outer one runs forward on the external surface of the scapula below the spine and gives branches to the supraspinatus, infraspinatus, and teres minor. The inner branch passes forward in a similar fashion on the costal surface of the scapula and supplies branches to the subscapularis.

3. The =anterior circumflex= (or prehumeral) =artery= (A. circumflexa humeri anterior) arises usually at the upper border of the teres major. It passes forward and outward between the two parts of the coraco-brachialis or between the latter and the humerus. It gives branches to the coraco-brachialis and deep pectoral and ends in the upper part of the biceps and in the mastoido-humeralis. It anastomoses with the posterior circumflex.

Not rarely it arises below the middle of the humerus and passes upward between the biceps and coraco-brachialis. In these cases a small branch for the coraco-brachialis is usually given off at the usual point of origin of the anterior circumflex.

4. The =deep brachial artery= (A. profunda brachii) is a large but short trunk which arises usually about the middle of the humerus. It passes backward and divides into several branches which supply the triceps, tensor fasciæ antibrachii, anconeus, and brachialis. A branch runs in the musculo-spiral groove with the radial nerve to the front of the elbow joint and anastomoses with the anterior radial. A slender branch passes down along the external border of the extensor carpi and supplies cutaneous twigs. Anastomoses occur with the ulnar and recurrent interosseous arteries.

The point of origin is inconstant and it is not uncommon to find two arteries instead. Often a large branch for the posterior deep pectoral muscle is detached close to the origin or arises from the brachial directly.

5. =Muscular branches= (Rami musculares) are distributed to the teres major, deep pectoral, coraco-brachialis, and biceps. The largest and least variable of these supplies the lower part of the biceps.

6. The =ulnar artery= (A. collateralis ulnaris superior) arises a little below the nutrient foramen of the humerus and passes downward and backward along the lower edge of the internal head of the triceps under cover of the brachial vein and the tensor fascia antibrachii. It gives branches to these muscles, the posterior superficial pectoral, the cubital lymph glands, panniculus, and skin. At the internal epicondyle of the humerus it is joined by the ulnar nerve and turns downward under the ulnar head of the flexor carpi medius. It continues with the vein and nerve under the deep fascia of the forearm between the ulnar and humeral heads of the perforans, and in the distal half of the region between the external and middle flexors of the carpus. It unites just above the carpus (under cover of the flexor carpi externus) with a branch of the radial artery, with which it forms the supracarpal arch. It detaches small collaterals to the muscles along which it passes and terminal twigs to the outer surface of the carpus.

7. The =nutrient artery of the humerus= (A. nutritia humeri) is a short vessel which enters the nutrient foramen of the humerus. It often arises from the ulnar.

8. The =anterior radial artery= (A. collateralis radialis inferior) passes downward and a little outward on the anterior face of the humerus under cover of the biceps and brachialis to the front of the elbow joint, where it is joined by the radial nerve. It then descends on the anterior surface of the radius under cover of the anterior extensor of the digit to the carpus, where it concurs in the formation of the rete carpi dorsale, anastomosing with the posterior radial and interosseous arteries. It supplies branches to the elbow joint, the biceps, brachialis, and the extensors of the carpus and digit. A cutaneous branch emerges between the distal end of the biceps and the brachialis.

THE POSTERIOR RADIAL ARTERY

The =posterior radial= or =median artery= (A. mediana) is the direct continuation of the brachial. It passes downward and slightly backward, at first on the inner surface of the humerus, and then over the capsule and internal lateral ligament of the elbow joint, under cover of the posterior superficial pectoral muscle.[163] Below the elbow it dips under the flexor carpi internus and passes down the inner part of the posterior surface of the radius. In the distal part of the forearm it inclines backward and is separated from the radius by the reinforcing band (Caput tendineum) of the superficial flexor of the digit and is continued by the large metacarpal artery.

It is accompanied by the median nerve, which lies in front of the artery at its origin, then crosses over it obliquely at the elbow joint and becomes posterior. In the arm the brachial vein lies behind and partly upon the artery; lower down there are usually two satellite veins, anterior and posterior. The chief collateral branches are as follows:

1. =Articular branches= for the elbow joint.

2. =Muscular branches= to the flexors of the carpus and digit. The largest of these arise at the proximal third of the forearm.

3. The =common interosseous artery= (A. interossea communis) is a vessel of considerable size which arises at the level of the interosseous space, through which it passes. Before entering the space it gives off a small branch, the volar interosseous (A. interossea volaris), which descends to the radial head of the perforans. In the space it supplies the nutrient arteries of the radius and ulna. Emerging from the space it gives off branches to the flexor carpi externus, a small =recurrent branch= (A. interossea recurrens) which passes upward on the outer surface of the ulna and anastomoses with the deep brachial and ulnar arteries, and is continued as the =dorsal interosseous artery=. This vessel (A. interossea dorsalis) passes down between the anterior and lateral extensors of the digit and concurs with the anterior radial in forming a network on the anterior surface of the carpus, the =rete carpi dorsale=. From the latter arise two small vessels, the =internal= and =external dorsal metacarpal arteries= (A. metacarpea dorsalis medialis, lateralis), which run distally in the grooves between the large and small metacarpal bones and anastomose with the volar metacarpal arteries.

4. The =artery of the rete carpi volare= (A. retis carpi volaris) is a small vessel which arises at the distal third of the forearm and passes downward to the posterior surface of the carpus, where it concurs with branches of the volar metacarpal arteries in forming the rete carpi volare.

5. The =external volar metacarpal artery= (A. metacarpea volaris lateralis) is a small vessel which arises just above the carpus under cover of the flexor carpi medius and anastomoses with the ulnar artery, forming the supracarpal arch. From the latter a branch descends with the outer branch of the median nerve, inclines outward toward the posterior border of the accessory carpal bone, and arrives at the head of the external metacarpal bone. Here it is connected with the internal volar metacarpal artery, usually by two transverse branches, thus forming the =deep volar= or =subcarpal arch= (Arcus volaris profundus). One of these branches lies between the subcarpal check ligament and the suspensory ligament; the other (not always present) lies beneath the latter on the large metacarpal bone. A small branch descends to the fetlock with the external metacarpal nerve. Below the arch the artery pursues a flexuous course downward on the posterior face of the large metacarpal bone alongside of the external small metacarpal and under cover of the suspensory ligament. At the distal third of the metacarpus it commonly unites with the corresponding vessel of the inner side to form a short trunk which passes backward through the angle of divergence of the branches of the suspensory ligament and joins the external digital or the common digital artery.

6. The =internal volar= or =small metacarpal artery= (A. metacarpea volaris medialis) is given off from the posterior radial at an acute angle, usually a little above the external one or by a common trunk with it. It passes down the inner side of the carpus behind the tendon of the flexor carpi internus and embedded in the posterior annular ligament. Arriving at the proximal end of the inner metacarpal bone it becomes more deeply placed and is connected with the external volar artery by one or two transverse branches as stated above. It then pursues a flexuous course downward alongside of the inner small metacarpal bone, like the corresponding external artery, with which it commonly unites as described above. It is larger than the external artery and supplies the nutrient artery to the large metacarpal bone.

The foregoing account describes the most common arrangement of the dorsal and volar metacarpal arteries. Variations in their origin and connections are common, but have no great surgical importance. Collateral branches are omitted for the same reason. In some cases the internal volar metacarpal is connected with the common digital or large metacarpal artery a little below the carpus by a branch passing obliquely across the inner border of the deep flexor tendon; this forms a superficial volar arch.

THE COMMON DIGITAL ARTERY

The =common digital= or =large metacarpal artery=[164] is the direct continuation of the posterior radial. It descends in the carpal canal along the inner side of the flexor tendons in company with the inner branch of the median nerve and a satellite vein. Continuing down the limb it preserves this relation to the tendons to the distal third of the metacarpus, where it inclines toward the middle line of the limb behind the suspensory ligament, and divides into the internal and external digital arteries. In the metacarpus the artery is related to the vein in front and the artery behind, and is covered by the fascia and skin. It furnishes collateral branches to the suspensory ligament, the flexor tendons, and the skin.

THE DIGITAL ARTERIES

The =internal= and =external digital arteries= (A. digitalis medialis, lateralis) are formed by the bifurcation of the common digital at the distal third of the metacarpus. They diverge, pass down over the outer surface of the corresponding sesamoid at the fetlock, and descend parallel with the borders of the deep flexor tendon to the volar grooves and foramina of the third phalanx. Entering the latter the two arteries unite in the semilunar canal and form the =terminal arch= (Arcus terminalis), from which numerous branches pass through the bone to the wall surface and ramify in the matrix of the wall and sole of the hoof. A number of branches emerge through the foramina at the lower border, where they anastomose with each other in arciform fashion. These branches were named by Spooner the inferior communicating arteries, and the anastomotic arch formed by them is termed the circumflex artery of the third phalanx (Chauveau) or the artery of the lower border of the third phalanx (Leisering).

Each artery is accompanied by a vein and by the digital nerves. Above the fetlock the artery is most deeply placed and is covered by the vein; the nerve is behind the vein. At the fetlock the artery has become superficial and is related to the vein in front and the posterior branch of the nerve behind. The anterior branch of the nerve crosses over the artery obliquely to the lateral aspect of the first phalanx. The vessels and nerves are crossed obliquely by a small tendinous band, the ligament of the ergot, which begins in the fibrous basis of the ergot at the back of the fetlock and extends downward and forward to end in the fascia at the side of the pastern joint.

In addition to branches to the joints, tendons and synovial sheath, ergot, and skin, the digital arteries give off:

1. The =artery of the first phalanx= or perpendicular artery (A. phalangis primæ), a short trunk which arises at a right angle about the middle of the first phalanx, and divides into dorsal and volar branches (Rami dorsales et volares). The dorsal (anterior) branch passes between the first phalanx and the extensor tendon and ramifies on the front of the digit, anastomosing with its fellow. The volar (posterior) branch dips in between the flexor tendons and the inferior sesamoidean ligaments and anastomoses with the opposite artery.

2. The =artery of the plantar cushion= (A. toricæ phalangis tertiæ) arises at the proximal border of the lateral cartilage and passes backward and downward to ramify in the plantar cushion and the matrix of the heels and frog.

3. The =dorsal artery of the second phalanx= (Ramus dorsalis phalangis secundæ) arises a little above the navicular bone, and passes forward under cover of the lateral cartilage and the extensor tendon to the front of the second phalanx, where it anastomoses with the opposite vessel. It gives branches to the skin, the tendon, the coffin joint, and the coronary matrix of the hoof.

4. The =volar artery of the second phalanx= (Ramus volaris phalangis secundæ) is smaller than the preceding, opposite to which it arises. It passes above the proximal border of the third sesamoid and unites with the opposite artery.

The arteries of the second phalanx form what is termed by Chauveau the =coronary circle=. The dorsal (anterior) part of the circle gives off commonly an artery (A. coronalis phalangis tertiæ) near either side of the extensor tendon, which divides into two branches. The central branch unites with that of the opposite side, while the other joins a branch of the artery of the plantar cushion. In this way is formed the =circumflex artery of the coronary cushion=, an anastomotic arch which lies on the extensor tendon at the coronet. In some cases descending branches of the arteries of the first phalanx concur in the formation of the arch.

5. The =dorsal artery of the third phalanx= or preplantar artery (A. dorsalis phalangis tertiæ) arises at the deep face of the wing of the third phalanx, passes outward through the notch or foramen there, and runs forward in the groove on the wall surface. It gives off ascending and descending branches, which ramify in the matrix of the wall of the hoof, anastomosing above with the circumflex artery of the coronary cushion and below with the circumflex artery of the third phalanx. Before passing through the wing it detaches a retrograde branch to the plantar cushion, and after emerging one which ramifies on the outer aspect of the lateral cartilage.

The terminal part of the digital artery after giving off the preceding vessel is sometimes termed the plantar.

BRANCHES OF THE THORACIC AORTA

In addition to the coronary arteries and the common brachiocephalic trunk (which have been described), the thoracic part of the aorta gives off branches to the thoracic walls and viscera and to the spinal cord and its membranes. The =visceral branches= (Rami viscerales) are the bronchial and œsophageal, which arise by a broncho-œsophageal trunk. The =parietal branches= (Rami parietales) are the intercostal and phrenic arteries.

1. The =broncho-œsophageal= (Truncus broncho-œsophageus) is a short, usually bulbous, trunk which arises at the sixth thoracic vertebra from the aorta or in common with the first aortic intercostal arteries. It passes (under cover of the vena azygos) down the right face of the aorta toward the bifurcation of the trachea and divides into bronchial and œsophageal branches.

(a) The =bronchial artery= (A. bronchialis) crosses the left face of the œsophagus to the bifurcation of the trachea, where it divides into right and left branches. Each enters the hilus of the corresponding lung above the bronchus, which it accompanies in its ramification. It supplies the lung tissue and also detaches twigs to the bronchial lymph glands and the mediastinum.

(b) The =œsophageal artery= (A. œsophagea) (Fig. 429) is a small vessel which passes backward above the œsophagus in the posterior mediastinum and anastomoses with the œsophageal branch of the gastric artery. It detaches twigs to the œsophagus and the mediastinal lymph glands and pleura, and also gives off two branches which pass between the layers of the ligaments of the lungs and ramify in the subpleural tissue. Very commonly there is another artery which runs backward ventral to the œsophagus.

In some cases there is no broncho-œsophageal trunk, the bronchial and œsophageal arising separately. In other cases the second aortic intercostal arises in common with them also.

2. The =intercostal arteries= (Aa. intercostales) (Figs. 428, 429) number eighteen pairs. The first arises from the deep cervical artery, the next three from the subcostal branch of the dorsal artery, and the remainder from the aorta. The aortic intercostals arise from the dorsal face of the aorta in pairs close together; the fifth and sixth usually spring from a common stem. Each passes across the body of a vertebra to the corresponding intercostal space, detaches twigs to the vertebræ and the pleura, and divides into dorsal and ventral branches. The =dorsal branch= (Ramus dorsalis) gives off a spinal branch (Ramus spinalis) which passes through the intervertebral foramen, gives twigs to the membranes of the spinal cord, perforates the dura, and reinforces the ventral spinal artery. A muscular branch passes to the muscles and skin of the back. The =ventral branch= (Ramus ventralis) is much the larger. It passes downward, at first almost in the middle of the intercostal space between the intercostal muscles, then gains the posterior border of the rib and is subpleural. Each is accompanied by a vein and nerve, the artery being in the middle and the vein in front. At the lower part of the space it unites with a ventral intercostal branch of the internal thoracic or the asternal artery. It supplies the intercostal muscles, the ribs and the pleura, and gives off perforating branches which pass out to the serratus magnus, the abdominal muscles, and the skin.

3. The =phrenic arteries= (Aa. phrenicæ) are two or three small vessels which arise at the hiatus aorticus from the ventral aspect of the aorta, often by a common trunk. They supply the crura of the diaphragm. In some cases they arise in common with an intercostal artery.

BRANCHES OF THE ABDOMINAL AORTA

The collateral branches of the abdominal part of the aorta are distributed chiefly to the walls and contents of the abdominal cavity, but some branches are supplied to the spinal cord and its membranes, and others extend into the pelvis and to the scrotum. The =visceral branches= are the cœliac, anterior mesenteric, renal, posterior mesenteric, and the spermatic or utero-ovarian. The =parietal branches= are the lumbar arteries.

I. The =cœliac artery= or =axis= (A. cœliaca) is an unpaired vessel, usually half an inch or less (ca. 1 cm.) in length, which arises from the ventral aspect of the aorta at its emergence from the hiatus aorticus. It divides on the dorsal surface of the pancreas into three branches—the gastric, hepatic, and splenic.

1. The =gastric artery= (A. gastrica sinistra) passes downward and forward in the gastro-phrenic ligament, gives off œsophageal and pancreatic branches, and divides above and behind the cardia into anterior and posterior branches.

(a) The =anterior branch= (Ramus cranialis) crosses the lesser curvature just to the right of the cardia and ramifies on the parietal surface of the stomach. The branches pursue a flexuous course toward the greater curvature and anastomose with the short gastric arteries and the gastric branch of the hepatic artery.

(b) The =posterior branch= (Ramus caudalis) is distributed in a similar fashion on the visceral surface.

(c) The =œsophageal branch= (Ramus œsophageus) passes through the hiatus œsophageus into the thoracic cavity above the œsophagus and anastomoses with the œsophageal branch of the broncho-œsophageal artery.

The gastric artery often arises by a common trunk with the splenic. The two terminal branches may arise separately, or the anterior from the splenic and the posterior from the hepatic. The œsophageal branch often arises from the splenic or the posterior gastric.

2. The =hepatic artery= (A. hepatica) is larger than the gastric. It passes forward to the right and ventrally on the dorsal surface of the pancreas, covered by the gastro-pancreatic fold, crossing obliquely under the vena cava, and reaches the inner border of the portal vein. It divides into three or four branches which enter the portal fissure of the liver and ramify within the gland with the portal vein and the hepatic duct. It gives off the following collateral branches:

(a) =Pancreatic branches= (Aa. pancreaticæ) are given off as the artery crosses the pancreas, in which it is partly embedded.

(b) The =pyloric artery= (A. gastrica dextra) arises above the first curve of the duodenum. It descends to the pylorus, sending branches to the first part of the duodenum and the pylorus, and anastomoses with the gastric and right gastro-epiploic arteries. It may arise from the gastro-duodenal.

(c) The =gastro-duodenal artery= (A. gastroduodenalis) passes to the second curve of the duodenum and divides into the right gastro-epiploic and the pancreatico-duodenal. The =right gastro-epiploic artery= (A. gastroepiploica dextra) crosses over the posterior surface of the duodenum and enters the great omentum, in which it runs to the left, parallel with the greater curvature of the stomach. It gives branches to the latter and to the omentum and forms an anastomotic arch with the left gastro-epiploic artery. The =pancreatico-duodenal artery= (A. pancreaticoduodenalis) divides into pancreatic and duodenal branches. The former (Ramus pancreaticus) supplies the middle part of the pancreas and is often replaced by a number of variable twigs. The duodenal branch (Ramus duodenalis) passes to the right along the lesser curvature of the duodenum and anastomoses with the first branch of the anterior mesenteric artery.

3. The =splenic artery= (A. lienalis) is the largest branch of the cœliac. It passes to the left (with the large satellite vein) on the left extremity of the pancreas and across the saccus cæcus of the stomach. Entering the suspensory ligament of the spleen, it runs in the hilus of the spleen to the apex, beyond which it is continued as the left gastro-epiploic. It gives off the following branches:

(a) =Pancreatic branches= (Rami pancreatici) to the left extremity of the pancreas.

(b) =Splenic branches= (Rami lienales), which plunge into the substance of the spleen.

(c) =Gastric branches= (Aa. gastricæ breves), which pass in the gastro-splenic omentum to the greater curvature of the stomach, where they bifurcate and anastomose with the gastric arteries.

(d) The =left gastro-epiploic artery= (A. gastroepiploica sinistra) is the continuation of the splenic artery. It passes to the right in the great omentum, parallel with the greater curvature of the stomach, and anastomoses with the right gastro-epiploic. It gives off branches to the greater curvature of the stomach, and twigs to the omentum.

II. The =anterior= or =great mesenteric artery= (A. mesenterica cranialis) arises from the ventral face of the aorta at the first lumbar vertebra. It is a large unpaired trunk, about an inch (ca. 2 to 3 cm.) in length, which passes ventrally between the vena cava and the left adrenal into the root of the great mesentery, when it divides into three branches—left, right, and anterior.[165]

1. The =left branch= gives off at once about fifteen to twenty =arteries of the small intestine= (Aa. intestinales).[166] These come off close together and pass in divergent fashion between the layers of the great mesentery, each dividing into two branches which anastomose with adjacent branches to form a series of arches. In the anterior part of the series secondary arches are formed by the union of branches given off from the primary set of arches. From the convex side of these arches terminal branches pass to the wall of the small intestine, in which they ramify and form a vascular network. They are accompanied by satellite veins and by nerves and lymph vessels. The first artery anastomoses with the pancreatico-duodenal, and the last with the ileal branch of the great mesenteric artery. Branches are supplied to the mesenteric lymph glands.

2. The =right branch= (A. ileo-cæco-colica) might be regarded as the continuation of the trunk. It runs downward and a little forward and to the right and gives off the ileal, the two cæcal, and the ventral colic arteries.

(1) The =ileal= or =ileo-cæcal artery= (A. ilea) passes in retrograde fashion along the terminal part of the ileum and unites with the last branch of the left division.

(2) The =external= or =inferior cæcal artery= (Ramus cæcalis lateralis) passes between the cæcum and the origin of the colon and runs on the external muscular band of the cæcum to the apex, where it anastomoses with the internal artery. Besides numerous collaterals to the cæcum, it gives off the artery of the arch, which passes along the lesser curvature of the base of the cæcum and runs on the outer face of the origin of the great colon.

(3) The =internal= or =superior cæcal artery= (Ramus cæcalis medialis) passes along the inner band to the apex of the cæcum, where it anastomoses with the external cæcal artery.

(4) The =ventral=, =right=, or =direct colic artery= (A. colica ventralis) runs along the opposed surfaces of the ventral (first and second) parts of the great colon to the pelvic flexure, where it unites with the dorsal colic artery. It supplies the ventral parts of the great colon and sends a branch to the base of the cæcum.

3. The =anterior branch= divides after a very short course into the dorsal and middle colic arteries.

(1) The =dorsal=, =left=, or =retrograde colic artery= (A. colica dorsalis) is a large vessel which passes along the dorsal (fourth and third) parts of the great colon to the pelvic flexure, where it joins the ventral colic artery.

(2) The =middle colic artery= or =first artery of the small colon= (A. colica media) is a much smaller vessel which passes to the origin of the small colon, enters the colic mesentery, and forms an arch by joining the first branch of the posterior mesenteric artery close to the lesser curvature of the bowel. It sends an anastomotic branch to the dorsal colic artery.

III. The =renal arteries= (Aa. renales), right and left, are relatively large vessels which arise from the aorta near the anterior mesenteric. The =right artery= is the longer of the two. It crosses over the dorsal surface of the vena cava to the right and somewhat forward. At the hilus it divides into several (five to eight) branches; some of these enter the gland at the hilus, while others pass to the ventral surface and enter there. The =left artery= is short and usually arises a little further back; it passes directly outward to the kidney and is then disposed like the right one. Small collateral branches are supplied to the ureters and the adrenals. The latter also receive small vessels directly from the aorta. The distribution within the kidney has been described.

Variations in the renal arteries are frequent. Two or more arteries may occur on one or both sides. Accessory arteries are more common on the left side and usually enter the posterior pole of the gland. They may arise from the aorta, the external iliac, or the circumflex iliac artery.

IV. The =posterior= or =small mesenteric artery= (A. mesenterica caudalis) is an unpaired vessel which arises from the ventral face of the aorta at the fourth lumbar vertebra, i. e., about five or six inches (ca. 12 to 15 cm.) behind the origin of the anterior mesenteric artery. It is much smaller than the latter and supplies the greater part of the small colon and rectum. It descends in the colic mesentery and divides into two branches. The anterior branch (A. colica sinistra) gives off three or four arteries which divide and form anastomotic arches close to the bowel. The first arch is formed by union with the middle colic branch of the anterior mesenteric. The posterior branch or anterior hæmorrhoidal artery (A. hæmorrhoidalis cranialis) passes backward in the upper part of the mesentery and the mesorectum and terminates near the anus by anastomosing with the internal pudic artery. Three or four of its anterior collateral branches form arches.

V. The =spermatic arteries= (Aa. spermaticæ internæ), right and left, are long slender arteries which arise from the aorta near the posterior mesenteric and supply the testicle and epididymis. Each passes backward in a narrow fold of peritoneum (Plica vasculosa) to the internal inguinal ring and descends through the inguinal canal to the scrotum. In the canal and scrotum it pursues a very flexuous course in the anterior border of the spermatic cord, surrounded by the pampiniform plexus of veins, and associated closely with the spermatic nerves and lymphatics and unstriped muscle-fibers. It passes between the epididymis and testicle, runs in flexuous fashion along the attached border of the latter, turns around the posterior pole, and runs forward on the free border to the anterior pole. The largest branches arise from its ventral part, pass tortuously up either side of the gland embedded in the tunica albuginea, and give off fine branches to the gland substance. Small collateral branches are detached to the ureter and the spermatic cord.

The =utero-ovarian arteries= in the female correspond to the preceding vessels, but are much shorter. Each is placed in the anterior part of the broad ligament of the uterus and divides into ovarian and uterine branches. The =ovarian branch= (A. ovarica) pursues a flexuous course to the ovary, which it supplies. The =uterine branch= (A. uterina cranialis) passes to the concave border of the cornu of the uterus, which it supplies, anastomosing with the uterine artery.

VI. The =lumbar arteries= (Aa. lumbales) are in series with the intercostal arteries and have a similar origin and distribution. There are usually six pairs of lumbar arteries, of which five arise from the aorta and the sixth from the internal iliac or the lateral sacral at the junction of the last lumbar vertebra and the sacrum. Each passes across the body of a lumbar vertebra to the intertransverse space, gives branches to the sublumbar muscles, and divides into dorsal and ventral branches. The =dorsal branch= (Ramus dorsalis), the larger of the two, passes upward to ramify in the extensor muscles of the spine and the skin of the loins; it gives off a =spinal branch= (Ramus spinalis) which comports itself like the corresponding branch of an aortic intercostal artery. The =ventral branch= (Ramus ventralis) runs outward in the intertransverse space, passes between the transversus and obliquus internus abdominis, gives branches to these muscles, and ends in the obliquus externus, the panniculus, and the skin of the flank.

THE INTERNAL ILIAC ARTERY

The =internal iliac= or =hypogastric arteries= (Aa. hypogastricæ) result from the bifurcation of the aorta under the fifth or sixth lumbar vertebra. They diverge at an angle of about 60 degrees, and each passes backward under the wing of the sacrum, then inclines downward on the pelvic surface of the shaft of the ilium, and divides a little above the psoas tubercle into iliaco-femoral and obturator arteries. The chief branches are as follows:

1. The last pair of =lumbar arteries= pass up through the foramina at the junction of the last lumbar vertebra and the sacrum.

2. The =internal pudic artery= (A. pudenda interna) arises near the origin of the internal iliac. It passes backward and somewhat downward, at first along the iliac head of the obturator internus, then above the superior ischiatic spine on the inner surface of the sacro-sciatic ligament, perforates the latter and runs for a variable distance in its substance or on its external face. It then re-enters the pelvic cavity, passes backward on the retractor ani to the ischial arch, and divides into the perineal and the artery of the bulb in the male, perineal and artery of the clitoris in the female. It is accompanied by the internal pudic nerve. Its chief branches are as follows:

(1) The =umbilical artery= is given off from the internal pudic about an inch (ca. 2 to 3 cm.) from the origin of that vessel. It is a very large artery in the fœtus, in which it curves downward and forward at the side of the bladder in the edge of the umbilical fold of peritoneum, passes through the umbilical opening, becomes a component of the umbilical cord and ramifies in the fœtal placenta. After birth it extends only to the vertex of the bladder and is much reduced. Its lumen is almost obliterated and its wall is very thick, giving the vessel a cord-like character, hence it is commonly termed the round ligament of the bladder.[167] It gives off small =vesical branches= (Aa. vesicates craniales) to the bladder, and twigs to the prostate and vasa deferens in the male. In the mare a small branch runs forward along the ureter into the broad ligament of the uterus.

(2) The =middle hæmorrhoidal= or =vesico-prostatic artery= (A. hæmorrhoidalis media) in the male arises usually near the prostate and runs backward lateral to the rectum. It supplies branches to the rectum, bladder, urethra, and accessory genital glands. In the female the homologous vessel is much larger and gives off the =posterior uterine artery= (A. uterina caudalis). This runs forward on the side of the vagina, to which it gives branches, and ramifies on the body of the uterus, anastomosing with the anterior and middle uterine arteries. (In some cases this artery arises from the internal iliac or the umbilical.)

(3) The =perineal artery= (A. perinei) is relatively small in the male. It passes upward at the side of the anus, which it supplies, and gives twigs to the bulbo-cavernosus muscle and the skin of the perineum. In the female it is large and is distributed to the anus and vulva, and gives a large branch to the vestibular bulb.

(4) The =artery of the bulb= (A. bulbi urethræ) may be regarded as the direct continuation of the internal pudic in the male. It lies at the side of the urethra above the ischial arch, dips under the bulbo-cavernosus muscle, and ramifies in the corpus spongiosum. Before doing so it gives off a small branch which turns around the ischial arch to reach the dorsum penis, and anastomoses with the deep branch of the obturator.

(4_a_) The =artery of the clitoris= (A. clitoridis) is the homologue in the female of the preceding vessel, but is much smaller. It passes to the ventral surface of the vulva with the internal pudic nerve, supplies the clitoris, and gives twigs to the vulva.

3. The =lateral sacral artery= (A. sacralis lateralis) arises at the lumbo-sacral articulation (Fig. 451). It passes backward under the wing of the sacrum, then along the pelvic surface of the bone below the inferior sacral foramina and the nerves emerging from them, and is continued by the lateral coccygeal artery. The branches are as follows:

(1) =Spinal branches= (Rami spinales) enter the vertebral canal through the foramen between the last lumbar vertebra and the sacrum and through the ventral sacral foramina. They give off branches to the spinal cord and its membranes which reinforce the ventral spinal artery, and others which emerge through the dorsal sacral foramina and supply the muscles and skin of the croup.

(2) The =middle coccygeal artery= (A. coccygea) is an unpaired vessel which arises from the right or left lateral sacral or from a lateral coccygeal artery. It passes backward on the pelvic surface of the sacrum to the median line and continues in that position along the tail between the depressor muscles, supplying these and the skin.

(3) The =posterior gluteal= or =ischiatic artery= (A. glutea caudalis) emerges through the upper part of the sacro-sciatic ligament and runs backward and downward on the latter under cover of the biceps femoris (Fig. 455). It gives branches to that muscle and to the semitendinosus and semimembranosus, and anastomoses with the obturator, deep femoral, and posterior femoral arteries.

(4) The =lateral coccygeal artery= (A. caudalis lateralis ventralis) continues the direction of the lateral sacral, but is much smaller than the preceding vessel. It passes back between the depressor and intertransversales muscles of the tail and divides into two branches which supply twigs to the muscles and skin.

4. The =ilio-lumbar artery= (A. iliolumbalis) arises at a right angle from the internal iliac and runs outward behind the sacro-iliac joint, crossing the ventral surface of the ilium under cover of the iliacus muscle. It gives branches to the ilio-psoas and longissimus, and terminates by turning around the external border of the ilium a little behind the external angle, giving branches to the gluteus medius and tensor fasciæ latæ (Fig. 455).

5. The =anterior gluteal artery= (A. glutea cranialis) is the largest branch of the internal iliac. It arises usually opposite to the preceding vessel and passes outward through the great sacro-sciatic foramen, dividing into several branches as it emerges; these enter the gluteal muscles (Fig. 455).

6. The =iliaco-femoral= or =external circumflex artery= (A. cireumflexa femoris lateralis) passes downward and outward across and beneath the shaft of the ilium, under cover of the iliacus muscle, and dips in between the rectus femoris and vastus externus. It is accompanied by two satellite veins. It gives collateral branches to the ilio-psoas, the glutei, and the tensor fasciæ latæ, supplies the nutrient artery of the ilium, and terminates in the quadriceps femoris (Fig. 455).

7. The =obturator artery= (A. obturatoria), the inner terminal branch of the internal iliac, passes downward and backward on the pelvic surface of the shaft of the ilium along the ventral border of the iliac head of the obturator internus, accompanied by the satellite vein and nerve, which lie in front of the artery (Fig. 451). On reaching the anterior border of the obturator foramen it dips under the obturator internus and passes obliquely through the foramen. In this part of its course it gives off a vesical branch and twigs to the obturator internus and the hip joint. After its emergence from the pelvic cavity it runs backward on the ventral face of the ischium, passes through the semimembranosus and, in the male, enters the crus penis, forming the arteria profunda penis. It anastomoses with the internal pudic, and usually with the external pudic by a branch which runs forward on the dorsum penis. Large collateral branches are detached to the adductors of the thigh, the biceps femoris and semitendinosus, forming anastomoses with the deep femoral and posterior femoral arteries. In the female the terminal part is small and enters the root of the clitoris.

ARTERIES OF THE PELVIC LIMB

The main arterial trunk of each pelvic limb descends as far as the lower border of the popliteus muscle, where it divides into the anterior and posterior tibial arteries. The different parts of the trunk receive special names which correspond to the several regions through which it passes. In the abdomen it is termed the =external iliac artery=, in the proximal two-thirds of the thigh it is called the =femoral artery=, while below this it is termed the =popliteal artery=.

THE EXTERNAL ILIAC ARTERY (Figs. 450, 451)

The =external iliac artery= (A. iliaca externa) arises from the aorta under the fifth lumbar vertebra, usually just in front of the origin of the internal iliac. It descends at the side of the pelvic inlet along the tendon of the psoas minor, crosses the insertion of that muscle, and reaches the level of the anterior border of the pubis, where it becomes the femoral artery. It is covered by the peritoneum and fascia, and is related behind to the corresponding vein. Its branches are as follows:

1. The =circumflex iliac artery= (A. circumflexa ilium profunda) arises from the external iliac at its origin or from the aorta directly. It passes across the iliac fascia toward the external angle of the ilium, near which it divides into two branches. The artery lies between the fascia and the peritoneum and is accompanied by two veins and the external cutaneous nerve of the thigh. The =anterior branch= gives twigs to the iliacus and psoas muscles, and passes downward and forward in the flank between the obliquus internus and transversus abdominis, in which it ramifies. The =posterior branch= perforates the abdominal wall and runs downward on the inner face of the tensor fasciæ latæ to the fold of the flank, supplying branches to that muscle, the panniculus, the precrural lymph glands, and the skin.

2. The =cremasteric artery= (A. spermatica externa) is a very small vessel which arises in a variable manner. It springs most often from the external iliac near its origin, but may come from the circumflex iliac, the aorta between the external and internal iliac, or the latter vessel. It accompanies the cremaster muscle to the inguinal canal, supplies twigs to that muscle, the tunica vaginalis, and other constituents of the spermatic cord (Fig. 450).

2_a_. The =middle uterine artery= (A. uterina media) of the female is regarded as the homologue of the preceding vessel. It has a similar origin, but is a much larger artery, which passes between the layers of the broad ligament and reaches the horn of the uterus close to its junction with the body. It is distributed to both, and anastomoses with the anterior and posterior uterine arteries.

THE FEMORAL ARTERY (Figs. 450, 451)

The =femoral artery= is the main arterial trunk of the thigh. It begins at the level of the anterior border of the pubis, from which it is separated by the femoral vein. It descends almost vertically in the femoral canal behind the sartorius muscle, covered at first by the internal femoral fascia and lower down by the gracilis. After passing over the insertion of the pectineus, it perforates the adductor muscle, crosses in the vascular groove of the posterior surface of the femur, and is continued between the two heads of the gastrocnemius as the popliteal artery. It is related at its origin to the sartorius in front, the femoral vein behind (which separates it from the pectineus), and the iliacus externally. Lower down it is related superficially to the deep inguinal lymph glands, and deeply to the vastus internus, while the saphenous nerve is in front of it, and the vein passes to its external face. The chief branches are as follows:

1. The =prepubic artery= (Truncus pudendo-epigastricus) arises from the femoral artery at its origin, usually by a common trunk with the deep femoral. It passes forward and a little inward and downward across the edge of the inguinal ligament, and then runs on the abdominal surface of the ligament to the inner part of the internal inguinal ring, where it divides over the upper border of the internal oblique muscle into the posterior abdominal and external pudic arteries.

(1) The =posterior abdominal artery= (A. epigastrica caudalis) passes along the external border of the rectus abdominis and anastomoses in the umbilical region with the anterior abdominal artery (Figs. 272). It supplies branches to the rectus and obliquus internus muscles.

(2) The =external pudic artery= (A. pudenda externa) descends on the inguinal ligament through the inner part of the inguinal canal and emerges at the inner angle of the external ring. In the male it divides into the =subcutaneous abdominal artery= and the =dorsal artery of the penis=. The former runs forward a short distance from the linea alba, and gives branches to the superficial inguinal lymph glands, the sheath, and the scrotum. The latter passes to the dorsum penis and ends at the glans as the arteria glandis. It gives off collateral branches (Rami profundi penis) to the corpus cavernosum, one of which usually passes backward and anastomoses with a branch of the obturator artery. Branches are also supplied to the superficial inguinal lymph glands, the prepuce, and the scrotum. In the female the =mammary artery= takes the place of the dorsal artery of the penis; it plunges into the base of the mammary gland.

2. The =deep femoral artery= (A. profunda femoris) arises either by a common trunk with the prepubic or a little distal to it (Fig. 451). It passes backward and downward across the inner face of the femoral vein, then below the pubis in the space between the hip joint and the pectineus muscle. On reaching the obturator externus it inclines more ventrally and outward, passes out between the internal border of the femur and the quadratus femoris, and ramifies in the biceps femoris and semitendinosus. It supplies large collateral branches to the adductor muscles and twigs to the deep inguinal lymph glands and the hip joint.

3. The =anterior femoral artery= (A. femoralis cranialis) arises a little distal to the preceding vessel from the opposite side of the femoral trunk. It passes forward, outward, and a little downward across the deep face of the sartorius, and dips in between the rectus femoris and vastus internus (Fig. 451). It is related externally to the ilio-psoas and the femoral nerve.

In some cases this artery is replaced by a large branch of the external circumflex, which passes between the ilio-psoas and rectus femoris and enters the interstice between the latter muscle and the vastus internus.

4. Innominate =muscular branches= (Rami musculares) of variable size and arrangement are given off to the muscles of this vicinity.

5. The =saphenous artery= (A. saphena) is a small vessel which arises from the femoral about its middle or from a muscular branch, and emerges between the sartorius and gracilis or through the latter to the inner surface of the thigh (Fig. 457). It is joined by the large saphenous vein and the saphenous nerve and passes down superficially on the fore part of the gracilis, continues on the deep fascia of the leg, and divides above the hock into two branches, which accompany the radicles of the vein. It gives off cutaneous twigs and anastomoses with the recurrent tibial artery.

In some cases this anastomosis does not occur. The artery may be larger and directly continuous with the internal tarsal artery.

6. The =articular branch= (A. genu suprema) is a slender artery which arises from the femoral just before it passes through the adductor. It descends along the posterior border of the vastus internus to the inner surface of the stifle joint, where it ramifies.

