A text-book of veterinary anatomy (part 3 of 5)

A text-book of veterinary anatomy (part 3 of 5)

Structure.—This muscle presents a distinct segmental arrangement. It is composed of a series of bundles, the fibers of which are directed forward and a little downward and outward. From these are detached two sets of tendons. The superficial tendons spring from the outer edge of the muscle. They are flat and are about half an inch in width. Each crosses two or three intercostal spaces, to be inserted on the posterior border of a rib. The deep tendons are detached from the dorsal part of the deep face of the muscle. Each passes backward across one or two intercostal spaces to its origin on the anterior border or outer surface of a rib. Small bursæ may be found between the ribs and tendons.

Relations.—Superficially, the dorsal serrati and the complexus; deeply, the external intercostals and the ribs. The lumbar origin is covered by the longissimus. The superior (deep) cervical and dorsal vessels cross the surface of the muscle at the first and second intercostal spaces respectively, and branches of the intercostal vessels and nerves emerge between the transversalis and longissimus; here a fascial layer dips in between the two.

Blood-supply.—Intercostal arteries.

Nerve-supply.—Dorsal branches of the thoracic nerves.

7. =Longissimus.=[46]—This is the largest and longest muscle in the body. It extends from the sacrum and ilium to the neck, filling up the space between the spinous processes medially and the lumbar transverse processes and the upper ends of the ribs below; consequently it has the form of a three-sided prism.

Origin.—(1) The internal angle, crest, and adjacent part of the ventral surface of the ilium; (2) the first three sacral spines; (3) the lumbar and thoracic spines and the supraspinous ligament.

Insertion.—(1) The lumbar transverse and articular processes; (2) the thoracic transverse processes; (3) the spinous and transverse processes of the last four cervical vertebræ; (4) the outer surfaces of the ribs, except the first.

Action.—Acting with its fellow, it is the most powerful extensor of the back and loins; by its cervical attachment it assists in extending the neck. By its costal attachment it may also assist in expiration. Acting singly, it flexes the spine laterally.

Structure.—This is quite complex. The posterior part of the muscle is greatly developed and constitutes the common mass of the loins. This is covered by a strong aponeurosis which blends with the supraspinous and sacro-iliac ligaments, and is attached to the crest and inner (sacral) angle of the ilium and the first and second sacral spines; it furnishes origin to the lumbar portion of the middle gluteus. In its course further forward the muscle receives fasciculi from the lumbar and thoracic spines, but diminishes somewhat in volume. At the withers it divides into two parts. The dorsal division (spinalis et semispinalis), reinforced by bundles from the first four thoracic spines, passes forward under the complexus to be inserted into the spines of the last four cervical vertebræ. The ventral division passes forward and downward underneath the serratus magnus to be inserted into the ribs and the transverse processes of the last four cervical vertebræ. Three sets of fasciculi may be distinguished, viz.: (1) spinal, which are superficial and internal; (2) transverse, attached to the transverse and articular processes, which are internal and deep; (3) costal, which are external.

Relations.—Superficially, the middle gluteus, the lumbo-dorsal fascia, the latissimus dorsi, dorsal serrati, serratus magnus, and complexus; deeply, the multifidus, intertransversales, external intercostals, levatores costarum, the ligamentum nuchæ, and its fellow of the opposite side (in the neck).

Blood-supply.—Dorsal, superior cervical, intercostal, and lumbar arteries.

Nerve-supply.—Dorsal branches of the thoracic and lumbar nerves.

8. =Multifidus dorsi= (semispinalis of the back and loins).—This is a long segmental muscle which covers the sides of the spinous processes of the vertebræ from the sacrum to the neck.

Origin.—(1) The lateral portion of the sacrum; (2) the articular processes of the lumbar vertebræ; (3) the transverse processes of the thoracic vertebræ.

Insertion.—The spinous processes of the first two sacral, the lumbar, thoracic, and last cervical vertebræ.

Action.—Acting with its fellow, it is an extensor of the spine; acting singly, it flexes it laterally.

Structure.—It is composed of a series of bundles which are directed obliquely forward and upward. Each fasciculus passes over several vertebræ to its insertion. In the posterior part of the series the bundles cross two or three vertebræ and are inserted into the summits of the spines. Anteriorly, the bundles have a more horizontal direction, cross three to five vertebræ, and are inserted into the sides of the spines considerably below their summits. A further complication consists in the fusion of several bundles into a common insertion.

Relations.—Superficially, the longissimus; deeply, the vertebral spines.

Blood-supply.—Intercostal and lumbar arteries.

Nerve-supply.—Dorsal branches of the thoracic and lumbar nerves.

9. =Intertransversales lumborum.=—These are very thin muscular and tendinous strata, which occupy the spaces between the transverse processes of the lumbar vertebræ except the fifth and sixth.

Action.—To assist in flexing the loins laterally.

Relations.—Superficially, the longissimus; deeply, the quadratus lumborum.

Blood-supply.—Lumbar arteries.

Nerve-supply.—Lumbar nerves.

THE FASCIA AND MUSCLES OF THE TAIL

The muscles of the tail are inclosed in the strong =coccygeal fascia=, which is continuous in front with the gluteal fascia and blends with the lateral sacro-iliac ligament. At the root of the tail it is loosely attached to the subjacent muscles, but further back it is intimately adherent to them. From its deep face are detached septa which pass between the muscles to become attached to the vertebræ.

1. =Coccygeus= (Compressor coccygis; ischio-coccygeus).—This is a flat, triangular muscle which lies chiefly between the sacro-sciatic ligament and the rectum.

Origin.—The pelvic surface of the sacro-sciatic ligament near the ischiatic spine.

Insertion.—The first four coccygeal vertebræ and the coccygeal fascia.

Action.—Acting together, to depress (flex) the tail, compressing it over the perineum; acting singly, to depress and incline it to the same side.

Structure.—The origin of the muscle is aponeurotic. Becoming fleshy, its fibers pass upward and backward and divide into two layers. The outer layer is attached to the vertebræ, the inner to the fascia; included between the two lie the intertransversales. When the tail is raised, the ventral edges of the muscles produce a distinct ridge at either side of the anus.

Relations.—Outwardly, the sacro-sciatic ligament and the semimembranosus; inwardly, the rectum and the sacro-coccygeus ventralis muscle. The internal pudic artery crosses the outer face of the origin of the muscle.

2. =Sacro-coccygeus superior= (M. sacro-coccygeus dorsalis medialis; erector s. extensor coccygis).—This muscle lies along the dorso-median aspect of the tail, in contact with its fellow.

Origin.—The last three sacral spines and some of the coccygeal spines.

Insertion.—The dorsal surface of the coccygeal vertebræ.

Action.—Acting together, to elevate (extend) the tail; acting singly, to elevate and incline it laterally.

Structure.—The muscle has a strong rounded belly. It is inserted by means of short tendons which fuse with those of the next muscle.

Relations.—Superficially, the coccygeal fascia; internally, its fellow; laterally, the sacro-coccygeus lateralis; deeply, the vertebræ.

3. =Sacro-coccygeus lateralis= (M. sacro-coccygeus dorsalis lateralis; curvator coccygis).—This muscle lies immediately lateral to the preceding.

Origin.—The sides of the sacral spines, with the multifidus, and the transverse processes of the sacral and coccygeal vertebræ.

Insertion.—The lateral surface of the coccygeal vertebræ, except the first four.

Action.—Acting with its fellow, to assist the preceding muscle in elevating the tail; acting singly, to incline it to the same side.

Structure.—This muscle appears to be a direct continuation of the multifidus dorsi. The belly is fusiform and receives reinforcing fasciculi from the transverse processes of the sacrum. This is succeeded by bundles of tendons, as many as four lying alongside of each other.

Relations.—Superficially, the lateral sacro-iliac ligament and the coccygeal fascia; dorsally, the sacro-coccygeus superior; ventrally, the intertransversales; deeply, the vertebræ and a branch of the lateral coccygeal artery and accompanying vein and nerve.

4. =Intertransversales caudæ= (Mm. intertransversarii caudæ).—These consist of muscular bundles which lie on the lateral aspect of the tail, between the preceding muscle and the sacro-coccygeus inferior. They begin on the lateral edge of the sacrum and occupy the spaces between the transverse processes, to which they are attached. They are, however, not arranged in a strict segmental manner.

Action.—Acting together, to fix the coccygeal vertebræ; acting singly, to assist in lateral flexion.

5. =Sacro-coccygeus inferior= (Mm. sacro-coccygei ventrales; depressor coccygis).—This muscle lies on the ventral aspect of the sacrum and coccyx. It is composed of two portions, described by Bourgelat and the German anatomists as separate muscles.

(a) The =outer portion= (M. coccygeus ventralis lateralis) is much the larger of the two. It arises from the outer part of the ventral surface of the sacrum, about as far forward as the third foramen, and is inserted into the transverse processes and ventral surface of the coccygeal vertebræ.

(b) The =inner portion= (M. sacro-coccygeus ventralis medialis) arises from the ventral surface of the sacrum internal to the preceding muscle and the first eight coccygeal vertebræ, and is inserted into the ventral surfaces of the coccygeal vertebræ.

Action.—Acting together, to depress (flex) the tail; acting singly, to incline it laterally also.

Structure.—The outer portion has a somewhat compressed belly, and receives bundles from the transverse processes of the coccygeal vertebræ. The inner portion is much smaller and shorter, reaching only about to the middle of the tail.

Relations.—Ventrally, the pelvic and coccygeal fasciæ; dorsally, the sacrum, coccygeal vertebræ, and the intertransversales; laterally, the sacro-sciatic ligament, the coccygeus, and the coccygeal fascia; medially, its fellow, the recto-coccygeus, and the middle coccygeal vessels. Branches of the lateral coccygeal vessels and nerves lie between the outer division of the muscle and the intertransversales.

Blood-supply.—Middle and lateral coccygeal arteries.

Nerve-supply.—Coccygeal nerves.

THE MUSCLES OF THE THORAX

These consist of seven muscles or sets of muscles, which are attached to the thoracic vertebræ, to the ribs and their cartilages, and to the sternum. Functionally, they are muscles of respiration.

1. =Levatores costarum.=—These constitute a series of small muscles which occupy and overlie the upper ends of the intercostal spaces.

Origin.—The transverse processes of the thoracic vertebræ.

Insertion.—The external surfaces of the upper ends of the ribs posterior to the vertebral origin.

Action.—To draw the ribs forward in inspiration.

Structure.—Arising by tendinous fibers, each muscle passes backward and outward and expands at its insertion. Some fibers pass over one rib and are inserted on a succeeding one. At the first and last spaces the muscle cannot be distinguished from the external intercostal, of which it is in reality only a specially developed part.

Relations.—Superficially, the longissimus dorsi; deeply, the ribs, internal intercostal muscles, and the intercostal vessels and nerves.

Blood-supply.—Intercostal arteries.

Nerve-supply.—Intercostal nerves.

2. =External intercostals= (Mm. intercostales externi).—Each of these occupies an intercostal space, from the levatores to the sternal extremity of the rib. They do not occupy the intercartilaginous spaces.

Origin.—The posterior borders of the ribs.

Insertion.—The anterior borders and external surfaces of the succeeding ribs.

Action.—To draw the ribs forward in inspiration.

Structure.—The fibers are directed downward and backward. There is a considerable admixture of tendinous tissue. The thickness of the muscles gradually diminishes toward the lower ends of the spaces.

Relations.—Superficially, the serratus magnus, latissimus dorsi, serratus anticus and posticus, longissimus dorsi, transversalis costarum, rectus thoracis, deep pectorals, obliquus abdominis externus, and panniculus; deeply, the internal intercostals and (in the upper part of the spaces) the intercostal vessels and nerves.

Blood-supply.—Intercostal and internal thoracic arteries.

Nerve-supply.—Intercostal nerves.

3. =Internal intercostals= (Mm. intercostales interni).—These occupy the entire length of the intercostal spaces, including their interchondral portion.

Origin.—The anterior borders of the ribs and their cartilages.

Insertion.—The posterior borders of the preceding ribs and cartilages.

Action.—To draw the ribs backward in expiration. It seems probable, however, that the intercartilaginous portion is inspiratory.[47]

Structure.—The direction of the fibers is oblique downward and forward. There is a smaller amount of tendinous tissue than in the external set, and the thickness diminishes from below upward. In the upper part of the spaces fibers sometimes cross a rib in a fashion similar to the subcostals of man. A thin aponeurosis separates the internal from the external intercostal muscle in each space.

Relations.—Superficially, the levatores costarum and the external intercostals; deeply, the endothoracic fascia and pleura, the transversus thoracis, diaphragm, transversus abdominis, and the internal thoracic and asternal vessels. In the upper part of the intercostal spaces the intercostal vessels and nerves lie between the internal and external intercostal muscle, but below they lie chiefly on the deep face of the inner muscle.

Blood-supply.—Intercostal and internal thoracic arteries.

Nerve-supply.—Intercostal nerves.

4. =Retractor costæ.=—This is a small triangular muscle which lies behind the last rib, chiefly under cover of the serratus posticus.

Origin.—The transverse processes of the first three or four lumbar vertebræ by means of the lumbar fascia.

Insertion.—The posterior border of the last rib.

Action.—To retract the last rib.

Structure.—The muscle arises by a thin aponeurosis. Its fibers are parallel to those of the adjacent internal oblique.

Relations.—Superficially, the serratus posticus and external oblique; deeply, the transversus abdominis.

Blood-supply.—Lumbar arteries.

Nerve-supply.—Lumbar nerves.

5. =Rectus thoracis= (M. transversus costarum; lateralis sterni).—This is a thin muscle which lies under cover of the deep pectoral muscles. It is directed obliquely backward and downward, and crosses the lower part of the first three intercostal spaces.

Origin.—The outer surface of the first rib, below the scalenus.

Insertion.—The cartilage of the third or fourth rib. The aponeurosis usually joins the rectus abdominis. It may reach the sternum.

Action.—It may assist in inspiration or concur with the rectus abdominis.

Relations.—Superficially, the deep pectoral muscles; deeply, the intercostal muscles and the ribs.

6. =Transversus thoracis= (Triangularis sterni).—This is a flat muscle situated on the thoracic surface of the sternum and the cartilages of the sternal ribs.

Origin.—The sternal ligament.