7. The =nutrient artery of the femur= (A. nutritia femoris) is given off at the middle of the femur and enters the nutrient foramen.

8. The =posterior femoral= or =femoro-popliteal artery= (A. femoris caudalis) is a large vessel which arises from the posterior face of the femoral just before the trunk passes between the two heads of the gastrocnemius (Fig. 455). It is very short and divides into two branches. The ascending branch passes upward and outward between the adductor (in front) and the semimembranosus (behind), and enters the biceps femoris, in which it ramifies. The descending branch passes downward and backward on the external head of the gastrocnemius, then curves upward between the biceps femoris and semitendinosus (crossed externally by the tibial nerve) and divides into branches to these muscles. A branch is detached from the convexity of the curve which passes down between the heads of the gastrocnemius, gives branches to that muscle and the flexor perforans, and is continued by a slender artery which accompanies the tibial nerve and unites with the recurrent tibial artery. A branch ascends alongside of the sciatic nerve between the biceps and semitendinosus and anastomoses with a descending branch of the obturator.

THE POPLITEAL ARTERY

This artery (A. poplitea) is the direct continuation of the femoral. It lies between the two heads of the gastrocnemius, at first on the posterior face of the femur, then on the femoro-tibial joint capsule. It then passes down through the popliteal notch under cover of the popliteus, inclines outward, and divides near the upper part of the interosseous space into anterior and posterior tibial arteries. The satellite vein lies along its inner side. Collateral branches are supplied to the stifle joint and the gastrocnemius and popliteus.

POSTERIOR TIBIAL ARTERY

The =posterior tibial artery= (A. tibialis posterior) is much the smaller of the two terminals of the popliteal. It lies at first between the tibia and the popliteus, then between that muscle and the deep and inner heads of the deep flexor. Lower down it descends along the tendon of the inner head, becomes superficial in the distal third of the leg, and passes behind the tendon. At the level of the tuber calcis it forms an ᔕ-shaped curve, runs downward along the deep flexor tendon accompanied by the plantar nerves, and divides at the lower part of the hock into internal and external plantar arteries. The collateral branches are as follows:

1. The =nutrient artery of the tibia= (A. nutritia tibiæ).

2. =Muscular branches= (Rami musculares) to the popliteus and deep flexor.

3. The =external tarsal artery= (A. tarsea lateralis) arises at the distal end of the leg and is distributed to the outer surface of the hock. It gives off a small recurrent branch which ascends along the external border of the gastrocnemius tendon and anastomoses with a branch of the posterior femoral artery.

4. The =recurrent tibial artery= (A. tibialis recurrens) is given off from the second part of the curve, ascends in relation to the tibial nerve, and anastomoses with the descending branch of the posterior femoral and with the saphenous artery.

5. Small branches (internal calcaneal) to the inner aspect of the hock.

6. The =plantar arteries=, internal and external (A. plantaris medialis, lateralis), are the small terminals of the posterior tibial. They descend along the sides of the tarsal sheath of the deep flexor with the plantar nerves to the proximal part of the metatarsus, where they unite with the perforating tarsal artery to form the =plantar arch= (Arcus plantaris). Four =plantar metatarsal arteries= proceed from this arch. The two slender =superficial plantar metatarsal arteries= (A. metatarsea plantaris superficialis medialis, lateralis) descend on either side of the deep flexor tendon with the plantar nerves and unite with the great metatarsal or with the corresponding digital artery. The two =deep= or =interosseous plantar metatarsal arteries= (A. metatarsea plantaris profundus medialis, lateralis) descend between the suspensory ligament and the corresponding small metatarsal bone and unite near the fetlock with the great metatarsal. The inner artery is the larger of the two and usually appears to be the continuation of the perforating tarsal. It supplies the nutrient artery of the large metatarsal bone. In other cases it is the direct continuation of the internal plantar artery, the external artery only uniting with the perforating tarsal.

ANTERIOR TIBIAL ARTERY

The =anterior tibial artery= (A. tibialis anterior) is much the larger of the two terminal branches of the popliteal. It passes forward through the upper part of the interosseous space and descends with two satellite veins on the outer part, of the front of the tibia, under cover of the tibialis anterior. At the lower part of the leg it deviates to the outer border of the tendon of this muscle, passes on to the capsule of the hock joint, gives off the perforating tarsal artery, and is continued asthe great metatarsal artery. It gives off =muscular branches= to the dorso-lateral group of muscles of the leg and =articular branches= to the hock. The =peroneal= branch (A. peronea) is a variable vessel which descends along the fibula under the lateral extensor; it gives off muscular branches and one which perforates the fascia and divides into ascending and descending cutaneous twigs.

The =perforating tarsal artery= (A. tarsea perforans) is given off under cover of the outer tendon of the peroneus tertius. It passes backward through the vascular canal of the tarsus with a satellite vein and unites on the upper part of the suspensory ligament with the plantar arteries (or only with the external plantar) to form the plantar arch.

In well-injected specimens it is seen that there is a fine arterial network (Rete tarsi dorsale) on the flexor surface of the hock, which is formed by twigs from the anterior tibial and lateral tarsal arteries. From it proceed two very slender =dorsal metatarsal arteries=. The inner one descends in the furrow between the inner small and large metatarsal bones, and anastomoses usually in the proximal part of the metatarsus with the internal superficial plantar metatarsal, uniting sometimes with the deep plantar metatarsal. The external vessel passes down under the periosteum on the anterior face of the large metatarsal bone and becomes lost or joins the large metatarsal artery.

Sometimes the anterior tibial artery passes undivided through the tarsus, gives off the internal deep plantar metatarsal, and is continued as a very large internal superficial plantar metatarsal along the deep flexor tendon, thus resembling the arrangement in the fore limb.

In a few cases the perforating tarsal is a large vessel, directly continuing the anterior tibial, and is continued by a large internal deep plantar metatarsal. The great metatarsal is then small. Other variations are common.

THE GREAT METATARSAL ARTERY

This artery (A. metatarsea dorsalis lateralis) is the direct continuation of the anterior tibial. It passes downward and outward under the extensor brevis and the tendon of the lateral extensor, at first on the joint capsule and then in the oblique vascular groove on the upper part of the large metatarsal bone. It then descends in the groove formed by the apposition of the large and external small metatarsals, inclines inward between the two, and divides on the lower part of the posterior face of the large metatarsal bone into the internal and external digital arteries. It is not usually accompanied by a vein. Beyond this the arterial arrangement is the same as in the thoracic limb.

THE VEINS[168]

PULMONARY VEINS

The terminal =pulmonary veins= (Vv. pulmonales), usually seven or eight in number, return the aërated blood from the lungs and open into the left atrium of the heart. They are destitute of valves. Their tributaries arise in the capillary plexuses in the lobules of the lungs, and unite to form larger and larger trunks which accompany the branches of the bronchi and pulmonary arteries. A very large vein is formed by the union at an acute angle of a trunk from each lung, where the latter are adherent to each other.

SYSTEMIC VEINS

THE VEINS OF THE HEART (Figs. 426, 427)

The =coronary sinus= (Sinus coronarius) is a very short bulbous trunk which receives most of the blood from the wall of the heart. It is situated just below the termination of the posterior vena cava and is covered in part by a thin layer of ventricular muscle-fibers. It opens into the right atrium just below the posterior vena cava. It is formed by the union of two tributaries. The =great cardiac= or =left coronary vein= (V. cordis magna) begins at the left side of the apex of the heart, ascends in the left longitudinal groove and turns backward in the coronary groove, in which it winds around the posterior border of the heart to the right side and joins the coronary sinus. The =middle cardiac= or =right coronary vein= (V. cordis media) begins on the right side of the apex, ascends in the right longitudinal groove and joins the coronary sinus, or opens separately into the atrium just in front of the orifice of the left vein, so that a common trunk (coronary sinus) does not then exist.

The =small cardiac veins= (Vv. cordis minores), three to five in number, are small vessels which return some blood from the right ventricle and atrium; they open into the latter near the coronary groove in spaces between the musculi pectinati.

THE ANTERIOR VENA CAVA (Fig. 429)

The =anterior vena cava= (V. cava cranialis) returns to the heart the blood from the head, neck, thoracic limbs, and the greater part of the thoracic wall. It is formed at the ventral part of the thoracic inlet by the confluence of the two jugular and two brachial veins. Its origin is attached to the first pair of ribs. It passes backward in the anterior mediastinum, at first median and ventral to the common carotid trunk, then deviates to the right of the anterior aorta, and opens into the right atrium opposite to the third rib. The demarcation between vein and atrium is not very distinct. It contains no valves except at the mouths of its radicles. Its length is about five to six inches (ca. 12 to 15 cm.) and its caliber about two inches (ca. 5 cm.) in a subject of medium size. Its right face is crossed by the right phrenic nerve, and on the left it is related to the anterior aorta and brachiocephalic artery. It receives, in addition to small pericardial and mediastinal veins, the following tributaries:

1. The =internal thoracic vein= (V. thoracica interna) is a satellite of the artery of that name. It opens into the anterior vena cava at the first rib.

2. The =vertebral vein= (V. vertebralis) corresponds to the homonymous artery. On the right side it terminates either in front of the deep cervical vein or by a short common trunk with it. On the left side it almost always unites with the deep cervical and vertebral vein to form a common trunk.

3. The =deep= or =superior cervical vein= (V. cervicalis profunda) corresponds to the artery. On the right side it leaves the artery at the first intercostal space, crosses the right face of the trachea and opens into the vena cava; it may form a common trunk with the dorsal or vertebral. On the left side there is nearly always a common trunk for all three.

4. The =dorsal vein= (V. costo-cervicalis) corresponds to the artery. On the right side it leaves the artery on entering the thorax, crosses the right face of the trachea, and opens into the vena cava in front of the deep cervical or by a common trunk with it. On the left side it almost always joins the deep cervical and vertebral to form a short common trunk which crosses the left face of the intrathoracic part of the brachial artery opposite the second rib and opens into the anterior vena cava.

THE VENA AZYGOS

The =vena azygos= (Fig. 429) is an unpaired vessel which arises at the level of the first lumbar vertebra by radicles coming from the psoas and the crura of the diaphragm; it is connected with the first lumbar vein. It passes forward along the right side of the bodies of the thoracic vertebræ, in contact usually with the thoracic duct, which separates the vein from the aorta. At the seventh vertebra it leaves the spine, curves downward and forward over the right side of the thoracic duct, trachea, and œsophagus, and opens into the right atrium opposite the third intercostal space. Its tributaries are:

1. The last fourteen =intercostal veins= (Vv. intercostales) of the right side. On the left side the last four to seven intercostal veins usually empty into the =vena hemiazygos=. This vessel runs on the left side of the aorta from the fourteenth to the eleventh thoracic vertebra, passes between the aorta and the spine, and joins the vena azygos. In its absence its tributaries join the vena azygos.

2. The =œsophageal vein= (V. œsophagea), satellite of the œsophageal artery, joins the vena azygos as it inclines downward.

3. The =bronchial vein= (Vena bronchialis) unites with the preceding to form a short common trunk (Ellenberger-Baum), or empties into the great coronary vein (Chauveau).[169]

VEINS OF THE HEAD AND NECK

JUGULAR VEINS

The =jugular veins= (Vv. jugulares), right and left (Fig. 431), arise behind the posterior border of the lower jaw about two and a half inches (ca. 6 to 7 cm.) below the temporo-maxillary articulation by the union of the superficial temporal and internal maxillary veins. Each passes downward and backward, at first embedded more or less in the parotid gland, and continues in the jugular furrow to the thoracic inlet, where it unites with its fellow and the two brachial veins to form the anterior vena cava. In the neck it is covered by the skin, fascia, and panniculus, and is superficial to the carotid artery, from which it is separated in the anterior two-thirds of the region by the omo-hyoideus muscle.[170] It contains valves at the mouths of its tributaries and has several pairs of semilunar valves variably disposed along its course. Its tributaries are as follows:

1. The =internal maxillary vein= (V. maxillaris interna) is larger than the external maxillary. It may be considered to begin as the continuation backward of the buccinator vein where the vessel crosses the alveolar border of the mandible (about two inches (ca. 5 cm.) behind the last molar tooth). It runs backward on the inner surface of the ramus below the external pterygoid and covered by the internal pterygoid muscle for a distance of about three inches (ca. 7 to 8 cm.), then inclines a little downward and runs ventral to the artery for about an inch (ca. 2 to 3 cm.). It crosses the external face of the artery at the posterior border of the jaw, and is joined by the superficial temporal vein to form the jugular. Its principal radicles are:

(1) The =dorsal lingual vein= (V. dorsalis linguæ), which is a satellite of the lingual nerve.

(2) The =inferior alveolar= or =dental vein= (V. alveolaris mandibulæ), a satellite of the corresponding artery. It often unites with the preceding.

(3) =Pterygoid veins= (Rami pterygoidei).

(4) The =deep temporal vein= (V. temporalis profunda) is a large vessel which receives tributaries from the temporalis muscle and emissaries from the parieto-temporal canal. It is connected with the anterior cerebral vein and usually with the meningeal veins by its frontal branch. The latter drains chiefly the lacrimal gland and passes behind the supraorbital process.

2. The =superficial temporal vein= (V. temporalis superficialis) is a satellite of the corresponding artery. It is formed by the confluence of the =anterior auricular= and =transverse facial veins=. The former receives the =superior cerebral vein= (V. cerebralis dorsalis), which is the emissary of the transverse sinus of the dura mater; it emerges from the parieto-temporal canal behind the postglenoid process. The =transverse facial vein= (V. transversa faciei) runs at first above the artery of like name, then plunges deeply into the masseter and unites in front with the facial vein. It is connected with the vena reflexa.

3. The =inferior masseteric= or =maxillo-muscular vein= (V. masseterica) joins the jugular at the upper border of the sterno-cephalicus tendon. It is a short trunk formed by the confluence at the posterior border of the jaw of masseteric and pterygoid veins. The former is usually connected by a large branch with the buccinator vein.

4. The =great auricular vein= (V. auricularis magna) is a satellite of the posterior auricular artery above, but joins the jugular a variable distance below and behind the point of origin of the artery.

5. The =inferior cerebral vein= (V. cerebralis ventralis) is an emissary of the cavernous sinus of the dura. It is a satellite of the internal carotid artery, receives the =condyloid vein=, and joins the jugular near the occipital vein or by a common trunk with it.

6. The =occipital vein= (V. occipitalis) arises in the fossa atlantis by the union of muscular and cerebrospinal branches. The former comes from the muscles of the poll and passes through the foramen transversarium. The latter is connected with the occipital sinus of the dura mater and emerges from the spinal canal by the intervertebral foramen.

7. The =external maxillary= or =facial vein= (V. maxillaris externa) arises by radicles which correspond in general to the branches of the artery of like name. It passes down over the cheek along the anterior border of the masseter muscle behind the artery, crossing over the parotid duct, which lies behind the vein lower down. Thus on the ramus and as they turn around its lower border the artery is in front, the vein in the middle, and the duct posterior. In the submaxillary space the vein is ventral to the artery for some distance, then parts company with the artery, runs straight backward along the lower border of the parotid gland, and opens into the jugular vein at the posterior angle of the gland.[171] The chief differences in the tributaries of the vein as compared with the branches of the corresponding artery are as follows:

The =labial veins= (Vv. labiales) form a plexus in the submucous tissue of the cheek from which two veins emerge. The upper one passes back and joins the buccinator vein. The lower one (V. labialis communis) joins the external maxillary vein.

Three veins connect with the external maxillary at the anterior border of the masseter.

The upper one is the =transverse facial=, which unites close to the end of the facial crest.

A little lower is the large valveless =vena reflexa= or =alveolar vein=. This passes back under the upper part of the masseter on the maxilla, turns around the tuber maxillare, perforates the periorbita, and joins the ophthalmic vein. It is relatively small at each end, but presents one or two large fusiform dilatations. It receives the following tributaries: (a) The =palatine vein= (V. palatina major), which separates from the palatine artery at the anterior palatine foramen and passes in the groove between the tuber maxillare and the palate bone. The palatine veins form a very rich plexus of valveless vessels in the submucosa of the hard palate, which consists of several layers anteriorly. (b) The =sphenopalatine vein= (V. sphenopalatina) forms a rich plexus of valveless vessels on the turbinal bones and the septum nasi. It is usually joined by the =infraorbital vein= (V. infraorbitalis) to form a short common trunk.

The venous plexuses are remarkably developed in certain parts of the nasal mucosa. On the septum a little below its middle and on the turbinals the veins are in several layers. The olfactory region does not share in this arrangement and the veins here are small and join the ethmoidal vein.

(c) The =ophthalmic vein= (V. ophthalmica) is a short trunk, connected in front with the vena reflexa and behind with the cavernous sinus through the foramen lacerum orbitale. It receives veins which correspond to the arterial branches.

The =buccinator vein= (V. buccinatoria) extends backward from the external maxillary along the lower border of the depressor labii inferioris and buccinator under cover of the masseter, passes between the ramus of the mandible and the tuber maxillare and is continued as the internal maxillary vein. It has a large fusiform dilatation and is valveless. It receives a large common labial vein from the labial plexus, and is usually connected with the masseteric vein.

The =lingual vein= (V. lingualis) is not a satellite of the artery. It is formed at the side of the lingual process of the hyoid bone by the confluence of several veins which come from the substance of the tongue. One or two considerable vessels run partly in the substance of the hyo-glossus, and another in the genio-hyoideus. The vein is at first covered by the mylo-hyoideus, perforates that muscle, runs back along the omo-hyoideus in relation to the submaxillary lymph glands, and joins the external maxillary near the posterior border of the jaw. Near its termination it receives the =sublingual vein=, or the latter may open into the external maxillary directly.

8. The =thyroid vein= (V. thyreoidea) is a large vessel which joins the jugular near the external maxillary vein. It receives anterior thyroid, laryngeal, and pharyngeal radicles, and sometimes a posterior thyroid vein.

9. =Muscular=, =tracheal=, and =œsophageal= veins.

10. The =cephalic vein= (Vena cephalica) enters the jugular near its termination. It will be described with the veins of the thoracic limb.

11. The =inferior cervical vein= (V. cervicalis ascendens) accompanies the artery. It may open into the brachial vein.

THE SINUSES OF THE DURA MATER

These (Sinus duræ matris) are blood-spaces between the meningeal and periosteal layers of the dura mater and are lined with endothelium. In many places the lumen is crossed by fibrous strands. They receive the veins of the brain, communicate with the meningeal and diploic veins, and with veins outside of the cranium; their connections with the latter are by means of small =emissary veins= (Emissaria). They convey the blood directly or indirectly to the jugular veins. Some are paired, others unpaired. They may be divided into dorsal and basilar systems. The =dorsal system= comprises the following:

The =superior longitudinal= or =sagittal sinus= (S. sagittalis superior) is situated in the upper border of the falx cerebri along the internal sagittal crest. It begins at the crista galli and ends at the tentorium osseum by dividing into two transverse sinuses. It receives the superior cerebral veins. Along each side are small pouches (Lacunæ laterales) into which the veins open. The lumen of the sinus is traversed by fibrous bands and is partially divided by a longitudinal septum.

The =transverse sinuses= (S. transversi), right and left, pass outward in the transverse grooves of the parietal bones, enter the parieto-temporal canals, and are continued by the superior cerebral veins to the superficial temporal veins. The two sinuses are connected by the sinus communicans.

The =inferior longitudinal= or =sagittal sinus= (S. sagittalis inferior) runs backward on the upper surface of the corpus callosum along the concave edge of the falx cerebri and joins the great cerebral vein (of Galen) to form the straight sinus.

The =straight sinus= (S. rectus) passes upward and backward between the cerebral hemispheres and in the tentorium cerebelli and joins the superior sagittal sinus. The point of meeting is the confluence of the sinuses (Confluens sinuum).

The =occipital sinuses= (S. occipitales) lie on either side of the vermis cerebelli. They empty anteriorly into the sinus communicans and communicate behind with the spinal veins.

The =superior petrosal sinuses= (S. petrosi superiores) pass in the tentorium cerebelli to end in the transverse sinuses.

The =basilar system= consists of the following:

The =cavernous sinuses= (S. cavernosi) lie in the inner grooves of the root of the temporal wings of the sphenoid bone at either side of the sella turcica. The two are connected by a wide traverse branch (Sinus intercavernosus) behind and below the posterior part of the pituitary body. Each is continuous in front with the ophthalmic vein and below with the inferior petrosal sinus. The third, sixth, and the ophthalmic and maxillary divisions of the fifth nerve lie along the outer wall of the sinus. The internal carotid artery traverses the sinus and is connected with its fellow by a transverse branch which lies in the corresponding part of the sinus. An oval opening in the floor communicates with the inferior petrosal sinus and transmits the internal carotid artery.

The sinus is not subdivided by strands of fibrous tissue as in man, but a few delicate bands attach the artery to its wall.

The =inferior petrosal sinuses= (S. petrosi inferiores)[172] lie along the borders of the basilar part of the occipital bone, inclosed in the thick dura which closes the foramen lacerum. The anterior part extends about half an inch (ca. 12 mm.) under the temporal wing of the sphenoid. Here communications exist with veins in the pterygo-palatine fossa. The posterior end is bulbous and lies in the condyloid fossa; it communicates with the condyloid vein issuing from the hypoglossal foramen and is drained by the inferior cerebral vein; it also receives an emissary vein from the parieto-temporal canal. The roof of the sinus is perforated by an oval opening at the carotid notch which opens into the cavernous sinus and transmits the internal carotid artery; the latter forms the first bend of its =S=-shaped curve in the petrosal and the second in the cavernous sinus.

The =basilar plexus= (Plexus basilaris) is a venous plexus situated on the upper surface of the basilar part of the occipital bone. It is connected through the hypoglossal foramen with the inferior petrosal sinus and communicates behind with the spinal veins.

THE VEINS OF THE CRANIUM

The =veins of the brain= (Venæ cerebri) do not in general accompany the cerebral arteries. They have very thin walls, no muscular coat, and no valves. They are arranged in two sets, superficial or cortical and deep or central. The superficial veins are more numerous and larger than the arteries. They lie on the surface of the brain in the pia mater and the subarachnoid space.

The =superior cerebral veins= drain the upper and outer part of the cerebral cortex. They receive veins from the medial surfaces of the hemispheres and open into the superior sagittal sinus. Their terminal parts are bulbous and their openings are directed obliquely forward, i. e., contrary to the direction of the bloodstream in the sinus.

The =inferior cerebral veins= lie on the inferior and external aspect of the hemispheres. They open into the inferior system of sinuses. The =middle cerebral vein= runs in the lateral fissure (of Sylvius) and opens into the cavernous sinus.

The =deep cerebral veins= issue from the central or ganglionic parts of the brain at the transverse fissure. They converge to form the =great cerebral vein= (of Galen) (V. magna cerebri). This passes upward and backward behind the splenium of the corpus callosum and is continued as the straight sinus to join the sagittal sinus.

The =superior cerebellar veins= ramify on the upper surface of the cerebellum. They open into the superior system of sinuses and the great cerebral vein. The =inferior cerebellar veins= are larger and go chiefly to the basilar plexus. The veins of the medulla and pons end in the inferior system of sinuses.

The =meningeal veins= (Venæ meningeæ) arise in capillary plexuses in the superficial and deep faces of the dura mater. Some end in the sinuses of the dura, others accompany the meningeal arteries.

The =diploic veins= (Venæ diploicæ) are anastomosing channels in the spongy substance of the cranial bones. Their walls are thin, consisting in many places only of the endothelium, and they have no valves. Some open inward into venous sinuses, others into extracranial veins.

SPINAL VEINS

Two =longitudinal spinal veins= or =sinuses= (Sinus columnæ vertebralis) extend along the floor of the vertebral canal, one on either side of the superior common ligament. They are continuous in front with the basilar plexus. They lie in the grooves on the bodies of the vertebræ and are connected by a series of transverse anastomoses which pass between the central part of the bodies of the vertebræ and the superior common ligament or in channels in the bone. They receive veins from the spinal cord, the dura mater, and the bodies of the vertebræ (Venæ basis vertebræ). Through the intervertebral foramina efferent vessels connect with the vertebral, intercostal, lumbar, and lateral sacral veins.

VEINS OF THE THORACIC LIMB

The =brachial vein= (V. brachialis) is a satellite of the extrathoracic part of the brachial artery. It arises at the inner side of the distal end of the shaft of the humerus and passes upward in the arm behind the artery under cover of a layer of fascia and the posterior superficial pectoral muscle. At the shoulder it is ventral to the artery, crosses the anterior border of the first rib, and concurs with its fellow and the two jugulars in the formation of the anterior vena cava. The roots of the vein are somewhat variable, but most often four radial veins in addition to a large oblique branch from the cephalic unite in its formation. Its tributaries correspond in general to the branches of the artery, but a few differences are worthy of notice. The =thoracico-dorsal vein= joins the brachial directly or opens into the deep brachial. The =external thoracic= or =“spur” vein= (V. thoracica externa) is a large vessel which arises in the ventral wall of the abdomen, passes forward (embedded more or less in the panniculus) along the outer border of the posterior deep pectoral muscle, and joins the brachial vein near the first rib. It often communicates with the subscapular vein.

The =cephalic vein= (V. cephalica) arises at the inner side of the carpus as the continuation of the internal superficial metacarpal vein. It runs upward on the deep fascia of the forearm at first in the furrow between the flexor carpi internus and the radius. Toward the middle of the forearm it inclines gradually forward on the inner surface of the radius, accompanied by a cutaneous branch of the median nerve, and arrives at the insertion of the biceps. Here it detaches a large branch (Ramus communicans) which passes upward and backward over the inner insertion of the biceps, the posterior radial artery, and the median nerve, and joins the brachial vein. The vein to this point is often termed the internal subcutaneous vein of the forearm (V. cephalica antebrachii). It is continued (as the V. cephalica humeri) in the furrow between the mastoido-humeralis and the anterior superficial pectoral with a branch of the inferior cervical artery, crosses the deep face of the cervical panniculus, and opens into the terminal part of the jugular or the brachial vein. It receives an =accessory cephalic vein= (V. cephalica accessoria), which arises from the carpal network, runs upward along the inner border of the extensor carpi, turns along the lower border of the brachialis, passes under the superficial tendon of the biceps and joins the cephalic.

The deep veins of the forearm are variable. Commonly two =posterior radial veins= (Venæ medianoradiales) accompany the artery of like name, one in front and one behind. A third vein arises by radicles emerging from the proximal part of the deep flexor; it joins the posterior satellite of the posterior radial artery or forms one of the roots of the brachial vein. The =common interosseous vein= joins the posterior satellite. The =anterior radial vein= (V. collateralis radialis) is a satellite of the artery. The =ulnar vein= is usually double at its proximal part, and communicates with the deep brachial vein.

There are three chief =metacarpal veins=. The =internal metacarpal vein= (V. metacarpea volaris superficialis medialis) arises from the volar venous arch above the fetlock. It is the largest vein of the region and lies in front of the large metacarpal artery. It separates from the artery at the proximal end of the metacarpus, passes upward on the inner part of the posterior surface of the carpus, under cover of the superficial layer of the posterior annular ligament, and is continued as the cephalic vein. It communicates at its proximal part with the origin of the posterior radial veins by a short but relatively large branch. The =external metacarpal vein= (V. metacarpea volaris superficialis lateralis) arises from the venous arch above the fetlock and passes upward behind the external border of the suspensory ligament in front of the external branch of the median nerve and accompanied by a small artery. At the proximal end of the metacarpus it is connected with the deep metacarpal vein by two transverse anastomoses which pass across the suspensory ligament. It then passes upward as a satellite of the external volar metacarpal artery and concurs in the origin of the ulnar and posterior radial veins. The =deep metacarpal vein= (V. metacarpea volaris profunda medialis) arises from the venous arch, passes forward between the two branches of the suspensory ligament, and ascends on the posterior surface of the large metacarpal bone. At the proximal end of the latter it communicates with the other metacarpal veins, ascends with the small metacarpal artery, and concurs in forming the radial veins.

The =venous arch= (Arcus venosus volaris), from which the metacarpal veins arise, is situated above the sesamoids of the fetlock between the suspensory ligament and the deep flexor tendon. It is formed by the junction of the two digital veins.

The =digital veins=, internal and external (V. digitalis medialis, lateralis), drain the venous plexuses of the foot. They arise at the upper edge of the lateral cartilages and ascend in front of the corresponding arteries.

It is convenient to recognize the following venous plexuses of the foot, which, however, communicate very freely:

1. The =coronary plexus= encircles the upper part of the foot. It is attached to the terminal part of the extensor tendon, the lateral cartilages, and the bulbs of the plantar cushion.

2. The =dorsal= (or laminal) =plexus= covers the dorsal or wall surface of the third phalanx in the deep layer of the matrix of the wall of the hoof. It forms the circumflex vein of the third phalanx or vein of the distal border of the third phalanx, which corresponds to the artery of like name.

3. The =volar plexus= is in the deep layer of the matrix of the sole of the hoof and on the deep surface of the lateral cartilages. It communicates around the inferior border of the third phalanx with the dorsal plexus and through the lateral cartilages with the coronary plexus.

The deep vein of the third phalanx accompanies the terminal part of the digital artery. It drains the =intraosseous plexus=.

THE POSTERIOR VENA CAVA (Figs. 270, 450)

The =posterior vena cava= (V. cava caudalis) returns almost all of the blood from the abdomen, pelvis, and pelvic limbs. It is formed by the confluence of the right and left common iliac veins at the fifth lumbar vertebra, above the terminal part of the aorta and chiefly to the right of the median plane.

The mode of origin is variable. In some cases there is a common trunk formed by the union of the two internal iliac veins so that the arrangement resembles the termination of the aorta. In other cases the internal iliac vein does not exist.

It passes forward on the ventral face of the psoas minor to the right of the abdominal aorta. At the last thoracic vertebra it separates from the aorta and runs forward between the right crus of the diaphragm and the pancreas till it reaches the liver. Here it inclines downward along the inner border of the right lobe and the parietal surface of the liver, largely embedded in the gland substance, and passes through the foramen venæ cavæ of the diaphragm. It then runs forward and somewhat downward between the mediastinal lobe and the main mass of the right lung at the upper margin of a special fold of the right pleura, accompanied by the right phrenic nerve, and opens into the posterior part of the right atrium. It receives the following tributaries:

1. The =lumbar veins= (Vv. lumbales) correspond to the arteries. Five pairs usually empty into the vena cava. Sometimes the corresponding veins of opposite sides unite to form a common trunk. The first communicate with the vena azygos.

2. The =spermatic veins= (Vv. spermaticæ internæ) (Fig. 450) accompany the arteries of like name. In the spermatic cord they form the pampiniform plexus about the artery and nerves. Their termination is variable. The right one commonly joins the vena cava near the renal vein, often by a common trunk with the left one. Frequently the left vein joins the left renal, and sometimes the right one ends similarly.

2_a_. The =utero-ovarian veins= are much larger than the preceding and are satellites of the arteries in the broad ligaments. The ovarian branch is plexiform near the ovary. The uterine branches form a rich plexus in the wall of the uterus. The trunk is very short.

3. The =renal veins= (Vv. renales), satellites of the arteries, are of large caliber and thin-walled. The right vein passes inward and backward on the ventral face of the kidney between the artery in front and the ureter behind. It joins the vena cava above the right adrenal. The left vein is somewhat longer. It passes inward at first like the right one, then bends around the posterior end of the adrenal, crosses the origin of the renal artery and opens into the vena cava a little further back than the right one. They receive veins from the adrenals, but some adrenal veins open directly into the vena cava.

4. The =hepatic veins= (Vv. hepaticæ) return the blood from the liver, and open into the vena cava as it lies in the groove in the liver. Three or four large vessels open into the vena cava just before it leaves the liver, and numerous small ones discharge into its embedded part.

5. The =phrenic veins= (Vv. phrenicæ), two or three in number, return the blood from the diaphragm. They are very large in comparison with the arteries, and join the vena cava as it lies in the caval opening.

In some cases there is a small middle sacral vein (V. sacralis media), a satellite of the artery. It opens into the angle of junction of the common iliac veins or into one of the latter.

THE PORTAL VEIN

The =portal vein= (V. portæ) is a large trunk which returns the blood carried to the viscera by the gastric, splenic, and mesenteric arteries. Its peripheral tributaries correspond closely with the branches of the arteries, but the terminal trunks do not. The vein is formed behind the pancreas and below the posterior vena cava by the confluence of the =anterior= and =posterior mesenteric= and =splenic veins=. It passes forward, traverses the posterior part of the pancreas very obliquely, inclines ventrally and a little to the right, and reaches the portal fissure of the liver. Here it divides into three branches which enter the liver and ramify in the substance of the gland like an artery, terminating in the lobular capillaries. From the lobules the blood passes into the hepatic veins and through these to the posterior vena cava. Thus the blood which is distributed to the stomach, nearly the entire intestinal tract, the pancreas, and the spleen, passes through two sets of capillaries prior to its return to the heart, viz., the capillaries of these viscera and of the liver.

1. The =anterior mesenteric vein= (V. mesenterica cranialis) is the largest of the portal radicles. It is situated to the right of the artery of like name, and its tributaries correspond in general to the branches of the artery. Usually a single colic vein corresponds to the two arteries of the right portions of the colon.

2. The =posterior mesenteric vein= (V. mesenterica caudalis) is the smallest of the radicles of the portal. It accompanies the artery in the colic mesentery and its rectal branches anastomose with those of the internal pudic vein.

3. The =splenic vein= (V. lienalis) is the very large satellite of the splenic artery. It is formed by the union of two radicles at the base of the spleen. On leaving the hilus of the spleen it passes inward between the anterior pole of the left kidney and the saccus cæcus of the stomach and above the left end of the pancreas, receives commonly the =posterior gastric vein= (V. gastrica caudalis), and unites with the anterior mesenteric at the posterior border of the pancreas.

The collateral tributaries of the portal veins are as follows:

(1) =Pancreatic veins= (Rami pancreatici).

(2) The =gastro-duodenal vein= (V. gastroduodenalis) corresponds mainly to the extrahepatic branches of the hepatic artery.

(3) The =anterior gastric vein= (V. gastrica cranialis) joins the portal at the portal fissure.

THE COMMON ILIAC VEINS (Fig. 456)

These (V. iliacæ communes) are two very large but short trunks which result from the union of the internal and external iliac veins of each side at the sacro-iliac articulation. The left one is the longer and crosses obliquely over the terminal part of the aorta. The chief tributaries of each are as follows:

1. The last =lumbar vein=.

2. The =circumflex iliac veins= (Vv. circumflexæ ilium profundæ) are the two satellites of each corresponding artery, on either side of which they are placed. They may open directly into the posterior vena cava or into the external iliac vein.

3. The =ilio-lumbar vein= (V. iliolumbalis) may open into the common iliac, the external iliac, or the internal iliac vein.

THE INTERNAL ILIAC VEINS

The =internal iliac= or =hypogastric veins= (Vv. hypogastricæ), right and left, are usually formed by the confluence of lateral sacral, gluteal, and internal pudic veins. The obturator vein may open into them also. They are short trunks and are smaller than the external iliac veins. The tributaries correspond in general to the branches of the three arteries of like names. The internal pudic veins receive affluents from the venous plexuses of the prepuce and penis in the male; of the mammary gland, vulva, vagina, and vestibular bulb in the female.

THE VEINS OF THE PELVIC LIMB

The =external iliac vein= (V. iliaca externa) (Figs. 450, 451) lies behind the corresponding artery at the brim of the pelvis. It is the upward continuation of the femoral vein, and unites at the sacro-iliac joint with the internal iliac to form the common iliac vein. Its tributaries are as follows:

1. The =obturator vein= (V. obturatoria) is a satellite of the artery and usually opens into the external iliac at the insertion of the psoas minor. Its radicles anastomose with those of the internal and external pudic veins (Figs. 451, 455).

2. The =iliaco-femoral= or =external circumflex veins= (Vv. circumflexæ femoris laterales) are the two satellites of the homonymous artery. They open a little higher than the obturator.

The =femoral vein= (Figs. 450, 451) lies behind the artery in the upper part of the thigh, external to it lower down. Its chief tributaries are:

1. A very large but short trunk formed by the union of the deep femoral and the external pudic vein. The =deep femoral vein= (V. profunda femoris) corresponds otherwise to the artery. The =external pudic vein= (V. pudenda externa) arises chiefly from a rich plexus of large veins situated above and along the sides of the penis and prepuce in the male, the mammary glands in the female. It passes through a foramen in the anterior part of the tendon of origin of the gracilis and runs outward in the subpubic groove behind the pectineus to unite with the deep femoral vein. The right and left veins are connected by a large transverse anastomosis and each has a large connection with the obturator vein. Each receives the =posterior abdominal vein= (V. epigastrica caudalis) which accompanies the artery of like name. The =subcutaneous abdominal vein= (V. abdominalis subcutanea) arises in the skin and panniculus of the ventral abdominal wall, anastomoses with the internal and external thoracic and deep abdominal veins, and joins the external pudic or posterior abdominal vein.

A small vein accompanies the external pudic artery in the inguinal canal.

2. The =anterior femoral vein= (V. femoris cranialis) accompanies the artery.

3. The =saphenous vein= (V. saphena) (Figs. 451, 458, 460) arises at the inner side of the flexion surface of the tarsus as the upward continuation of the internal metatarsal vein. Its course is distinctly visible. It ascends on the subcutaneous surface of the tibia and the popliteus muscle, inclosed between layers of the deep fascia, inclines a little backward to the proximal part of the leg, then deviates slightly forward, runs upward on the gracilis, passes between that muscle and the sartorius, and joins the femoral or the external pudic vein.[173] On the upper part of the capsule of the hock joint it forms an arch with the anterior tibial vein. The vein has numerous valves. The satellite artery is relatively small and lies in front of the vein as far as the junction with the recurrent tibial vein, which it accompanies on the leg. It receives the =recurrent tibial vein= (V. recurrens tibialis)[174] at the proximal fourth of the leg. This vessel arises at the inner surface of the tarsus and forms an arch with the posterior tibial vein at the level of the tuber calcis. It ascends in the furrow in front of the gastrocnemius tendon, inclines forward at the proximal third of the leg, and joins the saphenous vein at an acute angle. It has numerous valves. A smaller vein from the anterior face of the metatarsus joins the saphenous at the hock.

4. =Muscular branches= which correspond to the arteries.

5. The =posterior femoral= or =femoro-popliteal= vein (V. femoris caudalis) is a satellite of the artery. It receives the =recurrent tarsal= or =external saphenous vein= (V. tarsea recurrens), which arises at the outer side of the hock, ascends on the deep fascia of the external surface of the leg in front of the tendo Achillis, passes between the biceps femoris and semitendinosus, and joins the posterior femoral vein. It is connected with the recurrent tibial vein by a large anastomotic branch which crosses in front of the tuber calcis. Usually a branch from it ascends along the great sciatic nerve and anastomoses with the obturator vein.