Insertion.—The cartilages of the ribs, from the second to the eighth inclusive.

Action.—It draws the costal cartilages inward and backward, thus assisting in expiration.

Structure.—Each muscle has the form of a scalene triangle, of which the base is the strongly serrated external border. The muscle contains a good deal of tendinous tissue. The anterior bundles are directed forward and outward; the posterior backward and outward.

Relations.—Superiorly, the endothoracic fascia and pleura; inferiorly, the costal cartilages, the internal intercostal muscles, and the internal thoracic vessels.

Blood-supply.—Internal thoracic artery.

Nerve-supply.—The intercostal nerves.

7. =Diaphragm.=—This is a broad, unpaired muscle which forms a partition between the thoracic and abdominal cavities.[48] In outline it has some resemblance to a palm-leaf fan. In form it is dome-shaped, compressed laterally. On a median section it is seen to have a general direction downward and forward from the lumbar vertebræ to the xiphoid cartilage. The thoracic surface is strongly convex, and is covered by the pleura. The abdominal surface is deeply concave, and is covered for the most part by the peritoneum. The muscle consists of a peripheral fleshy portion, two muscular crura, and a tendinous center.

Attachments.—(1) =Costal part=: The cartilages of the ninth to the fifteenth ribs, and the last three ribs at an increasing distance from their sternal ends.

(2) =Sternal part=: The upper surface of the xiphoid cartilage.

(3) =Lumbar part=: (a) The =right crus= is attached to the inferior common ligament, and by this means to the first four or five lumbar vertebræ. (b) The =left crus= is attached in a similar fashion to the first and second lumbar vertebræ.

Action.—It is the principal muscle of inspiration and increases the longitudinal diameter of the chest. The contraction produces a general lessening of the curvature of the diaphragm. In the expiratory phase the costal part and crura lie directly on the body walls, so that the bases of the lungs are in contact with the tendinous center and sternal portion only. In ordinary inspiration the fleshy rim recedes from the chest-wall, so that the bases of the lungs move backward to a line about parallel with the curve formed by the cartilages of the asternal ribs, and about four or five inches (ca. 10 to 12 cm.) therefrom. It is stated that the inspiratory movement affects the tendinous center much less than the fleshy part, since the posterior vena cava is firmly attached to the former. It should be noted, however, that the direction of the thoracic part of the vena cava in the expiratory phase is oblique upward and backward. Thus it would seem that there is no anatomical reason why the diaphragm should not move as a whole in ordinary inspiration at least.

Structure.—The =costal part= (Pars costalis) consists of a series of digitations which meet, or are separated by a very narrow interval from, the transversus abdominis; between the two are the asternal vessels. From the tenth rib backward the attachments are to the ribs at an increasing distance above the costo-chondral junctions. Thus at the last rib the attachment is four to five inches (10 to 12 cm.) from the lower end. Anteriorly, the origin extends along the ninth costal cartilage to the xiphoid cartilage. From these points of origin the fibers curve inward and forward to join the tendinous center. The =right crus= (Crus dextrum) is about twice as thick as the left one and is also longer. It arises by a strong tendon from the lumbar vertebræ (by means of the inferior common ligament). The tendon is succeeded by a rounded belly which leaves the vertebral column, at the last thoracic vertebra. Passing downward and forward, its fibers spread out and join the tendinous center. The =left crus= (Crus sinistrum) arises by a thin tendon from the inferior common ligament at the first and second lumbar vertebræ. This is succeeded by a triangular belly which joins the central tendon. Between the crura and the attachment to the last rib the edge of the muscle crosses the ventral surface of the psoas muscles without attachment, forming the so-called lumbo-costal arch; here the thoracic and abdominal cavities are separated only by the serous membranes and some areolar tissue. The =tendinous center= (Centrum tendineum) resembles the periphery in outline, but is more elongated. It is partially divided into right and left halves by the descent of the crura into it. It is composed largely of radiating fibers, but many interlace in various directions; this is specially evident around the foramen venæ cavæ, which is encircled by fibers. A strong tendinous layer extends across below the hiatus œsophageus.

The diaphragm is pierced by three foramina. (1) The =hiatus aorticus= is an interval between the two crura and below the last thoracic vertebra. It contains the posterior aorta, vena azygos, and cisterna chyli. (2) The =hiatus œsophageus= (or foramen sinistrum) perforates the right crus near its junction with the tendinous center. It is situated a little to the left of the median plane and two or three inches below the thirteenth thoracic vertebra (in expiration). It transmits the œsophagus, the vagus nerves, and the œsophageal branch of the gastric artery. (3) The =foramen venæ cavæ= (s. dextrum) pierces the tendinous center about an inch to the right of the median plane, and about six inches below the twelfth thoracic vertebra (in expiration). The vena cava is firmly attached to the margin of the opening.[49]

Relations.—The thoracic surface is related to the endothoracic fascia, pleuræ, pericardium, the bases of the lungs, and the ribs in part. The abdominal surface is in great part covered by the peritoneum, and is related chiefly to the liver, stomach, spleen, pancreas, kidneys and adrenals, and the anterior flexures of the colon. The sympathetic and splanchnic nerves pass between the crus and the psoas muscles on each side. The asternal vessels perforate the edge of the muscle at the ninth costo-chondral joint.

Blood-supply.—Phrenic and asternal arteries.

Nerve-supply.—Phrenic nerves (from the fifth, sixth, and seventh cervical nerves).

THE ABDOMINAL MUSCLES

The =superficial fascia= covering the lateral and ventral walls of the abdomen is continuous dorsally with the lumbo-dorsal fascia, in front with the thoracic fascia, and behind with the gluteal fascia. In the inguinal region it forms part of the fascia of the penis or of the mammary glands. At the lower part of the flank it forms a fold which is continuous with the fascia of the thigh near the stifle joint. In this fold are the precrural lymph glands. Medially it blends with the linea alba. It contains the abdominal portion of the panniculus carnosus.

The =abdominal panniculus= (M. cutaneus maximus) covers a large part of the lateral surface of the abdomen and thorax. The general direction of its fibers is longitudinal. Its posterior extremity forms the basis of the fold of the flank. Its anterior extremity is inserted by a thin tendon into the internal tuberosity of the humerus, with the posterior deep pectoral muscle. Its dorsal edge may be indicated by a line drawn from the upper end of the thirteenth rib to the fold of the flank. Its ventral limit corresponds to a line drawn from the fold to a point about a handbreadth external to the umbilicus, and from here to a point a little above the level of the elbow. Behind the shoulder the fibers become oblique and blend with the scapular portion. The aponeurosis extends ventrally to the linea alba, dorsally to the supraspinous ligament. The muscle is intimately adherent to the skin, so that special care is necessary in removing the latter. Its deep face, on the other hand, is loosely attached to the underlying structures by a quantity of areolar tissue which is more or less loaded with fat (panniculus adiposus) in animals in good condition. The large external thoracic (“spur”) vein is partially embedded in the lower part of the muscle. Its action is to twitch the skin.

The =deep fascia= is represented chiefly by the =abdominal= tunic (Tunica flava abdominis). This is a sheet of elastic tissue which assists the muscles in supporting the great weight of the abdominal viscera. It is practically coextensive with the obliquus externus, which it covers. Ventrally it is thick, and is intimately adherent to the aponeurosis of the muscle. Laterally it becomes thinner and is more easily separated, although fibers from it dip in between the muscle-bundles. It is continued for some distance upon the intercostals and serratus magnus. Traced forward, it passes as a thin layer beneath the posterior deep pectoral muscle. Posteriorly it is attached to the external angle of the ilium. In the inguinal region it forms the deep fascia of the prepuce or of the mammary glands.

The =linea alba= is a median fibrous raphé which extends from the xiphoid cartilage to the symphysis pubis. It is formed chiefly by the junction of the aponeuroses of the oblique and transverse muscles, but partly by longitudinal fibers. A little behind its middle is a cicatrix which indicates the position of the umbilical opening of the fœtus.

1. =Obliquus abdominis externus= (great oblique; external oblique of the abdomen).—This is the most extensive of the abdominal muscles. It is a broad sheet, irregularly triangular in shape, widest behind. Its fibers are directed chiefly downward and backward.

Origin.—(1) The outer surfaces of the last fourteen ribs, and the fascia over the external intercostal muscles; (2) the lumbo-dorsal fascia.

Insertion.—(1) The linea alba and the prepubic tendon; (2) the external angle and shaft of the ilium; (3) the internal femoral fascia.

Action.—(1) To compress the abdominal viscera, as in defecation, micturition, parturition, and expiration; (2) to flex the trunk (arch the back); (3) acting singly, to flex the trunk laterally.

Structure.—The muscle is composed of a fleshy portion and an aponeurosis. The =muscular portion= lies on the lateral wall of the thorax and abdomen. It arises by a series of digitations, the anterior four of which alternate with those of the serratus magnus. The origin may be indicated by a slightly curved line (concave above) drawn from the lower part of the fifth rib to the external angle of the ilium. The fibers are directed downward and backward and terminate on the aponeurosis, except in the flank, where they are almost horizontal in direction. The line of junction is a curve (concave above) extending from the upper edge of the posterior deep pectoral muscle toward the external angle of the ilium. The =aponeurosis= is intimately attached to the abdominal tunic, and its fibers are largely interwoven ventrally with those of the aponeurosis of the internal oblique. By this fusion is formed the outer sheath of the rectus abdominis, which blends at the linea alba with that of the opposite side. In the inguinal region the aponeurosis divides into two chief layers; one of these curves upward and backward and is inserted into the external angle of the ilium and the prepubic tendon. Between these points the aponeurosis is much strengthened and is called the =inguinal= (Poupart’s) =ligament= (Ligamentum inguinale). This curves upward and somewhat forward, becomes thin, and blends with the iliac fascia. It forms the posterior wall of the inguinal canal. About an inch (ca. 2 to 3 cm.) in front of the pubis and about two inches (ca. 4 to 5 cm.) from the median plane the aponeurosis is pierced by a slit-like opening,[50] the =external inguinal ring= (Annulus inguinalis subcutaneus). This is the external orifice of the inguinal canal. Its long axis is directed outward and forward, and is about four inches (ca. 10 cm.) in length. The inner angle is rounded and is well defined by the junction of the inguinal ligament with the prepubic tendon, but the outer angle is not so sharply defined. The borders or pillars are constituted by arciform fibers of the aponeurosis of the external oblique (Crus mediale, laterale). The =femoral layer= of the aponeurosis (Lamina femoralis) passes on to the inner surface of the thigh, where it blends with the femoral fascia. A thin =iliac layer= (Lamina iliaca) passes over the outer margin of the iliacus to the external border of the ilium.

Relations.—Superficially, the skin, the panniculus carnosus, the abdominal tunic, and the posterior deep pectoral muscle; deeply, the ribs and their cartilages, the intercostal muscles, the internal oblique, the contents of the inguinal canal, and the sartorius and gracilis.

Blood-supply.—Intercostal and lumbar arteries.

Nerve-supply.—Intercostal and lumbar nerves.

2. =Obliquus abdominis internus= (small oblique; internal oblique of the abdomen).—This muscle is situated under the preceding one. Its fibers are directed downward, forward, and inward. It forms a triangular curved sheet with the base behind.

Origin.—The external angle of the ilium and the adjacent part of the inguinal (Poupart’s) ligament.

Insertion.—(1) The cartilages of the last four or five ribs; (2) the linea alba and the prepubic tendon.

Action.—Similar to that of the preceding muscle.

Structure.—Like the external oblique, it is composed of a fleshy portion and an aponeurosis. The fleshy portion is fan-shaped, and is situated chiefly in the flank. At its iliac origin it is covered by a glistening aponeurosis. Traced inward and downward along the abdominal surface of the inguinal ligament, the muscular origin is found to become much thinner, and also becomes loosely attached to the ligament. About four or five inches (ca. 10 to 12 cm.) from the linea alba the muscle separates from the ligament and forms the anterior wall of the inguinal canal. The abdominal orifice of the canal, the =internal inguinal ring=[51] (Annulus inguinalis abdominalis), is found here. It is normally a narrow slit, bounded in front by the edge of the internal oblique, and behind by the inguinal ligament. The =aponeurosis= is to a great extent blended with that of the external oblique, being, indeed, considerably interwoven with it ventrally. Where it covers the rectus abdominis it is attached to the tendinous inscriptions of that muscle.

Relations.—Superficially, the external oblique; deeply, the rectus abdominis, transversus abdominis, and the peritoneum.

Blood-supply.—Circumflex iliac, lumbar, and intercostal arteries.

Nerve-supply.—Ventral branches of the lumbar nerves.

3. =Rectus Abdominis.=—This muscle is confined to the ventral part of the abdominal wall; it extends from the lower part of the chest-wall to the pubis.

Origin.—The cartilages of the fifth to the ninth ribs inclusive, and the adjacent surface of the sternum.

Insertion.—The pubis, by means of the prepubic tendon.

Action.—Similar to that of the oblique muscles. It is specially adapted to flex the lumbo-sacral joints and the lumbar and thoracic parts of the spine.

Structure.—The fibers of the muscle are directed longitudinally. Nine to eleven transverse bands of fibrous tissue extend in an irregular manner across the muscle. These are termed =inscriptiones tendineæ=. They strengthen the muscle and serve to prevent separation of its fibers. The width of the muscle is greatest about its middle.

Relations.—Superficially, the aponeuroses of the oblique muscles (which constitute the external rectus sheath), and the posterior deep pectoral; deeply, the transversus, intercostals, the cartilages of the ribs, and the sternum. The posterior abdominal artery runs along the outer edge of the muscle posteriorly, and the anterior abdominal artery on or in its anterior part.

Blood-supply.—Anterior and posterior abdominal arteries.

Nerve-supply.—Intercostal and lumbar nerves.

4. =Transversus abdominis.=—This muscle, named from the general direction of its fibers, is a triangular curved sheet. Its lateral part is muscular, its ventral aponeurotic.

Origin.—(1) The inner surfaces of the distal ends or the cartilages of the asternal ribs, meeting the costal attachment of the diaphragm; (2) the transverse processes of the lumbar vertebræ, by means of the deep layer of the lumbo-dorsal fascia.

Insertion.—The xiphoid cartilage and the linea alba.

Action.—Similar to that of the oblique muscles.

Structure.—The =muscular part= is a sheet of parallel bundles of fibers, directed downward and inward. It is thickest over the cartilages of the ribs, and from here it thins out greatly toward the aponeurosis and the lumbar region. The fibers of the =aponeurosis= directly continue those of the fleshy part. Posteriorly it becomes extremely thin and blends with the aponeuroses of the oblique muscles. It covers the deep face of the rectus, so forming the internal rectus sheath.

Relations.—Superficially, the oblique and straight muscles, the retractor costæ, the cartilages of the asternal ribs, and the internal intercostal muscles; deeply, the transversalis fascia and the peritoneum. The =transversalis fascia= is little developed in the horse, and is very thin in emaciated subjects, but in animals in good condition it contains a good deal of fat. It blends with the iliac fascia and descends into the inguinal canal. The asternal artery runs along the interval between the origin of the transversus and the costal part of the diaphragm. The intercostal nerves pass down over the external surface of the muscle, to which they give branches. Branches of the first three lumbar nerves are similarly disposed further back.

Blood-supply.—Intercostal, lumbar, and asternal arteries.

Nerve-supply.—Intercostal and lumbar nerves.

5. =Cremaster externus.=—This small muscle may be regarded as a detached portion of the internal oblique, with which it blends at its origin (Figs. 272, 450).

Origin.—The iliac fascia, near the origin of the sartorius.

Insertion.—The tunica vaginalis communis.

Action.—To raise the tunica vaginalis, and with it the testicle.

Structure.—The muscle arises by a thin aponeurosis which is succeeded by a flat muscular belly about one and a half to two inches (ca. 4 to 5 cm.) in width.[52] It passes down the inguinal canal on the postero-external surface of the tunica vaginalis, to which it is rather loosely attached. On reaching the point where the tunic is reflected on to the tail of the epididymis, the muscle is firmly attached to the outer surface of the tunic by short tendinous fibers.

Relations.—The muscle lies between the peritoneum and the fascia transversalis in front and the iliac fascia and inguinal ligament behind. On reaching the internal ring it descends the inguinal canal on the postero-external surface of the tunica vaginalis communis.

Blood-supply.—External spermatic or cremasteric artery.

Nerve-supply.—External spermatic nerve.

=The Inguinal Canal.=—This term (Canalis inguinalis) is applied to an oblique passage through the posterior part of the abdominal wall.[53] It begins at the internal inguinal or abdominal ring, and extends obliquely downward, inward, and somewhat forward, to end at the external inguinal or subcutaneous ring. Its anterior wall is formed by the fleshy posterior part of the internal oblique muscle, and the posterior wall by the strong tendinous inguinal (Poupart’s) ligament. The average length of the canal, measured along the spermatic cord, is about four inches (ca. 10 cm.). The =internal inguinal ring= (Annulus inguinalis abdominalis) is bounded in front by the thin margin of the internal oblique muscle, and behind by the inguinal ligament. It is directed approximately from the edge of the prepubic tendon toward the external angle of the ilium. Its length is about four or five inches (ca. 10 to 12 cm.). The edge of the muscle is attached to the surface of the ligament here by delicate connective tissue, except where structures intervene between the walls of the canal. Consequently the limits of the ring are not very clearly defined. The =external inguinal ring= (Annulus inguinalis subcutaneus) is a well defined slit in the aponeurosis of the external oblique muscle, situated lateral to the prepubic tendon. Its long axis is directed from the edge of the prepubic tendon outward and forward, and its average length is about four inches (ca. 10 cm.). The canal contains in the male the spermatic cord, the tunica vaginalis, the external cremaster muscle, the external pudic artery and a small satellite vein, and the inguinal lymph vessels and nerves. In the female it contains the mammary vessels and nerves; in the bitch it also lodges the round ligament of the uterus, inclosed in a tubular process of peritoneum.

The two rings do not correspond in direction, so that the length of the canal varies greatly when measured at different points. The inner angle of the internal ring lies almost immediately above that of the external ring, but the outer angle is situated five to six inches (ca. 12 to 15 cm.) from that of the external ring. The inner angles of the external rings are well defined and distinctly palpable in the living subject; they are about three to four inches (ca. 8 to 10 cm.) apart.

=The Prepubic Tendon.=—The =prepubic tendon= is essentially the tendon of insertion of the two recti abdominis, but also furnishes attachment to the obliqui, the graciles, and the pectinei. It is attached to the anterior borders of the pubic bones, including the ilio-pectineal eminences. It has the form of a very strong thick band, with concave lateral borders which form the inner boundaries of the external inguinal rings. Its direction is oblique upward and backward.[54] Its structure is somewhat complex. Most of the fibers of the posterior part extend from one ilio-pectineal eminence to the other. The fibers which belong to the recti curve in to the median line. The aponeuroses of the internal oblique muscles are inserted into its abdominal surface, and the inguinal ligaments are attached to and continue across it in arciform fashion. The anterior part of the tendon of origin of the gracilis is fused with it ventrally, and many of the fibers of the pectineus arise from it. It gives off on either side a strong round band, the so-called =pubo-femoral= or =accessory ligament=, which is inserted into the fossa of the head of the femur with the round ligament (vide hip joint).

MUSCLES OF THE THORACIC LIMB

I. THE MUSCLES OF THE SHOULDER GIRDLE (Figs. 177, 178, 179, 186)

This group consists of those muscles which connect the thoracic limb with the head, neck, and trunk. The group naturally falls into two divisions—dorsal and ventral.[55]

A. DORSAL DIVISION

This division consists of two layers which overlie the proper muscles of the neck and back.

FIRST LAYER

1. =Trapezius.=—This is a flat, triangular muscle, the base of the triangle corresponding with the spine. It is divided by an aponeurotic portion into two divisions:

(=a=) =Trapezius cervicalis.=—Origin.—The funicular portion of the ligamentum nuchæ, from the second cervical to the third thoracic vertebra.

Insertion.—The spine of the scapula and the fascia of the shoulder and arm.

(=b=) =Trapezius thoracalis s. dorsalis.=—Origin.—The supraspinous ligament, from the third to the tenth thoracic vertebra.

Insertion.—The tubercle of the spine of the scapula.

Action.—Acting as a whole, to elevate the shoulder; the cervical portion draws the scapula forward and upward and the thoracic portion draws it backward and upward.

Structure.—The muscle arises by a short, thin aponeurosis, from which the fibers of the flat fleshy portion converge to the spine of the scapula and the aponeurosis which separates the two portions. The cervical fascia joins the ventral edge of the cervical portion to the mastoido-humeralis, or the two muscles may unite here.

Relations.—Superficially, the skin and fascia; deeply, the rhomboideus, latissimus dorsi, supraspinatus, infraspinatus, deltoid, splenius, serratus magnus, and anterior deep pectoral muscles, and the cartilage of the scapula.

Blood-supply.—Deep cervical and intercostal arteries.

Nerve-supply.—Spinal accessory nerve.

SECOND LAYER

This consists of two muscles—the rhomboideus and the latissimus dorsi.

2. =Rhomboideus.=—This consists of two portions:

(=a=) =Rhomboideus cervicalis s. cervicis.=—Origin.—The funicular portion of the ligamentum nuchæ, from the second cervical to the second thoracic vertebra.

Insertion.—The internal surface of the cartilage of the scapula.

(=b=) =Rhomboideus thoracalis s. dorsalis.=—Origin.—The spinous processes of the second to the seventh thoracic vertebra by means of the dorso-scapular ligament.

Insertion.—The inner surface of the cartilage of the scapula.

Action.—To draw the scapula upward and forward. When the limb is fixed the cervical portion will elevate the neck.

Structure.—The cervical portion is narrow, pointed at its anterior extremity, and lies along the funicular part of the ligamentum nuchæ, to which it is attached by short tendon bundles. The fibers are directed for the most part longitudinally. The thoracic portion is quadrilateral in shape, and its fibers are nearly vertical. Its deep face is intimately attached to the dorso-scapular ligament.

Relations.—Superficially, the skin and fascia (over a small area), the trapezius, and the cartilage of the scapula; deeply, the dorso-scapular ligament, the splenius, complexus, longissimus dorsi, and serratus anticus.

Blood-supply.—Dorsal and superior cervical arteries.

Nerve-supply.—Sixth cervical nerve.

3. =Latissimus Dorsi.=—This is a wide muscle which has the form of a right-angled triangle. It lies for the most part under the skin and panniculus, on the lateral wall of the thorax, from the spine to the arm.

Origin.—The lumbo-dorsal fascia—and by this means from the lumbar and thoracic spines as far forward as the highest point of the withers.

Insertion.—The internal tubercle of the humerus, in common with the teres major.

Action.—To draw the humerus upward and backward and flex the shoulder joint. If the limb is advanced and fixed, it draws the trunk forward.

Structure.—The muscle arises by a wide aponeurosis, which fuses with that of the serratus posticus and with the lumbo-dorsal fascia. The muscular portion is at first rather thin, but by the convergence of its fibers becomes thicker as it approaches the arm. The anterior fibers pass almost vertically downward over the dorsal angle of the scapula and its cartilage. The posterior fibers are directed downward and forward. The thick belly formed by the convergence of these passes under the triceps to end on the flat tendon of insertion, which is common to this muscle and the teres major.

Blood-supply.—Subscapular, intercostal, and lumbar arteries.

Nerve-supply.—Brachial plexus (eighth cervical and dorsal roots).

B. VENTRAL DIVISION

1. =Mastoido-humeralis= (M. brachiocephalicus; levator humeri).—This muscle extends along the side of the neck from the head to the arm. It is incompletely divisible into two portions.

Origin.—(1) The mastoid process of the petrous temporal bone and the occipital crest; (2) the wing of the atlas and the transverse processes of the second, third, and fourth cervical vertebræ.

Insertion.—The deltoid tuberosity and the curved rough line which extends from this to the distal extremity of the humerus.

Action.—When the head and neck are fixed, to draw the limb forward, extending the shoulder joint. When the limb is fixed, to extend the head and neck, if the muscles act together; acting separately, to incline the head and neck to the same side.

Structure.—As already mentioned, the muscle is capable of incomplete division into two parts, the line of division being indicated by the emergence of superficial branches of the ventral divisions of the cervical nerves. The mastoid portion (M. cleido-mastoideus) partly overlaps the other portion (M. cleido-transversarius), which lies dorsal to it. The former is attached to the mastoid process and the occipital bone by a broad tendon which fuses with that of the splenius and trachelo-mastoideus; it is also attached to the tendon of insertion of the sterno-cephalicus by aponeurosis. The dorsal portion is attached to the transverse processes by four fleshy digitations. The belly of the muscle is adherent superficially to the cervical fascia and the panniculus, and deeply to the subscapulo-hyoideus. In front of the shoulder its deep face is marked by a tendinous intersection of variable development.[56] Here the muscle becomes wider, covers the shoulder joint, passes between the brachialis and biceps, and is inserted by means of a wide tendon which it shares with the superficial pectoral muscle.

Relations.—Superficially, the skin, cervical fascia, the parotid gland, the panniculus, brachialis, and branches of the cervical nerves; deeply, the splenius, trachelo-mastoideus, rectus capitis anterior major, omo-hyoideus, serratus magnus, anterior deep pectoral and biceps muscles, the inferior cervical artery, the prescapular lymph glands, and branches of the cervical nerves. The ventral edge of the muscle forms the dorsal boundary of the jugular furrow. The dorsal border may be in contact with the cervical trapezius, or separated from it by a variable interval.

Blood-supply.—Inferior cervical, carotid, and vertebral arteries.

Nerve-supply.—Spinal accessory and cervical nerves.

The =pectoral fascia= is a thin membrane covering the surface of the pectoral muscles, to which it is, for the most part, pretty intimately attached. It detaches a layer which passes between the superficial and deep pectorals. At the posterior edge of the triceps another layer is given off, which passes on the outer surface of this muscle to blend with the scapular fascia; the deeper layer becomes continuous with the subscapular and cervical fasciæ.

The =pectoral muscles= form a large fleshy mass which occupies the space between the ventral part of the chest-wall and the shoulder and arm. They are clearly divisible into a superficial and a deep layer. The superficial layer may be subdivided into two portions by careful dissection; the deep layer is clearly made up of two muscles.

2. =Superficial pectoral= (M. pectoralis superficialis).

(=a=) =Anterior superficial pectoral= (portio clavicularis s. descendens; pectoralis anticus).—This is a short, thick, somewhat rounded muscle, which extends between the anterior part of the sternum and the front of the arm. It forms a distinct prominence on the front of the breast, which is easily recognized in the living animal.

Origin.—The cariniform cartilage of the sternum.

Insertion.—(1) The curved line of the humerus with the mastoido-humeralis; (2) the fascia of the arm.

Action.—To adduct and advance the limb.

Structure.—The belly of the muscle is convex on its superficial face, but deeply it is flattened where it overlaps the posterior superficial pectoral. Here the two muscles are usually pretty intimately attached to each other, and care must be exercised in making the separation. The tendon of insertion blends with that of the mastoido-humeralis and with the fascia of the arm. At the middle line of the breast a furrow occurs between the two muscles; laterally, another furrow, containing the cephalic vein, lies between the muscle and the mastoido-humeralis.

Relations.—Superficially, the skin, fascia, and panniculus; deeply, the posterior division, the deep pectoral, and the biceps. The cephalic vein lies in the groove between this muscle and the mastoido-humeralis.

(=b=) =Posterior superficial pectoral= (portio sternocostalis; pectoralis transversus).—This is a wide muscular sheet which extends from the ventral edge of the sternum to the fascia on the inner surface of the forearm.

Origin.—(1) The ventral edge of the sternum as far back as the sixth cartilage; (2) a fibrous raphé common to the two muscles.

Insertion.—(1) The fascia on the proximal third of the forearm; (2) the curved line of the humerus with the preceding muscle.

Action.—To adduct the limb and to tense the fascia of the forearm.

Structure.—It is thin and pale, and mixed with a good deal of fibrous tissue. The right and left muscles fuse at a median fibrous raphé. The tendon of insertion unites with the fascia on the inner side of the forearm for the most part; only a small part in front, about an inch in width, is attached to the humerus.

Relations.—Superficially, the skin, fascia, and the preceding muscle; deeply, the deep pectoral, the biceps, and the brachialis; at the elbow, the posterior radial vessels, the median nerve, and the inner and middle flexors of the carpus.