The =popliteal vein= (V. poplitea) lies along the inner side of the artery (Fig. 459). It is formed by the confluence of anterior and posterior tibial veins.

Two =anterior tibial veins= (Vv. tibiales anteriores) usually accompany the artery of like name; the outer vein is much the larger. In other cases there is a single large vein in the proximal part of the leg, two lower down. They arise from a number of anastomosing radicles on the front of the capsule of the hock joint, chiefly as the continuation of the perforating tarsal vein. The origin of the chief vein is connected with the saphenous by a large anastomotic branch.

The =posterior tibial vein= (V. tibialis posterior) is commonly double (Fig. 458). It arises at the level of the tuber calcis, where it has a communication with the recurrent tibial vein. It is a satellite of the artery.

The =internal= or =great metatarsal vein= (V. metatarsea dorsalis medialis) (Figs. 458, 460) arises from the venous arch above the sesamoids at the fetlock, but is practically the upward continuation of the internal digital vein. It ascends along the inner border of the deep flexor tendon, then in the groove on the inner aspect of the proximal part of the large metatarsal bone to the capsule of the hock joint, and is continued by the saphenous vein.

The =external metatarsal vein= (V. metatarsea plantaris lateralis) arises from the venous arch above the fetlock, but appears to be the upward continuation of the external digital vein. It ascends along the outer border of the deep flexor tendon in front of the plantar nerve, and is connected with the deep metatarsal vein at the proximal part of the metatarsus by a transverse branch. It then passes upward along the deep flexor tendon in relation to the plantar nerves and the inner tarsal artery and is continued by the recurrent tibial vein.

The =deep metatarsal vein= (V. metatarsea plantaris medialis) arises from the plantar venous arch, passes forward between the branches of the suspensory ligament, and ascends on the posterior face of the large metatarsal bone. At the proximal end of the metatarsus it is connected with the external metatarsal vein by a transverse branch. It then passes (as the perforating tarsal vein) through the vascular canal of the tarsus and forms the chief radicle of the anterior tibial vein.

The =plantar venous arch= (Arcus venosus plantaris) and the digital veins are arranged like those of the thoracic limb.

The Lymphatic System

The =lymphatic system= (Systema lymphaticum) is subsidiary to the venous part of the circulatory system, from which it arises in the embryo. It consists of the lymph vessels and glands.

The =lymph vessels= (Vasa lymphatica) contain a colorless fluid, the lymph, which contains numerous lymphocytes.[175] They resemble the veins in structure but have thinner walls and are provided with more numerous valves. The vessels are sacculated opposite the segments of the valves and have a characteristic beaded appearance when distended. The collecting lymph vessels do not usually form rich plexuses, as veins often do, their branching is more limited and less tree-like than that of the blood-vessels, and their caliber therefore increases less from the periphery toward their termination. All of the lymph is ultimately carried into the venous system by two trunks, the =thoracic duct= and the =right lymphatic duct=. Almost all of the lymph passes through at least one group of lymph glands before entering the blood-vascular system.

The =lymph glands= or =nodes= (Lymphoglandulæ) are intercalated in the course of the lymph vessels. They vary widely in size, some being microscopic, others several inches in length. In form they may be globular, ovoid and flattened, elongated, or irregular. In certain situations they are aggregated into groups, and a knowledge of the position of these and the territory drained into them is important. It is convenient, when possible, to indicate their position with regard to arteries on the course of which they are placed. In color they are usually gray or yellowish-brown in the dead subject, pink or reddish-brown during life, but this varies according to their position and functional state. The bronchial glands are often blackened by infiltration with carbon. The mesenteric glands are creamy or white while the chyle is passing through, but pink at other times. Vessels which carry lymph to a gland are called =afferent=; the =efferent= vessels which convey it away are larger and fewer. Each gland has a depression, the =hilus=, at which the blood-vessels enter and the efferent lymph vessels emerge.

=Lymph nodules= or =follicles= (Noduli lymphatici) are minute masses of lymphoid tissue which occur in certain mucous membranes. They may be solitary, as in the solitary glands of the intestine, or aggregated into masses or patches, as in the tonsils and the so-called Peyer’s patches.

The =lymph nodule= or =follicle= is the unit of structure of the lymph gland. It consists essentially of an artery surrounded by a =reticulum= of connective tissue, the meshes of which contain numerous =lymphocytes=. Surrounding this is a rich plexus of lymph vessels, forming the so-called =sinus=, inclosed in some cases by a fibrous capsule. The gland consists of a mass of follicles, inclosed in a fibrous =capsule=, from which =trabeculæ= pass in and unite the follicles. Beneath the capsule is the =peripheral sinus=, which consists of a very rich plexus of lymph vessels; to this the afferent vessels pass at various points of the surface. In the =cortical substance= the cells are in rounded masses, the cortical nodules, while in the =medullary substance= they lie around the arteries, forming the so-called medullary cords. The =medulla= is redder than the cortex, since it is more vascular; it contains the =central lymph sinuses=, which have a similar structure to the peripheral sinus.

The =hæmolymph glands= differ from the lymph glands in color and structure. They are of a deep red color, which is due to the high vascularity of the cortical substance. The peripheral sinuses especially are greatly developed and contain numerous red blood-cells. There is no clear division into cortical and medullary substance, and the trabeculæ contain smooth muscle-cells. Some have afferent and efferent lymph vessels and others do not. They resemble the spleen in some respects, but their significance is not yet clear. They are numerous in the ox and sheep, much fewer in the dog, and apparently are absent in the horse. They occur along the course of the aorta, in the perineal fat, at the portal fissure, and with the gastric and mesenteric lymph glands. In the ox they are also found under the trapezius muscle, under the skin of the upper part of the flank, and in other places less constantly.

The =tissue= or =lymph-spaces= are interstices of varying size between cells or in the meshes of connective tissue. They contain a fluid derived from the blood-plasma, which is usually called =lymph=. They are drained by the veins and lymph vessels. The large serous sacs are often included in this category.

The exact relationship between the lymphatic capillaries and the tissue spaces is still a matter of controversy. It is held by some that the lymph vessels are in direct communication with the tissue spaces, while others maintain that the lymphatics are complete closed tubes. Communication between the spaces and vessels is in general very free. Mall has shown that granules injected into the hepatic artery are returned by the lymphatics as well as by the veins, and intramuscular injections will enter the lymph vessels of the tendon in spite of the absence of lymphatics in muscle.

LYMPHATIC SYSTEM OF THE HORSE

THE THORACIC DUCT (Figs. 428, 429)

The =thoracic duct= (Ductus thoracicus) is the chief collecting trunk of the lymphatic system. It begins as an elongated irregular dilatation, the =cisterna= or =receptaculum chyli=, which is situated between the right side of the aorta and the right crus of the diaphragm at the first and second lumbar vertebræ. The duct enters the thorax through the hiatus aorticus and runs forward on the right of the median plane between the vena azygos and the aorta, covered by the pleura. At the sixth or seventh thoracic vertebra it inclines somewhat ventrally, crosses obliquely over the left face of the œsophagus, and passes forward on the left side of the trachea to the inlet of the thorax. The extrathoracic terminal part passes downward and forward a variable distance (3 to 4 cm.) on the deep face of the left scalenus muscle, bends inward and backward under the bicarotid trunk, and opens into the upper part of the origin of the anterior vena cava just behind the angle of junction of the jugular veins. The terminal bend is ampullate and sometimes divides into two very short branches which open close together.

Since the duct develops from a plexus of ducts in the embryo, considerable variation from the more usual course occurs. There is often a left duct which arises at the cisterna or at a variable point from the right duct, runs across the left intercostal arteries parallel to the latter, and unites with it over the base of the heart or further forward. The two are connected by cross-branches. In some cases the left duct is the larger, and there may indeed be none on the right side. Other variations are common.

The chief tributaries of the thoracic duct are as follows:

1. The two =lumbar trunks= (Trunci lumbales) are formed by the confluence of the efferent ducts of the lumbar glands, and commonly unite with each other and with the posterior intestinal trunk before opening into the cistern.

2. The =intestinal trunks= (Trunci intestinales), two or three in number, receive the efferents of the lymph glands of the intestine, stomach, liver, and spleen.[176]

In its course through the thorax the thoracic duct receives efferents from the intercostal, mediastinal, and bronchial glands. At the thoracic inlet it is joined by ducts from the prepectoral and right axillary glands, and by the left tracheal duct. The duct is provided with several pairs of valves; the best developed are at its termination.

THE RIGHT LYMPHATIC DUCT

This vessel (Ductus lymphaticus dexter)—when present—collects the lymph from the right side of the head, neck, and thorax, and from the right thoracic limb. It is most frequently absent, being represented by a number of short ducts which terminate in the thoracic duct, the right jugulo-brachial junction, or the origin of the anterior vena cava. When present in its typical form, it results from the confluence of efferent ducts from the right axillary and prepectoral lymph glands with the right tracheal duct. It lies on the deep face of the scalenus muscle above the terminal part of the right jugular vein. It is more or less ampullate and usually opens into the anterior vena cava to the right of the thoracic duct. It may be connected with the latter by considerable anastomoses and may join it.

The duct is very variable in form and in regard to its afferents. Often it is a very short, irregular, and bulbous trunk; in some cases it is about an inch and a half (ca. 3 to 4 cm.) in length and receives the tracheal duct at its terminal bend. The lymphatico-venous connections here need further study.

THE LYMPH GLANDS AND VESSELS OF THE HEAD AND NECK

1. The =submaxillary lymph glands= (Lg. submaxillares) (Figs. 437, 462) are arranged in two elongated groups in the submaxillary space along each side of the omo-hyoid muscles. The two groups are in apposition in front of the insertion of these muscles and diverge posteriorly in the form of a =V=, extending backward about four or five inches (ca. 10 to 12 cm.). They are covered by the skin and a thin layer of fascia and panniculus, and are therefore palpable. Anteriorly they are firmly attached to the mylo-hyoidei, but otherwise they are rather movable in the normal state. Each group is related externally to the external maxillary artery and the anterior belly of the digastricus, below to the external maxillary vein, and above to the lingual and sublingual veins.

They receive =afferent vessels= from the lips, nostrils, nasal region, cheeks, the anterior part of the tongue, the jaws, the floor of the mouth, and the greater part of the hard palate and nasal cavity. The =efferent vessels= pass to the anterior cervical and pharyngeal glands.

The superficial lymph vessels of the face converge to twelve to fifteen trunks which turn around the lower border of the jaw with the facial vessels. Those of the lips form plexuses at the commissures. The nasal mucous membrane is richly supplied with lymph vessels which accompany the veins; posteriorly they communicate with the subdural and subarachnoid spaces and send efferents to the pharyngeal and anterior cervical glands.

2. The =pharyngeal lymph glands= (Figs. 436, 437, 569) may consist of two groups. One lies on the lateral surface of the pharynx along the course of the external carotid artery. These glands are related externally to the stylo-maxillaris and digastricus and often to the submaxillary gland also, above to the guttural pouch. Other glands (Lg. retropharyngeales) are commonly found on the guttural pouch along the course of the internal carotid artery. They lie below the artery and are covered by the aponeurosis of the mastoido-humeralis and the cervical end of the submaxillary gland.

They receive afferent vessels from the cranium, the posterior part of the tongue, the soft palate, pharynx, guttural pouch, larynx, posterior part of the nasal cavity, and efferents from the submaxillary glands.

3. The =anterior cervical lymph glands= (Lg. cervicales craniales) are situated chiefly along the course of the common carotid artery in the vicinity of the thyroid gland, under cover of the cervical angle of the parotid gland. Some occur between the thyroid and the submaxillary salivary gland, others above and partly upon the thyroid. They are related deeply to the posterior part of the larynx, the trachea, the thyroid gland, and the œsophagus; below to the external maxillary vein and the outer border of the omo-hyoideus.[177]

These glands are variable. Often there are none in front of the thyroid and the group may extend back a considerable distance along the course of the carotid artery.

Their afferents are deep lymph vessels from the head, the pharynx, larynx, guttural pouch, and thyroid gland, and efferents from the submaxillary and pharyngeal glands. Their efferent vessels go to the middle and posterior cervical glands.

4. The =middle cervical lymph glands= (Lg. cervicales mediæ) form an inconstant group situated a little in front of the middle of the neck on the trachea below the carotid artery. The group is usually small and in some cases is absent, being replaced by a number of glands occurring at intervals along the course of the carotid artery. In other subjects the group consists of several glands of considerable size. They are intercalated in the course of the tracheal lymph ducts.

5. The =posterior cervical= or =prepectoral lymph glands= (Lg. cervicales caudales) form a large group below the trachea at the entrance to the thorax (Fig. 466). They occupy the interstices between the vessels and muscles and extend forward a variable distance on the ventral aspect of the trachea. They are covered by the panniculus and sterno-cephalicus. Their afferent vessels come from the head, neck, thorax, and thoracic limb. They receive efferent ducts of the anterior and middle cervical, prescapular, and axillary glands. Their efferents go to the thoracic duct on the left, to the right lymphatic duct on the right, or open directly into the vena cava.

6. The =prescapular= or =superficial cervical lymph glands= (Lg. cervicales superficiales) lie on the anterior border of the anterior deep pectoral muscle, in relation to the omo-hyoideus internally and the mastoido-humeralis externally (Figs. 431, 441, 466). They are on the course of the ascending branch of the inferior cervical artery. They receive afferents from the neck, breast, shoulder, and arm. Their efferents pass to the prepectoral glands.

The =tracheal ducts=, right and left (Ductus trachealis dexter, sinister), are collecting trunks for the lymph of the head and neck. They lie on the trachea in relation to the carotid arteries. The right one goes to the prepectoral glands or to the right lymphatic duct, the left one to the terminal part of the thoracic duct.

THE LYMPH GLANDS AND VESSELS OF THE THORAX

1. The =intercostal lymph glands= (Lg. intercostales) are small and are situated at the sides of the bodies of the thoracic vertebræ, in series corresponding to the intercostal spaces. They receive afferents from the vertebral canal, the spinal muscles, the diaphragm, intercostal muscles, and pleura. The efferent vessels go to the thoracic duct.

In the young subject these glands are more numerous than in the adult, and there are also glands along the dorsal face of the thoracic aorta which seem to disappear later.

2. The =anterior mediastinal lymph glands= (Lg. mediastinales craniales) (Fig. 428) are numerous. Some are situated on the course of the brachial arteries and their branches; on the right side they are related deeply to the trachea, on the left to the œsophagus also. They are variable in size and disposition and are continuous in front with the prepectoral glands. Other glands lie along the ventral face of the trachea on the anterior vena cava and the right atrium of the heart; these are continuous behind with the bronchial glands. A few glands usually occur along the dorsal surface of the trachea, and there is often one at the angle of divergence of the brachiocephalic trunk. The afferent vessels come chiefly from the pleura, the pericardium, the heart, the thymus or its remains, the trachea, and the œsophagus. Their efferent vessels pass to the prepectoral glands and the thoracic duct.

The glands along the ventral face of the trachea are frequently enlarged and usually pigmented in dissecting-room subjects. The left recurrent nerve lies above them on the ventral face of the trachea.

3. The =bronchial lymph glands= (Lg. bronchiales) are grouped around the terminal part of the trachea and the bronchi. One group lies on the upper surface of the bifurcation of the trachea and is continued a short distance backward under the œsophagus (Fig. 428). Others lie below the trachea and bronchi and also occupy the angle between the aortic arch and the pulmonary artery, concealing the left recurrent nerve and often covering the vagus in this part of its course. Small glands occur along the chief bronchi in the substance of the lungs (Lg. pulmonales). They receive the deep and most of the superficial lymph vessels of the lungs, and the efferents from the posterior mediastinal glands. Their efferent vessels go to the thoracic duct and the anterior mediastinal glands. The deep lymph vessels of the lung arise in plexuses which surround the terminal bronchi and accompany the bronchi to the root of the lung. The superficial vessels form a rich network under the pleura; most of them pass to the bronchial glands.

The bronchial glands are commonly pigmented except in young subjects and are often enlarged and indurated.

4. The =posterior mediastinal lymph glands= (Lg. mediastinales caudales) are usually small and are scattered along the posterior mediastinum above the œsophagus. They receive afferent vessels from the œsophagus, mediastinum, diaphragm, and liver. The efferents go to the bronchial and anterior mediastinal lymph glands, partly to the thoracic duct directly.

Sometimes one or two small glands are situated in the acute angle between the posterior vena cava and the diaphragm.

THE LYMPH GLANDS AND VESSELS OF THE ABDOMEN AND PELVIS

The =lymph glands of the abdomen= consist of two main groups, parietal and visceral. The =parietal glands= lie in the subperitoneal or subcutaneous tissue; they receive the lymph vessels from the abdominal and pelvic walls, from parts of the viscera, and from the proximal lymph glands of the pelvic limbs. The =visceral glands= lie on the walls of the viscera or in the peritoneal folds which connect the organs with the wall or with adjacent viscera. They receive all or most of the lymph vessels from the organs with which they are connected.

The =parietal glands= comprise the following:

1. The =lumbar lymph glands= (Lg. lumbales) lie along the course of the abdominal aorta and posterior vena cava (Fig. 450). Some are placed along the lower surface and sides of the vessels, others above. A few small glands may be found above the sublumbar muscles. The small nodes which are situated at the hilus of the kidneys are often termed the renal lymph glands. They receive afferent vessels from the lumbar wall of the abdomen and the paired viscera (kidneys, adrenals, genital organs), also the inguinal vessels and the efferents of the iliac glands. Their efferents go to the thoracic duct, constituting the lumbar trunks of origin of that vessel.

2. The =internal iliac lymph glands= (Lg. iliacæ internæ) are grouped about the terminal part of the aorta and the origins of the iliac arteries (Fig. 450). Their afferent vessels come chiefly from the pelvis, pelvic viscera, and tail, and they receive efferent vessels of the external iliac and deep inguinal glands.

3. The =external iliac lymph glands= (Lg. iliacæ externæ) form a group on either side on the iliac fascia at the bifurcation of the circumflex iliac artery (Fig. 450). Their afferent vessels come from the flank and abdominal floor, the outer surface of the thigh, and the precrural glands. The efferent vessels go to the lumbar and internal iliac glands.

4. The =sacral lymph glands= (Lg. sacrales) are small nodes situated along the borders of the sacrum and on its pelvic surface. They receive afferents from the roof of the pelvis and from the tail, and their efferent vessels pass to the internal iliac glands.

5. The =superficial inguinal lymph glands= (Lg. inguinales superficiales) lie on the abdominal tunic in front of the external inguinal ring (Fig. 457). They form an elongated group along the course of the subcutaneous abdominal artery, on either side of the penis in the male, above the mammary glands in the female; in the latter they are often termed =mammary=. Their afferents come from the inner surface of the thigh, the abdominal floor, the sheath and scrotum in the male, and the mammary glands in the female. The efferent vessels ascend through the inguinal canal and go to the deep inguinal and lumbar glands.

6. The =ischiatic lymph gland= (Lg. ischiadica) is a small node which may be found at the lesser sciatic notch. It receives lymph from the adjacent parts and from the popliteal glands, and sends efferents to the sacral and internal iliac glands.

The =visceral glands= include the following:

1. The =gastric lymph glands= (Lg. gastricæ) are situated along the course of the gastric arteries. Several occur along the attachment of the gastro-phrenic ligament. A group lies at the lesser curvature a short distance below the cardia. There is another small group on the visceral surface where the posterior gastric artery divides into its primary branches. Two or three small nodes are usually found on the ventral aspect of the pylorus. Other small glands are scattered along the course of the gastro-epiploic and short gastric arteries in the great and gastro-splenic omenta. The efferent vessels pass largely to the cœliac radicle of the thoracic duct, but along the left part of the great curvature they go to the splenic glands.

2. The =hepatic lymph glands= (Lg. hepaticæ) lie along the portal vein and hepatic artery and in the lesser omentum. Their efferent vessels go to the cœliac radicle of the cisterna chyli.

Many of the lymph vessels from the parietal surface of the liver pass in the falciform and lateral ligaments to the diaphragm and anastomose with its lymphatics. Some pass through the diaphragm with the vena cava and go to the mediastinal glands.

The =pancreatic lymph vessels= follow the course of the blood-vessels which supply the gland; most of them go to the splenic and hepatic glands.

3. The =splenic lymph glands= (Lg. lienales) lie along the course of the splenic blood-vessels. Their afferent vessels come from the subscapular network of the spleen, from the greater curvature of the stomach, and from the left part of the pancreas. The efferents pass to the cœliac radicle of the thoracic duct.

4. The =mesenteric lymph glands= (Lg. mesentericæ) are situated in the great mesentery near its root. They are numerous and hence lie close together. They receive a very large number of afferent vessels (400 to 500) from the small intestine. They have several considerable efferents which concur in the formation of the intestinal radicles of the cisterna chyli.

The lymph vessels of the intestine form three sets of capillary plexuses, viz., in the subserosa, submucosa, and mucosa. The lymph follicles, solitary and aggregate, lie in the zone of the plexus of the mucosa. The collecting vessels arise from the subserous plexus.

5. The =lymph glands of the cæcum= are numerous and are distributed along the course of the cæcal blood-vessels. Their efferents enter into the formation of an intestinal radicle of the cisterna chyli.

6. The =lymph glands of the great colon= are extremely numerous and are placed close together along the colic blood-vessels. Their efferent vessels are large and numerous. They converge to two large trunks which concur with those of the cæcum and small intestine to form an intestinal radicle of the cisterna chyli.

The intestinal radicles of the cisterna chyli are formed by the confluence of efferents from the intestinal lymph glands. The anterior trunk lies on the left side of the anterior mesenteric artery, passes between that vessel and the cœliac artery, turns sharply backward across the right renal vessels and opens into the cisterna. It is formed by the union of the cœliac trunk with efferents from the glands of the small intestine, cæcum and colon. It is about four inches (ca. 10 cm.) in length and is ampullate. The posterior trunk receives vessels from the small intestine and small colon. It usually opens into a trunk formed by the union of the right and left lumbar ducts. It is usually ampullate at its termination (Franck). The arrangement of these collecting trunks is, however, very variable.

7. The =lymph glands of the small colon= are situated in part on the wall of the bowel along the attachment of the mesentery, in part between the layers of the latter along the course of the blood-vessels. The efferent vessels go to the lumbar glands and to the posterior intestinal radicle of the thoracic duct.

The =lymph vessels of the rectum= pass chiefly to the lumbar and internal iliac glands.

8. The =anal lymph glands= (Lg. anales) form a small group on either side of the sphincter ani externus (Figs. 451, 453). They receive afferents from the anus, perineum, and tail. Their efferents go to the internal iliac glands.

THE LYMPH GLANDS AND VESSELS OF THE THORACIC LIMB

1. The =axillary lymph glands= (Lg. axillares), some ten to twelve in number, are grouped on the inner face of the distal part of the teres major and the tendon of the latissimus dorsi at the angle of junction of the external thoracic and subscapular veins with the brachial (Fig. 466). Their efferents include most of the lymph vessels of the limb, which come directly or as efferents from the cubital glands. They receive also lymph vessels from the thoracic wall. The efferents accompany the brachial blood-vessels and end in the prepectoral glands and the thoracic and right lymphatic ducts.

2. The =cubital lymph glands= (Lg. cubitales), usually eight to ten in number, form a discoid oval group at the inner side of the distal part of the shaft of the humerus (Figs. 441, 446). They lie behind the biceps muscle on the brachial vessels and median nerve and are covered by the deep fascia and the posterior superficial pectoral muscle. They receive as afferents most of the vessels from the limb below this point. Their efferents pass chiefly to the axillary glands, but in part to the prescapular glands also.

A number of superficial lymph vessels ascend with or near the subcutaneous veins (cephalic and accessory cephalic) and join the prescapular and prepectoral glands. Superficial vessels from the chest-wall and shoulder run across the latter to the prescapular glands. The superficial lymphatics of the pectoral region form a plexus which drains into the prepectoral and prescapular glands by a number of vessels which accompany the cephalic vein. The deep lymph vessels of the pectoral region run with the external thoracic vein to the axillary glands.

THE LYMPH GLANDS AND VESSELS OF THE PELVIC LIMB

1. The =precrural= or =subiliac lymph glands= (Lg. subiliacæ) are situated in the fold of the groin on the anterior border of the tensor fasciæ latæ, about midway between the point of the hip and the patella (Figs. 450, 451, 457). They lie on the course of the posterior branch of the circumflex iliac artery, and number usually about a dozen. They receive superficial lymph vessels from the hip, thigh, and flank. Their efferent vessels ascend with the posterior circumflex iliac vein, enter the abdomen near the external angle of the ilium, and join the external iliac lymph glands.

2. The =deep inguinal lymph glands= (Lg. inguinales profundæ) form a large group situated in the upper part of the femoral canal between the pectineus and sartorius muscles (Figs. 451, 457). They cover the femoral vessels and are related superficially to the inguinal ligament. They receive nearly all of the lymph vessels of the limb below them. Their efferent vessels ascend to the internal iliac glands.

3. The =popliteal lymph glands= (Lg. popliteæ), usually four to six in number, lie behind the origin of the gastrocnemius and between the biceps femoris and semitendinosus at the division of the posterior femoral artery into its primary branches (Fig. 455). They receive the deep lymph vessels of the distal part of the limb. Their efferent vessels chiefly follow the course of the femoral vessels to the deep inguinal glands, but one or two ascend in company with a vein along the great sciatic nerve and may enter a gland at the lesser sciatic notch. From this a vessel accompanies the internal pudic vein and joins the internal iliac glands.

Several superficial lymph vessels ascend with or near the internal metatarsal and saphenous veins, enter the femoral canal, and end in the deep inguinal glands.

THE FŒTAL CIRCULATION

The blood of the fœtus is oxygenated, receives nutrient matter, and gives off waste matter by close contiguity with the maternal blood in the placenta. The chief differences in the blood-vascular system as compared with that which obtains after birth are correlated with this interchange.

The =umbilical arteries=, right and left, are large vessels which arise from the internal iliac arteries and pass downward and forward in the umbilical folds of peritoneum on either side of the bladder to the umbilicus. Here they are incorporated with the umbilical vein and the urachus in the umbilical cord, ramify in the allantois, and end as the capillaries of the fœtal placenta. They conduct the impure blood to the placenta. After birth these vessels retract with the bladder to the pelvic cavity; their lumen becomes greatly reduced and the wall thickened so that they are cord-like and are termed the round ligaments of the bladder.

The =umbilical vein= receives the oxygenated blood from the placenta. Its radicles converge to form in the horse a single large trunk which separates from the other constituents of the umbilical cord on entering the abdomen and passes forward along the abdominal floor in the free border of the falciform ligament of the liver. It enters the latter at the umbilical fissure and joins the portal vein, so that the blood conveyed by it passes through the capillaries of the liver before entering the posterior vena cava.

In the ox and dog some of the blood in the umbilical vein is conveyed directly to the vena cava by the =ductus venosus= (Arantii). This vessel is given off within the liver from a venous sinus formed by the confluence of the portal and umbilical veins and passes directly to the posterior vena cava.

The =foramen ovale= is an opening in the septum between the atria, by which the latter communicate with each other. It is guarded by a valve (Valvula foraminis ovalis) which prevents the blood from passing from the left atrium to the right. After birth the foramen soon closes, but this part of the septum remains membranous, and there is a deep =fossa ovalis= in the right atrium which indicates the position of the former opening. In some cases the foramen persists to a variable extent in the adult without apparent disturbance of the circulation.

The =pulmonary circulation= is very limited in the fœtus, and most of the blood which enters the pulmonary artery passes through the =ductus arteriosus= to the aorta. This vessel is larger than the divisions of the pulmonary which go to the lungs and joins the left side of the aortic arch. After birth the pulmonary circulation undergoes promptly an enormous increase and the ductus is rapidly transformed into a fibrous cord—the =ligamentum arteriosum=.

The only arterial blood in the fœtus is that carried by the umbilical vein. This blood is mixed in the liver with the venous blood of the portal vein, and after passing through the capillaries of the liver is carried by the hepatic veins to the posterior vena cava. The latter receives also the venous blood from the posterior part of the trunk and the pelvic limbs. It is generally believed that the blood carried into the right atrium by the posterior vena cava passes largely, if not entirely, through the foramen ovale into the left atrium, while the blood flowing into it through the anterior vena cava passes into the right ventricle. On this basis the blood received by the left atrium consists chiefly of mixed blood from the posterior vena cava, since the small amount of blood conveyed by the pulmonary veins is venous. This mixed blood passes into the left ventricle and is forced into the systemic arteries. The venous blood from the anterior part of the body and the thoracic limbs is conveyed by the anterior vena cava to the right atrium, passes into the right ventricle, and is forced into the pulmonary artery. A small amount is carried to the lungs, but the bulk of it passes by the ductus arteriosus into the aorta behind the point of origin of the brachiocephalic trunk (anterior aorta), and is carried to the posterior part of the body, a large part passing by the umbilical arteries to the placenta.

THE BLOOD-VASCULAR SYSTEM OF THE OX

THE PERICARDIUM AND HEART

The =pericardium= is attached by two fibrous bands (Ligamenta sternopericardiaca) to the sternum opposite the facets for the sixth costal cartilages; these ligaments, right and left, are embedded in the mass of fat which separates the apex of the pericardium from the floor of the thorax.

The =heart= of the adult ox has an average weight of about 5½ to 6 pounds (ca. 2.5 to 2.7 kg.), or about 0.4 to 0.5 per cent. of the body-weight. Its length from base to apex is relatively longer than that of the horse and the base is smaller in both its diameters.

A shallow =intermediate groove= (Sulcus intermedius) extends from the coronary groove down the left side of the posterior border, but does not reach the apex. The amount of fat in and near the grooves is much greater than in the horse.

The heart is situated more to the left of the median plane than in the horse and is opposite to the third, fourth, and fifth ribs when it is contracted. It has extensive contact with the lateral wall of the thorax on the left side, but none on the right side, where a considerable thickness of lung covers the pericardium.[178]

Two bones, the =ossa cordis=, develop in the aortic fibrous ring. The right one is in apposition with the atrio-ventricular rings and is irregularly triangular in form. Its left face is concave and gives attachment to the right posterior cusp of the aortic valve. The right surface is convex from before backward. The base is superior. The posterior border bears two projections separated by a notch. It is usually a little more than an inch (ca. 4 cm.) in length. The left bone is smaller and is inconstant. Its concave right border gives attachment to the left posterior cusp of the aortic valve. There is a large fleshy moderator band in the right ventricle.

THE ARTERIES[179]

The great arterial trunks in the thorax resemble those of the horse in general disposition.

The =left coronary artery= is much larger than the right one; it gives off a branch which descends in the intermediate groove, and terminates by running downward in the right longitudinal groove. The =right artery=, after emerging from the interval between the right auricle and the pulmonary artery, divides into branches which are distributed to the wall of the right ventricle.

The =brachiocephalic trunk= (anterior aorta) is usually four or five inches (ca. 10 to 12 cm.) in length.

The =brachial arteries= give off in the thorax the following branches:

1. A common trunk for the subcostal, dorsal, superior or deep cervical, and vertebral arteries.

(1) The =subcostal artery= commonly arises separately, but may be given off as in the horse. It supplies the first three intercostal arteries.

(2) The =dorsal artery= is relatively small. It usually ascends in front of the first costo-vertebral joint and is distributed as in the horse.

(3) The =superior= or =deep cervical artery= may arise from a common stem (Truncus vertebro-cervicalis) with the vertebral, or may constitute a branch of that artery. It passes up between the first thoracic and last cervical vertebræ or between the sixth and seventh cervical and is distributed as in the horse.

(4) The =vertebral artery= passes along the neck as in the horse to the intervertebral foramen between the second and third cervical vertebræ, gives off a muscular branch, and enters the vertebral canal (Fig. 469). It runs forward on the floor of the canal—connected with its fellow by two or three transverse anastomoses—and divides in the atlas into two branches. The smaller internal division (cerebrospinal artery) passes forward to the floor of the cranium and concurs with the condyloid artery and branches of the internal maxillary in the formation of a large =rete mirabile=. The large external branch emerges through the intervertebral foramen of the atlas and ramifies in the muscles of the neck in that region, compensating for the smallness of the branches of the occipital artery. It also sends a branch to the rete mirabile. The collateral branches detached to the cervical muscles are large and compensate for the small size of the deep cervical artery. The collateral spinal branches of the vertebrals pass through the intervertebral foramina, divide into anterior and posterior branches, and form two longitudinal trunks which are connected by cross-branches so as to form irregular polygonal figures.

2. The =internal thoracic artery=.

3. The =inferior cervical artery= corresponds usually to the ascending branch of that vessel in the horse.

4. The =external thoracic artery= is large and usually gives off a branch which is equivalent to the descending branch of the inferior cervical artery of the horse.

THE COMMON CAROTID ARTERY

The =carotid arteries= usually arise from a common trunk about two inches (ca. 5 cm.) in length, but in exceptional cases are given off separately from the brachiocephalic. Each pursues a course similar to that of the horse and is accompanied by the small internal jugular vein, but is separated from the external jugular vein by the omo-hyoid and sterno-mastoid muscles. It divides at the digastricus into occipital, external maxillary, and external carotid arteries. In addition to tracheal, œsophageal, and muscular branches, it gives off the thyroid and laryngeal arteries. The =thyroid artery= (A. thyreoidea cranialis) bends around the anterior end of the thyroid gland, in which it ramifies. The accessory thyroid artery is usually absent. The =laryngeal artery= may arise with the thyroid.

1. The =occipital artery= is relatively small. It gives off:

(1) The =pharyngeal artery=, which, however, may arise from the external carotid.

(2) Several branches to the muscles (chiefly the flexors) and to the atlanto-occipital joint.

(3) The =condyloid artery= passes into the cranium through the anterior foramen in the condyloid fossa, and joins the vertebral in the formation of the rete mirabile about the pituitary gland. Before entering the cranium it gives off a branch to the pharyngeal lymph glands, and the middle meningeal artery; the latter passes through the foramen lacerum. Another branch enters the temporal canal and gives twigs to the temporalis muscle and the mucous membrane of the frontal sinus. A muscular branch emerges from the intervertebral foramen of the atlas. A diploic branch goes into the occipital condyle and squama, and emits twigs to the occipital muscles.

2. The =external maxillary artery= is smaller than that of the horse, but pursues a similar course. The =lingual artery= is large, and often arises separately from the common carotid; it gives off a branch to the submaxillary gland, and the =sublingual artery=. After turning around the jaw the facial gives off the two =labial arteries=. The =superior labial= is large; it usually gives off a branch which runs forward almost parallel with the lateral nasal. The =angular artery= is absent or rudimentary, and the =lateral= and =dorsal nasal arteries= spring from branches of the internal maxillary.

3. The =external carotid artery= passes upward between the stylo-hyoideus and the great cornu of the hyoid bone, turns forward across the external face of the latter, and divides into superficial temporal and internal maxillary arteries. It gives off the following collateral branches:

(1) Branches to the parotid and submaxillary glands.

(2) The =pharyngeal artery=, which, however, often arises from the occipital.

(3) The =posterior auricular=, which resembles that of the horse and sends a stylo-mastoid branch into the tympanum. It may arise from the superficial temporal.

(4) The =masseteric artery= resembles that of the horse, but is smaller.

The =superficial temporal artery= is large and presents the following special features: (1) Its transverse facial branch passes into the central part of the masseter. (2) It gives off a branch which corresponds to the posterior meningeal artery of the horse, enters the temporal canal, and ramifies in the dura mater, giving off twigs to the external ear, the temporal muscle, and the frontal sinus. (3) It usually gives off the anterior auricular artery. (4) Branches are supplied to the frontalis muscle and the eyelids, (5) A large branch passes around the outer side of the base of the horn-core, supplies the matrix of the horn, and anastomoses across the back of the frontal eminence with the artery of the opposite side.

The =internal maxillary artery= is less curved than in the horse and is entirely extraosseous, since the alar canal is absent. The principal differential features in its branching are as follows:

(1) The =ophthalmic artery= forms a rete mirabile within the periorbita. Its frontal branch enters the supraorbital canal and ramifies chiefly in the frontal sinus.

(2) Several branches take the place of the internal carotid artery. One of these enters the cranial cavity through the foramen ovale and several small ones pass through the foramen which represents the foramen rotundum and foramen lacerum orbitale of the horse. They concur with the vertebral and condyloid arteries in the formation of an extensive =rete mirabile= on the cranial floor around the sella turcica. From each side of the rete an artery arises which is distributed in general like the internal carotid of the horse.

(3) The =malar artery= is large; it arises by a common trunk with the infraorbital and gives off the =dorsal nasal= and the =angular artery of the eye=.

(4) The =infraorbital artery= is large and emerges from the infraorbital foramen to form the =lateral nasal artery=.

(5) The =palatine artery= is smaller than in the horse and usually arises by a common trunk with the sphenopalatine. It passes through the palatine canal and along the palatine groove, enters the nasal cavity through the incisive fissure, and does not go to the upper lip. It forms a rete mirabile about the naso-palatine canal and terminates in the mucous membrane of the anterior part of the nasal cavity.

ARTERIES OF THE THORACIC LIMB

The =brachial artery= pursues the same course in the arm as that of the horse. At the elbow it becomes the median.[180] The chief differential features in its branches are as follows:

1. The =subscapular artery= is almost as large as the continuation of the brachial. The =posterior circumflex artery= sends branches backward and downward into the triceps, taking the place in part of the deep brachial artery. The =thoracico-dorsal artery= supplies branches to the pectoral muscles and the triceps as well as the teres major and latissimus dorsi; it may arise directly from the brachial.

2. The =deep brachial artery= is small.

3. The =superior collateral ulnar artery= is often double, and does not extend to the carpus.

The =median artery= (posterior radial artery) descends along the inner part of the posterior surface of the radius and divides near the middle of the forearm into the radial and ulnar arteries. It gives off at the upper third of the forearm the =common interosseous artery=, a large vessel which anastomoses with the deep brachial, passes through the proximal interosseous space, and descends (as the dorsal interosseous) in the groove between the radius and ulna, and concurs in the formation of the rete carpi dorsale. At the distal end of the forearm it sends a branch through the distal interosseous space, which passes downward, assists in forming the rete carpi volare, and is continued in the metacarpus as the external deep volar metacarpal artery. This is a small vessel which passes down under the outer border of the suspensory ligament and assists in forming the deep volar arch near the fetlock.