3. =Deep pectoral= (M. pectoralis profundus).—This muscle is much thicker and more extensive in the horse than the superficial pectoral. It consists of two distinct portions.

(=a=) =Anterior deep pectoral= (portio prescapularis; pectoralis parvus).—This division is prismatic and extends from the anterior part of the lateral surface of the sternum to the cervical angle of the scapula.

Origin.—The anterior half of the lateral surface of the sternum and the cartilages of the first four ribs.

Insertion.—The aponeurosis which covers the supraspinatus at its dorsal end, and the scapular fascia.

Action.—To adduct and retract the limb; when the limb is advanced and fixed, to draw the trunk forward.

Structure.—The muscle is almost entirely fleshy. It describes a curve (convex anteriorly), passing at first forward, then upward over the front of the shoulder, a little to its inner side, and finally upward and backward along the anterior border of the supraspinatus. It is loosely attached to the latter muscle, and terminates in a pointed end which becomes more firmly attached near the cervical angle of the scapula.

Relations.—Superficially, the skin and fascia, the panniculus, superficial pectoral, trapezius, and mastoido-humeralis muscles, the cephalic vein, and the inferior cervical artery; deeply, the posterior deep pectoral, biceps, supraspinatus, omo-hyoideus, and serratus magnus muscles, the brachial vessels, and the branches of the brachial plexus of nerves.

(=b=) =Posterior deep pectoral= (portio humeralis s. ascendens; pectoralis magnus).—This is much the largest of the pectoral group in the horse. It is somewhat triangular or fan-shaped.

Origin.—(1) The abdominal tunic; (2) the xiphoid cartilage and ventral aspect of the sternum; (3) the cartilages of the fourth to the ninth ribs.

Insertion.—(1) The internal tuberosity of the humerus; (2) the external lip of the bicipital groove; (3) the tendon of origin of the coraco-brachialis.

Action.—To adduct and retract the limb; if the limb is advanced and fixed, to draw the trunk forward.

Structure.—This muscle is also almost entirely fleshy. Its posterior part is wide and thin, but as the muscle is traced forward, it becomes narrower and much thicker. It passes forward and slightly upward in a gentle curve to its insertion. The humeral insertion is just below that of the inner division of the supraspinatus. Part of the fibers are inserted by means of a tendinous band which binds down the tendon of the biceps and is attached to the external lip of the bicipital groove, and a small part is attached to the tendon of origin of the coraco-brachialis.

Relations.—Superficially, the skin, panniculus, and superficial pectoral; deeply, the abdominal tunic, the external oblique, the rectus abdominis et thoracis, the brachial vessels, and branches of the brachial plexus of nerves. The external thoracic vein lies along the outer or upper border.

Blood-supply.—Internal and external thoracic, inferior cervical, anterior circumflex, and intercostal arteries.

Nerve-supply.—Pectoral (or thoracic) nerves, from the brachial plexus.

4. =Serratus magnus= (M. serratus ventralis).—This is a large, fan-shaped muscle, situated on the lateral surface of the neck and thorax. It derives its name from the serrated ventral edge of its thoracic portion. It consists of a cervical and a thoracic portion.

(=a=) =Cervical part= (M. serratus cervicis; levator scapulæ hominis).

Origin.—The transverse processes of the last four or five cervical vertebræ.

Insertion.—The anterior triangular area on the costal surface of the scapula and the adjacent part of the cartilage.

(=b=) =Thoracic part= (M. serratus thoracis; serratus anterior hominis).

Origin.—The external surfaces of the first eight or nine ribs.

Insertion.—The posterior triangular area on the costal surface of the scapula and the adjacent part of the cartilage.

Action.—The two muscles form a sort of sling in which the trunk is suspended. Contracting together, they raise the thorax; contracting singly, the weight is shifted to the limb on the side of the muscle acting. The two parts can contract separately and are antagonistic in their action on the scapula. The cervical part draws the base of the scapula toward the neck, while the thoracic part has the opposite action; these effects concur in the backward and forward swing of the limb respectively. With the limb fixed, the cervical part extends (raises) the neck or inclines it laterally. The thoracic part may act as a muscle of forced inspiration.

Structure.—In the domesticated animals there is no such clear division of the muscle as is found in man and the apes. On account of the difference in action, however, it seems desirable to distinguish the two portions. The cervical part is thick and almost entirely fleshy. The thoracic part has on its superficial face a thick, tendinous layer which may sustain the weight of the trunk when the muscle substance relaxes. The ventral edge presents distinct digitations, the last four of which alternate with those of the obliquus externus abdominis, and are covered by the abdominal tunic. The fourth, fifth, and sixth digitations extend nearly to the distal ends of the ribs. The last digitation is small and may be absent. Exceptionally additional digitations may be attached to the tenth or eleventh rib or to the fascia over the intercostal muscles. The fibers converge to the insertion, which is thick and is intersected by elastic lamellæ derived from the dorso-scapular ligament.

Relations.—Superficially, the mastoido-humeralis, trapezius, deep pectoral, subscapularis, teres major, latissimus dorsi, panniculus carnosus, the abdominal tunic, the brachial vessels, and the long thoracic nerve; deeply, the splenius, complexus, longissimus, transversalis costarum, the ribs and external intercostal muscles, and branches of the superior cervical and dorsal arteries.

Blood-supply.—Superior cervical, dorsal, vertebral, and intercostal arteries.

Nerve-supply.—Brachial plexus.

II. THE MUSCLES OF THE SHOULDER

Under this head will be described those muscles which arise on the scapula and end on the arm; they may be divided into two groups—one covering the dorsum, the other the venter of the scapula.

The =superficial fascia= of the shoulder and arm contains the panniculus carnosus of this region, and may be considered to be continued on the inner side of the limb by the subscapular fascia.

The =thoracic= or =scapulo-humeral portion= of the =panniculus= (M. cutaneus scapulæ et humeri) arises by a thin aponeurosis from the ligamentum nuchæ at the withers. The fleshy portion begins over the upper part of the scapula and extends to the elbow. Its fibers have in general a dorso-ventral direction. It is continuous behind with the abdominal portion.

The =deep fascia= of the shoulder and arm (Fascia omobrachialis) is much more developed and important. It is strong and tendinous, and is intimately adherent to the muscles on the outer surface of the scapula, between which it detaches =intermuscular septa=, which are attached to the spine and borders of the scapula. The brachial portion is, for the most part, only loosely attached to the underlying muscles, for which it forms sheaths; it is attached to the humerus, especially to the lips of the bicipital groove and the deltoid tuberosity. It blends distally with the tendon of insertion of the biceps, and is continued by the antibrachial fascia.

A. EXTERNAL GROUP (Figs. 178, 179)

1. =Deltoid= (M. deltoideus; long abductor of the arm; scapular portion of the deltoid of man).—This lies partly on the triceps in the angle between the scapula and humerus, partly on the infraspinatus and teres minor.

Origin.—(1) The upper part of the posterior border of the scapula; (2) the spine of the scapula, by means of the strong aponeurosis which covers the infraspinatus.

Insertion.—The deltoid tuberosity of the humerus.

Action.—To flex the shoulder joint and abduct the arm.

Structure.—The origin of the muscle is partly aponeurotic, partly fleshy. The aponeurosis fuses with that of the infraspinatus; the posterior part is attached to the scapula immediately in front of the origin of the long head of the triceps. The belly of the muscle lies for the most part in a cavity formed in the triceps. It is widest about its middle.

Relations.—Superficially, the skin, fascia, panniculus, and mastoido-humeralis; deeply, the infraspinatus, teres minor, triceps, and brachialis muscles, and branches of the posterior circumflex artery and axillary nerve.

Blood-supply.—Subscapular artery (chiefly through the posterior circumflex).

Nerve-supply.—Suprascapular and axillary nerves.

2. =Supraspinatus.=—This muscle occupies the supraspinous fossa, which it fills, and beyond which it extends, thus coming in contact with the subscapularis.

Origin.—The supraspinous fossa, the spine, and the lower part of the cartilage of the scapula.

Insertion.—The inner and outer lips of the bicipital groove.

Action.—To extend the shoulder joint. It also assists in preventing dislocation.

Structure.—The surface of the muscle is covered by a strong aponeurosis, from the deep face of which many fibers arise. The muscle is thin at its origin from the cartilage, but becomes considerably thicker below. At the neck of the scapula it divides into two branches, between which the tendon of origin of the biceps emerges. These branches, fleshy superficially, tendinous deeply, are inserted into the lips of the bicipital groove. They are united by a fibrous membrane already mentioned in connection with the deep pectoral muscle; some fibers are attached to this membrane and the capsule of the shoulder joint.

Relations.—Superficially, the skin, fascia, panniculus, trapezius, and mastoido-humeralis; deeply, the scapula and its cartilage, the subscapularis muscle, and the suprascapular vessels and nerve; in front, the anterior deep pectoral muscle; behind, the spine of the scapula and infraspinatus muscle.

Blood-supply.—Suprascapular and posterior circumflex arteries.

Nerve-supply.—Suprascapular nerve.

3. =Infraspinatus.=—This muscle occupies the greater part of the infraspinous fossa.

Origin.—The infraspinous fossa and the scapular cartilage.

Insertion.—(1) The outer tuberosity of the humerus, distal to the outer insertion of the supraspinatus; (2) the posterior eminence of the outer tuberosity.

Action.—To abduct the arm and rotate it outward.[57] It also plays the part of a lateral ligament.

Structure.—This muscle is also covered by a strong aponeurosis, from which many fibers arise, and by means of which the deltoid is attached to the spine of the scapula. A thick tendinous layer partially divides the muscle into two strata, and, coming to the surface at the shoulder joint, constitutes the chief means of insertion. This tendon, an inch or more (3 cm.) in width, passes over the posterior eminence of the external tuberosity of the humerus; it is bound down by a fibrous sheet, and a =synovial bursa= is interposed between the tendon and the bone. When the long insertion is cut and reflected, the short insertion, partly tendinous, partly fleshy, is exposed.

Relations.—Superficially, the skin, fascia, panniculus, trapezius, and deltoid; deeply, the scapula and its cartilage, the shoulder joint and capsule, the long head of the triceps, the teres minor, and the nutrient artery of the scapula.

Blood-supply.—Subscapular artery.

Nerve-supply.—Suprascapular and axillary nerves.

4. =Teres minor.=—This is a much smaller muscle than the foregoing. It lies chiefly on the triceps, under cover of the deltoid and infraspinatus.

Origin.—(1) The rough lines on the distal and posterior part of the infraspinous fossa; (2) a small part of the posterior border of the scapula, about its middle; (3) a tubercle near the rim of the glenoid artery.

Insertion.—The deltoid tuberosity and a small area just above it.

Action.—To flex the shoulder joint and to abduct the arm; also to assist in outward rotation.

Structure.—The muscle is not rounded in the horse and ox, but flat and triangular. Its origin from the posterior border of the scapula is by means of an aponeurosis which also gives origin to fibers of the infraspinatus and triceps. A =bursa= is commonly found between the terminal part of the muscle and the capsule of the shoulder joint, and is often continuous with that of the infraspinatus.

Relations.—Superficially, the deltoid and infraspinatus muscles; deeply, the scapula, the shoulder joint, and the triceps muscle.

Blood-supply.—Subscapular artery (circumflex branches).

Nerve-supply.—Axillary nerve.

B. INTERNAL GROUP

1. =Subscapularis.=—This muscle occupies the subscapular fossa, beyond which, however, it extends both before and behind.

Origin.—The subscapular fossa.

Insertion.—The posterior eminence of the internal tuberosity of the humerus.

Action.—To adduct the humerus.

Structure.—The muscle is flat and triangular, with the base upward. The latter is thin and interdigitates with the scapular attachments of the serratus. Below this the belly thickens and becomes narrower. It is covered by an aponeurosis, and contains a considerable amount of tendinous tissue. The tendon of insertion is crossed by the tendon of origin of the coraco-brachialis; it is intimately adherent to the capsule of the shoulder joint, and may be regarded as replacing the internal ligament of the latter. A small bursa may occur here.

Relations.—Superficially, the scapula and shoulder joint, the supraspinatus, triceps, and teres major muscles; deeply, the serratus magnus muscle, the brachial vessels, and the chief branches of the brachial plexus. The subscapular vessels run along or near the posterior edge of the muscle.

Blood-supply.—Subscapular artery.

Nerve-supply.—Subscapular nerves (from the brachial plexus).

2. =Teres major= (Teres internus; adductor of the arm).—This muscle is flat, widest about its middle, and lies chiefly on the deep face of the triceps.

Origin.—The dorsal angle and the adjacent part of the posterior border of the scapula.

Insertion.—The tubercle on the inner surface of the shaft of the humerus, in common with the latissimus dorsi.

Action.—To flex the shoulder joint and adduct the arm.

Structure.—It is for the most part fleshy, but the origin consists of an aponeurosis which blends with that of the tensor fasciæ antibrachii. The insertion is by a flat tendon which fuses with that of the latissimus dorsi.

Relations.—Superficially, the triceps, infraspinatus, and deltoid muscles; deeply, the serratus magnus muscle. The subscapular vessels lie in a groove between the anterior edge of this muscle and the posterior border of the subscapularis; near the shoulder joint the posterior circumflex artery and the axillary nerve emerge between the two muscles. The deep face of the muscle is crossed by the thoracic branches of the brachial plexus, and by the branch of the subscapular artery which supplies the latissimus dorsi.

Blood-supply.—Subscapular artery.

Nerve-supply.—Axillary nerve.

3. =Coraco-brachialis= (Coraco-humeralis).—This muscle lies on the inner surface of the shoulder joint and the arm.

Origin.—The coracoid process of the scapula.

Insertion.—(1) A small area above the internal tubercle of the humerus; (2) the middle third of the anterior surface of the humerus.

Action.—To adduct the arm and to flex the shoulder joint.

Structure.—The long tendon of origin emerges between the subscapularis and the inner branch of the supraspinatus. It passes over the terminal part of the subscapularis and is provided with a =synovial sheath=. The muscular part spreads out and divides into two portions. The smaller and shorter portion is inserted into the proximal third of the shaft of the humerus, close to the origin of the lateral head of the triceps; the larger and longer portion is inserted into the middle third of the humerus, in front of the internal tubercle and the inner head of the triceps.

Relations.—Externally, the subscapularis muscle and the humerus; internally, the deep pectoral and brachialis muscles. The anterior circumflex artery and the nerve to the biceps usually emerge between the two insertions, and the brachial vessels lie along the posterior border of the muscle.

Blood-supply.—Anterior circumflex artery.

Nerve-supply.—Musculo-cutaneous nerve.

4. =Capsularis= (Scapulo-humeralis posticus s. gracilis).—This is a very small muscle, which lies on the back of the capsule of the shoulder joint.

Origin.—The scapula, close to the rim of the glenoid cavity.

Insertion.—The posterior surface of the shaft of the humerus, a short distance below the head.

Action.—It may perhaps tense the capsule of the shoulder joint and prevent its being pinched during flexion.

Structure.—It is fleshy and usually about the breadth of a finger. It may, however, consist of only a few bundles of fibers; sometimes it is double. Its attachment to the joint capsule is slight. It passes through the brachialis muscle to reach its insertion.

Relations.—Superficially, the teres minor and triceps muscles; deeply, the teres major and subscapularis muscles, and the capsule of the joint.

Blood-supply.—Posterior circumflex artery.

Nerve-supply.—Axillary nerve.

III. THE MUSCLES OF THE ARM

This group consists of five muscles which are grouped around the humerus. They arise from the scapula and the humerus, and are inserted into the forearm. They act on the elbow joint and the fascia of the forearm.

1. =Biceps brachii= (Coraco-radialis; flexor brachii).—This is a strong, somewhat rounded muscle, which lies on the anterior surface of the humerus.

Origin.—The tuberosity of the scapula.

Insertion.—(1) The bicipital tuberosity of the radius; (2) the internal lateral ligament of the elbow joint; (3) the fascia of the forearm and the tendon of the extensor carpi radialis.

Action.—To flex the elbow joint, to fix the shoulder, elbow, and carpus in standing, and to tense the fascia of the forearm.

Structure.—The muscle is inclosed in a double sheath of fascia, which is attached to the lips of the bicipital groove and the deltoid ridge of the humerus. The tendon of origin is moulded on the bicipital groove; it is very strong and dense and is partly cartilaginous. Its play over the groove is facilitated by the large =bicipital bursa= (Bursa intertubercularis). The synovial membrane covers not only the deep face of the tendon, but extends somewhat over the edges to the superficial face. A well-marked tendinous intersection runs through the belly of the muscle and divides distally into two portions. Of these, the short, thick one is inserted into the bicipital tuberosity and detaches fibers to the internal lateral ligament. The long tendon (Lacertus fibrosus) is thinner, blends with the fascia of the forearm, and ends by fusing with the tendon of the extensor carpi radialis.

Relations.—Externally, the mastoido-humeralis and brachialis muscles; internally, the posterior deep pectoral and the superficial pectoral muscles; in front, the anterior deep pectoral muscle; behind, the humerus, the coraco-brachialis muscle, the anterior circumflex and anterior radial vessels, and the musculo-cutaneous nerve.

Blood-supply.—Branches of the brachial and anterior radial arteries.

Nerve-supply.—Musculo-cutaneous nerve.

2. =Brachialis= (Humeralis obliquus s. externus; brachialis anticus).—This muscle occupies the musculo-spiral groove of the humerus.

Origin.—The proximal third of the posterior surface of the humerus.

Insertion.—The inner surface of the neck of the radius (under cover of the lateral ligament) and the arciform ligament.

Action.—To flex the elbow joint.

Structure.—The peculiar spiral course of this muscle gave rise to the name often applied to it—humeralis obliquus. Beginning on the posterior surface of the shaft, close to the head of the humerus, it winds over the lateral surface, crosses the biceps very obliquely, and finally reaches the inner side of the forearm by passing between the biceps and the extensor carpi. It is entirely fleshy, with the exception of its relatively slender tendon of insertion.

Relations.—Externally the skin and fascia, the teres minor, deltoid, triceps (lateral head), biceps, and mastoido-humeralis muscles. The anterior radial artery crosses the deep face of the muscle in its distal third, and the radial nerve accompanies the muscle in the distal half of the musculo-spiral groove.

Blood-supply.—Brachial artery.

Nerve-supply.—Radial nerve.

3. =Tensor fasciæ antibrachii= (Fig. 187)[58] (Scapulo-ulnaris; long extensor of the forearm; accessory anconeus of the latissimus dorsi).—This is a thin muscle which lies on the inner surface of the triceps.

Origin.—The tendon of insertion of the latissimus dorsi and the posterior border of the scapula.

Insertion.—(1) The fascia of the forearm; (2) a small eminence on the posterior border of the olecranon.

Action.—(1) To tense the fascia of the forearm and to extend the elbow joint.

Structure.—The origin consists of a very thin aponeurosis which blends with those of the caput longum and the latissimus dorsi. The muscular portion is quite thin in its anterior part, somewhat thicker behind, and is narrower than the aponeurotic origin. It is succeeded by an aponeurotic insertion, which ends chiefly by blending with the fascia of the forearm a little below the elbow. There is, however, a small but constant tendinous attachment to the olecranon.

Relations.—Externally, the panniculus, triceps (long and internal heads), the inner and middle flexors of the carpus, and the ulnar vessels and nerve; internally, the latissimus dorsi, serratus magnus, and posterior pectoral muscles.

Blood-supply.—Subscapular, ulnar, and deep brachial arteries.

Nerve-supply.—Radial nerve.

4. =Triceps brachii= (Figs. 178, 179, 187) (Brachial triceps; triceps extensor cubiti).—This, together with the preceding muscle, constitutes the large muscular mass which fills the angle between the posterior border of the scapula and the humerus. It is clearly divisible into three heads, as described below.

(=a=) =Long head= (Caput longum tricipitis; anconeus longus; caput magnum).—This, the largest and longest of the three heads, is a powerful, thick, triangular muscle, which extends from the posterior border of the scapula to the olecranon.

Origin.—The posterior border of the scapula.

Insertion.—The outer and posterior part of the summit of the olecranon.

Action.—(1) To extend the elbow joint; (2) to flex the shoulder joint.

Structure.—The muscle arises by a wide, strong aponeurosis from the posterior border of the scapula. From this the bundles of the fleshy portion converge to the short, strong tendon of insertion. A careful examination will show that the muscle is penetrated by a tendinous intersection from which many fibers take origin obliquely. The superficial face is covered by an aponeurosis which is specially developed at its distal part. A small =bursa= occurs under the tendon of insertion.

Relations.—Externally, the panniculus, deltoid, infraspinatus, teres minor, and the external head; internally, the tensor fasciæ antibrachii, teres major, latissimus dorsi, and posterior deep pectoral muscles, and the subscapular vessels; in front, the brachialis, and the inner head, the deep brachial and posterior circumflex vessels, and the axillary and radial nerves; behind, the skin and fascia.

Blood-supply.—Subscapular and deep brachial arteries.

Nerve-supply.—Radial nerve.

(=b=) =External head= (Caput laterale tricipitis s. anconeus lateralis s. externus; caput medium).—This is a strong, quadrilateral muscle, which lies on the outer surface of the arm. Its proximal third is covered by the deltoid and teres minor muscles; the remainder only by the thin panniculus and the skin.

Origin.—The deltoid tuberosity and the curved rough line which extends from it to the neck of the humerus.

Insertion.—(1) A small prominent area on the outer surface of the olecranon; (2) the tendon of the long head.

Action.—To extend the elbow joint.

Structure.—The origin consists of short tendinous fibers. The belly is thick, and is composed of parallel bundles which are directed obliquely downward and backward. They are inserted partly into the tendon of the long head and partly into the olecranon below and in front of that tendon.

Relations.—Externally, the deltoid, teres minor, and panniculus muscles; internally, the long and inner heads and the brachialis muscle. Branches of the circumflex vessels and axillary nerve emerge between the posterior edge of the muscle and the long head. The deep face of the muscle is related to the branches of the deep brachial artery and of the radial nerve.

Blood-supply.—Posterior circumflex and deep brachial arteries.

Nerve-supply.—Radial nerve.

(=c=) =Internal head= (Fig. 193) (Caput mediale tricipitis; anconeus medialis s. internus; caput parvum).—This is much the smallest of the three heads. It is situated on the inner surface of the arm, and extends from the middle third of the humerus to the olecranon.

Origin.—The middle third of the inner surface of the shaft of the humerus, behind and below the internal tubercle.

Insertion.—The inner and fore part of the summit of the olecranon, between the insertion of the long head and the origin of the ulnar head of the flexor perforans.

Action.—To extend the elbow joint.

Structure.—The muscle is fleshy except at its insertion, where it has a flat tendon, under which a small bursa may be found.

Relations.—Externally, the humerus, brachialis, anconeus, and the external head; internally, the posterior deep pectoral, coraco-brachialis, teres major, latissimus dorsi, and tensor fasciæ antibrachii muscles, the brachial and deep brachial vessels, and the median and ulnar nerves; behind, the long head, branches of the deep brachial vessels, and the radial nerve.

Blood-supply.—Deep brachial and ulnar arteries.

Nerve-supply.—Radial nerve.

5. =Anconeus= (M. anconeus parvus s. subanconeus).—This is a small fleshy muscle which covers the olecranon fossa and is covered by the triceps. It is somewhat difficult to separate from the outer head.

Origin.—The distal third of the posterior surface of the humerus.

Insertion.—The outer surface of the olecranon.

Action.—To extend the elbow joint, and to raise the capsule of the joint and prevent its being pinched during extension.

Structure.—It is almost entirely fleshy, and is adherent by its deep face to the joint capsule.

Relations.—Superficially, the triceps muscle; deeply, the humerus and the elbow joint.

Blood-supply.—Deep brachial artery.

Nerve-supply.—Radial nerve.

IV. FASCIÆ AND MUSCLES OF THE FOREARM AND MANUS

The forearm is covered on three sides by the muscles of this group, leaving the inner surface of the radius for the most part subcutaneous. The extensors of the carpus and digit lie on the anterior (dorsal) and external part of the region, while the flexors occupy the posterior (volar) surface.

The =fascia= of the =forearm= (Fascia antibrachii) forms a very strong and complete investment for all the muscles of the region. The =superficial fascia= is thin, and disappears at the carpus by fusing with the deep fascia. The =deep fascia= is very strong and tendinous in character. It furnishes insertion at its upper and inner part to the tensor fasciæ antibrachii and posterior superficial pectoral muscles; at its upper anterior and outer part, to aponeuroses from the mastoido-humeralis and biceps. It is attached at the elbow by its deep face to the outer tuberosities of the humerus and radius, to the ulna, and to the lateral ligaments. On the inner surface of the forearm it blends with the periosteum on the subcutaneous surface of the radius. It is closely adherent to the surface of the extensor muscles, but is rather loosely attached to the flexors. From its deep face are detached =intermuscular septa=, which form sheaths for the muscles and are attached to the underlying bones. The principal septa are: (a) One which passes between the common extensor (in front) and the lateral extensor and flexor carpi externus (behind); (b) one between the common extensor and the extensor carpi radialis; (c) one between the inner and middle flexors of the carpus.

The =carpal fascia= (Fascia carpi) is a direct continuation of that of the forearm. It is attached chiefly to the tuberosities at the distal end of the radius, to the accessory carpal (pisiform) bone, and to the lateral ligaments. In front it forms the so-called =anterior annular ligament= (Ligamentum carpi dorsale), bridging over the grooves and binding down the extensor tendons and their synovial sheaths. Behind it is greatly thickened and forms the =posterior annular= or =transverse ligament= of the carpus (Ligamentum carpi transversum). This stretches across from the accessory carpal bone to the internal lateral ligament and the proximal extremity of the inner metacarpal bone. It thus completes the carpal canal, in which lie the flexor tendons, their synovial sheath, and the principal vessels and nerves of the region.

The =fascia= of the =metacarpus= and =digit= (Fascia metacarpea et digitalis) is, in general, thinner than the preceding. It is attached to the tendons, ligaments, and the exposed bony prominences—especially to the small metacarpal bones. On the flexion surface of the fetlock joint it is much thickened by fibers passing transversely from one sesamoid bone to the other, forming an =annular ligament= which binds down the flexor tendons in the sesamoid groove or canal. Distal to this is a second thick quadrilateral sheet (Ligamentum vaginale) which covers and is adherent to the tendon of the flexor perforatus. It is attached on either side by two bands to the borders of the first phalanx, thus firmly binding down the flexor tendons. A little further down a crescentic =fibro-elastic sheet= covers the terminal expansion of the deep flexor tendon. It is attached on either side by a strong band to the side of the first phalanx about its middle.

A. EXTENSOR DIVISION

1. =Extensor carpi radialis= (M. extensor carpi radialis s. radialis dorsalis; extensor metacarpi magnus; anterior extensor of the metacarpus).—This is the largest muscle of the extensor division, and lies on the anterior (dorsal) surface of the radius.

Origin.—(1) The external condyloid crest of the humerus; (2) the coronoid fossa.

Insertion.—The tuberosity on the anterior (dorsal) surface of the proximal extremity of the large (third) metacarpal bone.

Action.—To extend and fix the carpal joint and to flex the elbow joint.

Structure.—The belly of the muscle is rounded, and runs out to a point at the distal third of the forearm. The tendon, which runs nearly the whole length of the fleshy portion, appears on the surface of the latter about its middle; here the muscle shows a distinctly pennate arrangement. The tendon passes through the middle groove at the distal extremity of the radius and over the capsule of the carpal joint, bound down by the anterior annular ligament and invested with a =synovial sheath=. The latter begins three to four inches (ca. 8 to 10 cm.) above the carpus and extends almost to the insertion of the tendon. In the lower half of the forearm the deep fascia blends with the tendon, and here the latter is joined by the long tendon of the biceps.

Relations.—Superficially, the skin, fascia, and the oblique extensor; deeply, the capsule of the elbow joint, the biceps tendon, the radius, the carpal joint capsule, the anterior radial artery, and the radial nerve; externally, the anterior or common extensor; internally, at the elbow, the brachialis and biceps.

Blood-supply.—Anterior radial artery.

Nerve-supply.—Radial nerve.

2. =Anterior= or =common digital extensor= (M. extensor digitalis communis; anterior extensor of the phalanges; extensor pedis).—This muscle lies external to the foregoing, which it resembles in general form, although less bulky.