The =radial artery= is smaller than the ulnar. It descends on the flexor carpi internus, passes over the postero-internal surface of the carpus, and is continued as the internal deep volar metacarpal. At the distal end of the forearm and at the carpus it furnishes branches to the retia carpi. Another branch (A. met. perforans proximalis) runs outward between the suspensory ligament and the large metacarpal bone, passes through the proximal foramen of the bone, and unites with the interosseous artery; before passing through the foramen it detaches the middle deep volar metacarpal artery (A. met. volaris prof. III.), which descends on the posterior face of the metacarpal bone, receives an anastomotic branch from the internal deep artery, and concurs in the formation of the deep volar arch above the fetlock.

The =ulnar artery=, the larger of the two divisions of the median, descends under cover of the flexor carpi internus without giving off any large branches in the forearm. It passes through the carpal canal and continues along the inner side of the deep flexor tendon as the =volar common digital artery=. At the distal third of the metacarpus this vessel is joined by a branch from the internal deep volar metacarpal artery, forming the superficial volar arch. Near this another branch of the common digital (or of the internal digital) passes around the outer border of the flexor tendons to the posterior face of the metacarpal bone and concurs with the deep volar metacarpal arteries in the formation of the deep volar arch. A branch from the arch passes forward through the inferior foramen of the metacarpal bone and joins the dorsal metacarpal artery which descends from the rete carpi dorsale in the anterior metacarpal groove.

The =volar common digital artery= (A. metacarpea volaris superficialis III.) passes into the interdigital space and divides into two =volar proper digital arteries=, which descend along the interdigital surfaces of the chief digits and pass through the foramina at the upper part of the interdigital surfaces of the third phalanges, enter the cavities in these bones, and ramify in a manner similar to the corresponding vessels in the horse. The volar common digital detaches a branch (A. interdigitalis perforans) which passes forward through the upper part of the interdigital space and anastomoses with the dorsal metacarpal artery.

The volar proper digital arteries (or the common digital) give off, in addition to other collaterals, branches which correspond to the arteries of the plantar cushion of the horse. These pass to the bulbs of the claws and anastomose with each other and with the inner and outer digital arteries, forming an arch from which numerous branches are distributed to the matrix of the hoofs.

The =dorsal metacarpal artery= (A. met. dorsalis III.) is a small vessel which arises from the rete carpi dorsale, descends in the groove on the anterior face of the metacarpal bone, and is joined by the inferior perforating metacarpal artery from the deep volar arch to constitute the dorsal common digital artery. This vessel (A. digitalis communis dorsalis III.) divides into two =dorsal proper digital arteries=.

The =internal digital artery= (A. digiti III. medialis) is the continuation of the internal deep volar metacarpal artery. It descends on the inner side of the inner digit and terminates at the bulb of the claw by anastomosing with the corresponding volar proper digital artery. It gives off a branch to the rudimentary digit and forms a transverse anastomosis behind the first phalanx with the volar common digital or its inner division.

The =external digital artery= (A. digiti IV. lateralis) arises from the deep volar arch, passes down on the outer side of the external digit and is distributed like the inner one.

BRANCHES OF THE THORACIC AORTA

The =bronchial= and =œsophageal= arteries often arise separately.

Ten pairs of =intercostal arteries= usually arise from the aorta. The other three come from the subcostal artery.

The two =phrenic arteries= are very variable in origin. They may come from the aorta, the cœliac, left ruminal, or an intercostal or lumbar artery.

BRANCHES OF THE ABDOMINAL AORTA

The =cœliac artery= is about four to five inches (ca. 10 to 12 cm.) in length. It passes downward and curves forward between the rumen and pancreas on the left and the right crus of the diaphragm and the posterior vena cava on the right. It gives off five chief branches.

1. The =hepatic artery= arises from the convex side of the curve of the cœliac artery as it crosses the posterior vena cava. It passes downward, forward, and to the right above the portal vein to the portal fissure, and gives off the following branches:

(1) Pancreatic branches.

(2) Dorsal and ventral branches to the liver. The ventral branch is the larger; it gives off the right gastric artery, which runs in the lesser omentum to supply the origin of the duodenum and the pylorus, anastomosing with the dorsal branch of the omaso-abomasal artery.

(3) The cystic artery supplies the gall-bladder.

(4) The gastro-duodenal artery divides into right gastro-epiploic and pancreatico-duodenal branches. The right gastro-epiploic artery anastomoses with the left gastro-epiploic. The pancreatico-duodenal artery anastomoses with the first intestinal branch of the anterior mesenteric artery.

2. The =right ruminal artery= (A. ruminalis dextra) is the largest branch, and usually arises by a short common trunk with the splenic. It runs downward and backward on the right face of the dorsal sac of the rumen to the posterior transverse fissure, in which it turns around to the left and anastomoses with branches of the left ruminal artery. It gives off a pancreatic branch, dorsal and ventral coronary arteries, branches to the great omentum, and ramifies on both surfaces of the rumen.

3. The =left ruminal artery= (A. ruminalis sinistra) runs downward on the anterior part of the right face of the rumen, enters the anterior furrow, in which it runs from right to left, and continues backward in the left longitudinal groove, anastomosing with branches of the right artery. It supplies chiefly the left face of the rumen, but not its posterior part. It usually gives off near its origin the =reticular artery= (A. reticularis); this rather small vessel passes forward on the dorsal curvature of the rumen and turns downward in the rumino-reticular groove, in the bottom of which it runs around ventrally to the right side. It gives off a branch which passes to the left of the cardia and along the lesser curvature of the reticulum to the neck of the omasum. The reticular branches anastomose with the omaso-abomasal and left ruminal arteries.

4. The =omaso-abomasal artery= (A. gastrica sinistra) appears as the continuation of the cœliac. It passes forward and downward to the greater curvature of the omasum and divides after a course of four or five inches into two branches. The =dorsal branch= curves sharply backward on the dorsal surface of the omasum, continues along the lesser curvature of the abomasum, and anastomoses with the hepatic artery. It supplies branches to the omasum and to the lesser curvature and pyloric part of the abomasum. The =ventral branch= (A. gastroepiploica sinistra) runs forward and downward over the anterior extremity of the omasum, passes backward in the great omentum close to the ventral curvature of the abomasum, and anastomoses with the right gastro-epiploic. A considerable branch from it curves around in front of the neck of the omasum to communicate with the reticular artery. Another branch runs back across the left side of the omaso-abomasal junction, and a third goes to the lesser (ventral) curvature of the omasum.

5. The =splenic artery= usually arises by a common trunk with the right ruminal artery. It passes forward and to the left across the dorsal curvature of the rumen and enters the hilus of the spleen.

The =anterior mesenteric artery= arises from the aorta just behind the cœliac, and has about the same caliber as the latter. It passes downward and a little to the right, and crosses the colon as it emerges from the spiral to run backward. After detaching twigs to the pancreas it gives off in succession the following branches:

1. A branch (A. colica media) passes to the colon as it emerges from the spiral arrangement. This artery is comparable to the middle colic or artery of the small colon of the horse; it runs backward along the terminal part of the colon, which it supplies.

2. The ileo-cæco-colic artery ramifies on the right face of the spiral part of the colon. It gives off the ileo-cæcal artery, which divides into ileal and cæcal arteries.

3. An artery (Ramus collateralis) runs in the mesentery in a curve along the ventral border of the coils of the colon. (This vessel is absent in the sheep.)

4. An artery which constitutes the direct continuation of the anterior mesenteric pursues a course in the mesentery corresponding to the series of mesenteric lymph glands.

The two preceding vessels are essentially the arteries of the small intestine, which they supply with the exception of its initial and terminal parts. The first gives off no considerable branches in its course along the ventral border of the coils of the colon, but on curving upward along the latter it anastomoses with the second artery and detaches numerous branches to the small intestine which form series of superposed anastomotic arches. It supplies, roughly speaking, about one-third of the small intestine and terminates by joining the ileal artery. The second artery gives off numerous branches which also form arches and supply about the first two-thirds of the small intestine, exclusive of the small part supplied by branches of the cœliac artery. Both arteries give branches to the lymph glands.[181]

The =posterior mesenteric artery= arises from the aorta near its termination. It is small and supplies branches to the terminal part of the colon and to the rectum.

The =renal arteries= arise from the aorta close together. The right one passes outward and forward across the dorsal face of the posterior vena cava to the hilus of the kidney. The left one runs backward, but necessarily varies in direction in conformity with the position of the kidney (q. v.).

The =spermatic arteries= resemble those of the horse.

The =utero-ovarian arteries= are small.

The five pairs of =lumbar arteries= derived from the aorta are distributed much as in the horse. The sixth usually comes from the internal iliac artery.

The =middle sacral artery= is a vessel about 5 mm. in diameter which continues the aorta. It arises from the dorsal face of the aorta at the angle of divergence of the internal iliacs, runs backward on the pelvic surface of the sacrum a little to the left of the median line, and is continued as the middle coccygeal artery. It gives off small collateral branches to the spinal cord and the muscles of the tail and the =lateral coccygeal arteries=. The latter may have a common trunk of origin, and each divides into dorsal and ventral branches. The =middle coccygeal artery= runs through the ventral (hæmal) arches of the coccygeal vertebræ. The coccygeal arteries are connected at pretty regular intervals by segmental anastomoses.

The =internal iliac arteries= are much longer than in the horse. Each passes backward on the sacro-sciatic ligament and divides about the middle of the pelvic wall into posterior gluteal and internal pudic branches. The chief differences in its distribution are: (1) A large trunk gives origin to the umbilical and middle uterine arteries. The =umbilical artery= is usually largely obliterated and its terminal branches receive their blood through anastomoses with the internal pudic. It gives off near its origin two small vessels, the ureteral artery (A. ureterica) and the deferential artery (A. deferentialis), which accompany the ureter and the vas deferens respectively. The =middle uterine artery= (A. uterina media) is very large. It is distributed chiefly to the cornu of the uterus, and compensates for the small size of the utero-ovarian artery. (2) The =ilio-lumbar= artery is relatively small and is distributed chiefly to the sublumbar muscles. It is sometimes replaced by branches of the circumflex iliac and gluteal arteries. (3) The =anterior gluteal artery= is commonly represented by several vessels. (4) The =obturator artery= is represented by several small branches which supply the obturator and adductor muscles. (5) The =iliaco-femoral= and =lateral sacral arteries= are absent. The absence of the latter is compensated by the middle sacral and gluteal arteries. (6) The =posterior gluteal= (or ischiatic) artery is large. It emerges through the lesser sciatic notch and ramifies in the biceps femoris and adjacent muscles. (7) The =internal pudic artery= (A. urethro-genitalis) is the direct continuation of the internal iliac. It gives off branches to the rectum, bladder, urethra, and genital organs. In the male it supplies the accessory genital glands and divides into dorsal and deep arteries of the penis; the a. dorsalis penis runs along the dorsum penis to the glans and gives twigs to the prepuce; the a. profunda penis gives off a perineal branch and enters the corpus cavernosum penis. In the female it gives off a large posterior uterine artery, which supplies the posterior part of the uterus and gives branches to the vagina and bladder. It ends as the a. clitoridis, which supplies the clitoris and adjacent parts.

ARTERIES OF THE PELVIC LIMB

The =external iliac artery= has the same course as in the horse. The circumflex iliac artery is large. A branch from it emerges between the abdominal and lumbar muscles near the external angle of the ilium and ramifies like the terminals of the ilio-lumbar artery of the horse.

The =femoral= and =popliteal arteries= pursue a similar course to those of the horse. The chief differences in their branches are as follows:

1. The =external pudic artery= is distributed chiefly to the scrotum in the male. In the cow it is usually termed the =mammary= and is very large, especially during lactation. Each divides at the base of the mammary gland into two branches which are distributed to the anterior and posterior parts (“quarters”) of the gland. A small branch accompanies the subcutaneous abdominal vein to the xiphoid region.

2. The =deep femoral artery= gives off an obturator branch which passes up through the obturator foramen to supply the obturator internus and compensates otherwise for the absence of the obturator artery.

3. The =anterior femoral artery= is large. It often gives off the =external circumflex artery= of the thigh, which perforates the proximal end of the quadriceps, gives branches to that muscle, the iliacus, glutei, and tensor fasciæ latæ.

4. The =saphenous artery= is large. It descends in front of the homonymous vein to the postero-internal surface of the hock, where it divides into two plantar branches. The =internal plantar artery= (A. plantaris medialis) is the direct continuation of the saphenous. It descends at first along the inner border of the superficial flexor tendon and is continued as the =internal superficial plantar metatarsal artery= along the inner side of the deep flexor tendon with the internal plantar nerve. It anastomoses at the proximal end of the metatarsus with the perforating metatarsal artery, assisting in the formation of the proximal plantar arch. Near the fetlock it concurs with the perforating branch of the dorsal metatarsal artery in the formation of the distal plantar arch. Below this it is continued as the =internal digital artery=. The =external plantar artery= (A. plantaris lateralis) is small. It descends along the outer border of the deep flexor tendon with the external plantar nerve, concurs with the perforating tarsal and the internal plantar arteries in the formation of the proximal plantar arch, and gives branches to the rete tarsi dorsale. Continuing downward along the deep flexor tendon as the =external superficial plantar metatarsal artery=, it assists in forming the distal plantar arch and becomes the =external digital artery=.

The =posterior tibial artery= is relatively small and is distributed chiefly to the muscles on the posterior surface of the tibia. Lower down it is replaced by the saphenous artery as described above.

The =anterior tibial artery= has the same course as in the horse. It is continued down the groove on the front of the metatarsal bone as the =dorsal metatarsal artery=. This is the chief artery of the region and is accompanied by two veins. It detaches the proximal perforating metatarsal artery, which passes through the proximal foramen of the metatarsal bone and concurs with the superficial plantar arteries in forming the proximal plantar arch. Near the distal end of the metatarsal bone it gives off the distal perforating metatarsal artery which passes back through the distal metatarsal foramen and assists in forming the distal plantar arch. The two arches are connected by the small =deep plantar metatarsal artery= which lies in the groove on the posterior face of the large metatarsal bone.

The =dorsal common digital artery= is the direct continuation of the dorsal metatarsal. It divides into two branches which unite in the interdigital space with the corresponding branches of the plantar common digital to form the proper digital arteries.

The =plantar common digital artery= descends from the distal plantar arch, anastomoses in the interdigital space with the internal and external digitals, and divides into two branches which join those of the dorsal common digital artery as before mentioned.

The foregoing is a brief statement of the more common arrangement of the vessels in the distal part of the limb, but minor variations are very common.

THE VEINS[182]

The =vena hemiazygos= usually takes the place of the vena azygos. It lies along the left side of the aorta and the bodies of the thoracic vertebræ, turns down across the left face of the aorta and left pulmonary artery, runs back over the left auricle and opens into the great coronary vein or the right atrium. It receives the intercostal veins.

Two =jugular veins= occur on either side. The =internal jugular vein= (V. jugularis interna) is a relatively small vessel which accompanies the carotid artery. It arises by occipital, laryngeal, and thyroid radicles, receives tracheal, œsophageal, and muscular branches, and joins the external jugular near its termination. It is sometimes absent, but in some cases it appears, on the other hand, to be large enough to interfere with venesection practised on the external jugular. The =external jugular vein= (V. jugularis externa) is very large and corresponds to the single jugular of the horse. It is separated from the carotid artery in the greater part of its course by the sterno-cephalicus and omo-hyoideus muscles.

The =inferior cerebral vein= usually does not unite with the occipital; the latter is continued by the internal jugular vein.

The =orbital veins= form a network between the periorbita and the muscles of the eyeball. This plexus communicates with the cavernous sinus and with the superior cerebral vein. It is also drained by the =frontal vein= which runs in the supraorbital canal and groove and joins the angular vein of the eye.

The =dorsal nasal vein= is usually double.

The =superior labial vein= usually joins the infraorbital.

The =vena reflexa= is absent and the radicles which are received by it in the horse go to the internal maxillary vein.

The =sublingual vein= is very large.

The =veins of the thoracic limb= differ chiefly in the distal part; the special features are as follows:

The =dorsal digital veins= ascend on the front of the digits and are connected with the other digital veins by transverse branches. They unite near the fetlock to form the =dorsal metacarpal vein=. This runs upward on the anterior face of the metacarpus and carpus, inclines to the inner surface of the radius, and joins the accessory cephalic or the cephalic vein.

The =volar digital veins= are larger than the dorsal. They lie on the interdigital surfaces of the digits and unite in the interdigital space to form a trunk which is a satellite of the volar common digital artery.

The =internal= and =external digital veins= lie in front of the corresponding arteries. They are connected with the volar digital vein by a large branch which passes between the flexor tendons and the first phalanx. At the distal end of the metacarpus each inclines forward and anastomoses with the volar common digital vein to form the volar venous arch. The inner vein is continued as the internal volar metacarpal vein along the inner border of the suspensory ligament, and becomes a satellite of the radial artery in the forearm, while the outer one is continued on the posterior face of the metacarpal bone by two irregular veins, the external and middle volar metacarpals. The latter anastomose freely with each other and with the inner vein. They unite below the carpus or join the volar common digital vein.

The =accessory cephalic vein= is the upward continuation of the dorsal metacarpal vein and is much larger than in the horse.

The =posterior vena cava= is partially embedded in the medial border of the liver. Its abdominal part has a thicker wall than in the horse. Its affluents correspond to the arteries of which they are satellites. The =renal veins= are large and thick-walled; they run obliquely forward and join the vena cava at an acute angle. The left one is much the longer.

Two =middle sacral= veins usually accompany the artery.

The =veins of the mammary glands= deserve special notice. They converge to a venous circle at the base of the udder, which is drained chiefly by two pairs of veins. The =subcutaneous abdominal vein= (anterior mammary or “milk” vein) is very large in animals of the dairy breeds and its course along the ventral wall of the abdomen is easily followed. It is usually flexuous. It emerges at the anterior border of the udder about two or three inches (ca. 5 to 8 cm.) from the linea alba, runs forward (deviating a little outward), dips under the panniculus, passes through a foramen in the abdominal wall about a handbreadth from the median plane, and joins the internal thoracic vein. The =external pudic vein= (middle mammary vein) is also of considerable size. It ascends in the inguinal canal as a satellite of the artery and joins the external iliac vein. The right and left veins are connected at the posterior border of the base of the udder by a large transverse branch. From the latter arises the =perineal vein= (posterior mammary vein), which runs medially upward and backward to the perineum, turns around the ischial arch, and joins the internal pudic vein. In the male these veins are relatively small.

The deep veins of the thigh and leg resemble those of the horse, but there is no recurrent tibial vein.

The =saphenous vein= is much smaller than in the horse.

The =recurrent tarsal vein= (external saphenous) is large. It is the upward continuation of the external plantar metatarsal vein and anastomoses with the anterior tibial and saphenous veins. It arises on the outer face of the hock, ascends at first in front of the tendo Achillis, then crosses the latter externally, passes up between the biceps femoris and semitendinosus, and joins the posterior femoral vein.

There are three chief metatarsal veins. The great =dorsal metatarsal vein= arises at the distal part of the metatarsus by the union of the dorsal digital vein and a large branch from the venous arch above the sesamoids. It ascends superficially between the long and lateral extensor tendons and becomes the chief radicle of the anterior tibial vein. The =internal plantar metatarsal vein= arises from the venous arch above the fetlock, ascends along the inner border of the suspensory ligament, passes through the vascular canal of the metatarsus and centro-tarsal (as the perforating tarsal) and joins the anterior tibial vein. The =external plantar metatarsal vein= is larger than the preceding. It passes superficially over the outer face of the hock and is continued by the recurrent tarsal vein.

The =digital veins= differ from those of the fore limb chiefly in that the dorsal vein is large and the plantar absent or small.

The =portal vein= is formed usually by the confluence of two radicles, gastric and mesenteric. It receives the right gastro-epiploic vein and veins of the pancreas. The gastric vein is the largest affluent. It is formed by the junction of three veins, the left of which receives the splenic vein. The anterior mesenteric vein is also formed by the confluence of three radicles; it usually receives the small posterior mesenteric vein. The portal tributaries are in general satellites of the corresponding arteries.

THE LYMPH VESSELS AND GLANDS

The =thoracic duct= arises from a small cisterna chyli and is very variable. It is exceptional to find a single trunk throughout, as is often the case in the horse. The duct is largely covered by fat and small lymph glands. There are often two ducts, one on either side of the aorta, which may join at a variable point or terminate close together at the junctions of the jugular and brachial veins. In other cases the duct is single for a variable distance, then bifurcates or divides into three or four branches, which are often connected by plexiform anastomoses.

The efferent vessels from the intestine converge to a large trunk which accompanies the anterior mesenteric artery and vein. It passes below the pancreas and receives usually a large duct which is formed by efferent vessels from the stomach, liver, and spleen; this duct sometimes opens directly into the cisterna. The gastro-intestinal trunk receives the collecting duct (truncus lumbalis) of the lumbar lymph vessels, forms a bend around the right renal vessels, and joins the cisterna.

The =lymph glands= in the ox are in general less numerous but larger than those of the horse, and in some situations a single large gland occurs instead of a group of smaller ones, as found in the latter animal.

The =submaxillary lymph glands= are usually two in number, right and left; each is situated under the external maxillary vein between the submaxillary salivary gland and the sterno-cephalicus muscle. Usually one or two hæmolymph glands are near it. There is generally a small gland on the deep face of the anterior part of the submaxillary salivary gland, and small nodes may be found further forward in the submaxillary space.

A large =subparotid lymph gland= lies on the postero-superior part of the masseter muscle, partly under cover of the upper end of the parotid gland. Several hæmolymph glands lie on its deep face.

The =pharyngeal lymph glands= number two or three on each side. Of these, two large =suprapharyngeal glands= (Lg. retropharyngeales) are situated about an inch apart between the dorsal wall of the pharynx and the ventral straight muscles (Figs. 281, 365). These are two to three inches long. Enlargement of them is likely to cause difficulty in swallowing and in respiration. Behind these are several hæmolymph glands. An =atlantal gland= is situated below the wing of the atlas on the spinal accessory nerve. It is discoid, oval in outline, and may be an inch and a half or more in length. It is partly covered by the upper end of the submaxillary salivary gland.

A smaller lymph gland (parapharyngeal) is usually found along the lower border of the carotid artery (i. e., ventral to the atlantal gland) on the lateral wall of the pharynx and under cover of the submaxillary salivary gland or at its posterior border.

Two or three small =anterior cervical lymph glands= commonly lie along the carotid artery a little further back.

The =middle cervical lymph glands= comprise a series of small nodes along the dorsal face of the trachea.

In the sheep and goat there is a lymph gland at the middle of the neck, in the angle between the spinalis and complexus, covered by the splenius. It receives vessels from the atlantal gland and sends efferents to the prescapular gland.

The =prescapular= or =superficial cervical lymph gland= is situated at the anterior border of the supraspinatus under cover of the mastoido-humeralis and omo-transversarius. It is elongated and may be an inch or more in width and four or five inches long. Two occur in exceptional cases.

A chain of hæmolymph glands lies along the front of the shoulder, covered above by the trapezius. Others are frequently found (in the calf especially) on the surface of the omo-transversarius at the point where it passes under the mastoido-humeralis (Forgeot).

The =posterior cervical= or =prepectoral lymph glands= number commonly three or four on each side. One or two are placed on the brachial vessels, one below the brachial vein at the first rib, and one at the junction of the common trunk of the external and subcutaneous thoracic veins with the brachial.

The =intercostal lymph glands= are situated, as in the horse, at the upper parts of the intercostal spaces. Associated with them are hæmolymph glands.

The =sternal lymph glands= are situated along the course of the internal thoracic artery at the lower part of the intercostal spaces. The largest is close to the thoracic inlet. Several glands occur in the fat about the apex of the pericardium.

4. Several =anterior mediastinal lymph glands= are placed along the œsophagus and the trachea.[183] Hæmolymph glands occur here also.

Usually three large =bronchial lymph glands= are present (Fig. 289). One is situated at the origin of each chief bronchus and the third is under the trachea at the origin of the special bronchus of the apical lobe of the right lung. Another may lie between the aorta and the left branch of the pulmonary artery. These glands are often pigmented. Other small nodes are situated on the bronchi within the lungs.

The =posterior mediastinal lymph glands= comprise two or three of large size placed along the dorsal wall of the œsophagus. There may be a single gland about eight inches (ca. 20 cm.) long, the anterior half of which lies on the œsophagus (Fig. 289).[184] A small gland is situated in the acute angle formed by the posterior vena cava and the diaphragm.

A single =axillary lymph gland= is situated on the distal part of the teres major on the course of the vein from the latissimus dorsi.

The =lumbar lymph glands= form an irregular series scattered along the abdominal aorta and posterior vena. A bean-shaped =renal lymph gland= occurs at the hilus of each kidney. Hæmolymph glands occur along the course of the aorta.

The =internal iliac lymph glands= comprise a group of five or six at the termination of the aorta and the origin of the vena cava. A discoid gland, two inches or more in diameter, occurs near the side of the pelvic inlet at the angle of divergence of the circumflex iliac artery from the external iliac. A small gland is sometimes found at the origin of the prepubic artery.

The =external iliac lymph glands= number commonly two on either side, and are situated near the point of the hip in front of the anterior branch of the circumflex iliac artery.

The =superficial inguinal lymph glands= are situated centrally below the prepubic tendon and in the narrow space between the origins of the graciles. In the male there are usually two or three on either side of the penis behind the sigmoid flexure. In the cow they are =supramammary=, i. e., are situated above the posterior margin of the base of the udder; two large glands which are in apposition with each other medially are constant, and above these there are often two smaller glands.

An =ischiatic lymph gland=, which is discoid and usually about an inch in diameter, is situated on the lower part of the sacro-sciatic ligament near the lesser sciatic notch under cover of the biceps femoris.

An =anal lymph gland= is situated on the retractor ani on either side.

The =gastric lymph glands= are numerous and comprise: (1) a chain along the course of the right ruminal artery and two or three in the left groove of the rumen; (2) several on the reticulum above and below the junction with the omasum; (3) a series along the course of the superior omasal vessels and the lesser curvature of the abomasum; (4) an extensive chain along the ventral face of the omasum and the great curvature of the abomasum at the attachment of the great omentum.

Two or three large =hepatic= or =portal lymph glands= are found at the upper part of the portal fissure of the liver.

No lymph glands occur at the hilus of the spleen. The efferent vessels from the spleen appear to go to glands along the course of the cœliac artery.

A large discoid =cœliac lymph gland= lies on the pancreas and the cœliac artery, covering the latter as it gives off its primary branches.

The =mesenteric lymph glands= are large and numerous. They form a chain in the lower part of the mesentery along the course of the great mesenteric artery. They are chiefly elongated and narrow, and some have a length of six to eight inches (15 to 20 cm.) or more. Sometimes several unite and form a narrow band about two feet in length. A large gland occurs at the origin of the spiral part of the colon, a group is found near the anterior mesenteric trunk, and others are scattered along the coils of the colon, which are best seen on the right side. A chain of small lymph glands extends along the terminal part of the colon and the dorsal face of the rectum.

The =precrural lymph gland= is situated on the aponeurosis of the external oblique at the anterior border of the tensor fasciæ latæ a short distance above the stifle. It is elongated like the prescapular gland and may be six inches (ca. 15 cm.) long. Several subcutaneous hæmolymph glands occur in this vicinity; others are situated further forward on the flank not far from the last rib. There is usually a round, flattened lymph gland about an inch in diameter on the upper part of the quadriceps femoris under cover of the tensor fasciæ latæ, and one or more small ones occur on the surface of the latter muscle.

No =deep inguinal lymph glands= are present.

A single =popliteal lymph gland= an inch or more in length is situated somewhat lower down on the gastrocnemius than in the horse, behind the tibial and peroneal nerves.

CIRCULATORY SYSTEM OF THE PIG

THE HEART

The =pericardium= is attached to the sternum from a point opposite to the third rib as far as the xiphoid cartilage, and also to the sternal part of the diaphragm.

The =heart= is small in proportion to the body-weight, especially in fat animals. It is relatively short and wide. The apex is blunt and is marked by a notch (Incisura apicis). Its long axis is more oblique, but it is less asymmetrical with regard to the median plane, than in the horse or ox. The lower border of the left auricle (appendix) is marked by several notches and is situated at a lower level than the right one. The right longitudinal groove is placed far back and there is no posterior sulcus.

THE ARTERIES

The =pulmonary vessels= present no remarkable features.

The =aorta= resembles that of the horse and ox in its course and relations, but the arch is more strongly curved. There is no common brachiocephalic trunk (anterior aorta).

The =brachiocephalic artery= arises first from the aortic arch and passes forward below the trachea to the first rib. Here it gives off the =common carotid arteries=, and is continued around the first rib as the right brachial.

The =superior cervical=, =vertebral=, and =dorsal arteries= usually arise by a common trunk.

The =inferior cervical artery= is large; its ascending branch gives off the =posterior thyroid artery= and =parotid branches=.

The =internal= and =external thoracic arteries= give branches to the pectoral mammary glands; the external may be absent.

The =left brachial artery= arises from the aortic arch just above the brachiocephalic. It curves forward and downward and turns around the anterior border of the first rib. Its superior cervical, vertebral, and dorsal branches usually arise separately.

The =carotid arteries= arise from the brachiocephalic close together or by a very short common trunk. They terminate in occipital and internal and external carotid divisions.

The =occipital artery= resembles that of the horse.

The =internal carotid artery= usually arises by a common trunk with the occipital. After giving off a large meningeal branch it passes through the foramen lacerum and forms with the opposite artery a rete mirabile which resembles that of the ox, but is smaller and is not connected with the vertebral and condyloid arteries.

The =external carotid artery= has the same course and termination as in the horse. Its branches present the following special features: (1) The =lingual artery= is relatively large and supplies the pharyngeal artery, the sublingual, muscular branches, and branches to the soft palate, submaxillary gland, and larynx. (2) The =external maxillary artery= is small and is distributed chiefly to the submaxillary gland, the pharyngeal lymph glands, the masseter and the panniculus. It does not extend upon the lateral surface of the face. (3) The =posterior auricular artery= is long and relatively large; it gives off the posterior meningeal.

The =internal maxillary artery= pursues a flexuous course between the ramus of the mandible and the pterygoid muscles. Its branches offer the following special features: (1) The =inferior alveolar= (or dental) is large; branches from it emerge through four or five mental foramina and take the place of the inferior labial. (2) The =buccinator artery= is also large and extends to the lips. (3) The =infraorbital artery= extends to the snout and replaces the superior labial largely and the lateral nasal in part. The malar branch compensates largely for the absence of the lateral and dorsal nasal. (4) The =palatine artery= is small.

The =intercostal arteries= number fourteen to sixteen in either side; of these ten to twelve arise from the aorta, usually by short common trunks. Frequently an intercostal artery is given off from that of an adjacent space.

The =bronchial= and =œsophageal arteries= usually arise separately.

The =cœliac artery= is half an inch to an inch long. It supplies a branch to the left crus of the diaphragm and divides into two primary branches, gastro-hepatic and splenic. The =gastro-hepatic artery= is the larger. It gives off pancreatic branches, the anterior gastric artery, branches to the lesser curvature of the stomach, pyloric and gastro-duodenal arteries. The latter divides into pancreatico-duodenal and right gastro-epiploic. The anterior gastric usually supplies the œsophageal artery. The continuing trunk (A. hepatica propria) gives off a cystic branch and divides in the portal fissure into four branches which supply the liver. The =splenic artery= gives off the posterior gastric (usually), branches to the stomach above the cardia, twigs to the pancreas, short gastric arteries to the left part of the great curvature, and splenic branches, and is continued as the left gastro-epiploic artery.

The posterior gastric may arise from the gastro-hepatic or in the angle of divergence of the two primary divisions of the cœliac. The œsophageal branch may come from the splenic.

The =anterior mesenteric artery= is long like that of the ox. It gives twigs to the pancreas, the ilio-cæcal and two colic arteries, and is continued in the mesentery as the artery of the small intestine. This gives off numerous branches which form a series of arches along the mesenteric lymph glands. From these is formed a rich network which gives off innumerable fine branches placed close together. The colic arteries enter the axis of the spiral coil of the colon and anastomose at its apex. Their branches also form rich networks.

A =phrenico-abdominal artery= arises on either side a little in front of the renals. It divides into branches which go to the costal part of the diaphragm and the abdominal muscles.

The =renal= and =spermatic arteries= present no special characters.

The =posterior mesenteric artery= arises near the termination of the aorta. It is small and is distributed like that of the ox.

Six pairs of =lumbar arteries= arise from the aorta. The seventh comes from the middle sacral.

The terminal branches of the aorta resemble those of the ox.

The arteries of the shoulder, arm, and forearm resemble in general those of the ox.

The main facts as to the =metacarpal= and =digital arteries= are as follows: The rete carpi dorsale is formed essentially by the terminals of the interosseous artery of the forearm. It gives rise to three =dorsal metacarpal arteries= which descend in the corresponding interosseous spaces and unite with branches of the volar metacarpals to form three =common digital arteries=. Each of these divides into two =proper digital arteries=, which descend along the interdigital surfaces of the digits. On the volar surface are two =volar arches=, superficial and deep, from which three =superficial= and three =deep volar metacarpal arteries= arise. The deep arteries unite near the distal end of the space between the principal metacarpal bones to form a stem which unites with the middle (third) superficial artery. The superficial arteries unite to form an arch from which =proper digital arteries= are given off to the axial aspect of the small digits, and a common digital which supplies two =volar proper digital arteries= to each of the chief digits.

The arteries of the hip, thigh, and leg are arranged much as in the ox. A few special features may be noted.

The =ilio-lumbar artery= gives off a branch to the quadriceps femoris. It may also supply the posterior abdominal artery, which otherwise arises from the deep femoral.

The =deep femoral artery= is given off above the level of the pubis. The posterior abdominal and external pudic arteries may arise from it by a short common trunk or separately. The =external pudic= gives branches to the prepuce but not to the penis. The =popliteal artery= gives off the peroneal.

The =femoral artery= gives off a short trunk which divides into anterior femoral and external circumflex arteries, the latter being much the larger.

The =saphenous artery= is large. It descends on the inner surface of the leg and hock and concurs with the perforating tarsal artery in forming the proximal plantar arch.

The =posterior tibial artery= is small, being replaced distally by the saphenous. It gives branches to the muscles on the posterior face of the tibia and supplies the nutrient artery of that bone.

The =anterior tibial artery= is continued as the dorsalis pedis on the flexion surface of the tarsus. This gives off the =perforating tarsal artery=, which passes back through the vascular canal of the tarsus and unites with the terminal branches of the saphenous to form the proximal plantar arch.

The =metatarsal= and =digital arteries= resemble in arrangement the corresponding vessels of the fore limb.

THE VEINS

The veins resemble in general those of the ox. Thus there is a hemiazygos vein and two jugular veins, the internal one being relatively larger than in the ox. A few differential features may be noted.

The =buccinator vein= resembles that of the horse, and unites with the =vena reflexa= to form a short common trunk which joins the facial.

The =facial vein= resembles in general that of the horse. The =dorsal nasal vein= is large, receives the veins from the snout, runs backward in the groove of the nasal bone and joins the frontal vein; it is connected with its fellow by a transverse branch, and anastomoses freely with the malar and facial.

The veins of the distal parts of the limbs naturally present differences which are correlated with those of the arteries.

THE LYMPH VESSELS AND GLANDS

The =thoracic duct= often divides near its termination into two branches which unite to form an ampulla. The latter suddenly contracts and opens into the terminal part of the left jugular vein.

The =submaxillary lymph glands= are situated in the space between the omo-hyoid and internal pterygoid muscles in relation to the lower part of the anterior border of the submaxillary salivary gland. There are commonly two on each side, one large, the other small.

The =parotid lymph glands= (Fig. 309) are reddish-brown in color. There are usually four of considerable size on either side. One is situated at the upper part of the posterior border of the masseter, partly covered by the parotid gland. Another large subparotid gland lies below the base of the ear. Ventral to this are two smaller glands, one above and one below the external jugular vein.

The =pharyngeal= (or retropharyngeal) =lymph glands= (Fig. 310) are situated on the dorsal wall of the pharynx above the external carotid artery and below and behind the paramastoid process. There are usually two of considerable size on either side.

The =middle cervical lymph glands= form a group on the mastoido-humeralis on the course of the external jugular vein.

The =prescapular lymph glands= are situated at the anterior border of the anterior deep pectoral muscle undercover of the trapezius and omo-transversarius.

The =prepectoral lymph glands= are reddish in color and usually three in number. The largest is placed centrally under the trachea; the others are situated on the brachial vessels as they turn around the first rib.

The =axillary lymph glands= are usually absent, but very small nodes may be found near the insertion of the latissimus dorsi. Cubital glands are not present.

The =thoracic lymph glands= comprise: (1) a gland of considerable size on the first segment of the sternum; (2) about half a dozen reddish glands along the upper face of the thoracic aorta; (3) several glands along the lower face of the trachea; (4) the =bronchial lymph glands=; one of these lies on the bifurcation of the trachea and another at the apical bronchus of the right lung (Fig. 371).

The =lumbar lymph glands= are scattered along the abdominal aorta and the vena cava. Those placed near the hilus of the kidney are often designated =renal=.

The =internal iliac lymph glands= are rounded and relatively large. They comprise: (1) three or four glands situated on and between the origin of the circumflex iliac and the external and internal iliac arteries; (2) a gland in the angle of divergence of the internal iliac arteries.

The =external iliac lymph glands= are small, two or three in number, and lie in front of the circumflex iliac vessels near the point of the hip.

The visceral lymph glands of the abdomen comprise:

1. =Gastric lymph glands= on the lesser curvature of the stomach.

2. Several small =hepatic lymph glands= along the portal vein.

3. An elongated =splenic lymph gland= on the splenic vessels near the dorsal end of the spleen.

4. A series of =mesenteric lymph glands= situated along the anastomotic arches formed by the vessels of the small intestine.

5. Several =cæcal lymph glands= situated along the first part of the cæcal vessels.

6. Two series of =colic lymph glands= which accompany the arteries of the spiral part of the colon, and are exposed by separating the coils of the bowel. Small glands are placed in the colic mesentery and above the rectum.

7. The =anal lymph glands= are situated on either side on the retractor ani.

A small =ischiatic lymph gland= is found on the sacro-sciatic ligament near the lesser sciatic notch.

The =precrural lymph glands= are situated on the aponeurosis of the external oblique below the external angle of the ilium.

The =superficial inguinal lymph glands= are situated behind the external inguinal ring. No deep inguinal lymph glands are present.