Origin.—(1) The front of the distal extremity of the humerus, just external to the coronoid fossa; (2) the external tuberosity on the proximal extremity of the radius, the external lateral ligament of the elbow, and the external border of the radius at the junction of its proximal and middle thirds; (3) the external surface of the shaft of the ulna; (4) the fascia of the forearm.

Insertion.—(1) The extensor (or pyramidal) process of the third phalanx; (2) the anterior surface of the proximal extremities of the first and second phalanges.

Action.—To extend the digital and carpal joints, and to flex the elbow joint.

Structure.—The muscle is a compound one, representing the common extensor, together with vestiges of the proper extensors of the digits. Usually at least =two heads= may be distinguished. The principal or humeral head (Caput humerale) arises from the front of the extensor epicondyle of the humerus in common with the extensor carpi. Its belly is fusiform, and terminates in a point near the distal third of the radius. The tendon appears on the surface of the muscle about the middle of the belly, the arrangement being pennate. The tendon passes downward through the outer of the two large grooves on the front of the distal end of the radius, and over the capsule of the carpal joint. Passing down over the front of the metacarpus, it inclines gradually inward, reaching the middle line of the limb near the fetlock. A little below the middle of the first phalanx it is joined by the branches of the suspensory ligament, and the tendon thus becomes much wider. Two =synovial membranes= facilitate the play of the tendon. The proximal one is a =sheath= which begins about three inches (ca. 7 to 8 cm.) above the carpus, and terminates at the proximal end of the metacarpus. At the fetlock a =bursa= occurs between the tendon and the joint capsule, but otherwise the two are adherent. The smaller head, arising chiefly from the radius and ulna, is often divisible into two parts (Fig. 443). The larger of these (Caput radiale, muscle of Phillips)[59] arises from the outer tuberosity and outer border of the radius, and from the lateral ligament of the elbow joint. The flat belly is succeeded by a delicate tendon, which accompanies the principal tendon over the carpus (included in the same sheath), and then passes outward to fuse with the tendon of the lateral extensor. Usually a slip is detached which is inserted on the proximal extremity of the first phalanx, or ends in the fascia here. The smaller and deeper division (Caput ulnare, muscle of Thiernesse)[60] is usually somewhat difficult to isolate. It arises from the ulna close to the interosseous space. It has a small rounded belly and is provided with a delicate tendon which may fuse with the principal tendon or may be inserted into the capsule and the fascia in front of the fetlock joint.

Relations.—The chief relations of the belly of the muscle are: superficially, the skin and fascia; deeply, the elbow joint, the radius and ulna, the extensor carpi obliquus, and the anterior radial vessels and radial nerve; in front and internally, the extensor carpi radialis; behind, the lateral extensor and the interosseous vessels.

Blood-supply.—Radial and interosseous arteries.

Nerve-supply.—Radial nerve.

3. =Lateral digital extensor= (M. extensor digitalis lateralis s. digiti quinti proprius; lateral extensor of the phalanges; extensor suffraginis).—This muscle is much smaller than the preceding, behind which it is situated.

Origin.—The external tuberosity of the radius and the lateral ligament of the elbow joint, the shaft of the ulna, the outer border of the radius, and the intermuscular septum.

Insertion.—An eminence on the front of the proximal extremity of the first phalanx.

Action.—To extend the digit and carpus.

Structure.—The muscle is pennate, and is inclosed in a sheath formed by the deep fascia, from which many fibers arise. The belly is thin and fusiform and terminates at the lower third of the forearm. From here the tendon (at first small and round) passes downward through the groove on the outer tuberosity of the distal end of the radius, then over the carpus, and, gradually inclining toward the front, but not reaching the middle line of the limb, it passes over the metacarpus and fetlock to its insertion. Two =synovial membranes= occur in connection with the tendon. A =sheath= envelops the tendon, beginning about three inches (ca. 6 to 8 cm.) above the carpus, and reaching to the proximal end of the metacarpus. At the fetlock a small bursa lies between the tendon and the joint capsule, but otherwise the tendon is adherent to the capsule. The tendon becomes flat and much larger below the carpus, having received the tendon of the radial head of the anterior extensor and a strong band from the accessory carpal bone.

Relations.—Superficially, the skin and fascia; deeply, the outer face of the radius and ulna; in front, the common extensor, the oblique extensor, and the interosseous artery; behind, the external flexor of the carpus and the deep flexor of the digit.

Blood-supply.—Interosseous artery.

Nerve-supply.—Radial nerve.

4. =Extensor carpi obliquus= (M. abductor pollicis longus et extensor pollicis brevis; oblique extensor of the metacarpus; extensor metacarpi obliquus).—This is a small muscle which curves obliquely over the distal half of the radius and the carpus.

Origin.—The external border and adjacent part of the anterior surface of the radius (the attachment area beginning at a point above the middle of the bone and extending down to its lower fourth).

Insertion.—The head of the inner (second) metacarpal bone.

Action.—To extend the carpal joint.

Structure.—The muscle is pennate and has a flat belly which curves downward, forward, and inward over the distal part of the radius. The tendon continues the direction of the muscle, crossing obliquely over the tendon of the extensor carpi radialis; it then occupies the oblique groove at the distal end of the radius, and crosses the inner face of the carpus. It is provided with a =synovial sheath=.

Relations.—Superficially, the skin and fascia, the lateral extensor, and the common extensor; deeply, the radius, the extensor carpi radialis, the carpal capsule, and the internal lateral ligament.

Blood-supply.—Interosseous and anterior radial arteries.

Nerve-supply.—Radial nerve.

B. FLEXOR DIVISION

1. =Flexor carpi internus= (M. flexor carpi radialis s. radialis volaris; internal flexor of the metacarpus).—This muscle lies on the inner surface of the forearm, immediately behind the inner border of the radius.

Origin.—The flexor (internal) epicondyle of the humerus, below and behind the lateral ligament.

Insertion.—The proximal end of the internal (second) metacarpal bone.

Action.—To flex the carpal joint and to extend the elbow.

Structure.—The muscle has a short tendon of origin, which is succeeded by a somewhat flattened, fusiform belly. The tendon of insertion begins near the distal fourth of the radius. It is provided with a synovial sheath which begins two or three inches (ca. 5 to 8 cm.) above the carpus and extends almost to the insertion of the tendon.

Relations.—Superficially, the skin and fascia, the posterior superficial pectoral and the tensor fasciæ antibrachii; deeply, the elbow joint, the radius, the deep flexor, the flexor carpi medius, the posterior radial vessels, and the median nerve. At the elbow the artery and nerve lie in front of the muscle, but below they dip beneath it.

Blood-supply.—Posterior radial artery.

Nerve-supply.—Median nerve.

On removing the deep fascia on the inner surface of the elbow the student may notice a small muscle lying along the lateral ligament. This is the =pronator teres=, which is usually not present or a mere vestige in the horse. It arises by a small, flat tendon from the flexor epicondyle of the humerus, and is inserted into the internal lateral ligament of the elbow. On account of its small size and the fact that the forearm is fixed in the position of pronation, the muscle can have no appreciable function. It is usually represented by the long portion of the internal lateral ligament.

2. =Flexor carpi medius= (M. flexor carpi ulnaris s. ulnaris medialis; oblique or middle flexor of the metacarpus).—This muscle lies on the inner and posterior aspect of the forearm partly under, partly behind, the preceding muscle. It arises by two heads—humeral and ulnar.

Origin.—(1) The flexor (internal) epicondyle of the humerus just behind the preceding muscle; (2) the inner surface and posterior border of the olecranon.

Insertion.—The upper edge of the accessory carpal (pisiform) bone.

Action.—To flex the carpal joint, and to extend the elbow.

Structure.—The humeral head is much the larger, constituting, in fact, the bulk of the muscle. It is flattened, curved, and tapers at both ends. The ulnar head, much smaller and very thin, is covered by an aponeurosis from which many of its fibers arise. It joins the large head a little above the middle of the forearm. The tendon of insertion is short and strong; it blends with the posterior annular ligament of the carpus.

Relations.—Superficially, the tensor fasciæ antibrachii, superficial pectoral, and flexor carpi internus, the skin and fascia, and cutaneous branches of the ulnar nerve; deeply, the superficial and deep flexors of the digit. In the distal half of the forearm the ulnar vessels and nerve lie between the outer edge of this muscle and the external flexor of the carpus.

Blood-supply.—Ulnar and posterior radial arteries.

Nerve-supply.—Ulnar and median nerves.

3. =Flexor carpi externus= (M. extensor carpi ulnaris[61] s. ulnaris lateralis; external flexor of the metacarpus).—This muscle lies on the outer face of the forearm, behind the lateral extensor of the digit.

Origin.—The extensor (external) epicondyle of the humerus, behind and below the lateral ligament.

Insertion.—(1) The outer surface and upper edge of the accessory carpal bone; (2) the proximal extremity of the outer (fourth) metacarpal bone.

Action.—To flex the carpal joint and to extend the elbow.

Structure.—The belly of the muscle is flattened and is intersected by a good deal of tendinous tissue. There are two tendons of insertion. The short tendon is inserted into the accessory carpal bone. The long tendon is detached just above the carpus; it is smaller and rounded; it passes downward and a little forward through a groove on the outer surface of the accessory carpal bone, enveloped by a =synovial sheath=, to reach its insertion on the outer metacarpal bone. A synovial pouch lies under the origin of the muscle at the elbow joint, with the cavity of which it communicates.

Relations.—Superficially, the skin, fascia, and cutaneous branches of the ulnar nerve; deeply, the elbow joint, the ulna, and the flexors of the digit; in front, the lateral extensor of the digit; behind, the middle flexor of the carpus, the ulnar head of the deep flexor, and the ulnar vessels and nerve.

Blood-supply.—Interosseous, ulnar, and median arteries.

Nerve-supply.—Radial nerve.

4. =Superficial digital flexor= or =flexor perforatus= (M. flexor digitalis sublimis; superficial flexor of the phalanges).—This muscle is situated in the middle of the flexor group, chiefly under cover of the middle flexor of the carpus.

Origin.—(1) The flexor epicondyle of the humerus, between the flexor carpi internus and the deep flexor of the digit; (2) a ridge on the posterior surface of the radius, below its middle and near the internal border.

Insertion.—(1) The eminences on the proximal extremity of the second phalanx behind the lateral ligaments; (2) the distal extremity of the first phalanx, also behind the lateral ligaments.

Action.—To flex the digit and carpus, and to extend the elbow.

Structure.—The fleshy portion of the muscle or =humeral head= takes origin from the humerus. The =radial head= (Caput tendineum) consists of a strong fibrous band, usually termed the =radial= or =superior check ligament=, which fuses with the tendon near the carpus. The belly of the muscle is intersected by tendinous strands, and fuses more or less with that of the deep flexor. Near the carpus it is succeeded by a strong, thick tendon which passes down through the carpal canal, formed by the posterior ligament of the carpus, the accessory carpal bone, and the posterior annular ligament (Ligamentum carpi transversum). Here it is enveloped by a synovial sheath, in common with the deep flexor. This =carpal sheath= (Vagina carpea) begins three or four inches (8 to 10 cm.) above the carpus, and extends downward nearly to the middle of the metacarpus. Below the carpus the tendon becomes flattened and broader. Near the fetlock it forms a ring through which the tendon of the deep flexor passes (Fig. 150). Here the two tendons are bound down in the sesamoid groove by the posterior annular ligament, which fuses more or less with the superficial flexor tendon. At the distal end of the first phalanx the tendon divides into two branches which diverge to reach their points of insertion, and between these branches the tendon of the deep flexor emerges (Fig. 189). A second =synovial sheath=, the =digital= or =sesamoidean= (Vagina digitalis), begins at the distal third of the metacarpus, three to four inches (8 to 10 cm.) above the fetlock, and extends to the middle of the second phalanx.

Relations.—The belly of the muscle is related superficially to the ulnar head of the deep flexor, the flexor carpi medius, and, at its origin, to the ulnar vessels and nerve; deeply to the humeral head of the deep flexor. The tendon is related superficially to the skin and fascia; deeply to the deep flexor tendon.

Blood-supply.—Posterior radial artery.

Nerve-supply.—Median nerve.

5. =Deep digital flexor= or =flexor perforans= (M. flexor digitalis profundus; deep flexor of the phalanges).—The fleshy part of this muscle lies on the posterior surface of the radius, under cover of the preceding muscles. It is the largest muscle of the flexor group.

Origin.—(1) The flexor (internal) epicondyle of the humerus; (2) the inner surface of the olecranon; (3) the middle of the posterior surface of the radius and a small adjacent area of the ulna.

Insertion.—The semilunar crest of the third phalanx, and the adjacent surface of the lateral cartilage.

Action.—To flex the digit and carpus, and to extend the elbow.

Structure.—This muscle consists of three principal heads. The =humeral head= (Caput humerale) constitutes the bulk of the muscle. It is marked by tendinous intersections, and is separable into three secondary heads. The =ulnar head= (Caput ulnare, ulnaris accessorius) is much smaller, and is at first superficially situated between the outer and middle flexors of the carpus, the =radial head= (Caput radiale, radialis accessorius) is the smallest, and is not always present; it is situated on the distal two-thirds of the posterior surface of the radius, under the humeral head. Each of these heads is provided with a tendon. The principal tendon—that of the humeral head—begins about three or four inches (8 to 10 cm.) above the carpus. It is soon joined by the small tendon of the ulnar head, which begins about the middle of the forearm. The tendon of the radial head fuses with the principal tendon close to the carpus. The conjoined tendon passes downward through the carpal canal, being included in the =carpal synovial sheath= with the superficial flexor tendon, as previously described. Continuing downward, it is joined about the middle of the metacarpus by a strong fibrous band, the so-called =inferior= or =subcarpal check ligament= (Caput tendineum). This is a direct continuation of the posterior ligament of the carpus. Below this the tendon passes through the ring formed by the perforatus, then in succession over the sesamoid groove, the inferior sesamoidean ligaments, and the tendon surface of the third sesamoid, to its insertion (Figs. 150 and 151). Its terminal part is much widened. From the distal third of the metacarpus to the distal end of the second phalanx it is inclosed in the =digital synovial sheath= described in connection with the perforatus. The =navicular bursa= or =bursa podotrochlearis= is found between the tendon and the third sesamoid or navicular bone. The terminal part of the tendon is bound down by the fibrous sheet described with the fascia.