The =popliteal lymph glands= are small normally and may escape notice.

A few nodules occur in front of the distal part of the tendo Achillis.

CIRCULATORY SYSTEM OF THE DOG

THE PERICARDIUM AND HEART

The =pericardium= is attached to the sternal part of the diaphragm by a fibrous band, and is connected with the sternum only by the mediastinal pleura.

The =heart= is almost globular in form, the apex being much blunter than in the other animals, and marked by a notch. Its long axis is almost parallel with the sternum and the apex (covered by the pericardium) is directed against the sternal part of the diaphragm. Its weight averages about 0.8 to 1 per cent. of the body-weight.[185]

The base extends forward to a transverse plane through the lower ends of the third pair of ribs. The apex lies opposite the sixth costal cartilage in expiration. On the right side the pericardium is in direct contact with the chest-wall over a narrow triangular area (base anterior) from the fourth to the sixth intercostal space inclusive. On the left side the area of contact is smaller and is at the fourth, fifth, and sixth cartilages.

The two coronary arteries may arise by a common trunk. Each divides into circumflex and descending branches. The left artery is twice as large as the right one.

The great coronary vein ascends in the left longitudinal groove to the coronary groove, in which it winds around the posterior face of the heart to the right side and opens into the right atrium below the posterior vena cava. Near its termination it receives one or two veins which ascend along the right longitudinal furrow. Several small veins from the wall of the right ventricle open into the right atrium separately or by a common trunk.

THE ARTERIES

There is no common brachiocephalic trunk (anterior aorta). Two large vessels arise from the convexity of the aortic arch; these are the =brachiocephalic= and =left brachial arteries=. The =brachiocephalic= gives off the two =carotid arteries=, no common trunk being present. It frequently also supplies the posterior thyroid artery. The intrathoracic branches of the brachial arteries are as follows:

A common trunk gives off the =dorsal=, =subcostal=, and =superior cervical arteries=. The dorsal artery passes out in front of the first rib, and the superior cervical through the first intercostal space. The latter is relatively small.

The =vertebral artery= may arise either in front of or behind the trunk above mentioned; in the latter case it crosses the inner face of the trunk. It passes along the neck in typical fashion to the third cervical vertebra and divides into three branches. The largest of these supplies the muscles in this region, compensating for the absence of branches of the superior cervical artery. The second passes between the second and third cervical vertebræ into the vertebral canal and unites with the opposite artery and a branch of the occipital artery to form the basilar. The third branch is the small continuation of the parent trunk; it passes to the wing of the atlas and anastomoses with the occipital.

The =inferior cervical artery= gives off the superficial cervical which ascends in front of the supraspinatus.

The =internal thoracic artery= is large and sends perforating branches to the pectoral mammary glands.

The =external thoracic artery= arises outside of the thorax and sends branches to the mammary glands.

The =common carotid arteries=, left and right, arise in that order from the brachiocephalic. In the neck the right one lies on the trachea, the left on the œsophagus. Collateral branches are the posterior thyroid (which may, however, arise from the right or left brachial or the inferior cervical), pharyngeal, thyro-laryngeal muscular, tracheal, and glandular (to the submaxillary gland). Each carotid divides under the wing of the atlas into occipital and internal and external carotid branches.

The =occipital artery= is small. It gives off =condyloid= and =posterior meningeal= branches, passes up through the notch on the anterior margin of the wing of the atlas, and is distributed to the muscles of this region. Its =cerebrospinal= branch passes through the intervertebral foramen, and unites in the vertebral canal with its fellow and a branch from the vertebral artery to form the basilar. A retrograde branch anastomoses with the vertebral as in the horse.

The =internal carotid artery=, also small, passes to the foramen lacerum posterius, enters the carotid canal and forms a bend before entering the cranium through the carotid foramen. It forms a plexus which is connected by branches with the middle meningeal and ophthalmic arteries. It then perforates the dura mater and enters into the formation of the arterial circle (of Willis).

The =external carotid artery= is the direct continuation of the common carotid. It passes along the lateral wall of the pharynx, emerges from beneath the digastricus, and divides into superficial temporal and internal maxillary arteries. It gives off the large flexuous =lingual artery= which has a course similar to that of the horse. The =external maxillary=, smaller than the lingual, passes along the upper border of the digastricus, gives off the sublingual, gains the anterior border of the masseter, and divides into superior and inferior labial and the angularis oris. The last-named vessel passes forward on the cheek between the labials to the angle of the mouth. The =sublingual artery= passes at first along the upper border of the digastricus and continues between the mylo-hyoideus and the ramus of the mandible. The =posterior auricular artery= arises at the anterior border of the digastricus, gives branches to the salivary glands and the adjacent muscles, and ascends on the convex face of the concha. It divides into two branches which return along the borders of the external ear.

The =superficial temporal artery=, after giving off the =anterior auricular artery= and a small =transverse facial artery=, turns forward under the temporal fascia toward the eye, and divides into upper and lower branches which supply the frontal region and the eyelids. It also supplies branches to the parotid gland and the masseter and temporalis muscles.

The =internal maxillary artery= pursues a course similar to that of the horse. The mental branches of the =inferior alveolar= (or dental) =artery= are of considerable size and are distributed in the lower lip and gums. Two or three =deep temporal arteries= are present. The anterior one may arise from the buccinator; it gives off the middle meningeal, which sends a branch to the carotid plexus. The =ophthalmic artery= gives off a branch which enters the cranium through the foramen lacerum orbitale and connects with the internal carotid; this is termed the internal ophthalmic and gives off the arteria centralis retinæ. The superficial branches of the =infraorbital artery= replace the dorsal and lateral nasal arteries, and compensate for the small size of the superior labial.

The =brachial artery= in its course in the arm presents no special features. At the elbow it passes between the biceps and the pronator teres, descends (as the median) under the flexor carpi radialis about a third of the way down the forearm, and divides into radial and ulnar arteries. Among its collateral branches are: (1) The =subscapular artery=, which passes up between the subscapularis and teres major, turns around the posterior angle of the scapula and terminates in the supraspinatus, deltoid, trapezius, and mastoido-humeralis. In about half the cases it gives off the =anterior circumflex=, which often arises with the =posterior circumflex=. Its other branches resemble those of the horse. (2) The =anterior circumflex artery= (in about half the cases). (3) The =deep brachial= arises about a third of the way down the arm. (4) The =bicipital artery= (for the biceps) is given off at the lower part of the arm. (5) The =proximal collateral radial artery= (not present in the horse) arises at the lower fourth of the arm, crosses over the terminal part of the biceps, descends on the extensor carpi radialis, and concurs with a branch of the volar interosseous in forming the rete carpi dorsale. It often supplies the bicipital artery. (6) The =anterior radial artery= (A. collateralis radialis distalis) is very small. (7) The =common interosseous artery= is given off a little below the elbow. It supplies branches to the flexor muscles and the =dorsal interosseous artery=, which passes through the interosseous space, gives branches to the extensor muscles, and by its terminal twigs concurs in the formation of the rete carpi dorsale. The direct continuation of the trunk is the =volar interosseous artery=, which descends under cover of the pronator quadratus, gives off a branch to the rete carpi dorsale and the =fifth volar metacarpal artery=, and terminates by joining the volar branch of the radial artery to form the deep volar arch. (8) The =volar antibrachial artery= (Ramus volaris antibrachii) arises below the interosseous and descends at first under the flexor carpi radialis, then between the heads of the deep flexor, and divides into ascending and descending branches.

The =radial artery=, the smaller terminal of the brachial, descends along the inner border of the radius, and divides near the carpus into dorsal and volar branches. The =dorsal branch= assists in forming the rete carpi dorsale. The larger =volar branch= descends behind the inner border of the carpus and joins the end of the volar interosseous in forming the =deep volar arch=. From this arch three =deep volar metacarpal arteries= descend in the second, third, and fourth intermetacarpal spaces and concur with the corresponding superficial volar and the dorsal arteries to form the common digitals.

The =ulnar artery= descends on the deep flexor of the digit, and sends a connecting branch to the radial above the carpus. Near the middle of the metacarpus it gives off the =first volar metacarpal artery= and divides into the =second=, =third=, and =fourth superficial volar metacarpal arteries=. The first volar artery unites with the corresponding dorsal artery to form the =first common digital artery=, which supplies proper digital arteries for the opposed surfaces of the first and second digits. The superficial volar arteries descend in the spaces between the other metacarpal bones, unite near the metacarpo-phalangeal joints with the corresponding deep volar and dorsal metacarpal arteries to form three =common digital arteries=. Each of the latter divides after a short course into two =proper digital arteries=, which run along the opposed surfaces of the second to the fifth digits. The external digital artery of the fifth digit comes from the =superficial volar arch= formed by the union of a branch of the volar interosseous with the fourth superficial volar metacarpal artery.

The rete carpi dorsale gives off four =dorsal metacarpal arteries= which descend in the intermetacarpal spaces and unite near the metacarpo-phalangeal joints with the volar arteries to form the common digitals.

The =thoracic aorta= supplies the last nine or ten pairs of intercostal arteries, but no anterior phrenic arteries. It gives off two or more œsophageal branches, in addition to the broncho-œsophageal, which arises close to or with the sixth intercostal and ramifies in the usual manner.

The =abdominal aorta=, after giving off the external iliac arteries, continues about half an inch to an inch (ca. 1 to 3 cm.) under the last lumbar vertebræ, gives off the internal iliacs, and is continued by the middle sacral artery. This small vessel runs backward under the sacrum and coccygeal vertebræ and gives off branches in segmental fashion.

The =cœliac artery= gives off the hepatic artery and forms a short =gastro-splenic= trunk. The hepatic artery gives off several hepatic branches, and the pyloric or right gastric artery, which passes along the lesser curvature of the stomach, anastomoses with the (left) =gastric artery=, and is continued by the gastro-duodenal. This divides near the pylorus into right gastro-epiploic and pancreatico-duodenal. The (left) gastric artery passes to the lesser curvature of the stomach and ramifies chiefly on the left part of the stomach, giving off a branch which anastomoses with the pyloric branch of the hepatic. The =splenic artery= gives off pancreatic branches, and reaches the lower part of the spleen. It gives off the left gastro-epiploic, and a branch which passes to the dorsal end of the spleen, which it supplies, besides giving off twigs to the left extremity of the stomach.

The =anterior mesenteric artery= arises close behind the cœliac. It gives off a common trunk for two colic arteries and the ileo-cæco-colic. The former supply the transverse and the anterior part of the left or descending colon, while the latter divides into branches for the ileum, cæcum, and first part of the colon. The trunk is continued as the artery of the small intestine, giving off fourteen to sixteen branches (Aa. jejunales), which form a series of anastomotic arches in the mesentery near the bowel.

Two =phrenico-abdominal arteries= (Aa. phrenicæ caudales) come off from the aorta behind the anterior mesenteric, and divide into phrenic and abdominal branches. Each phrenic artery diverges from its fellow in descending on the abdominal surface of the corresponding crus of the diaphragm to the sternal part. The inner branches anastomose with those of the opposite artery, the outer branches with intercostal arteries chiefly. The abdominal arteries give branches to the lumbar muscles, the renal fat and capsules, and ramify in the oblique abdominal muscles.

The =renal= and =spermatic arteries= offer no special features.

The =utero-ovarian artery= divides near the ovary into three or four branches which supply the ovary and uterus, anastomosing with the uterine artery.

The =posterior mesenteric artery= is small. It divides into two branches which supply the terminal part of the colon and the anterior part of the rectum.

The =circumflex iliac artery= usually arises from the terminal part of the aorta.

The =external iliac artery= usually gives off no collateral branches.

The =femoral artery= has the usual course. The deep femoral gives off the posterior abdominal and external pudic arteries by a very short common trunk or separately. In the female the external pudic divides into anterior and posterior branches; the former runs forward, supplies branches to the inguinal lymph glands, the mammary glands and skin, and anastomoses with the mammary branch of the internal thoracic artery; the posterior branch pursues a flexuous course between the thighs to the vulva, where its terminal branches anastomose with the internal pudic artery. The anterior femoral may arise by a short common trunk with the external circumflex. The former, accompanied by the anterior branch of the femoral nerve, dips in between the rectus femoris and vastus internus. The external circumflex artery passes forward between the sartorius internally and the rectus femoris and tensor fasciæ latæ externally and supplies branches to these muscles and the glutei. In addition to muscular branches of considerable size, an articular branch (A. genu suprema) arises a little below the middle of the thigh and runs downward and forward to the inner face of the stifle joint. The =saphenous artery= is large. It arises from the inner surface of the femoral a little below the middle of the femur, descends superficially to the upper part of the leg and divides into two branches. The smaller =anterior branch= (Ramus dorsalis) passes obliquely downward and forward across the inner surface of the tibia to the flexion surface of the hock and terminates in three =superficial dorsal metatarsal arteries=. The =posterior branch= (Ramus plantaris) passes down on the inner face of the gastrocnemius and the long digital flexor. It gives off a branch (A. tarsea lateralis) which descends obliquely to the external surface of the tarsus, and at the posterior face of the tarsus detaches the =internal= and =external plantar arteries=; these descend on either side of the deep flexor tendon and unite with the perforating metatarsal artery to form the proximal plantar arch. The artery continues down the middle of the plantar surface of the metatarsus and divides near the metacarpo-phalangeal joints into three =superficial plantar metatarsal arteries=. These vessels unite with three =deep plantar metatarsal arteries= which descend from the proximal plantar arch and with branches from the dorsal metatarsal arteries. From these anastomoses four digital arteries result; of these, the central two have a common digital trunk.

The =popliteal= and =posterior femoral arteries= present nothing of special interest.

The =posterior tibial artery= is small, being replaced largely by the saphenous. It supplies twigs to the flexor muscles at the upper part of the leg.

The =anterior tibial artery= descends on the anterior face of the tibia and tarsus and is continued as the perforating metatarsal artery, which passes through the upper part of the space between the second and third metatarsal bones and concurs with the plantar branches of the saphenous in the formation of the plantar arch. Besides muscular and articular branches, the anterior tibial supplies the external or fifth dorsal metatarsal artery for the outer side of the fifth digit. At the proximal part of the metatarsus it gives off three =deep dorsal metatarsal arteries= which descend in the intervals between the metatarsal bones and concur with the superficial dorsal and the plantar arteries in the formation of the =digital arteries=. The latter resemble in general arrangement the corresponding arteries of the thoracic limb.

The =internal iliac artery= (Fig. 491) runs backward and a little outward across the ilio-psoas, and on reaching the ilium divides into parietal and visceral branches. The =parietal branch= is the larger. It runs backward on the lateral wall of the pelvis, passes out through the lesser sciatic notch, and breaks up into branches which supply the muscles in this region like the posterior gluteal (or ischiatic) artery of the horse. It gives off the following branches: (1) The =ilio-lumbar artery= passes outward between the ilio-psoas and the shaft of the ilium and ramifies in the gluteus medius, giving branches to the ilio-psoas and tensor fasciæ latæ; it may arise from the internal iliac. (2) The =anterior gluteal artery= passes out behind the posterior superior spine of the ilium and supplies branches to the glutei. (3) =Muscular branches= go to the obturator internus, coccygeus, and retractor ani. (4) The =lateral coccygeal artery= (A. caudalis lateralis superficialis) passes back at first on the outer face of the coccygeus and continues beneath the skin along the side of the tail. The =visceral branch= or =internal pudic artery= passes back below the parietal branch on the lateral face of the rectum, retractor ani, and coccygeus. Its chief collateral branches are: (1) The =umbilical artery=, which pursues a flexuous course and supplies twigs to the bladder, ureter, and vas deferens. In the bitch it gives off a large =uterine artery= which ramifies chiefly in the body and neck of the uterus and the vagina and anastomoses with the utero-ovarian. (2) The =middle hæmorrhoidal artery= arises near the ischial arch, passes upward and forward on the lateral surface of the rectum, gains its dorsal surface and anastomoses with the posterior mesenteric; it supplies the rectum and the anus, together with its muscles and glands. (3) A small =perineal artery= is detached to the perineum. In the male the trunk turns around the ischial arch as the =artery of the penis= (A. penis). This vessel, after giving off the =deep artery of the penis= (A. profunda penis), which supplies the =artery of the bulb= (A. bulbi urethræ) and enters the corpus cavernosum, is continued as the =dorsal artery of the penis= (A. dorsalis penis) along the dorsum penis. In the female the terminal branches of the trunk go to the vulva, vestibular bulb, and clitoris.

THE VEINS

The arrangement of the veins is, of course, correlated in general with the arterial system, but a few special features are worthy of mention.

The =anterior vena cava= is formed by the junction of short right and =left brachiocephalic= (or subclavian) =veins=, and each of the latter results from the confluence of =jugular= and =brachial veins=.

The =vena azygos= resembles that of the horse, and receives at the ninth or tenth thoracic vertebra a =vena hemiazygos=.

Two =jugular veins= are present on each side. The =external jugular vein= is formed by the union of external and internal maxillary veins at the posterior border of the submaxillary gland. The two external jugulars are commonly united by a transverse branch below the cricoid cartilage. Each passes along the neck on the sterno-cephalicus, covered only by the skin and panniculus, dips under the cleido-cervicalis, and joins the internal jugular.

The =external maxillary vein= arises on the lateral nasal region by the junction of the dorsal nasal vein with the angularis oculi. Near the infraorbital foramen it receives the lateral nasal vein, and a little lower the superior labial. In its course along the anterior border of the masseter it receives the vena reflexa, which arises in the pterygo-palatine fossa by the junction of infraorbital, sphenopalatine, and palatine radicles, together with a branch from the cavernous sinus. At the lower border of the mandible it is joined by the inferior labial vein, which receives the buccinator vein. The lingual vein is connected with its fellow by a superficial transverse branch at the insertion of the sterno-hyoidei. The sublingual and submental veins terminate in a variable manner, but often form a common trunk which joins the lingual.

The =internal maxillary vein= arises from the pterygoid plexus, formed chiefly by dorsal lingual, inferior alveolar, deep temporal, pterygoid, and meningeal tributaries. It receives the superior cerebral, auricular, superficial temporal, transverse facial, and masseteric veins, and often a trunk formed by the union of the inferior cerebral and occipital veins.

The =internal jugular vein= results usually from the junction of laryngeal and thyroid veins, but in some cases it is formed by the confluence of the inferior cerebral and occipital veins.

The =brachial= and =radial veins= are satellites of the arteries.

The =ulnar vein= is usually double. It unites below the carpus with a branch of the interosseous vein to form the superficial venous arch.

The =cephalic vein= accompanies the ulnar artery in the forearm and joins the superficial venous arch below.

The =accessory cephalic vein= arises from the union of three dorsal metatarsal veins. It joins the cephalic about the middle of the forearm.

There are three short =volar metacarpal veins= which open into the superficial venous arch. They are formed above the metacarpo-phalangeal joints by the junction of the =volar digital veins=, of which there are two for each of the chief digits. The volar vein of the first digit joins the superficial venous arch.

Each of the chief digits has two =dorsal digital veins=, while the first digit has one.

The =posterior vena cava=, its collateral affluents, and common iliac radicles present no special features of importance.

The =internal iliac vein= corresponds in regard to its tributaries with the branches of the artery, except that it is not divided into dorsal and ventral branches.

The =external iliac=, =femoral=, and =popliteal veins= with their collateral tributaries are satellites of the arteries.

The =anterior tibial vein= is usually double, and the =posterior tibial vein= is very small.

The =internal saphenous vein= is the upward continuation of the internal plantar metatarsal vein. It communicates by a large branch with the dorsal metatarsal vein, and ascends the leg as a satellite of the saphenous artery and its plantar (posterior) branch. The =recurrent tarsal= or =external saphenous vein= is larger. It is formed at the lower part of the leg by the union of dorsal and external plantar metatarsal veins, crosses the outer surface of the leg obliquely upward and backward, ascends behind the gastrocnemius and joins the posterior femoral vein.

The =metatarsal= and =digital veins= resemble in general the corresponding vessels of the fore limb.

THE LYMPH VESSELS AND GLANDS

The =cisterna chyli= is large and fusiform. The =thoracic duct= may be single throughout, but often divides anteriorly into two branches, which may then unite and form a dilatation which receives the left tracheal duct and the vessels from the fore limb. The primitive plexiform arrangement persists in varying degree. The duct terminates in the left brachiocephalic vein.

The =tracheal ducts= accompany the internal jugular veins. The right duct opens into the right brachiocephalic vein.

The =submaxillary lymph glands= (Fig. 486) are commonly three in number on each side. They are situated in the angle between the masseter and the submaxillary salivary gland, in relation to the external maxillary vein and covered only by the skin and panniculus. The number may be reduced to two or one with a corresponding increase in size.

A small round =parotid= or =auricular lymph gland= is situated superficially between the upper part of the posterior border of the masseter and the parotid gland.

The =pharyngeal= (or retropharyngeal) =lymph glands=, one or two in number on each side, lie on the pharynx chiefly under cover of the submaxillary salivary gland.

The =prescapular= or =superficial cervical lymph glands= lie on the serratus magnus at the anterior border of the supraspinatus, embedded in a mass of fat. Usually two or three are present on each side, but there may be only one. They are oval and are about an inch long in a dog of medium size when two are present.

The =axillary lymph gland= lies in a mass of fat on the inner face of the lower part of the teres major. It is discoid.

The =cubital lymph gland= is inconstant. It is situated on the deep face of the long head of the triceps or on the latissimus dorsi.

The =mediastinal lymph glands= are small and quite variable in arrangement. Usually one or two glands are found on the course of the internal thoracic vessels at the second segment of the sternum.

The =bronchial lymph glands= are commonly four in number. The largest is situated in the angle of divergence of the chief bronchi, two lie on the origin of the right apical bronchus, and the fourth is between the aortic arch and the anterior face of the left bronchus. They are commonly pigmented.

The =lumbar lymph glands= are small.

The =mesenteric lymph glands= are represented chiefly by a long flattened mass (formerly known as the pancreas Aselli) which lies in the mesentery along the course of the anterior mesenteric artery and vein. It is succeeded by three or four glands which lie along the portal vein and appear to receive vessels from the stomach, spleen, liver, pancreas, and great omentum. A few very small nodes occur along the colon.

The =iliac lymph glands= are relatively large and are situated at the termination of the aorta and the origin of the iliac arteries. Commonly four are present. Of these, the two largest lie on the psoas minor on each side in relation to the aorta and vena cava respectively. The other two are in the angle of divergence of the internal iliac arteries.

The =superficial inguinal lymph glands= are situated in the subcutaneous fat behind the external inguinal ring. They are relatively large, especially when there is only one on each side; two smaller ones may be found instead.

The =popliteal lymph gland=—usually unique, but sometimes double—lies in a pad of fat on the upper part of the gastrocnemius between the biceps femoris and semitendinosus. It may project back so as to be superficial and palpable.

NEUROLOGY

THE NERVOUS SYSTEM

The nervous system is a complex mechanism by which the organism is brought into functional relation with its environment, and its various parts are coördinated. For purposes of gross description it is divided primarily into two parts, central and peripheral.

The =central nervous system= (Systema nervorum centrale) comprises (a) the =spinal cord= (Medulla spinalis), and (b) the =brain= (Encephalon).

The =peripheral nervous system= (Systema nervorum periphericum) includes (a) the =cerebral= and =spinal nerves= with their =ganglia=, and (b) the =sympathetic nervous system=.

The division into central and peripheral parts is quite arbitrary and is employed purely as a matter of convenience of description. The fibers of which the nerves are composed either arise or end within the central system, and therefore constitute an integral part of the latter. The structural and functional unit of the nervous system is the =neurone=, which consists of the =cell-body=, usually termed the =nerve-cell=, and all its =processes=. The processes arise as outgrowths from the cell-body and conduct impulses to or from the cell. They vary greatly in length, some being less than a millimeter long, while others extend from a cell in the spinal cord to the distal end of a limb. A nerve is composed of such processes, usually inclosed in a protecting and insulating sheath, and united into bundles by connective tissue. The neurones are embedded in a peculiar supporting tissue termed =neuroglia=. This consists of a very intricate feltwork of glia-fibers, many of which are connected with the small glia-cells or astrocytes. In addition the nervous tissue is invested closely by a vascular layer of connective tissue, the pia mater, from which ingrowths extend into the nervous substance proper.[186]

To the naked eye the central nervous system appears to be composed chiefly of two kinds of substance, white and gray. The =white matter= (Substantia alba) is dead white in color and is soft in the natural state. It consists largely of medullated nerve-fibers, packed closely together and arranged more or less clearly in large or small bundles or tracts (Funiculi, fasciculi). The =gray matter= (Substantia grisea) is usually brownish-gray in color, often with a tinge of pink. It is softer than the white substance and much more vascular. It is composed chiefly of cell-bodies and non-medullated processes. In some situations it is modified to form the =gelatinous substance= (Substantia gelatinosa), which is pale yellowish-gray and jelly-like.

=Ganglia= are gray masses found on the dorsal roots of the spinal nerves and on the course of many nerves. They are commonly ovoid in form and are inclosed in a fibrous capsule. They are composed largely of the cell-bodies of neurones, but have connected with and passing through them nerve-fibers (processes) which extend peripherally and centrally. In origin and function the spinal ganglia belong properly to the central system, but it is customary to include them with the peripheral part in gross anatomical descriptions.

=Nerves= are conducting trunks composed of bundles of parallel nerve-fibers. They are enveloped in a fibro-elastic sheath, the epineurium, which contains the blood and lymph vessels. They are classified according to their central connections as =cranial= or =cerebral=, =spinal=, and =sympathetic=. The groups of cells of which the nerve-fibers are processes are termed the =nuclei of origin or termination= of the nerves, according as the latter conduct impulses in a peripheral or central direction. On the same basis the nerve-fibers are designated =efferent= and =afferent= respectively. A bundle of fibers which passes from one nerve-trunk to another is called an =anastomotic branch= (Ramus anastomoticus). In some situations the exchange of branches between adjacent nerves is so free as to constitute a =nerve-plexus= (Plexus nervorum). The term =ramus communicans= is properly restricted to branches which connect the ventral divisions of the spinal nerves with adjacent ganglia or nerves of the sympathetic system. The terminal twigs of the nerves are designated according to their distribution as =muscular branches= (Rami musculares), =cutaneous nerves= (Nervi cutanei), and =articular nerves= (Nervi articulares). The muscular branches are motor in function, the cutaneous and articular sensory, but all contain vasomotor fibers which control the caliber of the blood-vessels.

THE MENINGES

The central organs of the nervous system are inclosed in three meninges or membranes. From without inward these are: (1) the dura mater, (2) the arachnoidea, and (3) the pia mater.

THE DURA MATER

The =dura mater= is a dense resistant membrane of white fibrous tissue; in the fresh state it is bluish-white in color. On account of the difference in its arrangement within the cranium from that in the spinal canal it is customary to describe it as consisting of two parts, cerebral and spinal; these portions are continuous with each other at the foramen magnum.

The =cerebral dura mater= (Dura mater encephali) is adherent to the interior of the cranium, and may be regarded as forming an internal periosteum for the bones here as well as being an envelope of the brain. Its outer surface is connected with the bony wall of the cranial cavity by numerous fine fibrous strands and by blood-vessels; hence it appears rough in many places when separated from the wall. The degree of adhesion varies greatly at different points. It is most firmly attached at the various projections, e. g., the internal sagittal crest, the tentorium osseum, the petrosal crest; also at the base and the foramen magnum. Before the sutures are closed the dura is connected with the sutural ligaments and through them is continuous with the external periosteum. The cranial nerves receive sheaths from the dura, which is thus continuous without the cranium with the epineurium and periosteum. Along the roof and sides (except as noted above) the adhesion is relatively slight, and lymph-spaces are said to exist between the dura and the bone. The internal surface of the dura is smooth and glistening, since it is lined by endothelium and is moistened by a fluid resembling lymph; it forms the outer boundary of the subdural space. In accordance with its double function the dura is composed of two layers, which are, however, intimately adherent to each other in most places in the adult. The venous sinuses are channels between the two layers and are lined by endothelium. They have been described with the other vessels.

Several folds or septa are given off from the inner surface of the dura; these project into the cavity between the gross subdivisions of the brain. These septa are: (1) the falx cerebri; (2) the tentorium cerebelli; and (3) the diaphragma sellæ.

The =falx cerebri= is a sickle-shaped median partition which projects downward in the great longitudinal fissure between the cerebral hemispheres. It is attached above to the internal sagittal crest, in front to the crista galli, and behind to the tentorium osseum. Its superior border is convex and separates into two layers which inclose the superior longitudinal sinus. Its lower border is concave and lies over the corpus callosum. The falx is thick above, but much thinner below, and in some places cribriform.

In the horse the posterior part of the falx cerebri does not extend to the corpus callosum, and hence the hemispheres are here in contact and adherent to each other over a small area. In the ox and pig the falx does not descend so far as in the horse, while in the dog it completely separates the hemispheres.

The =tentorium cerebelli= is a crescentic transverse fold which separates the cerebellum from the posterior poles of the cerebral hemispheres. It is attached above to the tentorium osseum and laterally to the petrosal crests. Its lower border is thin, concave, and free; it forms the upper and lateral boundaries of an opening (Incisura tentorii) which incloses the mid-brain.

The =diaphragma sellæ= is a small circular fold which roofs over the sella turcica and the pituitary body. It is perforated centrally by an opening (Foramen diaphragmatis) for the infundibulum.

The falx cerebelli, a sickle-shaped fold which projects into the median notch between the cerebellar hemispheres in man, is not present in the domesticated animals. There is instead merely a slight thickening of the dura.

The =spinal dura mater= (Dura mater spinalis) forms a tube around the spinal cord from the foramen magnum to the second or third segment of the sacrum. It is separated from the periosteum of the spinal canal by a considerable =epidural space= (Cavum epidurale) which is occupied by fatty connective tissue and veins. It is held in position chiefly by the sheaths which it furnishes to the roots of the spinal nerves, and in its anterior part by two ligaments; the latter connect it with the inferior atlanto-occipital membrane and with the odontoid process. It is large in proportion to its contents, but its diameter is not uniform. It is largest in the atlas, small in the thoracic region, and becomes very small in its terminal part, where it incloses the delicate filum terminale of the spinal cord.

The =subdural space= (Cavum subdurale) is the cavity between the inner surface of the dura mater and the arachnoidea. It is a mere capillary space which contains just sufficient fluid to moisten its surfaces; this fluid is usually regarded as lymph, which is replenished by filtration through the walls of the blood-vessels. The space is in communication with the lymph-paths of the nerve-sheaths.

THE ARACHNOIDEA

The =arachnoidea= is a very delicate and transparent membrane situated between the dura and pia mater. Its outer surface forms the inner wall of the subdural space and is covered by a layer of endothelium continuous with that of the opposed surface of the dura mater. Between it and the pia mater is the =subarachnoid space= (Cavum subarachnoideale) which contains the cerebrospinal fluid. An inner surface can scarcely be said to exist, since deeply the membrane becomes a reticulum of fine fibers which traverse the subarachnoid space and are attached to the pia mater. The arachnoidea furnishes sheaths to the cerebral nerves from their superficial origins to a variable but usually short distance beyond the emergence from the dursal sac. In the case of the optic nerve this sheath extends to the eyeball.

The =arachnoidea of the brain= (Arachnoidea encephali), except in the case of the great longitudinal and transverse fissures, does not dip into the sulci on the surface of the brain. On the summits of the gyri it is so closely attached to the pia mater that the two form practically a single membrane. Its outer part bridges over the sulci, and here the subarachnoid space is partially divided up by the loose arachnoid tissue into intercommunicating cavities. In certain situations the arachnoidea is separated from the pia by spaces of considerable depth and extent. These enlargements of the subarachnoid space are termed =subarachnoid cisterns= (Cisternæ subarachnoidales). In them the subarachnoid tissue does not form a close network, but consists of a relatively small number of long, thread-like strands which traverse the cavity.

The chief cisternæ are: (1) the cisterna magna, which is at the angle formed between the posterior face of the cerebellum and the upper surface of the medulla oblongata. It communicates with the fourth ventricle through the foramen of Magendie (in man); and behind with the wide subarachnoid space of the spinal cord; (2) the cisterna pontis on the ventral surface of the pons; (3) the cisterna basalis, which lies at the base of the cerebrum and is divided by the optic chiasma into two parts (cisterna chiasmatis, cisterna interpeduncularis); (4) the cisterna fossæ lateralis, situated at the lower part of the lateral fissure (of Sylvius), and continuous with the cisterna basalis.

Along the upper border of the falx cerebri the arachnoidea bears bulbous excrescences, the =arachnoid granulations= or =Pacchionian bodies=. These are inclosed in thin evaginations of the dura mater and project into the sagittal sinus or the parasinoidal sinuses along either side of it. In some cases they are sufficiently large to exert pressure on the bone and produce in it depressions of variable depth.

The =spinal arachnoidea= (Arachnoidea spinalis) is directly continuous with that of the brain at the foramen magnum. It forms a relatively wide tube around the spinal cord, so that the latter (inclosed in the pia) is surrounded by a very considerable quantity of cerebrospinal fluid. The spinal subarachnoid space is traversed by fewer arachnoid trabeculæ than is the case in the cranium. It is partially subdivided by three imperfect septa. One of these, the septum dorsale, is median and dorsal. The other two, the ligamenta denticulata, are lateral and will be described with the pia mater.

PIA MATER

The =pia mater= is a delicate and very vascular membrane which invests closely the surface of the brain and spinal cord and sends processes into their substance. It also furnishes sheaths to the nerves, which blend outside of the dural sac with the epineurium.

The =pia mater of the brain= (Pia mater encephali) follows accurately all the inequalities of the surface, dipping into all the fissures and sulci of the cerebrum, and into the larger fissures of the cerebellum. Its external surface, which forms the inner boundary of the subarachnoid space, is covered by a layer of endothelium. From its deep face numerous trabeculæ are given off which penetrate into the substance of the brain and cord, forming a path for the blood-vessels, and concurring with the neuroglia in forming the supporting tissue of the nervous substance proper. The larger blood-vessels of the brain lie within the subarachnoid space, but the smaller vessels ramify in the pia, forming rich plexuses. The twigs which penetrate into the gray matter are inclosed in pial sheaths. They are end-arteries, i. e., constitute the entire supply of the district which they enter and do not anastomose with adjacent vessels.

Two important folds of the pia extend into the two great transverse fissures of the brain; these processes overlie the third and fourth ventricles and are known as the =telæ chorioideæ= of these cavities. They constitute paths for the deeper vessels and their edges contain vascular convolutions which are known as chorioid plexuses. They will receive further consideration more appropriately later.

The =spinal pia mater= (Pia mater spinalis) is thicker and denser than that of the brain. It has a strong outer layer of fibrous tissue, most of the fibers of which are longitudinal. The inner layer is vascular and adheres closely to the surface of the cord because it sends numerous processes into it. It sends a fold into the median ventral fissure and also helps to form the median dorsal septum of the cord. Along the median ventral line it forms a band-like thickening, the linea splendens, along which the ventral spinal artery runs. On each side the pia mater gives off a strong longitudinal band, the =ligamentum denticulatum=, which is connected externally with the dura mater. The inner or pial border extends in a line between the dorsal and ventral roots of the nerves. The outer or dural border is denticulated and to a large extent free. The denticulations are attached to the dura between the nerve-roots.

NERVOUS SYSTEM OF THE HORSE

THE SPINAL CORD

The =spinal cord= (Medulla spinalis) is the part of the central nervous system which is situated in the vertebral canal. It extends from the foramen magnum to about the middle of the sacrum.

In the horse its average length is about 76 to 78 inches (ca. 190 to 195 cm.), and its weight about 8½ to 9 ounces (ca. 240 to 255 grams).

It is approximately cylindrical, but more or less flattened dorso-ventrally. There is no line of demarcation between it and the medulla oblongata, but for practical purposes the division is usually assumed to be at the posterior border of the foramen magnum. Its posterior part tapers rapidly to a point, forming the =conus medullaris=. This is prolonged for a short distance by the slender =filum terminale=.

Forty-two pairs of spinal nerves are connected with the sides of the spinal cord. They are classified as eight cervical, eighteen thoracic, six lumbar, five sacral, and five coccygeal. According to the attachments of these series of nerves the spinal cord is divided into =cervical=, =thoracic=, =lumbar=, and =sacral= parts.

In a horse about 16½ hands high these parts measured 65 cm. (ca. 26 in.), 86 cm. (ca. 34.4 in.), 27 cm. (ca. 10.8 in.), and 15 cm. (ca. 6 in.) respectively (Dexler).

In the embryo these divisions correspond primitively to the regions of the vertebral column, but later, through unequal growth of the cord and spine, the correspondence between the two is not at all exact in the anterior regions and is lost in the last two. The lumbar part of the cord in the horse ends at the junction of the fifth and sixth lumbar vertebræ, so that the roots of the last lumbar nerve must run backward the length of the last lumbar vertebra to reach the intervertebral foramen through which it emerges. The conus medullaris reaches only to the anterior part of the sacral canal, so that the roots of the sacral and coccygeal nerves extend backward in the spinal canal for a considerable distance, forming a leash of bundles, in the center of which lie the conus medullaris and the filum terminale. This arrangement is expressively designated the =cauda equina=.

Each pair of spinal nerves is attached by its root-fibers to a certain length of the cord, and the latter is, therefore, regarded as consisting of as many =segments= as there are nerves. It is to be noted, however, that there is no line of demarcation between the segments other than the intervals between the root-fibers of adjacent nerves.

The segments are of different lengths; the longest are the third to the sixth cervical, which measure 11, 10, 10, and 8.5 cm. respectively. The spinal nerves are in general designated according to the vertebræ behind which they emerge from the vertebral canal. In the neck, however, there are eight pairs of nerves and only seven vertebræ; here the first nerve emerges through the intervertebral foramen of the atlas and the eighth between the last cervical and the first thoracic vertebra.

In the greater part of the thoracic region the spinal cord is fairly uniform in size, but there are two conspicuous wide enlargements which involve the segments with which the nerves of the limbs are connected. The =cervical enlargement= (Intumescentia cervicalis) begins gradually in the fifth cervical vertebra and subsides in the second thoracic. Its maximum transverse diameter is about an inch (ca. 25 mm.) and its dorso-ventral nearly half an inch (ca. 12 mm.). The =lumbar enlargement= (Intumescentia lumbalis) is situated in the fourth and fifth lumbar vertebræ. It is a little narrower than the cervical enlargement, and its dorso-ventral diameter is also slightly smaller. Behind this the cord tapers rapidly to form the =conus medullaris=. The tip of the latter is continued by a delicate glistening strand, the =filum terminale=, which is composed largely of fibrous tissue continued from the pia mater, covered by arachnoid.

The surface of the spinal cord is divided into two similar halves by a =dorsal median groove= (Sulcus medianus dorsalis) and a =ventral median fissure= (Fissura mediana ventralis). On either side of the former is the =dorso-lateral groove= (Sulcus dorsalis lateralis) at which the fibers of the dorsal nerve-roots enter the cord; it is faint except at the enlargements, and is represented by two grooves in the first cervical segment. The ventral root-fibers as they emerge from the cord do not form a continuous series, but arise from a zone (Area radicularis ventralis) 3 to 5 mm. in width a little lateral to the ventral median fissure, and no groove is found here. In the greater part of the cervical region and the anterior part of the thoracic region there is a shallow =dorsal paramedian groove= (Sulcus intermedius dorsalis) a short distance lateral to the median groove. These grooves indicate the division of the white matter of the cord into columns to be described later.

Examination of cross-sections of the spinal cord shows that it is a bilaterally symmetrical structure, incompletely divided into right and left halves by a ventral fissure and a dorsal septum. The =ventral median fissure= (Fissura mediana ventralis) is narrow and penetrates nearly to the middle of the dorso-ventral diameter of the cord. It is occupied by a fold of pia mater. The =dorsal median septum= (Septum medianum dorsale) is a partition which descends from the posterior median groove to about the middle of the cord. It apparently consists of condensed neuroglia with an admixture of pial tissue. The two halves of the cord are connected by commissures of gray and white matter. The =gray commissure= (Commissura grisea) is a transverse band of gray matter at the ventral end of the dorsal septum. It is divided into dorsal and ventral parts by the central canal of the cord. The =white commissure= (Commissura alba) is a bridge of white matter which connects the ventral columns of the cord over the dorsal end of the ventral median fissure, and constitutes a conducting path from one side to the other.

The =central canal= of the cord (Canalis centralis), the spinal vestige of the lumen of the embryonal neural tube, is a minute passage which tunnels the gray commissure. It opens at its anterior end into the posterior part of the fourth ventricle of the brain, and its terminal part in the conus medullaris forms a slight dilatation, the ventriculus terminalis. It is lined by epithelium and is surrounded by a layer of modified neuroglia (Substantia grisea centralis).

The =gray matter= of the spinal cord as seen in cross-sections resembles roughly a capital =H= the cross-bar of the letter being formed by the gray commissure. Each lateral part is considered as consisting of =dorsal= and =ventral gray columns= or =horns= (Columna grisea dorsalis, ventralis). In the greater part of the cord the dorsal column or horn is elongated and narrow and tapers to a point which extends almost to the surface of the cord at the attachment of the dorsal root-fibers of the spinal nerves. Its apex or tip consists of gray matter which is lighter in color and less opaque than that of the rest of the horn and is termed the =substantia gelatinosa= (Rolandi). The ventral column or horn is short, thick and rounded, and is separated from the surface of the cord by a thick layer of white matter, through which the fibers of the ventral roots of the spinal nerves pass. From the middle of the cervical region to the lumbar region there is a medial projection of gray matter on the lower part of the dorsal column; this is the =nucleus dorsalis= or Clarke’s column. In the anterior part of the cord there is an outward projection of the gray matter at the base of the ventral horn; this is termed the =lateral column= or =horn=. The demarcation between the gray and white matter is in many places indistinct; this is especially the case laterally, where processes of gray matter extend into the white substance, producing what is known as the =formatio reticularis=.

Cross-sections of the spinal cord present the following gross regional characters: (1) The cervical cord near the medulla is compressed dorso-ventrally, its width is about 18 mm. and its greatest thickness about 8 mm. It has dorsally a deep median sulcus and a distinct dorso-lateral sulcus. Lateral grooves are also present. The dorsal cornua are strongly everted. Each has an expanded head, which comes very close to the surface of the cord, and has an extensive cap of substantia gelatinosa. The neck is distinct. The ventral cornua are short and blunt and diverge very little. The gray commissure is about in the middle of the section and 2.5 mm. in length. According to Dexler the column between the median and lateral grooves dorsally is the funiculus cuneatus, the funiculus gracilis being very small and not showing on the surface in this region. In the middle of the cervical region the diameters are about 16 mm. and 10 mm. respectively. The ventral surface is somewhat flattened. The dorsal cornua have pointed ends and turn decidedly outward. The ventral cornua are short and thick and are directed very slightly outward; their ends are about 4 mm. from the ventral surface. The gray commissure is just above the middle of the section and is about 2 mm. long. The cervical enlargement measures about 25 mm. transversely and 12 mm. vertically. The dorsal cornua are smaller than the ventral and have a large cap of substantia gelatinosa. The ventral cornua are short and thick, curve strongly outward, and are about 4 mm. from the ventral surface. Each bears a prominence on its inner side near the base. The gray commissure is considerably above the middle of the section and is about 4 mm. long. (2) In the middle of the thoracic region the cross-section is biconvex, the ventral surface being the more strongly curved. The transverse diameter is about 15 mm. and the dorso-ventral about 10 mm. The gray columns are close together, the gray commissure being only about 1 mm. in length, and lying considerably above the middle of the section. The dorsal cornua are short and have slightly enlarged ends. The ventral cornua have a uniform diameter, turn very little outward, and end about 3 mm. from the ventral surface. (3) The lumbar enlargement is much flattened, especially dorsally. The transverse diameter is about 22 mm. and the dorso-ventral 9 to 10 mm. The cornua are very large. The ventral cornua are thick and rounded and turn sharply outward; they end about 2 mm. from the ventral surface. The dorsal cornua are smaller and shorter and do not diverge so strongly. The gray commissure is about in the middle of the section and is about 3 mm. long. In the third lumbar vertebra the cord is about 3 mm. narrower and thicker, and both surfaces are about equally convex. The dorsal cornua are smaller, considerably everted, and constricted in the middle. The ventral cornua are very short and do not turn outward. (4) In the first sacral vertebra the cord is almost round and is 5 to 6 mm. in diameter; the cornua are relatively very large and the commissure has the form of a high intermediate mass.

The ventral horn contains large cells, the axones of which emerge as the fibers of the ventral nerve-roots (Fila radicularia). The axones of many cells cross to the opposite side in the white commissure and pass out in a ventral root of that side, or enter the white matter and pass forward and backward, associating various segments of the cord. Some pass to the ventral horn of the opposite side at the same or at different levels. Others pass to the periphery of the cord, join the cerebellospinal fasciculus, and extend to the cerebellum. Scattered through the gray matter are many smaller cells with axones which pursue a short course and serve to connect different parts of the gray matter.

The =white matter= of the spinal cord is divided into three pairs of =columns=. The =dorsal columns= (Funiculi dorsales) lie on either side of the dorsal median septum and extend outward to the dorso-lateral groove and the dorsal gray column. The =ventral columns= (Funiculi ventrales) are situated on either side between the median fissure and the ventral gray columns. They are connected above the fissure by the white commissure. The =lateral columns= (Funiculi laterales) lie external to the gray columns on either side; their limits are indicated superficially by the dorso-lateral groove and the emergence of the ventral root-fibers. The paramedian groove (where present) indicates a subdivision of the dorsal column into two fasciculi or tracts; the inner of these is the =fasciculus gracilis= (Goll’s column); the outer, the =fasciculus cuneatus= (Burdach’s column).

The amounts of gray and white matter vary greatly in different parts of the cord both absolutely and relatively. In cross-section the absolute areas of both are greatest in the enlargements. The relative area of gray matter is smallest in the thoracic region (except at its anterior end), and increases from the lumbar enlargement backward.

Investigations have shown that in man the columns of white matter are subdivided into fasciculi or tracts, which constitute definite conducting paths of greater or less length. Our knowledge of the tracts in the domesticated animals is very limited, and it is quite unsafe to make inferences from the arrangement in man. As evidence of this it may be noted that the ventral cerebrospinal or direct pyramidal tract of man cannot be recognized as such.

The dorsal white columns consist essentially of two sets of axones. The afferent or sensory axones which come from the cells of the spinal ganglia enter as the dorsal roots of the spinal nerves and divide into two branches in the vicinity of the dorsal gray column. The anterior (ascending) branches form the direct sensory path to the brain and extend in the fasciculus cuneatus and fasciculus gracilis or corresponding tracts to nuclei in the medulla oblongata. The posterior (descending) branches extend backward for varying distances and give off numerous collaterals to cells of the gray column, thus forming part of the mechanism for the mediation of reflex action. Some collaterals cross in the white commissure to the opposite side. Many of these fibers are collected in the comma-shaped tract between the fasciculus gracilis and cuneatus. The second set of axones arises from the smaller cells of the gray column. They enter the white matter, divide into anterior and posterior branches, forming the fasciculi proprii or ground bundles of the cord. Some branches cross to the opposite side. The function of this set of axones is chiefly to associate various levels of the cord.

The lateral columns contain some axones of the dorsal nerve-roots, which (in man) are grouped in the marginal tract of Lissauer, situated just dorsal to the apex of the dorsal horn. The cerebellospinal fasciculus (direct cerebellar tract of Flechsig) extends along the periphery of the lateral column. It contains the axones of the cells of the nucleus dorsalis (Clarke’s column), which proceed to the medulla oblongata and enter the cerebellum by the restiform body. The rubrospinal tract of Monakow appears to take the place of the lateral cerebrospinal fasciculus or crossed pyramidal tract of man. It lies at the inner side of the cerebellospinal fasciculus. Its fibers arise in the nucleus ruber of the mid-brain, cross the median plane (decussation of Forel), and pass backward in the tegmentum and medulla oblongata to the lateral column of the cord. It is a path for motor impulses coming from the cerebral cortex and the cerebellum. The lateral fasciculus proprius or ground-bundle is deeply situated at the side of the gray columns. The bulk of its fibers are axones of cells of the posterior cornu which divide into anterior and posterior branches. They are intersegmental paths which associate different levels of the gray matter of the cord. The significance of the remaining fibers is not yet known.

The ventral white columns do not contain a ventral cerebrospinal or direct pyramidal tract, as in man. There is a small tract (Fasciculus intracommissuralis ventralis) above the white commissure, which separates it from the rest of the ventral column. It extends to the middle of the thoracic region. It consists of intersegmental fibers, and contains in the anterior part of the cervical cord in the sheep and goat both crossed and direct pyramidal fibers. The descending cerebellospinal fasciculus extends from the cerebellum to the lumbar region. In the cervical region it occupies a semilunar area which reaches almost to the surface ventro-laterally. Scattered fibers belonging to it lie also in the medial part of the ventral column. Posteriorly it diminishes in size and comes to occupy a position next to the ventral median fissure, corresponding to the sulco-marginal fasciculus of man (Dexler).

THE BRAIN

The =brain= or =encephalon= is the part of the central nervous system that is situated in the cranial cavity. It is the enlarged and highly modified cephalic part of the primitive neural tube. It conforms rather closely in size and shape to the cavity in which it lies.

Its average weight without the dura mater is about 23 ounces (ca. 650 gm.), and forms about ⅐ of 1 per cent. of the body-weight.

It is desirable to examine the general external configuration of the brain before studying its various parts in detail.[187]

When divested of its membranes and vessels (Figs. 499, 500), its ventral surface or base presents the median =brain stem=, which is continuous with the spinal cord without any natural line of demarcation; it divides in front into two branches, the cerebral peduncles, each of which disappears into the mass of the corresponding cerebral hemisphere. The brain stem consists of three parts. The =medulla oblongata= is the posterior part which extends forward as the direct continuation of the spinal cord. The =pons= (Varolii) is a transversely elongated mass which appears to turn up on either side into the cerebellum. The =cerebral peduncles= extend forward from the pons and diverge to plunge into the ventral part of the cerebral hemispheres. The area between them is the =interpeduncular= space. It is largely covered by the =pituitary body= or =hypophysis=, a yellowish-brown, discoid structure, which is connected with the base of the cerebrum by a delicate tube called the =infundibulum=.[188] On drawing the pituitary body gently aside, the infundibulum is seen to be attached to a slight gray eminence, the =tuber cinereum=. Behind this is the =mammillary body=, a well-marked round prominence. The posterior part of the space is perforated by numerous openings for the passage of small arteries, and hence is termed the =locus perforatus posticus=. Two large bands of white matter, the =optic tracts=, cross the anterior ends of the cerebral peduncles and unite at the =optic chiasm= or =commissure=, forming the anterior boundary of the interpeduncular space. Above and in front of the chiasm the hemispheres are separated by the median =longitudinal fissure=. In contact with the anterior extremity of each hemisphere is the =olfactory bulb=, an oval enlargement adapted to the ethmoidal fossa of the cranium. This appears as a gray swelling on a wide flat band, the =olfactory peduncle=, which is continued behind by two divergent bands, the =olfactory tracts= or =striæ=. The =internal tract= disappears after a very short course on to the inner surface of the hemisphere. The =external tract= is larger and longer; it runs backward, inclines at first outward and then curves inward and disappears on the concealed or tentorial surface of the hemispheres. It is separated from the lateral cerebral gyri by a distinct groove (Sulcus rhinalis). Along the inner side of the stria are two eminences. The anterior of these is the =trigonum olfactorium=, a gray elevation situated in the angle of divergence of the inner and outer striæ. Behind this is a depression, the =fossa transversa=,[189] which is continued across the external stria and sharply limits the second and much larger eminence, the =pyriform lobe=.

The superficial origins of most of the cranial nerves are visible on the base of the brain.

The =olfactory nerve-fibers= join the convex surface of the olfactory lobe and give it a shaggy appearance in specimens which have been removed intact—a difficult proceeding.

The =second= or =optic nerves= converge to the optic chiasm.

The =third= or =oculomotor nerve= arises from the inner part of the cerebral peduncle.

The =fourth= or =trochlear nerve= may be seen emerging between the pons and the hemisphere, but its connection with the brain is not visible.

The =fifth= or =trigeminal nerve= is connected with the lateral part of the pons.

The =sixth= or =abducent nerve= arises just behind the pons and lateral to the pyramid of the medulla.

The =seventh= or =facial= and the =eighth= or =auditory nerves= arise close together just behind the pons proper on the extremity of the corpus trapezoideum.

The =ninth= or =glosso-pharyngeal=, the =tenth= or =vagus=, and the =eleventh= or =spinal accessory nerves= are connected by a linear series of roots with the lateral aspect of the ventral surface of the medulla. The spinal part of the accessory nerve comes forward along the edge of the medulla to join its medullary root.

The =twelfth= or =hypoglossal nerve= arises from the posterior part of the medulla along the lateral edge of the pyramid.

The parts that are visible when the brain is viewed from above are the cerebral hemispheres, the cerebellum, and part of the medulla oblongata. The =cerebral hemispheres= form an ovoid mass, and are separated from each other by the median =longitudinal fissure=, in which the falx cerebri is situated. Their surfaces are marked by thick folds, the =gyri cerebri=, separated by =sulci=. The upturned ends of the =olfactory bulbs= are seen in front of the frontal poles of the hemispheres. The occipital poles of the hemispheres overlie the anterior part of the cerebellum, from which they are separated by the =transverse fissure= and the tentorium cerebelli contained in it. The =cerebellum= is a much smaller rounded mass which conceals the greater part of the medulla oblongata. Its surface is divided into a middle lobe, the =vermis=, and two lateral =hemispheres=. It is marked by numerous gyri and narrow sulci which have in general a transverse direction. The posterior third of the =medulla oblongata= is not covered by the cerebellum.

The brain is developed from the expanded cephalic part of the neural tube of the embryo. The process comprises a series of thickenings, flexures, and unequal growth and expansion of various parts of the tube. In the higher animals the result is that the tubular character of the brain is not very evident, since the lumen comes to consist of four irregular cavities, the ventricles, which are connected by narrow passages. The tube is first subdivided by two constrictions into three brain vesicles, termed respectively the hind-brain or rhombencephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon. The hind-brain gives rise to three secondary segments and the fore-brain to two. The annexed table indicates the origin of the principal structures of the fully developed brain from the primitive vesicles. It has become customary to describe the brain with reference to its embryological relations.

TABLE INDICATING THE DERIVATION OF THE PRINCIPAL PARTS OF THE BRAIN PRIMARY SEGMENTS.│ Secondary │ DERIVATIVES. │ CAVITIES. │ SEGMENTS. │ │ Rhombencephalon │ │ │ (posterior │Myelencephalon │Medulla oblongata│Fourth ventricle vesicle) │ │ │ „ │Metencephalon │Pons │ „ „ │ „ │Cerebellum │ „ │Isthmus │Anterior │ „ │ rhombencephali │ cerebellar │ „ │ │ peduncles │ „ │ „ │Anterior │ „ │ │ medullary velum│ Mesencephalon │ │Corpora │ (middle │Mesencephalon │ quadrigemina │Cerebral aqueduct vesicle) │ │ │ „ │ „ │Cerebral │ „ │ │ peduncles │ Prosencephalon │ │{Optic thalami │Posterior part of (anterior │Diencephalon │ } │ third ventricle vesicle) │ │ │ „ │ „ │{Subthalamic │ „ │ │ tegmenta │ „ │ „ │{Pineal body │ „ │ │ │Anterior part of │ │ │ third │ │{Pituitary body │ ventricle. „ │Telencephalon │ } │ Lateral │ │ │ ventricles and │ │ │ olfactory │ │ │ continuations. „ │ „ │{Optic nerves and│ „ │ │ retinæ │ „ │ „ │{Cerebral │ „ │ │ hemispheres │ „ │ „ │{Olfactory tracts│ „ │ │ and bulb │

THE RHOMBENCEPHALON

THE MEDULLA OBLONGATA

The =medulla oblongata= lies on the basilar part of the occipital bone. It is quadrilateral in outline, but much wider in front than behind, and compressed dorso-ventrally. Its length, measured from the root of the first cervical nerve to the pons, is about two inches (ca. 5 cm.).

Its =ventral surface= is convex in the transverse direction. It is marked by a =median fissure= (Fissura mediana ventralis) which is continuous behind with the similar fissure of the spinal cord. The posterior part of the fissure is faintly marked, but in front it becomes deeper and ends in a small depression (Foramen cæcum) behind the central part of a transverse band, the =corpus trapezoideum=. On either side of the fissure is a rounded tract, the =pyramid= (Pyramis), which is bounded externally by a faint lateral groove (Sulcus intermedius ventralis). The pyramids join the pons in front; behind they become narrower and disappear into the substance of the medulla, in which their fibers intercross, forming the =decussation of the pyramids= (Decussatio pyramidum).[190]

The superficial origin of the =sixth cranial nerve= (N. oculomotorius) is just lateral to the anterior end of the pyramid. The =corpus trapezoideum= is a transverse band which extends across the surface immediately behind the pons. It is crossed by the pyramids, which cut off a small central part. The lateral part extends out to the roots of the seventh and eighth nerves on either side. Behind the outer part of the corpus trapezoideum there is a variably developed rounded eminence, the =tuberculum faciale=. The root-fibers of the twelfth or hypoglossal nerve form an oblique linear series lateral to the posterior part of the pyramid.

The =dorsal surface= is largely concealed by the cerebellum and forms the greater part of the floor of the fourth ventricle. The =dorsal median fissure= (Fissura mediana dorsalis), the direct continuation of the corresponding groove of the spinal cord, extends forward to about the middle of the surface. Here the =restiform bodies=, which constitute the lips of the fissure, diverge to form the lateral boundaries of a triangular depression; this is the posterior part of the rhomboid fossa or floor of the fourth ventricle of the brain. The dorso-lateral fissure winds outward and forward to the lateral aspect of the medulla, where it presents the roots of the =ninth=, =tenth=, and =eleventh cranial nerves=. External to it is a distinct oval eminence on the anterior part of the lateral column, termed the =tuberculum cinereum=. The central canal of the cord is continued in the posterior part of the medulla, inclines dorsally, and opens in the posterior angle of the fourth ventricle. Hence it is customary to distinguish a closed and an open part of the medulla. The dorsal aspect of the latter, which is concealed at present, will be considered later in the description of the fourth ventricle.

The lateral surface is narrow behind, wider and rounded in front. From it the root-fibers of the ninth, tenth, and eleventh cranial nerves arise in a linear series, and alongside of it the spinal part of the eleventh nerve passes forward to join the medullary root. Close inspection reveals the presence of striæ which curve obliquely downward and backward from the surface of the restiform body toward the hypoglossal root-fibers; these are the =external arcuate fibers= (Fibræ arcuatæ externæ). The recess between the lateral aspect of the medulla and the cerebellum is occupied by an irregular mass of villous projections of the pia mater, containing tufts of vessels; this is the =chorioid plexus of the fourth ventricle=, and is the lateral edge of the =tela chorioidea= of the ventricle. On raising the chorioid plexus it is seen that the tela chorioidea is attached to the dorsal aspect of the medulla, and reinforces here the wall of the fourth ventricle; also that the restiform body terminates in front by entering the base of the cerebellum, forming its posterior peduncle.

In the medulla the fiber tracts of the spinal cord either terminate in the nuclei of the gray matter or undergo changes in their relative position, and new tracts appear. The gray matter is highly modified and forms masses which have no homologues in the cord. The central canal of the closed part of the medulla is surrounded by a thicker layer of gray matter than is the case in the spinal cord. As the medulla opens out this gray matter is naturally spread in the floor of the fourth ventricle. The dorsal horns of gray matter become wide, spread apart, and are broken up to a great extent in the formatio reticularis. Two elongated masses of cells appear above the central gray matter; these are the =nucleus gracilis= and =nucleus cuneatus=, and in them the fasciculi of like name gradually end. In front of these are the =terminal nuclei= of the afferent or sensory cranial nerves and of the sensory portions of the mixed nerves. The ventral horns are succeeded by the nuclei of =origin= of the efferent or motor cranial nerves and the motor root-fibers of the mixed nerves. Of the twelve pairs of cranial nerves, the last eight are connected with nuclei in the medulla and pons. The =posterior olivary nucleus= (Nucleus olivaris caudalis) is a conspicuous gray mass which lies above the pyramid on each side. On cross-sections it appears as a wavy layer of gray matter which is folded on itself so as to inclose a mass of white matter. It is about 2 cm. long in the horse, but is smaller in circumference than in man and hence does not cause any very distinct external enlargement (olivary eminence), as in the latter. It is connected with the opposite side of the cerebellum by the cerebello-olivary fibers. At its inner side are two small accessory olivary nuclei. The =pyramidal tracts=, which are small in ungulates, send most of their fibers across to the opposite side in the posterior part of the medulla, forming the =pyramidal decussation=. Some fibers, however, continue in the ventral column of the same side of the cord, and others are connected with the nuclei of the motor nerve-roots. From the nucleus gracilis and nucleus cuneatus fibers arise which are traceable forward to the thalamus. These are the =internal arcuate fibers= (Fibræ arcuatæ internæ), which curve across the median plane below the central gray matter and form with those of the opposite side the =decussation of the fillet= (Decussatio lemniscorum). Beyond the decussation the fibers form an important longitudinal tract which extends forward in the mid-brain. This is the =fillet= or =lemniscus=, the chief continuation of the sensory conducting path from the dorsal roots of the spinal nerves. The =external arcuate fibers=, some of which were seen on the lateral aspect of the medulla, have a similar origin. Part of them (Fibræ dorsales) pass directly to the dorsal aspect of the restiform body of the same side; others (Fibræ ventrales) cross to the opposite side, descend close to the ventral fissure, and then curve upward and forward to the restiform body. The decussation of the arcuate fibers forms the distinct median =raphé= seen on cross-sections of the medulla anterior to the pyramidal decussation. The restiform body, situated dorso-laterally, contains, in addition to the arcuate fibers, the =cerebello-olivary fasciculus= before mentioned, and the =cerebellospinal fasciculus= or direct cerebellar tract. The =dorsal longitudinal fasciculus= corresponds to the ventral ground-bundle of the spinal cord, displaced dorsally by the decussation of the pyramids and fillet. In the posterior part of the medulla it is not marked off from the fillet, along the dorsal edge of which it lies. From the level of the hypoglossal nucleus forward it is distinct and can be traced as a conspicuous tract in the ventral margin of the gray matter of the floor of the fourth ventricle and of the central gray matter of the mid-brain. Below the restiform body and related externally to the external arcuate fibers, there is a considerable bundle of longitudinal fibers, the =spinal root= (Tractus spinalis) =of the fifth nerve=; internal to it is the terminal nucleus of the sensory root of the nerve.

THE PONS

The =pons= (Varolii) is that part of the brain stem which lies between the medulla and the cerebral peduncles; it is marked off from these ventrally by anterior and posterior grooves. Viewed from below it is elongated transversely, convex in both directions, and presents a wide shallow median groove (Sulcus basilaris), which lodges the basilar artery. Laterally a large part of its mass curves upward and backward into the base of the cerebellum, forming the =middle cerebellar peduncle= (Brachium pontis). The superficial origin of the trigeminal (fifth) nerve is at the lateral limit of the ventral surface. Transverse striations indicate the course of its superficial (ventral) fibers which connect the two sides of the cerebellum. The dorsal surface is blended on either side with the overlying anterior peduncles of the cerebellum; the central free portion forms the anterior part of the floor of the fourth ventricle, and will be considered in the account of that cavity.

On cross-section the pons is seen to be composed of dorsal and ventral parts. The =dorsal part= (Pars dorsalis pontis) consists superficially of a layer of gray matter covered by the ependyma of the fourth ventricle. Beneath this the median raphé of the medulla is continued into the pons, dividing it into similar halves. In the anterior part of the pons the fillet divides into an inner and outer part, the =medial= and =lateral fillets= (Lemniscus medialis, lateralis); the latter arches outward to reach the outer side of the anterior cerebellar peduncle. The =dorsal longitudinal fasciculus= becomes sharply defined into a round bundle which lies close to the raphé under the gray matter of the floor of the fourth ventricle. In cross-section the =formatio reticularis= forms a large area below the superficial gray matter and the longitudinal bundles. Dorso-laterally is the large rounded section of the =anterior cerebellar peduncle=. Lower down is a large bundle, the =sensory root of the fifth nerve=. In front of this is the =motor nucleus= of the same nerve, external to which is its =motor root=. The =ventral part= of the pons (Pars basilaris pontis) is composed of transverse and longitudinal fibers, and a large amount of gray matter which is broken up into small masses (Nuclei pontis) by the intersection of the fibers. The =transverse fibers= are gathered laterally into a compact mass which turns upward and backward and enters the central white matter of the cerebellum, forming the =middle cerebellar peduncle=. Centrally the fibers are arranged in bundles which intercross. The transverse fibers are chiefly of two kinds. Some arise from the Purkinje cells of the cerebellar cortex and pass either to the opposite side of the cerebellum or turn at the raphé and run forward and backward in the brain stem. Others are axones of cells of the nuclei pontis, and pass to the hemispheres of the cerebellum. The corpus trapezoideum is mainly the central continuation of the cochlear division of the auditory nerve. Above it is the small =anterior olivary nucleus= (Nucleus olivaris nasalis). The =longitudinal fibers= of the ventral part of the pons consist chiefly of the =cerebrospinal= or =pyramidal fasciculi=. These come from the ventral part (pes) of the cerebral peduncles and are situated laterally, interspersed among the deep transverse fibers in the anterior part of the pons. Toward the posterior part the bundles incline toward the median plane and become collected into a compact mass which appears superficially at the posterior border. Many fibers come from the cerebral cortex and terminate in the nuclei of the gray matter of the pons; they may be designated =corticopontile fibers=.

THE CEREBELLUM

The =cerebellum= is situated in the posterior fossa of the cranium, and is separated from the cerebral hemispheres by the transverse fissure and the tentorium cerebelli which occupies it. It overlies the pons and the greater part of the medulla, from which it is separated by the fourth ventricle. Its average weight is about two ounces (ca. 60 gm.) or about 9 per cent. of the weight of the entire brain. Its shape is approximately globular but very irregular. It is somewhat compressed dorso-ventrally and its transverse diameter is the greatest.

The =anterior surface= faces upward and forward and is covered partially by the tentorium cerebelli. The =posterior surface= is almost vertical. The =ventral surface= or =base= lies over the fourth ventricle, and is connected by three pairs of peduncles with the medulla, pons, and mid-brain.

It is customary to recognize three gross divisions of the cerebellum, viz., the median vermis and two lateral hemispheres. The =vermis= (cerebelli) is curved in a circular manner so that its two extremities are close together or even in contact on the ventral surface. The anterior extremity is termed the =lingula=; it lies between the cerebellar peduncles and gives attachment to the =anterior medullary velum=, a thin lamina which forms the anterior part of the roof of the fourth ventricle. The posterior extremity, the =nodulus=, gives attachment to the =posterior medullary velum= which covers the posterior recess of the fourth ventricle. The =hemispheres= (Hemisphæria cerebelli) are clearly separated from the vermis by two deep paramedian fissures. They lie in the lateral depressions of the cerebellar compartment of the cranium.

In tracing the fissures from behind forward it will be noticed that they are nearly sagittal as far as the anterior surface, where they diverge widely, so that the vermis forms all of the fore part of the cerebellum.

The surface of the cerebellum is further cut up into numerous =gyri= or =folia= by narrow and relatively deep =sulci=, many of which approach a transverse direction. Certain of the sulci are more pronounced than the others, and by means of them it is possible to define groups of gyri. Such groups are termed =lobes=, and have received specific names, derived chiefly from the systematic descriptions of the human cerebellum.

The lobes of the vermis are readily distinguished on a median section. Enumerated from the anterior to the posterior extremity they are: (1) lingula, (2) lobus centralis, (3) lobus ascendens, (4) lobus culminis, (5) lobus clivi, (6) tuber vermis, (7) pyramis, (8) uvula, (9) nodulus. Each hemisphere is cut into laterally by two sulci which mark off two sagittal discoid masses, termed by Ziehen tabulations. The external tabulation consists of four or five lobules, the lowest of which is regarded as the flocculus. The inner part of the hemisphere is divided into three or four lobes. In the absence of a satisfactory morphological basis it seems undesirable to deal with the lobation of the cerebellum in further detail.

The =cerebellar peduncles=, three on each side, join the central white matter of the cerebellum at the base. The =posterior peduncle= is the restiform body of the medulla, a large rounded tract derived from the lateral and ventral columns of the cord. Near the middle of the medulla it inclines outward, forms the lateral wall of the fourth ventricle, and ends by entering the central white matter of the cerebellum. The =middle peduncle= is formed, as previously seen, by the brachium pontis. The =anterior peduncles= (Brachia conjunctiva) pass forward on either side on the dorsal surface of the pons, forming the lateral boundary of the fore part of the fourth ventricle. They disappear under the corpora quadrigemina into the substance of the mid-brain. At the point of disappearance the =trochlear= (fourth) =nerve= emerges from the mid-brain. In some cases two or three bundles of fibers (Fila lateralia pontis) arise in the angle between the middle and anterior peduncle, curve obliquely forward and downward over the outer aspect of the latter, and spread out on the ventral face of the cerebral peduncle just in front of the pons.

On sagittal section the cerebellum is seen to consist of a layer of =cortical gray matter= (Substantia corticalis) and the =medullary white matter=. The white matter consists of a large =basal mass= (Corpus medullare), which is joined by the peduncles, and gives off primary laminæ to the lobules; from these secondary and tertiary laminæ arise, the latter entering the gyri. The arrangement on sagittal section is tree-like, hence the classical term “arbor vitæ” which is applied to it. The central gray matter consists of groups of cells which form small =nuclei= embedded in the central white substance.

As noted above the central gray matter does not form a large nucleus, the corpus dentatum, which is so conspicuous an object on sagittal sections of the cerebellar hemisphere in man.

The principal connections established by the peduncular fibers of the cerebellum are as follows: The posterior peduncle (Corpus restiforme) is composed of afferent and efferent fibers which connect the cerebellum with the medulla and spinal cord. The =cerebellospinal fasciculus= or =direct cerebellar tract=, which arises from the cells of the nucleus dorsalis (Clarke’s column) of the cord, ends in the cortex of the vermis; many of its fibers cross to the opposite side. Numerous =arcuate fibers= from the nucleus gracilis and nucleus cuneatus of the same and opposite sides establish connections with =cells= of the cerebellar cortex. =Olivocerebellar= fibers (chiefly afferent) connect with the olivary nucleus of the same and of the opposite side of the medulla oblongata. The =nucleo-cerebellar fasciculus= comprises fibers derived from the nuclei of the fifth, eighth, and tenth cranial nerves (Edinger). The =descending cerebellospinal fasciculus= consists of fibers which terminate in relation with cells of the ventral horns of the spinal cord. The chief facts concerning the middle peduncle have been mentioned in the description of the pons. The anterior peduncle is essentially an efferent tract, the fibers of which pass forward to the tegmentum of the cerebral peduncle, the subthalamic region, and the thalamus. After the peduncles disappear under the corpora quadrigemina, they converge and many of their fibers intercross, forming the =decussation of the superior peduncle=. A considerable number of fibers end in the nucleus ruber. Thence impulses are transmitted in two directions: first, by =thalamo-cortical fibers= to the cerebral cortex; second, by the =rubrospinal tract= through the brain stem and lateral columns of the cord to the ventral horn cells. The ventro-lateral cerebellospinal fasciculus (Gowers’ tract) is an ill-defined tract which connects the spinal cord with the cerebellum. Its fibers appear to be axones of cells of the posterior horns of the cord; they pass in the lateral column of the cord, become scattered in passing through the reticular formation of the medulla and pons, and enter the cerebellum by way of the anterior medullary velum.

THE FOURTH VENTRICLE

The =fourth ventricle= (Ventriculus quartus) is the cavity of the rhombencephalon; it communicates with the central canal of the spinal cord behind, and through the aqueduct with the third ventricle in front. It is somewhat rhomboid in outline, elongated from before backward and narrowest behind. It is lined completely by an epithelium (Ependyma) and contains a small amount of fluid.

Its =floor= (Fossa rhomboidea) is formed by the =medulla= and =pons= and is marked by three longitudinal furrows which converge behind. It is widest and deepest a little in front of its middle. The posterior part narrows to a point at the opening of the central canal, and on account of its appearance in man it has been termed the calamus scriptorius. The =median sulcus= (Sulcus medianus) extends the entire length of the floor and is deepest toward the ends. The =limiting sulci= (Sulci limitantes) begin on either side of the opening of the central canal and extend forward as the lateral limits of the rhomboid fossa. Just beyond the middle of the fossa they expand into a shallow depression, the =anterior fovea= (Fovea nasalis). On either side of the median sulcus and margined by the limiting sulcus is a slightly rounded column, the =eminentia medialis=. Opposite the fovea this presents an elongated prominence, the =colliculus facialis=, so named because it overlies the bend formed by the fibers of origin of the facial nerve. External to the limiting sulcus is a long fusiform elevation, the =area acustica=, from which a band of fibers (Striæ acusticæ) winds over the anterior end of the restiform body to the superficial origin of the cochlear nerve.

The =lateral wall= is formed by the restiform body and the anterior peduncle of the cerebellum.

The =roof= is formed in its middle part by the vermis of the cerebellum, covered by the epithelium before mentioned. There is commonly a dorsal recess (Recessus tecti s. fastigium) between the extremities of the vermis. The anterior part of the roof is formed by a thin lamina of white substance, the =anterior medullary velum= (or valve of Vieussens), which extends backward from the corpora quadrigemina, and is attached on either side to the anterior peduncles of the cerebellum. Its anterior part is relatively thick and contains the decussation of the fibers of the trochlear nerves. Posteriorly it blends with the white matter of the cerebellum. A thin lamina of white matter, the =posterior medullary velum=, backed by pia mater, completes the roof posteriorly. After removal of the cerebellum the line of attachment (Tænia ventriculi quarti) to the medulla is seen; it begins centrally over the opening of the central canal, runs forward on the inner face of the restiform body, and turns outward behind the brachium pontis. The thick part which stretches over the posterior angle of the ventricle is termed the =obex=. The posterior part of the ventricle forms three recesses, of which two are lateral and the third median and posterior. The =lateral recesses= communicate with the subarachnoid space by distinct openings (Aperturæ laterales). The layer of pia which strengthens the roof here is named the =tela chorioidea= of the fourth ventricle. It is triangular in outline and closely adherent to the velum. It forms three fringed masses which contain vascular convolutions and are designated the =median= and =lateral chorioid plexuses= of the fourth ventricle. They appear to lie within the ventricle, but are really excluded from the cavity by the epithelial lining, which they invaginate.

THE MESENCEPHALON

The =mesencephalon= or =mid-brain= connects the rhombencephalon with the fore-brain. In the undissected brain it is covered dorsally by the cerebral hemispheres. It consists of a dorsal part, the corpora quadrigemina, and a larger ventral part, the cerebral peduncles, which are visible on the base of the brain. It is traversed longitudinally by a narrow canal, the cerebral aqueduct, which connects the fourth ventricle with the third.

The =corpora quadrigemina=[191] are four rounded eminences which lie under the posterior part of the cerebral hemispheres. They consist of two pairs, separated by a transverse groove. The =anterior pair= (Colliculi nasales) are larger and much higher than the posterior pair. They are gray in color, almost hemispherical, and are separated by a narrow furrow which leads forward to the subpineal fovea. A wide groove intervenes between them and the optic thalami. The =posterior pair= (Colliculi caudales) are relatively small and are paler than the anterior pair. They are marked by a wide median depression, and are limited below by a transverse furrow (Sulcus postquadrigeminus), at either side of which the =trochlear= (fourth) =nerve= emerges. Laterally each is prolonged to the inner geniculate body by a band of white matter termed the inferior brachium.[192]

The =cerebral peduncles= or crura cerebri (Pedunculi cerebri) appear on the base of the brain as two large rope-like stalks which emerge from the pons close together and diverge as they extend forward to enter the cerebrum. At the point of disappearance the =optic tract= winds obliquely across the peduncle. About half an inch further back a small tract (Tractus peduncularis transversus) curves across the peduncles, and behind this near the median line is the superficial origin of the =oculomotor= (third) =nerve=. The triangular depression between the diverging peduncles is the =interpeduncular space= (Fossa interpeduncularis). It is covered to a large extent by the =pituitary body=, a discoid brown mass which is connected with the base of the brain by a hollow stalk, the =infundibulum=. The posterior part of the space is pierced by numerous minute openings which transmit blood-vessels, and is therefore termed the =locus perforatus posticus=. The objects in the space belong to the diencephalon, and will be described later. The lateral aspect of the peduncle is marked by a groove (Sulcus lateralis mesencephali) which indicates the division into a dorsal part, the =tegmentum=, and a ventral part the =basis pedunculi=; these are separated by a layer of dark gray matter, the =substantia nigra=. The triangular area (Trigonum lemnisci) above the lateral groove is faintly marked by fibers passing obliquely upward and backward to the anterior cerebellar peduncle; these belong to the fillet or lemniscus, an important tract that connects the thalamus and corpora quadrigemina with the sensory reception nuclei of the opposite side of the medulla.