Relations.—The belly of the muscle is related posteriorly to the perforatus and the middle flexor of the carpus; internally, to the internal carpal flexor, the radial check ligament, and the posterior radial vessels and median nerve; externally, to the external flexor of the carpus; anteriorly, to the radius and ulna and branches of the posterior radial artery and median nerve. Below the carpus the tendon is accompanied by the vessels and nerves of the digit. It may also be noted that the muscle is not entirely covered by the other flexors; it comes in contact with the skin and fascia on the postero-external aspect of the proximal half of the forearm, and also on the outer aspect of the distal fourth.

Blood-supply.—Posterior radial and ulnar arteries.

Nerve-supply.—Median and ulnar nerves.

METACARPAL MUSCLES

The five muscles of this group are either reduced to vestiges or modified greatly in structure.

1, 2. =Lumbricales= (internus et externus).—These are two small muscles which lie on either side of the flexor tendons above the fetlock. They arise from the deep flexor tendon, and are lost in the fibrous tissue which lies under the nodule of horn or ergot at the fetlock (Fig. 447). Their action is inappreciable. The size of these muscles is subject to much variation. Often very little muscular tissue can be found, but the small tendon is constantly present.

Blood-supply.—Metacarpal arteries.

Nerve-supply.—Median and ulnar nerves.

3, 4, 5. =Interossei.=—These are three in number in the horse, and are situated chiefly in the metacarpal groove. Two, the =internus= and =externus=, are very small muscles, each of which arises from the corresponding small metacarpal bone near its proximal extremity, and is provided with a delicate tendon which is usually lost in the fascia at the fetlock (Fig. 150). They have no appreciable action. Their blood- and nerve-supply is the same as that of the preceding muscles.

The =interosseus medius= is so much modified that it is usually termed the =suspensory=, or =superior sesamoidean ligament=. It contains little muscular tissue, being transformed very largely into a strong tendinous band, bifurcate below, and having for its chief function the supporting of the fetlock. It has been described, in deference to common usage, with the ligaments.

FASCIÆ AND MUSCLES OF THE PELVIC LIMB

THE FASCIÆ

The =iliac fascia= (Fascia iliaca) covers the ventral surface of the iliacus and psoas muscles, over which it is tightly stretched (Fig. 450). It is attached internally to the tendon of the psoas minor, externally to the inguinal (Poupart’s) ligament and the external angle of the ilium. Its anterior part is thin. Posteriorly, it is in part attached to the ilium, in part becomes continuous with the pelvic fascia. It furnishes surfaces of origin for the sartorius, cremaster externus, and transversus abdominis muscles.

The =pelvic fascia= (Fascia pelvis) lines the cavity (parietal layer) and is reflected on the viscera at the pelvic outlet (visceral layer). Laminæ are detached from it to strengthen the various peritoneal folds.

The =superficial fascia= of the =gluteal region= is continuous with the aponeurosis of the panniculus. It covers and partly blends with the deep fascia. The =gluteal fascia= (Fascia glutea) covers the superficial muscles of the region, and detaches intermuscular septa, which pass between the muscles. It is attached to the sacral spines, the dorsal sacro-iliac ligament, and the angles of the ilium, and is continuous in front with the lumbo-dorsal fascia, behind with the coccygeal fascia. Its deep face gives origin to fibers of the superficial and middle glutei, the biceps femoris, and the semitendinosus, so that care is necessary in dissecting it off these muscles. The chief =intermuscular septa= are: (1) One which passes between the superficial gluteus and the biceps femoris; (2) one between the biceps and semitendinosus, from which a lamella is detached which passes between the middle and posterior portions of the biceps and is attached to the tuber ischii; (3) one between the semitendinosus and semimembranosus, which is attached to the sacro-sciatic ligament and tuber ischii; it furnishes origin for fibers of the long head of the semimembranosus.

The =fascia lata= is continuous with the preceding, and covers the muscles on the outer surface and front of the thigh. It is tendinous and very strong, and for the most part easily separable from the underlying muscles. It furnishes insertion to the tensor fasciæ latæ and to the biceps femoris (in part), by both of which it is tensed. At the stifle it is attached to the patella and the inner and outer straight ligaments. Internally it is continuous with the internal femoral fascia. It furnishes the following =intermuscular septa=: (1) One which passes between the vastus externus and biceps femoris to be attached to the external trochanter of the femur; (2) two which pass between the three branches of the biceps femoris; (3) a fourth between the biceps femoris and semitendinosus.

The =internal femoral fascia= (Fascia femoralis medialis) covers the superficial muscles on the inner surface of the thigh. At its upper part it is joined by part of the aponeurosis of the external oblique muscle (Lamina femoralis) (Fig. 450). The posterior part is thin. It is continuous with the fascia lata in front and the crural fascia below. At the stifle it fuses with the tendons of the sartorius and gracilis.

The =crural fascia=, or fascia of the leg (Fascia cruris), consists of three layers. Two of these invest the entire region and may, therefore, be termed the =common fasciæ=. The superficial layer is a continuation of the fasciæ of the thigh, while the second layer may be regarded chiefly as a continuation of the tendons of the superficial muscles of the hip and thigh (biceps femoris, semitendinosus, tensor fasciæ latæ, sartorius, and gracilis). The two layers frequently fuse, and are attached chiefly to the patellar ligaments and the crest and internal surface of the tibia. About the middle of the leg the two layers unite behind the deep flexor of the digit and form a strong band which passes downward in front of the tendons of the gastrocnemius and superficial flexor, to be attached with the latter to the anterior and inner part of the tuber calcis. This may be regarded as an accessory or tarsal tendon of insertion of the biceps femoris and semitendinosus. The third layer forms sheaths for the muscles, furnishing origin in part to their fibers. Two important =intermuscular septa= are detached, viz.: (1) One which passes between the anterior and lateral digital extensors to be attached to the fibula and the external border of the tibia; (2) one between the lateral extensor and the deep flexor.

The =tarsal fascia= (Fascia tarsi) fuses with the ligaments and bony prominences of the region. It is strong and tendinous in front, and joins the tendon of the anterior extensor below the joint. Laterally, it is thin and fuses with the ligaments. Posteriorly, it is very thick and strong, forming an annular ligament which stretches from the internal lateral ligament to the fibular tarsal bone and the plantar ligament. This converts the groove at the back of the hock into a canal, in which are the deep flexor tendon with its synovial sheath and the plantar nerves. In front of the tarsus and above and below it there are three =annular ligaments= (Ligamenta transversa). The proximal one binds down the tendons of the anterior extensor, peroneus tertius, and tibialis anterior on the distal end of the tibia. The middle one stretches from the fibular tarsal bone to the outer tendon of the peroneus tertius, forming a loop around the tendon of the anterior extensor. The distal band stretches across the proximal extremity of the large metatarsal bone and incloses the tendons (and sheaths) of the two extensors of the digit.

The =metatarsal= and =digital fasciæ= do not differ materially from those of the corresponding regions of the thoracic limb.

THE MUSCLES

I. THE SUBLUMBAR MUSCLES (Figs. 450, 456)

The muscles of this group are not confined to the sublumbar region, but extend beyond it both before and behind. Their chief function is to flex the hip joint. Two, however,—the psoas minor and the quadratus lumborum,—have not this action.

1. =Psoas minor= (s. parvus).—This is a fusiform, flattened, pennate muscle, which lies along the ventro-lateral aspect of the bodies of the last three thoracic and the lumbar vertebræ.

Origin.—The bodies of the last three thoracic and first four or five lumbar vertebræ, and the vertebral ends of the sixteenth and seventeenth ribs.

Insertion.—The psoas tubercle on the shaft of the ilium.

Action.—To flex the pelvis on the loins, or to incline it laterally.

Structure.—The muscle arises by a series of digitations which pass backward and outward to be inserted at an acute angle on the tendon. The latter lies along the outer border of the fleshy portion and is flattened. It appears on the surface of the muscle at the third lumbar process and increases gradually in width until it reaches the pelvic inlet, where it becomes narrower.

Relations.—The ventral surface of the thoracic portion of the muscle is related to the pleura, crura of the diaphragm, and sympathetic nerve. In the abdomen, the chief ventral relations are the peritoneum, the vena cava (right side), the aorta and left kidney (left side), the sympathetic nerves, and the ureters. Dorsally, the chief relations are the vertebræ, the psoas major, and lumbar nerves. Near its insertion the tendon is crossed internally by the external iliac artery, and externally by the femoral nerve.

Blood-supply.—Intercostal and lumbar arteries.

Nerve-supply.—Lumbar nerves.

2. =Psoas major= (s. magnus).—This is much larger than the preceding muscle, by which it is partly covered. It is triangular, with the base anterior.

Origin.—The ventral surfaces of the vertebral ends of the last two ribs and the transverse processes of the lumbar vertebræ.

Insertion.—The internal trochanter of the femur, by a common tendon with the iliacus.

Action.—To flex the hip joint and to rotate the thigh outward.

Structure.—The origin of the muscle is fleshy, the belly being in general flattened, thick in its middle, thin at its edges. The thoracic portion is small, the abdominal part much thicker and wider, extending laterally beyond the extremities of the lumbar transverse processes. From the lumbo-sacral articulation it lies in a deep groove formed in the iliacus (with which it is partly united), becomes smaller and rounded, and passes downward and backward to terminate by a strong tendon common to it and the iliacus.[62]

Relations.—Dorsally, the last two ribs and thoracic vertebræ, the lumbar vertebræ, the internal intercostals, quadratus lumborum, longissimus dorsi, and iliacus, and the lumbar vessels and nerves; ventrally, the pleura and peritoneum, the iliac fascia, inguinal ligament, the diaphragm, psoas minor, and sartorius, the kidneys, the spleen, the intestine (duodenum, cæcum, etc.) and the circumflex iliac vessels.

Blood-supply.—Lumbar and circumflex iliac arteries.

Nerve-supply.—Lumbar and femoral nerves.

3. =Iliacus.=—This muscle covers the ventral surface of the ilium external to the sacro-iliac articulation, and extends outward beyond the external border of the bone, underneath the middle gluteus.

Origin.—The ventral surface of the ilium external to the ilio-pectineal line, the ventral sacro-iliac ligament, the wing of the sacrum, and the tendon of the psoas minor.

Insertion.—The internal trochanter of the femur, by a common tendon with the psoas major.

Action.—To flex the hip joint and to rotate the thigh outward.

Structure.—The belly of the muscle is so deeply grooved for the psoas major as to give the appearance of being completely divided into outer and inner portions. When the psoas is removed, it is seen, however, that the two heads are not entirely separated. The outer, larger head arises from the wing of the ilium chiefly; the inner, smaller head arises chiefly from a small area on the shaft of the ilium, between the psoas tubercle and the depression for the inner tendon of the rectus femoris, and from the tendon of the psoas minor. The two portions inclose the psoas major in front of the hip joint.

Relations.—Dorsally, the ilium, sacrum, sacro-iliac articulation, the gluteus medius, the ilio-lumbar and external circumflex vessels; ventrally, the iliac fascia, inguinal ligament, the psoas major, sartorius, and abdominal muscles. At the hip joint the chief relations are: internally, the femoral vessels, the femoral nerve, and the sartorius muscle; externally, the rectus femoris and tensor fasciæ latæ; in front, the abdominal muscles; behind, the hip joint.

Blood-supply.—Lumbar, circumflex iliac, and deep femoral arteries.

Nerve-supply.—Lumbar and femoral nerves.

4. =Quadratus lumborum.=—This thin muscle lies on the outer part of the ventral surfaces of the lumbar transverse processes.

Origin.—The ventral surface of the upper part of the last two ribs and the lumbar transverse processes.

Insertion.—The ventral surface of the wing of the sacrum and the ventral sacro-iliac ligament.

Action.—Acting together, to fix the last two ribs and the lumbar vertebræ; acting singly, to produce lateral flexion of the loins.

Structure.—The muscle is pennate, and is curved with the convexity outward. It is thin, largely mixed with tendinous fibers, and is, in general, little developed in the horse in comparison with some of the other animals (e. g., dog, sheep).

Relations.—Ventrally, the psoas major and the last intercostal and first three lumbar nerves; dorsally, the last two ribs, the lumbar transverse processes, and the lateral branches of the lumbar arteries.

Blood-supply.—Lumbar arteries.

Nerve-supply.—Lumbar nerves.

5. =Intertransversales lumborum= (M. intertransversarii lumborum).—These are described with the other spinal muscles. (See p. 238.)

II. THE EXTERNAL MUSCLES OF THE HIP AND THIGH

Under this head will be described the muscles of the outer surface of the pelvis and thigh, and those which form the posterior contour of the latter. They are given in the order in which they may be conveniently examined.

1. =Tensor fasciæ latæ= (Fig. 178).—This is the most anterior muscle of the superficial layer. It is triangular in form, with its apex at the external angle of the ilium.

Origin.—The external angle of the ilium.

Insertion.—The fascia lata, and thus indirectly to the patella, the external straight ligament, and the crest of the tibia.

Action.—To tense the fascia lata, flex the hip joint, and extend the stifle joint.

Structure.—The muscle arises by a rather small head, about two inches (ca. 5 cm.) wide, on the antero-inferior eminence of the external angle of the ilium. Below this the belly spreads out and terminates about midway between the point of the hip and the stifle in the aponeurosis. The belly fuses to a considerable extent with the superficial gluteus. The aponeurosis fuses with the fascia lata, and detaches a lamina which passes with the tendon of insertion of the superficial gluteus to the external border of the femur.

Relations.—Externally, the skin and fascia; internally, the obliquus abdominis externus, the iliacus, superficial gluteus, rectus femoris, and vastus externus, branches of the circumflex iliac, ilio-lumbar, and iliaco-femoral arteries, and the anterior gluteal nerve; anteriorly, the precrural lymph glands. A considerable quantity of connective tissue is found between the deep face of the muscle and the abdominal wall.

Blood-supply.—Circumflex iliac, ilio-lumbar, and iliaco-femoral arteries.

Nerve-supply.—Anterior gluteal nerve.

2. =Gluteus superficialis= (Figs. 178, 199) (Superficial gluteus; gluteus externus).—This muscle lies behind and partly underneath the tensor fasciæ latæ. It is triangular and consists of an anterior and a posterior head united by the gluteal fascia.