The =aqueduct of the cerebrum= or of Sylvius (Aquæductus cerebri) is the canal which extends through the mid-brain from the fourth to the third ventricle. It is surrounded by a layer of gray matter (Stratum griseum centrale), in the ventral part of which are the nuclei of origin of the oculomotor and trochlear nerves, and laterally nuclei of the mesencephalic roots of the trigeminal nerves.

THE DIENCEPHALON

The =diencephalon= or =inter-brain= comprises the thalamus and a number of other structures grouped about the third ventricle, the cavity of this division of the brain.[193] To expose its dorsal aspect, the greater part of the cerebral hemispheres, the corpus callosum, the fornix, the hippocampus, and the tela chorioidea of the third ventricle must be removed.

The =thalamus= (or optic thalamus) is the principal body in this part of the brain. It is a large ovoid gray mass placed obliquely across the dorsal face of each cerebral peduncle, so that the long axes of the two thalami would meet in front about at a right angle. Medially they are fused to a large extent, and around the area of adhesion they are separated by a sagittal circular space, the third ventricle. The dorsal surface is convex in both directions, and is separated from the overlying hippocampus by the tela chorioidea (or velum interpositum). Laterally it is separated from the nucleus caudatus by an oblique groove in which there is a band of white matter termed the =stria terminalis= or tænia semicircularis. Internally it is bounded by a narrow band, the =stria medullaris=, on which is a delicate ridge of ependyma termed the =tænia thalami=. The striæ unite posteriorly and blend with the stalk of the pituitary body. Near this point they present a small enlargement caused by the nucleus habenulæ. Anteriorly there is a small eminence, the =anterior tubercle=. The posterior part of the thalamus has the form of a rounded ridge which is continuous laterally with the optic tract.[194] Behind the point of origin of the tract, in the angle between the thalamus and the cerebral peduncle, is the =internal geniculate body= (Corpus geniculatum mediale), a well defined oval prominence.

The outer surface is separated from the lenticular nucleus by the =internal capsule=, an important mass of white matter composed of fibers passing to and from the cerebral cortex. These fibers go to form a large part of the ventral portion (basis) of the cerebral peduncle. From the entire external surface of the thalamus fibers pass into the internal capsule and radiate to reach the cerebral cortex; similarly fibers coming from the cortex converge in the internal capsule to enter the thalamus. This arrangement is termed the thalamic radiation. Ventral to the thalamus proper is the =subthalamic tegmental region=. This is the continuation of the tegmental part of the cerebral peduncle into the diencephalon. It contains the =red nucleus= (Nucleus ruber) an important ganglion on the course of the motor tracts. It receives numerous fibers from the cerebral cortex and the corpus striatum. From it fibers proceed to the thalamus and to the spinal cord; the fibers to the cord (Tractus rubro-spinalis of Monakow) cross to the opposite side and extend back in the tegmentum to the lateral columns of the cord. Lateral to the red nucleus a conspicuous lenticular area of dark gray matter is visible on cross-sections of the subthalamic region; this is the =subthalamic nucleus= (Nucleus hypothalamicus s. corpus Luysi), which consists of pigmented nerve-cells scattered through a dense network of fine medullated fibers, and is richly supplied with capillary blood-vessels. The two nuclei are connected by a transverse commissure (Commissura hypothalamica), which crosses the floor of the third ventricle above the mammillary body.

The =pineal body= or =epiphysis= is a small ovoid or fusiform red-brown mass situated in a deep central depression between the thalami and corpora quadrigemina. It is variable in size, but is commonly about 10 to 12 mm. long and 6 mm. wide. It is attached at the postero-superior quadrant of the third ventricle by a short stalk, in which is a small recess of that cavity. Its base blends in front with the junction of the striæ medullares of the thalamus. Immediately under the posterior part of the stalk is a short transverse band of white matter, the =posterior commissure=.

The pineal body is inclosed in a fibrous capsule from which numerous trabeculæ pass inward, dividing the organ into spaces occupied by round epithelial cells of the same origin as the ependyma of the ventricle.

The =mammillary body= (Corpus mammillare) is a white round elevation a little larger than a pea which projects ventrally at the anterior end of the median furrow of the interpeduncular space. While it is a single body in external form in the horse, sections show that it is double in structure and contains a nucleus of gray matter on either side.

Three sets of fibers are connected with the mammillary body. The anterior pillar of the fornix curves down in the lateral wall of the third ventricle to the body and many of the fornix fibers end in it. A bundle (Fasciculus thalamo-mammillaris) passes upward and backward from it into the anterior part of the thalamus, and a tract (Fasciculus pedunculo-mammillaris) extends back in the floor of the third ventricle to the tegmentum of the mid-brain.

The =pituitary body= or =hypophysis= was mentioned as covering part of the interpeduncular space. It is oval in outline, flattened dorso-ventrally, and nearly an inch (ca. 2 cm.) in width. It is attached by a delicate tubular stalk, the =infundibulum=, to the =tuber cinereum=, a small gray prominence situated between the optic chiasm in front and the mammillary body behind.

The pituitary body consists of two parts which can be distinguished readily on sections by their color. The glandular (or anterior) lobe is brown in color and is inclosed in a fibrous capsule. It is glandular in character and there is good ground for the view that it is an organ of internal secretion. Besides the chief cells which stain lightly, it contains large deeply staining chromophile cells. It arises as an outgrowth from the primitive mouth cavity. The cerebral (or posterior) lobe is pale and is connected with the infundibulum so as to form a rather flask-shaped arrangement. It is almost entirely inclosed by the glandular part. It arises as an outgrowth from the primitive diencephalon, but loses most of its earlier nervous character.

The optic chiasm and tracts form the anterior boundary of the interpeduncular space. The =optic chiasm= or =commissure= (Chiasma opticum) is formed by the convergence of the optic nerves and the crossing of the major part of the fibers of the nerve of one side to the tract of the opposite side. From the chiasm each =optic tract= (Tractus opticus) curves over the cerebral peduncle outward, backward, and upward to the posterior part of the thalamus and the internal geniculate body; some fibers reach the anterior quadrigeminal body.

All the fibers in the chiasm are not derived from the optic nerves. The posterior part contains fibers which pass from one tract to the other and are connected with the internal geniculate bodies; this bundle is called Gudden’s commissure (Commissura inferior). Above it is Meynert’s commissure (Commissura superior), the fibers of which enter the subthalamic body.

The =third ventricle= (Ventriculus tertius) is the narrow annular space between the thalami. It communicates by means of the aqueduct with the fourth ventricle behind, and in front it is continuous with the lateral ventricle on each side through the interventricular foramen. Its floor is formed by the structures of the interpeduncular space and to a small extent by the tegmentum of the cerebral peduncles. The roof is formed in the strict sense only by the ependyma, above which is a fold of pia mater, termed the =tela chorioidea= of the third ventricle or =velum interpositum=. The roof is invaginated by two delicate =chorioid plexuses= which appear to lie within the ventricle, although they are excluded from the cavity by the epithelium. When the tela is removed, the delicate ependyma of the roof is torn away with it, leaving the line of attachment to the stria medullaris to constitute the tænia thalami. The =anterior wall= is formed by the =lamina terminalis= (s. cinerea), a thin layer of gray matter which extends upward from the optic chiasm to the corpus callosum. A distinct rounded band of white matter extends across its posterior face, bulging into the ventricle. This is the =anterior commissure= (Commissura nasalis) of the cerebrum; its fibers extend to the olfactory bulb and to the pyriform lobe. A similar but more slender =posterior commissure= (Commissura caudalis) crosses the posterior wall above the entrance to the aqueduct; the connections of its fibers are not yet clearly known. The =interventricular foramen= (of Monroe) is situated on either side of the anterior part of the ventricle and leads outward and slightly upward between the anterior pillar of the fornix and the anterior tubercle of the thalamus. The cavity presents three =recesses= or diverticula, of which two are ventral and the third is supero-posterior. The =optic recess= (Recessus opticus) lies above the optic chiasm. Just behind it is the =infundibular recess= (Recessus infundibuli) which extends through the infundibulum to the pituitary body. The =pineal recess= (Recessus pinealis) is in the stalk of the pineal body.

THE TELENCEPHALON

The =telencephalon= or =end-brain= comprises two principal parts, the cerebral hemispheres and the optic part of the hypothalamus. The latter has been considered as a matter of convenience in the description of the diencephalon.

THE CEREBRAL HEMISPHERES

The =cerebral hemispheres= (Hemisphæria) form the greater part of the fully developed brain. Viewed from above (Fig. 498) they form an ovoid mass, of which the broader end is posterior, and the greatest transverse diameter is a little behind the middle. The two hemispheres are separated by a deep median cleft, the =longitudinal fissure= of the cerebrum, which is occupied by a sickle-shaped fold of dura mater, the falx cerebri. In front the separation is complete, and it appears to be behind also, but here the two hemispheres are attached to each other over a small area by the pia mater. When the hemispheres are gently drawn apart, it is seen that the fissure is interrupted in its middle part at a depth of a little more than an inch (ca. 3 cm.) by a white commissural mass, the =corpus callosum=; this connects the hemispheres for about half of their length. The =transverse fissure= separates the hemispheres from the cerebellum, and contains the tentorium cerebelli.

The =convex= or =dorso-lateral surface=[195] conforms closely to the cranial wall. The =medial= or =internal surface= is flat and sagittal and bounds the longitudinal fissure; to a large extent it is in contact with the falx cerebri, but behind the great cerebral vein the two hemispheres are in contact and are attached to each other over a small area as noted above. In well hardened specimens there is usually an impression for the vein in front of the area of adhesion. The =base= or =ventral surface= (Fig. 499) is irregular. Its anterior two-thirds is adapted to the cerebral fossa of the cranial floor. Crossing this area in front of the optic tract is a depression, the =transverse fossa= (vallecula or fossa of Sylvius), which leads outward to the =lateral fissure= (fissure of Sylvius), and lodges the middle cerebral artery. In front of the fossa there is a considerable rounded elevation known as the =trigonum olfactorium=. The trigonum and the inner part of the fossa are pierced by numerous openings for the passage of small blood-vessels and are equivalent to the locus perforatus anticus of man. Behind the outer part of the fossa is the rounded anterior end of the =pyriform lobe=. Traced backward the lobe curves upward and inward over the optic tract and the thalamus to the tentorial aspect of the hemisphere; its continuation, the hippocampus, forms part of the floor of the lateral ventricle, and will be examined later.

The posterior part or =tentorial area= is flattened, faces inward and backward as well as downward, and rests largely on the tentorium cerebelli; on its anterior part there is a shallow depression adapted to the corpora quadrigemina and the pineal body.

The =frontal pole= or anterior extremity (exclusive of the olfactory bulb) is compressed laterally, and the =occipital pole= or posterior extremity forms a blunt point.

The hemisphere comprises: (1) the =pallium= or =mantle=, which consists of an outer layer of gray matter, the =cortex= (Substantia corticalis), covering a large mass of white matter (Centrum semiovale); (2) the =rhinencephalon= or olfactory portion of the brain; (3) the =corpus callosum= and =fornix=, the great commissural white masses; (4) the =lateral ventricle= and certain important structures associated therewith.

The =pallium= is thrown into numerous folds, the =gyri cerebri=, which are separated by =sulci= or =fissures= of varying depth. The general pattern of the gyri and sulci is similar in normal brains of the same species, but the details are very variable and are never alike on the two hemispheres of the same brain. In the horse the arrangement is complicated by the existence of numerous short accessory fissures which cut into the gyri at right angles and tend to confuse the observer. The principal fissures and sulci of the convex surface (Figs. 497, 507) are as follows:

1. The =lateral fissure= (Fissura lateralis Sylvii) ascends on the lateral surface of the hemisphere as the continuation of the fossa transversa in front of the pyriform lobe. After crossing the external olfactory tract it divides into three branches; of these one passes upward, one runs obliquely forward and upward, and the third is directed upward and backward.

2. The =suprasylvian fissure= (F. suprasylvia)[196] is long and divides a large part of the convex surface of the hemisphere into dorsal and lateral portions. It begins on the dorso-medial border near its anterior end and, inclining gradually downward, passes back to end on reaching the tentorial surface. It is continuous internally with the transverse fissure usually and in front with the presylvian fissure.

3. The =presylvian fissure= (F. præsylvia) is the continuation forward of the preceding. It passes forward, outward, and downward almost to the frontal pole, and then inclines backward to end at the groove which marks the upper limit of the rhinencephalon (Sulcus rhinalis).

4. The =marginal fissure= (F. marginalis) extends along the dorso-medial border. It begins a little in front of the middle of the border and turns around the occipital pole to end on its tentorial aspect.

5. The =entomarginal fissure= (F. entomarginalis) lies internal to the dorso-medial border. It does not extend quite as far forward as the marginal fissure, from which it is separated by a narrow gyrus.

6. The =ectomarginal fissure= (F. ectomarginalis) lies about midway between the marginal fissure and the posterior part of the suprasylvian fissure.

7. The =sulcus rhinalis= is a very distinct furrow on the lower part of the lateral surface which marks off the olfactory part of the brain (rhinencephalon) from the rest of the hemisphere. It is undulating and is highest where it is crossed by the lateral fissure (of Sylvius).[197]

On the medial surface (Fig. 508) the main fissures and sulci are as follows:

1. The =calloso-marginal fissure= (F. calloso-marginalis) is extensive and well defined. It is approximately parallel to the dorso-medial border of the hemisphere, from which it is about half an inch distant. It begins in front a short distance below and in front of the anterior bend (genu) of the corpus callosum and forms a =C=-shaped curve, its posterior part extending on the tentorial surface to a point behind the depression for the corpora quadrigemina. It separates the marginal gyri above from the gyrus fornicatus, which extends down to the corpus callosum.

2. The =transverse fissure= (F. transversa) begins a little behind the middle of the calloso-marginal fissure, passes obliquely upward and forward to the dorso-medial border—into which it cuts deeply—and usually joins the suprasylvian fissure.[198]

3. The =sublimbic fissure= (F. sublimbica) curves over the gyrus fornicatus a short distance above the corpus callosum. Its middle part is commonly indistinct, and it is often divided into anterior and posterior parts.

4. The =callosal sulcus= (Sulcus corporis callosi) separates the corpus callosum from the gyrus fornicatus.

The =hippocampus= (or cornu Ammonis) is a gyrus which curves from the deep face of the pyriform lobe around the thalamus and forms the posterior part of the floor of the lateral ventricle. It can be displayed by cutting away the brain stem up to the optic tract and the interventricular foramen. Viewed from below the hippocampal gyrus is seen to form a semicircular curve from the apex of the pyriform lobe to the angle of divergence of the posterior pillars of the fornix, i. e., to a point under the central part of the corpus callosum. It is separated deeply by the hippocampal fissure from the =gyrus dentatus=. Along the concave margin of the latter is a band of white matter, termed the =fimbria=, which is the prolongation of the greater part of the posterior pillar of the fornix into this region. The ventricular surface of the hippocampus (Fig. 511) is covered with a thin layer of white matter, the =alveus=, which is also derived from the posterior pillar of the fornix, and is therefore continuous with the fimbria. The two hippocampi are connected at their highest parts by transverse fibers which constitute the =hippocampal commissure=.

The interval between the hippocampus and fimbria on the one hand and the brain stem on the other is a lateral continuation of the great transverse fissure of the brain, and is termed the =chorioid fissure=. It is occupied by a fold of pia mater, the =tela chorioidea= of the third ventricle, or =velum interpositum=. This fold is triangular in outline and its apex reaches to the interventricular foramen. Its base is continuous at the transverse fissure with the pia which covers the surface of the brain. Its middle part lies over the epithelial roof of the third ventricle, as has been seen (Fig. 506). The lateral borders will be seen on the floor of the lateral ventricles, where they form thick rounded bands containing convolutions of blood-vessels, known as the =chorioid plexuses= of the lateral ventricles (Fig. 511).

When the tela is pulled out one may easily get the impression that the lateral ventricle communicates with the exterior by means of the chorioid fissure. Such is not the case, since the chorioid plexus is covered by the epithelial lining of the ventricle, which has been torn away.

The =rhinencephalon= or olfactory part of the brain comprises the olfactory bulb, peduncle and tracts or roots, the trigonum olfactorium, the area parolfactoria, and the pyriform lobe.[199]

The =olfactory bulb= (Bulbus olfactorius) is an oval enlargement which curves upward in front of the frontal pole of the hemisphere. Its convex superficial face fits into the ethmoidal fossa and receives numerous olfactory nerve-fibers through the cribriform plate; hence it is very difficult to remove the bulb intact. It contains a considerable =cavity= (Ventriculus bulbi olfactorii) which is connected with the lateral ventricle by a small canal in the middle olfactory tract. The deep face is largely in contact with the frontal pole of the hemisphere and is connected with the olfactory peduncle.

The gray matter of the bulb is external and is thickest on the convex anterior surface. The posterior surface consists to a large extent of fibers which are the axones of the mitral cells of the deep layer of the gray substance and go to form the peduncle and striæ.

The =olfactory peduncle= (Tractus olfactorius) is a very short but wide band of white substance which arises in the olfactory bulb and extends back to be continued by the olfactory tracts.

The =olfactory tracts= or =striæ= (Striæ olfactorii) are three in number. The =external tract= (Stria lateralis) is much the largest and most distinct. It passes backward, upward, and outward, widens out and joins the pyriform lobe. It is clearly defined dorsally by the sulcus rhinalis and is marked off from the trigonum olfactorium by the sulcus arcuatus. The =intermediate tract= (Stria intermedia) is short, ill defined, and flat; it contains the canal which opens into the anterior horn of the lateral ventricle. The =internal tract= (Stria medialis) is small, short, and not well defined; it bends over to the parolfactory area (of Broca) on the inner face of the hemisphere below the genu of the corpus callosum.

The =trigonum olfactorium= is the prominent gray area situated in the angle of divergence of the internal and external olfactory striæ. It is bounded externally by the external olfactory stria, from which it is defined by the arcuate sulcus. It is continuous with the =area parolfactoria= on the medial surface; behind the latter a band descends from the rostrum of the corpus callosum and is continuous below with the anterior perforated space; it is termed the =subcallosal gyrus= or peduncle of the corpus callosum.

The =pyriform lobe= (Lobus piriformis) is the well-marked prominence on the base lateral to the optic tract and cerebral peduncle, from which it is separated by a deep fissure. Its nipple-like apex lies behind the fossa transversa and covers the optic tract. The external surface is marked by one or two sulci (lobi piriformis). The lobe contains a cavity, the ventral horn of the lateral ventricle.

The fibers of the olfactory tracts go to the pyriform lobe and hippocampus, the trigonum olfactorium, the area parolfactoria, the subcallosal gyrus, and part of the gyrus fornicatus. The central connections of the olfactory apparatus are complex and are not yet fully understood. The anterior cerebral commissure contains fibers which pass from the olfactory bulb of one side by way of the inner tracts to the bulb of the opposite side; also fibers which cross in it from the inner tract of one side to the pyriform lobe of the opposite side. Many fibers pass by way of the septum pellucidum, fornix, and fimbria to the hippocampus. Other fibers pass in the anterior pillar of the fornix to the mammillary body and thence to the thalamus by the thalamo-mammillary bundle.

The =corpus callosum= is the great transverse commissure which connects the two cerebral hemispheres through about half of their length. On median section (Fig. 508) it is seen to be arched from before backward, white in color, and composed substantially of transverse fibers. The middle part or =truncus= (Truncus corporis callosi) slopes downward and forward and is thinner than the ends. The anterior thickened end, the =genu=, bends downward and backward and thins out to form the =rostrum=; the latter is continuous with the lamina terminalis. The posterior end, the =splenium=, also thick, lies at a considerably higher level than the genu. The =upper surface= is convex in its length, concave transversely. Its central part forms the floor of the longitudinal fissure. It is covered by a thin layer of gray matter (Induseum griseum), in which are strands of longitudinal fibers (Striæ longitudinales); the latter are arranged in median and lateral bundles (Stria medialis, striæ laterales).[200] The =lower surface= has the reverse configuration, and forms the roof of the lateral ventricles. Along the median line the septum pellucidum is attached to it. The fibers of the corpus callosum (with the exception of the longitudinal striæ) run transversely and spread out laterally in all directions in the central white matter of the hemispheres to the cortex, forming the =radiation of the corpus callosum= (Radiatio corporis callosi).

The =fornix= is a bilateral structure composed of white fibers which arch chiefly over the thalamus and the third ventricle. It is described as consisting of a body and two pairs of pillars. The =body= (Corpus fornicis) is formed by the fusion of the two arches of which the fornix is composed. It is triangular and overlies the anterior parts of the thalami and the third ventricle. The upper surface gives attachment to the septum pellucidum and on either side forms part of the floor of the lateral ventricle. The lateral border is related to the chorioid plexus of the lateral ventricle and forms the inner boundary of the interventricular foramen. The =anterior pillars= or =columns= (Columnæ fornicis) (Fig. 512) are two slender round bundles which emerge from the body in front of the interventricular foramen and diverge slightly as they curve downward and backward to the mammillary body.

From the mammillary body the greater part of these fornix fibers are continued to the thalamus by the thalamo-mammillary fasciculus (or bundle of Vicq d’Azyr). Others pass to the cerebral peduncle. A portion of the fibers cross to the opposite thalamus and cerebral peduncle.

The =posterior pillars= (Crura fornicis) are much larger bands which diverge widely from the posterior angles of the body. Each curves outward and backward over the thalamus (from which it is separated by the tela chorioidea) and is chiefly continued as the =fimbria= along the concave border of the hippocampus. The pillars give off fibers to form the alveus or white matter which covers the ventricular face of the hippocampi, and between them are transverse fibers which constitute the hippocampal commissure.

The =septum pellucidum= is the median partition between the two lateral ventricles. Its convex upper border blends with the corpus callosum and its concave lower border joins the fornix. Its anterior part is received into the genu of the corpus callosum. Traced backward it diminishes in height and the two edges meet at an acute angle at the splenium.

The septum consists of two layers (Laminæ septi pellucidi) which are in direct contact with each other. They consist of nerve-fibers and gray matter. The latter exists in considerable amount in the thicker part of the septum adjacent to the anterior pillars of the fornix. Many of the fibers of the septum pass up through the corpus callosum to the gyrus fornicatus. Others are connected with the subcallosal gyrus and the parolfactory area.

The =lateral ventricle= (Ventriculus lateralis) is the irregular cavity in the interior of each cerebral hemisphere.[201] Each communicates with the third ventricle through the =interventricular foramen= (of Monro), and by a small canal with the cavity of the olfactory bulb. It is usual to describe the ventricle as consisting of three parts, viz., the central part or body, and anterior and inferior horns. The =anterior horn= (Cornu nasale) is the part in front of the interventricular foramen; it communicates ventrally with the olfactory cavity. The =central part= (Pars centralis) extends back to the splenium of the corpus callosum. It opens into the third ventricle through the interventricular foramen, which is situated between the fornix and the anterior part of the thalamus. The =inferior horn= (Cornu ventrale) curves downward and forward into the pyriform lobe. The roof of the ventricle is formed by the corpus callosum and the inner wall is the septum pellucidum. After removal of the roof the floor is seen to be formed chiefly by two bodies. The anterior one is the =caudate nucleus= (Nucleus caudatus), a somewhat pear-shaped gray eminence, the long axis of which is directed obliquely upward, backward, and outward. Its anterior large end is termed the head, and the posterior long tapering end the tail. The posterior body, the =hippocampus=, is white on its ventricular surface, which is strongly convex. It curves outward and backward and then turns downward and forward to join the pyriform lobe. The two bodies are separated by an oblique groove which is occupied by the =chorioid plexus= of the lateral ventricle. This is the thickened edge of a fold of pia mater, the tela chorioidea (or velum interpositum), which lies between the hippocampus and the thalamus. It contains convolutions of small blood-vessels and in old subjects there may be calcareous concretions in it. The plexuses of the two sides are continuous through the interventricular foramen. On drawing the chorioid plexus backward a narrow white band, the =stria terminalis= (or tænia semicircularis), is seen along the margin of the caudate nucleus, where it bounds the intermediate groove. The plexus partially covers a wider white band which is blended with the white substance of the hippocampus; this is the =posterior pillar of the fornix= and its continuation, the =fimbria=.

The =corpus striatum=[202] is the great basal ganglion of the hemisphere. It is situated in front of the thalamus and the cerebral peduncle, and its anterior rounded end appears on the base of the hemisphere at the trigonum olfactorium. It is composed of two masses of gray matter, the caudate and lenticular nuclei, separated incompletely by tracts of white matter which are known collectively as the internal capsule. The =caudate nucleus= (Nucleus caudatus) is the dorso-medial and larger of the two gray masses; it has been seen in the examination of the floor of the lateral ventricle. The =lenticular nucleus= (Nucleus lentiformis) lies ventro-laterally, over the trigonum olfactorium and the fossa transversa. It is related externally to a layer of white matter termed the =external capsule=, which separates it from a stratum of gray substance known as the =claustrum=. The two nuclei are fused in front, and further back they are connected by strands of gray matter which intersect the internal capsule.

The =amygdaloid nucleus= (Nucleus amygdalæ) (Fig. 506) is an ovoid mass of gray matter situated external to the ventral horn of the lateral ventricle and below the posterior part of the lenticular nucleus. Some fibers of the stria terminalis are connected with it.

The =internal capsule= (Capsula interna) is a broad band of white matter situated between the thalamus and caudate nucleus internally and the lenticular nucleus externally. A sagittal section through the brain shows that it is in great part directly continuous with the basis or ventral part of the cerebral peduncle. It contains most of the so-called =projection fibers= of the hemisphere, which connect the cerebral cortex with nuclei of other and more posterior parts of the brain. When the fibers of the internal capsule are traced forward it is evident that they spread out in all directions to reach the cerebral cortex. This arrangement, in which the fibers of the corpus callosum participate, is termed the =corona radiata=.

The internal capsule also contains fibers which connect the corpus striatum with the thalamus. These are termed the thalamo-striate and strio-thalamic fibers respectively, according to the direction in which they conduct impulses.

The fibers of the stria terminalis connect the amygdaloid nucleus with the septum pellucidum and trigonum olfactorium. It is therefore probably part of the complex connections between the primary and secondary olfactory centers.

THE CRANIAL NERVES

The =cranial= or =cerebral nerves= (Nn. cerebrales) comprise twelve pairs which are designated from before backward numerically and by name. Their number, names, and functional characters are given in the subjoined table:

I. Olfactory Sensory (Small) II. Optic Sensory (Sight) III. Oculomotor Motor IV. Trochlear Motor V. Trigeminal Mixed VI. Abducent Motor VII. Facial Mixed VIII. Auditory Sensory (Hearing and Equilibration) IX. Glosso-pharyngeal Mixed X. Vagus or Pneumogastric Mixed XI. Spinal accessory Motor XII. Hypoglossal Motor

THE FIRST OR OLFACTORY NERVE

The =olfactory nerve= (N. olfactorius) is peculiar in that its fibers are not aggregated to form a trunk, but are connected in small bundles with the olfactory bulb. They are non-medullated, and are the central processes of the olfactory cells which are situated in the olfactory region of the mucous membrane of the nasal cavity. This region is distinguished by its brown color and comprises most of the lateral mass of the ethmoid, a small adjacent area of the superior turbinal, and the corresponding surface of the septum nasi. The nerve-bundles are inclosed in sheaths derived from the membranes of the brain and pass through the foramina of the cribriform plate to join the convex surface of the olfactory bulb. Some fibers come from the vomero-nasal organ of Jacobson.

THE SECOND OR OPTIC NERVE

The =optic nerve= (N. opticus) is composed of fibers which are the central processes of the ganglion cells of the retina. The fibers converge within the eyeball to the optic papilla, where they are collected into a round trunk, the optic nerve. The nerve thus formed pierces the chorioid and sclera, emerges from the posterior part of the eyeball, and passes backward and inward to the optic foramen. After traversing the latter it decussates with its fellow of the opposite side to form the optic chiasma or commissure. In the orbit the nerve is slightly flexuous and is embedded in the fat behind the eyeball and surrounded by the retractor oculi muscle. Its intraosseous part is an inch or more (ca. 3 cm.) long. The =sheath= of the nerve is formed by prolongations of the membranes of the brain, and includes continuations of the subdural and subarachnoid spaces.

The greater part of the fibers of the optic nerve cross in the chiasma to the tract of the opposite side. In the tract the fibers proceed to (1) the internal geniculate body, (2) the posterior part of the thalamus, and (3) the anterior quadrigeminal body (indirectly). The fibers which go to the internal geniculate body appear to belong to Gudden’s commissure and to be nonvisual in function. The visual fibers, which come from the outer part of the retina of the same side and the inner part of the retina of the opposite side, terminate about cells in the anterior quadrigeminal body and the part of the thalamus which corresponds to the pulvinar and external geniculate body of man. From the cells of the former fibers pass to the nuclei of the motor nerves of the eyeball, and complete the reflex arc. Fibers proceed from the cells of the thalamus to the visual area of the cortex in the occipital part of the hemisphere.

THE THIRD OR OCULOMOTOR NERVE

The =oculomotor nerve= (N. oculomotorius) arises by several radicles from the basal surface of the cerebral peduncle, a little lateral to the interpeduncular furrow. It turns sharply outward and forward, crosses over the cavernous sinus, and continues above the maxillary nerve and in company with the ophthalmic nerve to the foramen lacerum orbitale. It emerges through the foramen with the latter nerve and the abducens and divides into two branches. The =superior branch= is short and divides into twigs which supply the rectus superior, retractor, and levator palpebræ superioris. The =inferior branch= (Figs. 438, 439) is larger and much longer. It supplies the motor fibers to the ciliary ganglion (which lies directly on this branch in the horse) and short branches to the rectus internus and rectus inferior, and continues forward on the latter to end in the inferior oblique muscle.

The deep origin of the fibers of the oculomotor nerve is in the oculomotor nucleus situated in the gray matter of the floor of the cerebral aqueduct in the region of the anterior corpora quadrigemina.

THE FOURTH OR TROCHLEAR NERVE

The =trochlear= or pathetic (N. trochlearis) is the smallest of the cranial nerves. It arises from the anterior cerebellar peduncle just behind the corpora quadrigemina, curves outward and forward, pierces the tentorium cerebelli, and passes forward along the outer border of the maxillary nerve (Figs. 504, 505). It emerges from the cranium through a small foramen immediately above the foramen lacerum orbitale or through the latter and passes forward along the inner wall of the orbit to end in the posterior part of the superior oblique muscle of the eyeball (Fig. 439).

The fibers of the fourth nerve spring from a nucleus in the gray matter of the floor of the cerebral aqueduct behind the oculomotor nucleus. The fibers run backward in the tegmentum, then turn upward and inward and undergo total decussation with those of the opposite nerve in the anterior part of the anterior medullary velum. In addition to this peculiarity it is the only nerve which is connected with the dorsal aspect of the brain.

THE FIFTH, TRIGEMINAL, OR TRIFACIAL NERVE

The =trigeminal nerve= (N. trigeminus) is the largest of the cranial series. It is connected with the lateral part of the pons by a large =sensory root= and a smaller =motor root= (Fig. 499).

The =sensory root= (Portio major) extends forward through a notch on the lower part of the petrosal crest and widens out to join the semilunar ganglion.

The =semilunar= (or Gasserian) =ganglion= (Ganglion semilunare) is a crescent-shaped mass of nerve-fibers and cells which overlies the antero-external part of the foramen lacerum basis cranii, and is partly embedded in the dense fibrous tissue which occupies the foramen except where vessels and nerves pass through. Its long axis, which is about an inch (2.5 cm.) in length, is directed forward and inward and its convex anterior face gives rise to the ophthalmic, the maxillary, and the sensory part of the mandibular division of the nerve. The surface of the ganglion is irregularly striated. It is connected by filaments with the adjacent carotid plexus of the sympathetic, and sends delicate twigs to the dura mater.

The fibers of the sensory root arise from the ganglion as axones of the ganglion cells, and the fibers of the nerves which extend peripherally from the ganglion are dendrites of the cells. The sensory root-fibers enter the tegmentum of the pons and divide into anterior and posterior branches, which terminate about the cells of the sensory nucleus of termination of the trigeminus. This nucleus extends from the pons to the sixth cervical segment of the spinal cord (Dexler). The posterior branches of the fibers are collected into a compact bundle, the spinal tract or root of the trigeminus, which lies lateral to the substantia gelatinosa in the medulla. The central connections of the sensory part of the trigeminus are very extensive. The most important paths are: (1) Axones of cells of the sensory nucleus and the substantia gelatinosa pass chiefly as arcuate fibers across the raphé to the thalamus, whence impulses are transmitted by thalamo-cortical fibers to the cerebral cortex. In ungulates a distinct tract extends from the anterior part of the sensory nucleus to the thalamus of the same side (Wallenberg). It is probable that collaterals of the arcuate fibers go to the motor nuclei of the fifth, seventh, ninth, and tenth cranial nerves. (2) Axones of cells of the sensory nucleus enter the posterior cerebellar peduncle of the same side and reach the cerebellar cortex. (3) Collaterals are distributed to the nuclei of origin of the hypoglossal and of the motor part of the trigeminal and facial nerves.

The =motor root= (Portio minor) extends forward beneath the sensory root and the semilunar ganglion and is incorporated with the mandibular division of the nerve. Its fibers arise chiefly from the so-called masticatory nucleus, which is situated in the pons near the inner face of the sensory nucleus; a few of these fibers come from the nucleus of the opposite side and cross in the raphé. Other fibers, which constitute the mesencephalic root, arise from cells in the outer part of the central gray matter of the mid-brain.

It is evident from the foregoing statements that the trigeminus has essentially the same arrangement as a typical spinal nerve. It divides into three branches.

I. The =ophthalmic nerve= (N. ophthalmicus) (Figs. 438, 439, 440) is purely sensory and is the smallest of the three branches of the trigeminus. It arises from the inner part of the front of the semilunar ganglion, passes forward along the outer side of the cavernous sinus, and is blended with the maxillary nerve for some distance. It enters the foramen lacerum orbitale with the third and sixth nerves and divides into three branches.

1. The =lacrimal nerve= (N. lacrimalis) runs forward on the rectus superior and the levator palpebræ superioris and ramifies chiefly in the lacrimal gland and the upper eyelid. A branch (Ramus zygomatico-temporalis) exchanges twigs with the zygomatic branch of the maxillary nerve, perforates the periorbita, and emerges from the orbital fossa behind the supraorbital process; it forms a plexus with branches of the auriculo-palpebral and frontal nerves, and ramifies in the skin of the temporal region.

2. The =frontal nerve= (N. frontalis), also termed the supraorbital, runs forward almost parallel with the superior oblique muscle, at first within, then outside of, the periorbita. It passes through the supraorbital foramen with the artery of like name and ramifies in the skin of the forehead and upper eyelid, forming a plexus with the lacrimal and auriculo-palpebral nerves. It divides into three branches.

3. The =naso-ciliary nerve= (N. naso-ciliaris), also termed the palpebro-nasal, runs forward along the inner side of the optic nerve between parts of the retractor muscle and divides into two branches. Of these the =ethmoidal nerve= (N. ethmoidalis) is the continuation of the parent trunk. It accompanies the ethmoidal artery through the foramen of like name into the cranial cavity and crosses the lower part of the ethmoidal fossa. Leaving the cranium through an opening in the cribriform plate close to the crista galli, it enters the nasal cavity and ramifies in the mucous membrane of the septum nasi and the superior turbinal. The =infratrochlear nerve= (N. infratrochlearis) runs forward to the inner canthus and ramifies in the skin in this region; it detaches twigs to the conjunctiva and caruncula lacrimalis, and a long branch which supplies the third eyelid and the lacrimal ducts and sac. The naso-ciliary nerve furnishes the =sensory= or =long root= (Radix longa) of the ciliary ganglion.

The =ciliary ganglion= is placed on the inferior branch of the oculomotor nerve. It is usually not larger than a millet-seed, and is best found by following the nerve to the inferior oblique muscle back to its origin. The ganglion receives (a) =sensory fibers= from the naso-ciliary nerve; (b) =motor fibers= from the oculomotor nerve; and (c) =sympathetic fibers= from the sphenopalatine plexus. It detaches filaments which unite with twigs from the ophthalmic and maxillary nerves and from the sphenopalatine ganglion to form the ciliary plexus. From the latter emanate five to eight delicate =short ciliary nerves= (Nn. ciliares breves), which pursue a somewhat flexuous course along the optic nerve, pierce the sclera near the entrance of that nerve, and run forward between the sclera and chorioidea to the circumference of the iris. Here the branches of adjacent nerves anastomose to form a circular plexus (Plexus gangliosus ciliaris), from which filaments go to the ciliary body, iris, and cornea.

The circular fibers of the iris and the ciliary muscle are innervated by fibers derived from the oculomotor nerve, the radial fibers of the iris by the sympathetic.

II. The =maxillary nerve= (N. maxillaris), also termed the superior maxillary, is purely sensory and is much larger than the ophthalmic. It extends forward from the semilunar ganglion in the middle cranial fossa in the large groove on the root of the temporal wing of the sphenoid. It is related internally to the cavernous sinus and superiorly to the ophthalmic nerve, with which it is blended for some distance. It emerges through the foramen rotundum, passes forward in the pterygo-palatine fossa above the internal maxillary artery and embedded in fat, and is continued in the infraorbital canal as the infraorbital nerve (Fig. 439). Its branches are as follows:

1. The =zygomatic nerve= (N. zygomaticus s. subcutaneus malæ), also termed the orbital branch, arises before the maxillary nerve reaches the pterygo-palatine fossa (Figs. 438, 439, 514). It pierces the periorbita and divides into two or three delicate branches which pass along the surface of the external straight muscle to the external canthus and ramify chiefly in the lower lid and the adjacent skin. Anastomoses are formed with branches of the lacrimal nerve.

2. The =sphenopalatine nerve= (N. sphenopalatinus) is given off in the pterygo-palatine fossa from the lower border of the maxillary nerve (Fig. 439). It is broad and flat and forms a plexus in which several small =sphenopalatine ganglia= are interposed. It divides into posterior nasal, and greater and lesser palatine nerves. (1) The =posterior nasal nerve= (N. nasalis aboralis) passes through the sphenopalatine foramen, in which it bears one or more minute ganglia, enters the nasal cavity, and divides into internal and external branches (Figs. 440, 464). The internal branch (N. septi narium) runs forward in the submucous tissue of the lower part of the septum nasi, gives twigs to the mucous membrane here and to the vomero-nasal organ (of Jacobson), passes through the palatine cleft and ramifies in the anterior part of the hard palate. The external branch (Ramus lateralis) ramifies in the mucous membrane of the inferior turbinal and the middle and inferior meatus nasi. (2) The =greater= or =anterior palatine nerve= (N. palatinus major s. anterior) (Figs. 438, 439) is the largest of the three branches. It runs forward in the palatine canal and groove and ramifies in the hard palate and gums. It also supplies twigs to the soft palate, and gives off branches which pass through the accessory palatine foramina to supply the mucous membrane of the inferior meatus.

The branches of the two nerves anastomose in the hard palate and form a plexus about the branches of the palatine arteries.

(3) The =lesser= or =posterior palatine nerve= (N. palatinus minor s. posterior), also termed the staphyline, is much the smallest of the three branches (Figs. 438, 439). It passes downward and forward with the palatine vein in the groove at the inner side of the tuber maxillare and ramifies in the soft palate.

The sphenopalatine ganglia and plexus (Fig. 515) lie on the perpendicular part of the palate bone and the pterygoid process under cover of the maxillary nerve. The afferent fibers of the plexus and ganglia come chiefly from the branches of the sphenopalatine nerve and the nerve of the pterygoid canal. Interspersed in these are several minute ganglia and one or more larger ones. The =nerve of the pterygoid canal= or =Vidian nerve= (N. canalis pterygoidei) is formed by the union of sympathetic fibers with the superficial petrosal branch of the facial nerve. It passes forward at first between the Eustachian tube and the sphenoid bone, enters the canal between the pterygoid bone and process, and joins the posterior part of the plexus. It is probable that it furnishes the motor fibers to the levator palati and palatinus muscles. Efferent filaments go to the periorbita and the ophthalmic vessels, and others accompany the branches of the maxillary nerve, around which they have a plexiform arrangement.

3. The =infraorbital nerve= (N. infraorbitalis) is the continuation of the maxillary trunk. It traverses the infraorbital canal, emerges through the infraorbital foramen, and divides into nasal and superior labial branches. Along its course it gives off =superior alveolar= or =dental branches= (Rami alveolares maxillæ), which supply the teeth, alveolar periosteum, and gums.

The delicate posterior alveolar branches are given off in the pterygo-palatine fossa, pass through small foramina in the tuber maxillare, and supply the posterior molar teeth and the maxillary sinus. The middle alveolar branches are given off in the infraorbital canal, and constitute the chief nerve-supply to the cheek teeth and the maxillary sinus. The anterior or incisor branch runs forward in the anterior alveolar or incisor canal and supplies branches to the canine and incisor teeth. The foregoing unite with each other to form the superior dental plexus, from which the dental and gingival branches are given off.

The =external nasal branches= (Rami nasales externi), two or three in number, accompany the levator labii superioris proprius and ramify in the dorsum nasi and the nasal diverticulum.

The large =anterior nasal branch= (Ramus nasalis anterior) passes over the nasal process of the premaxilla under cover of the dilatator naris inferior, gives branches to the nasal mucous membrane, and terminates in the skin of the upper lip.

The =superior labial branch= (Ramus labialis dorsalis) is the largest of the terminals of the infraorbital nerve. It passes downward and forward under the levator nasolabialis and, after supplying the skin of the anterior part of the cheek, forms a rich terminal ramification in the skin and mucous membrane of the upper lip. It anastomoses with the superior buccal branch of the facial nerve.

III. The =mandibular nerve= (N. mandibularis), also termed the inferior maxillary branch, is formed by the union of two roots; of these the large sensory root comes from the semilunar ganglion, and the small motor root is the pars minor of the trigeminus. It emerges from the cranium through the oval notch of the foramen lacerum, between the temporal wing of the sphenoid bone and the muscular process of the petrous temporal, and gives off the following branches:

1. The =masseteric nerve= (N. massetericus) (Figs. 437, 515) passes outward through the sigmoid notch of the mandible and enters the deep face of the masseter muscle, in which it ramifies.

2. The =deep temporal nerves= (Nn. temporales profundi) (Fig. 437), two or three in number, arise by a common trunk with the masseteric. They supply the temporal muscle.

3. The =buccinator nerve= (N. buccinatorius) passes obliquely forward through the anterior part of the external pterygoid muscle, then between the internal pterygoid and the tuber maxillare (Fig. 514). It continues forward in the submucous tissue of the cheek along the lower border of the buccinator and divides into branches which ramify in the mucous membrane and glands of the lips in the vicinity of the commissure. It supplies small branches to the external pterygoid and temporal muscles and detaches numerous collateral twigs to the mucous membrane of the cheek and to the buccal glands. It also communicates with the inferior buccal branch of the facial nerve.

4. The =pterygoid nerve= (N. pterygoideus) arises in common with the preceding, passes forward on the guttural pouch, and divides into branches for the pterygoid muscles (Fig. 515).

The =otic ganglion= (G. oticum) is situated near the origin of the pterygoid and buccinator nerves, and is related internally to the tensor palati and the Eustachian tube. It receives motor fibers from the pterygoid nerve and sensory fibers by the small superficial petrosal nerve from the tympanic plexus, through which communications are made with the facial and glosso-pharyngeal nerves. Sympathetic fibers are derived from the plexus on the internal maxillary artery. Efferent filaments go to the tensor palati, tensor tympani, and pterygoid muscles, and to the Eustachian tube.

The ganglion is small and somewhat difficult to demonstrate. In many cases it is replaced by a number of minute ganglia interspersed in a fine plexus.

5. The =superficial temporal nerve= (N. temporalis superficialis) (Figs. 437, 514, 515) runs outward across the pterygoideus externus, passes between the parotid gland and the neck of the ramus of the mandible, turns around the latter, and divides into two branches. The upper branch (Ramus transversus faciei) accompanies the transverse facial vessels and ramifies in the skin of the cheek. The larger inferior branch unites with the inferior buccal division of the facial nerve.

Before its division the nerve gives off twigs to the guttural pouch, the parotid gland, the external ear, and the skin of the external auditory meatus and the membrana tympani. Branches from it concur with filaments from the cervical branch of the facial nerve in the formation of the auricular plexus.

6. The =inferior alveolar= or =dental nerve= (N. alveolaris mandibulæ) (Figs. 436, 437, 514) arises with the lingual by a common trunk which passes forward at first on the external pterygoid muscle, then inclines ventrally between the internal pterygoid and the ramus of the mandible. The lingual and alveolar separate at an acute angle, and the latter enters the mandibular foramen and courses in the canal within the ramus (Fig. 516). Emerging at the mental foramen, it terminates by dividing into six to eight inferior labial and mental branches, which ramify in the lower lip and chin. Before entering the bone, the nerve detaches the =mylo-hyoid nerve= (N. mylo-hyoideus), which runs downward and forward between the ramus and the mylo-hyoid muscle; it supplies that muscle, the anterior belly of the digastricus, and the skin of the anterior part of the submaxillary space. The dental and gingival branches detached from the nerve within the mandible are arranged like the corresponding nerves of the upper jaw.

7. The =lingual nerve= (N. lingualis) arises by a common trunk with the inferior alveolar or dental nerve (Figs. 436, 437). After separating from the latter it runs downward and forward, lying at first between the ramus of the mandible and the internal pterygoid muscle, then on the inner face of the mylo-hyoid. On reaching the root of the tongue it divides into superficial and deep branches. The =superficial branch= (Ramus superficialis) runs forward on the stylo-glossus and accompanies the submaxillary duct on the deep face of the sublingual gland. It supplies the mucous membrane of the tongue and the floor of the mouth. At the root of the tongue it gives off a recurrent branch to the isthmus faucium, which communicates with the lingual branch of the glosso-pharyngeal nerve. The larger =deep branch= (Ramus profundus) turns around the lower edge of the hyo-glossus, passes upward and forward between that muscle and the genio-glossus, and continues forward on the latter to the tip of the tongue. It gives branches to the mucous membrane and the fungiform papillæ of the tongue, and anastomoses with branches of the hypoglossal nerve and with the superficial branch. Minute ganglia occur on the finer branches of the lingual nerve. The =chorda tympani= branch of the facial joins the lingual nerve at the origin of the latter and is incorporated with it in its distribution to the tongue.

THE SIXTH OR ABDUCENT NERVE

The =abducent nerve= (N. abducens) emerges from the brain behind the pons and just external to the pyramid (Fig. 499). It passes forward across the pons, pierces the dura mater, and accompanies the third and ophthalmic nerves, below which it emerges through the foramen lacerum orbitale. In the orbit it divides into two branches; the larger of these enters the external rectus, the smaller supplies the superior and external parts of the retractor muscle of the eyeball. Within the cranium it receives filaments from the carotid plexus of the sympathetic.

The fibers of the abducent nerve are axones of the large multipolar cells of the abducent nucleus which is situated beneath the eminentia teres of the floor of the fourth ventricle. The nucleus lies within the loop formed by the fibers of origin of the facial nerve. It is connected with (a) the anterior olive; (b) the oculomotor nucleus of the opposite side; (c) the motor area of the cerebral cortex by means of the pyramidal tract of the opposite side.

THE SEVENTH OR FACIAL NERVE

The =facial nerve= (N. facialis) has its superficial origin at the lateral part of the corpus trapezoideum, immediately behind the pons (Fig. 499). It passes outward in front of the eighth nerve and enters the internal auditory meatus. At the bottom of the meatus the two nerves part company, the facial coursing in the facial canal of the petrous temporal bone. The canal and nerve are at first directed outward between the vestibule and the cochlea, then curve backward and downward in the posterior wall of the tympanum to end at the stylo-mastoid foramen. The bend formed by the nerve is called the =knee= (Geniculum n. facialis) and bears at its highest point the round =geniculate ganglion= (G. geniculi).

The nerve consists of two parts, motor and sensory. The =motor part= constitutes the bulk of the nerve. Its deep origin is from the cells of the =facial nucleus=, which is situated in the medulla above the facial tubercle. On leaving the nucleus the root-fibers pass upward and inward, incline forward close to the median plane, and then bend sharply downward to the point of emergence. The highest point of the bend is subjacent to the gray matter of the eminentia teres in the floor of the fourth ventricle, and the abducent nucleus lies in the concavity of the curve. The small =sensory part= (N. intermedius) consists of axones of cells of the geniculate ganglion, which is interposed on the facial nerve as it bends downward in the facial canal. The fibers of this part after entering the medulla, pass to the nucleus of termination which it shares with the ninth and tenth nerves. The peripheral fibers from the geniculate ganglion constitute the chorda tympani.

After its emergence through the stylo-mastoid foramen (Fig. 514) the nerve passes downward, forward, and outward on the guttural pouch under cover of the parotid gland, and crosses between the origin of the superficial temporal and internal maxillary arteries internally and the superficial temporal vein externally. It then crosses the posterior border of the ramus of the mandible ventral to the transverse facial artery and about an inch and a half (ca. 3.5 to 4 cm.) below the articulation of the jaw. Emerging from beneath the parotid gland upon the masseter muscle, it receives the lower branch of the superficial temporal nerve, and divides into superior and inferior buccal branches. The following collateral branches are given off, the first five being detached within the facial canal, and the others between the stylo-mastoid foramen and the border of the jaw.

1. The =great superficial petrosal nerve= (N. petrosus superficialis major) arises from the geniculate ganglion.[203] It passes through the petrosal canal, contributes a filament to the tympanic plexus, receives the great deep petrosal nerve from the carotid plexus of the sympathetic, emerges through the foramen lacerum, and is continued as the Vidian nerve to the sphenopalatine plexus and ganglia (Fig. 515).

2. A delicate branch (R. anastomoticus cum plexu tympanico) emerges from the geniculate ganglion and unites with a filament issuing from the tympanic plexus to form the =small superficial petrosal nerve= (N. petrosus superficialis minor); this ends in the otic ganglion.

3. The =stapedial nerve= (N. stapedius) (Fig. 515) is a short filament detached from the facial nerve as it turns down in the facial canal. It innervates the stapedius muscle.

4. The =chorda tympani= (Fig. 515) is a small nerve which arises a little below the preceding and pursues a recurrent course in a small canal in the mastoid part of the temporal bone to reach the tympanic cavity. It traverses the latter, passing between the handle of the malleus and the long branch of the incus. Emerging through the petro-tympanic fissure, the nerve passes downward and forward, crosses beneath the internal maxillary artery, and joins the lingual nerve. It sends twigs to the submaxillary and sublingual glands, and through its incorporation with the lingual nerve furnishes fibers to the mucous membrane of the anterior two-thirds of the tongue which are believed to mediate the sense of taste.

5. Anastomotic filaments unite with the auricular branch of the vagus near the stylo-mastoid foramen.

6. The =posterior auricular nerve= (N. auricularis posterior) arises from the facial at its emergence from the facial canal (Fig. 514). It runs upward and backward with the posterior auricular artery under cover of the parotid gland and supplies the posterior auricular muscles and the skin of the convex surface of the external ear. It anastomoses with branches of the first and second cervical nerves.

7. The =internal auricular nerve= (N. auricularis internus) springs from the facial close to or in common with the preceding (Fig. 514). It ascends in the parotid gland just behind the styloid process of the conchal cartilage, passes through an opening in the cartilage, and ramifies in the skin of the concave surface of the ear.

8. The =digastric branch= (R. digastricus) (Fig. 514) arises from the facial below the auricular nerves. Its branches innervate the posterior belly of the digastricus, the stylo-hyoideus, and the occipito-hyoideus. At its origin it gives off a small branch which forms a loop around the great auricular artery or its posterior branch and rejoins the trunk.

9. The =auriculo-palpebral nerve= (N. auriculo-palpebralis) (Fig. 514) arises from the upper edge of the facial near the posterior border of the ramus. It ascends in the parotid gland behind the superficial temporal artery, and terminates in anterior auricular and temporal branches. The =anterior auricular branches= form with branches of the trigeminus the =anterior auricular plexus=. They innervate the anterior auricular and parotido-auricularis muscles. The =temporal branch= runs forward and inward over the temporal muscle to the inner canthus of the eye, forms a plexus with the terminal branches of the ophthalmic nerve, and is distributed to the orbicularis oculi, corrugator supercilii, and levator nasolabialis.

10. The =cervical branch= (R. colli) (Fig. 435) arises from the ventral border of the facial opposite to the preceding nerve. It emerges obliquely through the parotid gland, passes downward and backward on or near the jugular vein, and anastomoses with the cutaneous branches of the cervical nerves. It gives branches to the parotido-auricularis and the cervical panniculus. In its course along the neck the nerve is reinforced by twigs from the cutaneous branches of the second to the sixth cervical nerves.

11. Small branches are detached to the guttural pouch and the parotid gland. The latter (Rami parotidei) concur with branches of the superficial temporal nerve in forming the =parotid plexus=.

The facial nerve usually terminates after a short course on the surface of the masseter by dividing into two buccal branches (Figs. 435, 461).

1. The =superior buccal nerve= (N. buccalis dorsalis) passes forward on the upper part of the masseter, dips under the zygomaticus, and continues along the lower border of the dilatator naris lateralis. It then runs under the last-named muscle and anastomoses with branches of the infraorbital nerve, and is distributed to the muscles of the cheek, upper lip, and nostril.

2. The =inferior buccal nerve= (N. buccalis ventralis) crosses the masseter obliquely and continues forward along the depressor labii inferioris. It is connected by variable anastomotic branches with the superior nerve. It gives collateral branches to the panniculus, buccinator, and depressor labii inferioris, and ramifies with the terminal branches of the inferior alveolar nerve in the lower lip.

The buccal nerves are subject to much variation in regard to their course, anastomoses, and relations to the sensory components derived from the superficial temporal nerve. Their distribution is constant. The point at which the branch of the superficial temporal nerve joins the facial is variable.

THE EIGHTH OR AUDITORY NERVE

The =auditory nerve= (N. acusticus) is connected with the lateral aspect of the medulla just behind and external to the facial (Fig. 499). It has two roots, vestibular and cochlear (Radix vestibularis et cochlearis).

The auditory nerve consists of two distinct parts which might well be regarded as separate nerves. The cochlear part mediates the sense of hearing, while the vestibular part is not auditory in function, but is concerned in the sense of the position of the body and the mechanism of equilibration.

The nerve passes outward to the internal auditory meatus, which it enters behind the facial nerve. In the meatus it divides into two nerves, of which the upper is the vestibular and the lower is the cochlear nerve.

1. The =vestibular nerve= (N. vestibuli) is distributed to the utriculus, the sacculus, and to the ampullæ of the semicircular canals, of the internal ear. In the internal auditory meatus the nerve is connected by filaments with the geniculate ganglion of the facial nerve. At the bottom of the meatus it bears the =vestibular ganglion= (G. vestibulare), from the cells of which the fibers of the nerve arise.

2. The =cochlear nerve= (N. cochleæ) detaches a filament to the sacculus, passes through the lamina cribrosa to the labyrinth, and is distributed to the organ of Corti in the cochlea.

The fibers of the vestibular nerve arise from the vestibular ganglion as central processes (axones) of the bipolar cells of the ganglion. The peripheral processes (dendrites) of the cells form arborizations about the deep ends of the hair cells of the maculæ and cristæ acusticæ of the utriculus, sacculus, and semicircular canals. The fibers enter the medulla, pass between the restiform body and the spinal tract of the trigeminus, and spread out to end in the vestibular nucleus in the floor of the fourth ventricle. Among the central connections of the vestibular nerve are: (1) fibers which connect its nucleus with centers in the cerebellum (chiefly of the opposite side); (2) the vestibulo-spinal tract, which conveys impulses to the motor cells of the ventral columns of the spinal cord; (3) fibers which connect the nucleus with those of the abducent nerve of the same side, the third and fourth nerves, and the motor part of the trigeminus of both sides.

The fibers of the cochlear nerve are the central processes of the bipolar cells of the spiral ganglion of the cochlea. The peripheral processes of these cells end in relation to the hair cells of the organ of Corti. Some of the nerve-fibers enter the ventral cochlear nucleus in the medulla close to the superficial origin of the nerve; others end in the dorsal nucleus of the tuberculum acusticum at the lateral angle of the floor of the fourth ventricle. From the ventral nucleus fibers pass in the corpus trapezoideum to the anterior olivary nucleus of the same and of the opposite side. Thence tracts pass to the nuclei of the motor nerves of the eye, and through the lateral fillet to the posterior quadrigeminal body and the internal geniculate body. The axones of the cells of the dorsal nucleus pass largely (as the striæ acusticæ) over the restiform body and across the floor of the fourth ventricle toward the median plane. They then turn ventrally, cross to the opposite side, and are continued by the lateral fillet. From the mid-brain a tract proceeds to the cortex of the temporal lobe of the hemisphere.

THE NINTH OR GLOSSO-PHARYNGEAL NERVE

The =glosso-pharyngeal nerve= (N. glossopharyngeus) is attached to the anterior part of the lateral aspect of the medulla by several filaments (Fig. 499). The root-bundles enter the furrow ventral to the restiform body; they are separated by a short interval from the origin of the facial nerve, but are not marked off behind from the roots of the vagus. The bundles converge laterally to form a nerve which perforates the dura mater and emerges through the foramen lacerum posterius just in front of the tenth nerve (Fig. 515). As it issues from the cranium the nerve bears a considerable ovoid gray enlargement, the =petrous ganglion= (G. petrosum).[204] It then curves downward and forward over the guttural pouch and behind the great cornu of the hyoid bone, crosses the deep face of the external carotid artery, and divides into pharyngeal and lingual branches (Fig. 437). The collateral branches are as follows:

1. The =tympanic nerve= (N. tympanicus) (Fig. 515) arises from the petrous ganglion and passes upward between the petrous and tympanic parts of the temporal bone to reach the cavity of the tympanum. Here it breaks up into branches to form, along with branches from the carotid plexus of the sympathetic, the =tympanic plexus=. From the plexus branches pass to the mucous membrane of the tympanum and the Eustachian tube. The continuation of the nerve issues from the plexus and unites with a filament from the geniculate ganglion of the facial to form the small superficial petrosal nerve; this runs forward and ends in the otic ganglion.

Filaments also connect the petrous ganglion with the jugular ganglion of the vagus nerve and with the superior cervical ganglion of the sympathetic.

2. A considerable branch runs backward on the guttural pouch, contributes filaments to the pharyngeal plexus, and concurs with twigs from the vagus and the sympathetic in forming the carotid plexus on the terminal part of the carotid artery and on its chief branches. In this plexus is the small ganglion intercaroticum.

The =pharyngeal branch= (R. pharyngeus) (Fig. 437) is the smaller of the two terminal branches. It runs forward across the deep face of the great cornu of the hyoid bone and concurs with the pharyngeal branches of the vagus and with sympathetic filaments in forming the pharyngeal plexus; from this branches pass to the muscles and mucous membrane of the pharynx.

The =lingual branch= (R. lingualis) is the continuation of the trunk (Fig. 437). It runs along the posterior border of the great cornu of the hyoid bone in front of the external maxillary artery and dips under the hyo-glossus muscle. It gives collateral branches to the soft palate, isthmus faucium, and tonsil, and ends in the mucous membrane of the posterior part of the tongue, where it supplies gustatory fibers to the vallate papillæ. A considerable branch unites with a twig from the lingual nerve.

The glosso-pharyngeal is a mixed nerve, containing both motor and sensory fibers. The latter constitute the bulk of the nerve and include those which mediate the special sense of taste. They are processes of the cells of the petrous ganglion. The central processes of the ganglion cells enter the medulla, pass upward and inward through the formatio reticularis, and end in the nucleus of termination in the floor of the fourth ventricle. The motor fibers arise from dorsal and ventral efferent nuclei in the medulla. The glosso-pharyngeal shares these nuclei with the vagus and has practically the same central connections as that nerve (q. v.).

THE TENTH, VAGUS, OR PNEUMOGASTRIC NERVE

The =vagus= or =pneumogastric= (N. vagus) is the longest and most widely distributed of the cranial nerves; it is also remarkable for the connections which it forms with adjacent nerves and with the sympathetic. It is attached to the lateral aspect of the medulla by several filaments which are in series with those of the ninth nerve in front and the eleventh nerve behind (Fig. 499). The bundles converge to form a trunk which passes outward, pierces the dura mater, and emerges from the cranium through the foramen lacerum posterius (Fig. 515). In the foramen the nerve bears on its lateral aspect the elongated flattened =jugular ganglion= (G. jugulare).

The ganglion communicates with (a) the tympanic nerve, (b) the petrous ganglion of the ninth nerve, (c) the spinal accessory, and (d) the hypoglossal. It also gives off the =auricular branch= (R. auricularis), which runs forward below the petrous ganglion and passes through a small canal in the petrous temporal bone to gain the facial canal. Here it gives filaments to the facial and emerges with that nerve through the stylo-mastoid foramen. It ascends behind the external auditory meatus, dips under the rotator longus muscle, and passes through a foramen in the conchal cartilage to ramify in the integument which lines the meatus and the adjacent part of the ear.

Beyond the ganglion the vagus runs backward and downward with the spinal accessory in a fold of the guttural pouch (Fig. 437). Then the two nerves separate, allowing the hypoglossal to pass between them, and the vagus descends with the internal carotid artery and crosses the inner face of the origin of the occipital artery. Here it is joined by the cervical trunk of the sympathetic, and the two nerves continue along the dorsal aspect of the common carotid artery in a common sheath (Fig. 433). At the root of the neck the vagus separates from the sympathetic, and from this point backward the relations of the right and left vagi differ somewhat and must be described separately.

The =right vagus= (Fig. 429) enters the thorax in the angle of divergence of the right brachial artery and the truncus bicaroticus. It then passes backward and slightly upward, crossing obliquely the outer surface of the brachiocephalic artery and the right face of the trachea. Reaching the dorsal surface of the latter near the bifurcation, it divides into dorsal and ventral branches.

The =left vagus= (Fig. 428) enters the thorax on the ventral face of the œsophagus, crosses obliquely under the left brachial artery, and passes back on the external surface of that vessel in company with a large cardiac nerve.[205] Separating from the latter, the vagus continues backward on the left face of the aorta, inclines to the upper surface of the left bronchus, and divides into dorsal and ventral branches.

The dorsal and ventral branches unite with the corresponding branches of the opposite nerve, thus forming =dorsal= and =ventral œsophageal trunks= (Truncus (œsophageus dorsalis, ventralis). These run backward in the posterior mediastinum, above and below the œsophagus respectively, and enter the abdominal cavity through the hiatus œsophageus; they supply branches to the œsophagus and anastomose with each other. The dorsal trunk receives the major part of its fibers from the right vagus. After entering the abdomen it passes to the left of the cardia, gives branches to the visceral surface of the stomach, and ends in the cœliac and subsidiary plexuses. The ventral trunk passes to the lesser curvature of the stomach and ramifies on the parietal surface of the stomach; it forms here the anterior gastric plexus, from which branches are supplied also to the first part of the duodenum and to the liver.

The collateral branches of the vagus are as follows:

1. The =pharyngeal branch= (R. pharyngeus) is given off in relation to the superior cervical ganglion, turns around the internal carotid artery, and runs downward and forward on the guttural pouch to the dorsal wall of the pharynx (Fig. 437). Here its branches concur with the pharyngeal branch of the ninth nerve and with filaments from the spinal accessory and the sympathetic in forming the =pharyngeal plexus=. This supplies numerous twigs to the pharynx, and a larger branch which passes along the side of the œsophagus and ramifies in its cervical part.

According to Ellenberger and Baum the pharyngeal plexus receives filaments also from the digastric, superior laryngeal, hypoglossal, and first cervical nerves. The branches of the plexus form secondary intermuscular and submucous plexuses, in which there are numerous minute ganglia.

2. The =superior= or =anterior laryngeal nerve= (N. laryngeus cranialis) is larger than the preceding and arises a little behind it (Fig. 437). It crosses the deep face of the origin of the external carotid artery, runs downward and forward over the lateral wall of the pharynx behind the hypoglossal nerve, and passes through the foramen below the anterior cornu of the thyroid cartilage. Its terminal branches ramify in the mucous membrane of the larynx, the floor of the pharynx, and the entrance to the œsophagus; they anastomose with those of the recurrent. At its origin the nerve gives off its small =external branch= (R. externus); this descends to the crico-thyroid muscle, which it supplies, and sends filaments to the crico-pharyngeus also. It may arise from the trunk of the vagus or from the pharyngeal branch.

At the point of origin of the superior laryngeal nerve there is a plexiform widening which is regarded by some authors as the homologue of the ganglion nodosum of man. From it a filament arises which, after a short course, rejoins the vagus or enters the sympathetic trunk. Stimulation of its central end causes a reduction of the blood-pressure, and it is therefore termed the depressor nerve (N. depressor).

3. The =recurrent nerve= (N. recurrens), also termed the inferior or posterior laryngeal nerve, differs on the two sides in its point of origin and in the first part of its course. The =right nerve= (Fig. 429) is given off opposite the second rib, turns around the dorso-cervical artery from without inward, runs forward on the lower part of the lateral surface of the trachea, and ascends in the neck on the ventral face of the common carotid artery. The =left nerve= (Fig. 428) arises from the vagus where the latter begins to cross the aortic arch. It passes back over the ligamentum arteriosum, winds around the concavity of the aortic arch from without inward, runs forward on the lower part of the left face of the trachea, and continues in the neck in a similar position to the right nerve.

It is worthy of note that the left nerve passes beneath the bronchial lymph glands as it winds around the aorta; also that in the next part of its course it lies between the left surface of the trachea and the deep face of the aorta, and is then related to lymph glands which lie along the ventral aspect of the trachea. The left recurrent is often incorporated in part of its course in the anterior mediastinum with a deep cardiac nerve. Further, the left nerve lies at first ventral to, and then upon, the œsophagus in the neck. The right recurrent is given off from or in common with a considerable trunk which connects the vagus with the first thoracic ganglion of the sympathetic. The arrangement here is commonly more or less plexiform, and from it one or two cardiac nerves arise.

The terminal part of each nerve (Fig. 517) lies on the dorsal surface of the trachea, in relation to the œsophagus internally and the carotid artery above.[206] It passes between the crico-arytenoideus posterior and the crico-pharyngeus; the =terminal branches= supply all the muscles of the larynx except the crico-thyroid, and communicate with branches of the superior laryngeal nerve. =Collateral branches= are given off to the cardiac plexus (Rr. cardiaci), to the trachea (Rr. tracheales), to the œsophagus (Rr. œsophagei), and to the inferior cervical ganglion of the sympathetic.

4. =Cardiac branches= (Rr. cardiaci), usually two or three in number, are given off from each vagus within the thorax (Figs. 428, 429). These concur with the cardiac branches of the sympathetic and recurrent nerves to form the cardiac plexus, which innervates the heart and great vessels.

5. Small =tracheal= and =œsophageal branches= (Rr. tracheales et œsophagei) are given off from both vagi in the thorax. These concur with branches from the recurrent nerves and the inferior cervical and anterior thoracic ganglia of the sympathetic in forming the =posterior tracheal= and =œsophageal plexuses=, from which twigs go to the trachea, œsophagus, heart, and large vessels.

6. =Bronchial branches= (Rr. bronchiales) are detached at the roots of the lungs and unite with sympathetic filaments in forming the =pulmonary plexuses=. From the latter numerous branches proceed in a plexiform manner along the bronchi and vessels into the substance of the lungs.

The vagus and glosso-pharyngeal nerves are so closely associated in origin and central connections that they may be described together in this respect.

The =sensory fibers= arise from the petrous and jugular ganglia, and their central parts enter the lateral aspect of the medulla and divide into anterior and posterior branches like the fibers of the dorsal roots of the spinal nerves. Most of the fibers end in arborizations about the cells of the vago-glosso-pharyngeal nucleus of termination, which consists of two parts. Of these the =dorsal sensory nucleus= (Nucleus alæ cinereæ) is situated in the posterior part of the floor of the fourth ventricle and in the adjacent part of the closed portion of the medulla near the median plane. The other part is termed the =nucleus of the solitary tract=, and is so named because its cells are grouped about the bundle (Tractus solitarius) formed by the posterior divisions of the afferent nerve-fibers. It ends about the level of the pyramidal decussation. The secondary central connections are similar to those of the sensory part of the trigeminus.

The =motor fibers= (and those of the medullary part of the accessory) arise from the =dorsal motor nucleus= and the =ventral motor nucleus=. The cells of the former lie in groups along the ventro-medial side of the dorsal sensory nucleus. The latter, also termed the nucleus ambiguus, is situated more deeply in the lateral part of the formatio reticularis.

THE ELEVENTH OR SPINAL ACCESSORY NERVE

The =spinal accessory nerve= (N. accessorius) is purely motor. It consists of two parts which differ in origin and function.

The =medullary part= arises from the lateral aspect of the medulla by several rootlets which are behind and in series with those of the vagus (Fig. 499). The =spinal part= arises from the cervical part of the spinal cord by a series of fasciculi which emerge between the dorsal and ventral roots. The bundles unite to form a trunk which is very small at its origin at the fifth segment of the cord, but increases in size when traced toward the brain, since it continually receives accessions of fibers. It passes through the foramen magnum and joins the medullary part. The trunk thus formed sends its medullary fibers to the tenth and ninth nerves and emerges through the foramen lacerum posterius. It then runs backward and downward with the vagus in a fold of the guttural pouch, separates from that nerve, crosses the deep face of the submaxillary gland and the occipital artery, and divides in the recessus atlantis into dorsal and ventral branches.

It is connected by anastomotic branches with the vagus and hypoglossal nerves and the superior cervical ganglion of the sympathetic, and contributes a branch to the pharyngeal plexus.

The =dorsal branch= (R. dorsalis) (Figs. 433, 517, 518) receives a twig from the second and third cervical nerves and turns around the atlantal tendon of the splenius under cover of the mastoido-humeralis. It then passes obliquely through the latter muscle and continues backward on the splenius and the cervical part of the serratus, inclines upward across the anterior deep pectoral and the supraspinatus, and enters the deep face of the trapezius, in which it ramifies.

The =ventral branch= (R. ventralis) (Fig. 436) is smaller. It enters the sterno-cephalicus muscle behind the cervical angle of the parotid gland.

The fibers of the spinal part of the accessory arise from the ventro-lateral cells of the ventral gray column of the cord as far back as the fifth cervical segment. The fibers of the medullary part come chiefly from the nucleus ambiguus in common with the motor fibers of the vagus.

THE TWELFTH OR HYPOGLOSSAL NERVE

The =hypoglossal nerve= (N. hypoglossus) is purely motor and innervates the muscles of the tongue (Fig. 437). Its root-fibers arise from the ventral face of the medulla in linear series about 3 to 4 mm. lateral to the posterior half of the pyramid (Fig. 499). The filaments converge to three or four bundles which perforate the dura mater and unite to form the trunk. The latter emerges through the hypoglossal foramen (Fig. 515) and runs downward and backward between the guttural pouch and the capsule of the atlanto-occipital articulation for a distance of a little less than an inch (ca. 2 cm.). It then passes between the tenth and eleventh nerves, turns downward and forward, crosses the external face of the external carotid artery, and continues over the pharynx parallel with the great cornu of the hyoid bone and behind the external maxillary artery. It then crosses beneath the artery, runs forward on the external face of the hyo-glossus muscle, and divides into its terminal branches (Rami linguales). The smaller branch supplies the stylo-glossus, hyo-glossus, and lingualis. The larger branch ramifies on the genio-glossus and supplies the remaining muscles. Anastomoses occur with branches of the lingual nerve.

In the first part of its course the nerve communicates with the superior cervical ganglion and with the ventral branch of the first cervical nerve, and gives filaments to the pharyngeal branch of the vagus and the pharyngeal plexus.

The fibers of the nerve arise from the hypoglossal nucleus, an elongated group of large multipolar cells situated chiefly under the posterior part of the floor of the fourth ventricle close to the median plane. The two nuclei are connected by commissural fibers. The other central connections include: (a) communications by the medial longitudinal fasciculus with the nuclei of termination of other cranial nerves; (b) cortico-nuclear fibers which come from the cortex by way of the internal capsule and the pyramids and go largely to the nucleus of the opposite side; (c) fibers which join the dorsal longitudinal bundle of Schütz, a tract which underlies the floor of the fourth ventricle and is traceable forward below the cerebral aqueduct.

THE SPINAL NERVES

The =spinal nerves= (Nervi spinales) are arranged in pairs, of which there are usually forty-two in the horse. They are designated according to their relations to the vertebral column as =cervical= (8), =thoracic= (18), =lumbar= (6), =sacral= (5), and =coccygeal= (5). Each nerve is connected with the spinal cord by two roots, dorsal and ventral (Fig. 497).

The =dorsal= (or superior) =root= (Radix dorsalis) is the larger of the two. Its fibers (Fila radicularia) spread out in fan shape and join the cord in a linear series along the dorso-lateral groove. Laterally the fibers converge to form a compact bundle, on which is a gray nodular enlargement, the =spinal ganglion= (Ganglion spinale). Beyond the ganglion the dorsal root joins the ventral root to constitute the nerve. The ganglia are external to the dura mater, and are situated in the intervertebral foramina, except in the case of the sacral and coccygeal nerves, the ganglia of which lie within the vertebral canal. Those of the coccygeal nerves are intradural.

The ganglia vary greatly in size; that of the first cervical nerve is scarcely as large as a hemp seed, while that of the eighth cervical is about 2 cm. long and 1 cm. wide. On the large roots connected with the cervical and lumbar enlargement of the cord there are multiple ganglia of varying sizes interposed in the course of the root-bundles. The fibers of the dorsal roots arise from the cells of the spinal ganglia; each cell gives off a fiber which enters the spinal cord and another which passes into the nerve.

The =ventral= (or inferior) =root= (Radix ventralis) contains fewer fibers than the dorsal root, except in the case of the first cervical nerve. It arises from the ventral surface of the spinal cord (Fig. 495) by means of numerous small bundles of fibers which do not form a linear series but emerge from the cord over an area three to five millimeters in width (ventral root zone). The fibers are processes of the large cells of the ventral gray columns of the spinal cord. There is no ganglion on the ventral root.

In the cervical, thoracic, and anterior lumbar regions the bundles of both roots pass through separate openings in linear series in the dura mater before uniting into a root proper. Further back the bundles of each root unite within the dura. In the anterior part of the cervical region and in the thoracic part of the cord there are intervals of varying length between adjacent roots, but in some places the fibers of adjacent roots overlap and an exchange of fibers may be observed. Many of the roots are directed almost straight outward or incline slightly backward, but the posterior lumbar, sacral, and coccygeal roots and nerves run backward to reach the foramina through which they emerge. The distance thus to be traversed increases from before backward, so that these nerves form a tapering sheaf around the conus medullaris and filum terminale in the last lumbar vertebra and the sacrum which is known as the cauda equina.

The =size= of the spinal nerves varies greatly. The largest are connected with the cervical and lumbar enlargements.

In or immediately after its emergence from the intervertebral foramen each spinal nerve gives off a small =meningeal= or =recurrent branch= (Ramus meningeus). This is joined by a bundle of fibers from the ramus communicans and enters the vertebral canal, in which it is distributed. Each nerve then divides into two primary branches, =dorsal= and =ventral= (Ramus dorsalis, ventralis). The =dorsal branches= are smaller than the ventral, except in the cervical region. They are distributed chiefly to the muscles and skin of the dorsal part of the body. The =ventral branches= supply in general the muscles and skin of the ventral parts of the body, including the limbs. Each nerve or its ventral branch is connected with an adjacent ganglion of the sympathetic system by at least one small short branch known as a =ramus communicans=. Many nerves have two and some have three such rami. A nerve may be connected with two ganglia, and a ganglion may be connected with two nerves.

The dorsal root is sensory or afferent, i. e., it conveys impulses to the central system. Its fibers are axones of the cells of the spinal ganglion. The ventral root is motor or efferent, and conveys impulses toward the periphery. Its fibers are axones of the large cells in the ventral gray columns of the spinal cord. The common trunk or nerve formed by the union of the two roots contains both kinds of fibers, as do also their primary divisions. In addition to these fibers, which are distributed to the skeletal muscles and the skin, the spinal nerves contain fibers derived from the sympathetic system through the rami communicantes; these go to the glands and unstriped muscle and are designated secretory and vasomotor fibers.

THE CERVICAL NERVES

The =cervical nerves= (Nervi cervicales) (Figs. 431, 433, 518, 523) number eight pairs. Of these the first emerges through the intervertebral foramen of the atlas, the second through that of the axis, and the eighth between the last cervical and the first thoracic vertebræ.

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