Origin.—(1) The external angle and the adjacent part of the external border of the ilium (anterior head); (2) the gluteal fascia (posterior head).

Insertion.—The third (external) trochanter of the femur.

Action.—To abduct the limb, flex the hip joint, and tense the gluteal fascia.

Structure.—The anterior head of the muscle is not completely separable (except artificially) from the tensor fasciæ latæ. The attachment to the border of the ilium is by means of an intermuscular septum, which passes beneath the thick outer border of the gluteus medius. The posterior head arises from the deep face of the gluteal fascia, and so indirectly from the dorsal sacro-iliac ligament. The two heads unite and terminate on a strong flat tendon, which is inserted into the edge of the third trochanter of the femur, beneath the biceps femoris.

Relations.—Superficially, the skin, fascia, and biceps femoris; deeply, the gluteus medius, iliacus, rectus femoris, and branches of the iliaco-femoral artery; in front, the tensor fasciæ latæ; behind, the biceps femoris.

Blood-supply.—Gluteal and iliaco-femoral arteries.

Nerve-supply.—Anterior gluteal nerve.

3. =Gluteus medius= (Figs. 178, 179) (Middle gluteus; gluteus maximus).—This is a very large muscle which covers the dorsal surface of the ilium and the greater part of the lateral wall of the pelvis, and extends forward also on the lumbar part of the longissimus.

Origin.—(1) The aponeurosis of the longissimus, as far forward as the first lumbar vertebra; (2) the gluteal or dorsal surface and internal and external angles of the ilium; (3) the dorsal and lateral sacro-iliac and sacro-sciatic ligaments, and the gluteal fascia.

Insertion.—(1) The summit of the trochanter major of the femur; (2) the crest below the trochanter; (3) the outer aspect of the trochanteric ridge.

Action.—To extend the hip joint and abduct the limb. By its connection with the longissimus a muscular mass is formed which is one of the chief factors in rearing, kicking, and propulsion.

Structure.—The anterior extremity of the muscle is relatively thin, and lies in a depression on the surface of the longissimus, from the strong aponeurosis of which the fibers take origin. The pelvic portion of the muscle is very voluminous, and forms the bulk of the muscular mass which gives the haunch its rounded contour. This part of the muscle is intersected by several tendinous sheets. One of these is particularly distinct, and is attached to the gluteal line on the ilium. This divides the muscle incompletely into superficial and deep strata. The superficial part is inserted by a strong tendon into the summit of the great trochanter, and by a pointed fleshy mass with a tendinous border into the trochanteric ridge. The deep part (Gluteus accessorius) is smaller, and arises entirely from the ilium between the gluteal line and the external angle (Fig. 455). It has a strong flat tendon which passes over the convexity of the trochanter to be inserted into the crest below it. The convexity is covered with cartilage, and the =trochanteric bursa= (Bursa trochanterica) is interposed between the tendon and the cartilage.[63]

Relations.—Superficially, the skin, lumbo-dorsal and gluteal fasciæ, the tensor fasciæ latæ, gluteus superficialis, and biceps femoris; deeply, the longissimus, the ilium, sacro-iliac and sacro-sciatic ligaments, the gluteus profundus, iliacus and rectus femoris, the iliaco-femoral artery, the gluteal and internal pudic vessels and nerves, and the great sciatic nerve.

Blood-supply.—Gluteal, ilio-lumbar, lumbar, and iliaco-femoral arteries.

Nerve-supply.—Gluteal nerves.

4. =Gluteus profundus= (Deep gluteus; gluteus internus; scansorius).—This much smaller, quadrilateral muscle lies under the posterior part of the preceding muscle, and extends over the hip joint, from the superior ischiatic spine to the anterior part of the great trochanter (Fig. 455).

Origin.—The superior ischiatic spine and the adjacent part of the shaft of the ilium.

Insertion.—The edge of the anterior part or convexity of the trochanter major of the femur.

Action.—To abduct the thigh and to rotate it inward.

Structure.—The muscle is short and thick and contains numerous tendinous intersections. The fibers are directed almost transversely outward over the capsule of the hip joint and converge at the convexity of the trochanter.

Relations.—Superficially, the gluteus medius and branches of the gluteal vessels and nerves; deeply, the shaft of the ilium, the hip joint, and the rectus femoris and capsularis.

Blood-supply.—Gluteal artery.

Nerve-supply.—Anterior gluteal nerve.

5. =Biceps femoris= (Gluteo-biceps s. paramero-biceps).—This large muscle lies behind and in part upon the superficial and middle glutei. It extends in a curved direction from the sacral and coccygeal spines to the outer surface of the stifle and leg (Figs. 178, 199, 200, 201, 202).

Origin.—(1) The dorsal sacro-iliac ligament, the gluteal and coccygeal fasciæ, and the intermuscular septum between this muscle and the semitendinosus; (2) the tuber ischii.

Insertion.—(1) A tubercle on the posterior surface of the femur near the external trochanter; (2) the anterior surface and external straight ligament of the patella; (3) the tibial crest; (4) the anterior and internal surface of the tuber calcis.

Action.—The action is somewhat complex, because the muscle is composed of three portions, has several points of insertion, and acts on all the joints of the limb except those of the digit. The general action is to extend the limb, as in propelling the body, rearing or kicking, and to abduct it. The anterior part, by its attachment to the posterior surface of the femur and to the patella, would extend the stifle and hip joints and abduct the limb. The middle part, being inserted chiefly on the tibial crest and the external straight ligament, would extend the hip, and, with the semitendinosus, flex the stifle. The posterior part, by virtue of its attachment to the tuber calcis, assists in extending the hock.

Structure.—The muscle has two heads of origin:[64] The long or vertebral head arises chiefly from the dorsal and lateral sacro-iliac ligaments, the coccygeal fascia, and the intermuscular septum. There is often a large =bursa= between this head and the trochanter major. The short or ischiatic head arises by a strong tendon from the ventral spine on the tuber ischii. They unite, and a short tendon is detached from the deep face of the muscle, to be inserted into the posterior surface of the femur near the third trochanter (Fig. 455), a =bursa= being interposed between the tendon and the bone. The muscle then divides into three portions, which terminate on a strong aponeurosis over the junction of the thigh and leg. The anterior branch is directed toward the patella, the middle toward the tibial crest, while the posterior one assists in the formation of the posterior contour of the limb. The aponeurosis blends with the deep layer of the fascia cruris, as already described. A synovial =bursa= occurs under the patellar insertion.

Relations.—Superficially, the skin and fascia; deeply, the sacro-iliac and sacro-sciatic ligaments, the coccygeal fascia, the femur, the obturator, gemellus, quadratus femoris, adductor, semimembranosus, vastus externus, and gastrocnemius muscles, branches of the lateral sacral, gluteal, obturator, femoral and deep femoral vessels, the great sciatic, tibial, peroneal, and posterior gluteal nerves; in front, the superficial and middle glutei; behind and internally, the semitendinosus.

Blood-supply.—Gluteal, obturator, and femoro-popliteal arteries.

Nerve-supply.—Posterior gluteal and great sciatic nerves.

6. =Semitendinosus= (Biceps rotator tibialis).—This is a long muscle which extends from the first two coccygeal vertebræ to the proximal third of the inner surface of the tibia. It lies at first behind the biceps, then passes downward on the back of the thigh, between that muscle and the semimembranosus (Figs. 178, 179, 199). It has two heads of origin.

Origin.—(1) The transverse processes of the first and second coccygeal vertebræ, the coccygeal fascia, and the intermuscular septum between this muscle and the biceps femoris; (2) the ventral surface of the tuber ischii.

Insertion.—(1) The tibial crest; (2) the crural fascia and the tuber calcis.

Action.—To flex the stifle and rotate the leg inward; also to extend the hip and hock joints, acting with the biceps and semimembranosus in propulsion of the trunk, rearing, etc.

Structure.—The long or vertebral head is small at its origin, but becomes larger by the accession of fibers arising on the intermuscular septum. Below the tuber ischii it is joined by the short head, which arises partly by fleshy fibers, partly by a common tendon with the biceps. The muscle then passes downward on the back of the thigh, and terminates on a wide tendon on the inner surface of the proximal third of the leg. A distinct band passes forward to be inserted on the tibial crest (a =bursa= lying between the tendon and the tibia), part fuses with the fascia of the leg, while the remainder joins the biceps tendon and concurs in the formation of the tendinous band, which, as before described, terminates on the tuber calcis (Fig. 458). A bursa may occur under the long head where it passes over the tuber ischii.

Relations.—Externally, the skin and fascia, the biceps, and the internal head of the gastrocnemius; internally, the coccygeal fascia, the sacro-sciatic ligament, the semimembranosus; anteriorly, the biceps femoris, branches of the femoral artery, and the great sciatic nerve.

Blood-supply.—Posterior gluteal, obturator, and femoro-popliteal arteries.

Nerve-supply.—Posterior gluteal and great sciatic nerves.

7. =Semimembranosus=[65] (Figs. 179, 186, 199, 451).—This muscle lies on the inner surface of the preceding muscle and the gastrocnemius, and has two heads of origin.

Origin.—(1) The posterior border of the sacro-sciatic ligament; (2) the ventral surface of the tuber ischii.

Insertion.—The internal epicondyle of the femur, behind the lateral ligament.

Action.—To extend the hip joint and to adduct the limb.

Structure.—The long head, small and pointed above, extends toward the root of the tail, fusing with the sacro-sciatic ligament. Passing downward, it becomes larger and covers in part the posterior aspect of the tuber ischii. A =bursa= may be found here. Below this it joins the short head, which is much larger. The large belly so formed passes downward and forward, covered in great part by the gracilis, and terminates on a short, flat tendon of insertion at the distal end of the femur.

Relations.—The upper part of the muscle assists in forming the lateral boundary of the pelvic outlet. It is related posteriorly and externally to the skin and fascia and the semitendinosus; internally, to the anus and its muscles, the vulva in the female, and the internal pudic artery and nerve (Figs. 452, 453). Below the pelvis the chief relations are: externally, the semitendinosus, biceps, and gastrocnemius, branches of the obturator, femoral, and femoro-popliteal arteries, and the great sciatic nerve and its chief branches; internally, the crus penis and ischio-cavernosus muscle (in the male), and the gracilis; in front, the adductor and the femoral vessels; behind, the skin and fascia.

Blood-supply.—Obturator and femoral arteries.

Nerve-supply.—Great sciatic nerve.

IV. THE INTERNAL MUSCLES OF THE THIGH

The muscles of this group are arranged in three layers.

FIRST LAYER

1. =Sartorius= (Figs. 180, 202, 203).—This long and rather narrow muscle is the most anterior one of the first layer. It extends from the sublumbar region to the lower and inner part of the stifle.

Origin.—The iliac fascia and the tendon of the psoas minor.

Insertion.—The internal straight ligament of the patella and the adjacent part of the tuberosity of the tibia.

Action.—To flex the hip joint and adduct the limb.

Structure.—The muscle is very thin at its origin, but becomes thicker and narrower as it passes downward and a little backward. It terminates near the stifle joint on an aponeurosis which blends with that of the gracilis and with the fascia of the leg.

Relations.—Superficially, the inguinal (Poupart’s) ligament, the abdominal muscles, the skin and fascia, and the saphenous vessels and nerve; deeply, the ilio-psoas, quadriceps femoris, and adductor. It forms the anterior boundary of the femoral canal, in which the femoral artery and vein and the deep inguinal lymph glands are situated (Fig. 451).

Blood-supply.—Femoral artery.

Nerve-supply.—Femoral and saphenous nerves.

2. =Gracilis= (Figs. 186, 199, 202, 203).—This is a wide, flat, quadrilateral muscle, situated behind the sartorius, which it exceeds greatly in extent.

Origin.—The middle third of the pelvic symphysis, the prepubic tendon and pubo-femoral ligament, and the ventral surface of the pubis behind the prepubic tendon.

Insertion.—The internal straight ligament of the patella, the internal surface of the tibia in front of the lateral ligament of the stifle joint, and the fascia of the leg.

Action.—To adduct the limb. It may also rotate it inward.

Structure.—The muscle arises by a strong tendon, chiefly in common with the opposite muscle. Its direct attachment to the ventral surface of the pelvis is not so extensive as a superficial inspection would suggest. The origin of the muscle presents anteriorly a round perforation for the passage of the external pudic vein. The belly is composed of parallel bundles, and is marked by a superficial furrow which, however, does not indicate a muscular division. It terminates on the inner surface of the stifle on a thin wide tendon which blends in front with that of the sartorius, below with the crural fascia.

Relations.—Superficially, the skin and fascia, the penis or mammary gland, and the saphenous vessels and nerve; deeply, the pectineus, adductor, semimembranosus and semitendinosus, and, at the middle of the femur, the femoral vessels; anteriorly, the sartorius. In the upper third of the thigh the sartorius and gracilis are separated by a triangular interval (femoral triangle), in which lie the deep inguinal lymph glands and the femoral vessels.

Blood-supply.—Femoral and deep femoral arteries.

Nerve-supply.—Obturator and saphenous nerves.

SECOND LAYER

1. =Pectineus= (Figs. 180, 451, 456).—This muscle is fusiform and extends from the anterior border of the pubis to the middle of the inner border of the femur.

Origin.—The prepubic tendon, the pubo-femoral ligament, and the anterior border of the pubis.

Insertion.—The middle of the internal border of the femur, near the nutrient foramen.

Action.—To adduct the limb and flex the hip joint.

Structure.—The belly is cylindrical and contains little fibrous tissue. Its origin is perforated by the pubo-femoral ligament, and is thus divided into two unequal parts. The large upper part arises mainly from the prepubic tendon—only a very small part gaining direct attachment to the pubis. The small lower part does not reach the bone. The insertion is pointed and tendinous.

Relations.—Superficially, the gracilis; deeply, the femur, the vastus internus, the terminal tendon of the psoas major and iliacus, and the deep femoral artery; anteriorly, the sartorius, the femoral vessels, the saphenous nerve, and the deep inguinal lymph glands; posteriorly, the adductor, obturator externus, and quadratus femoris, and the obturator nerve (anterior division).

Blood-supply.—Femoral and deep femoral arteries.

Nerve-supply.—Femoral and obturator nerves.

The femoral canal is exposed in the dissection of the preceding muscles (Figs. 200, 451). It is bounded anteriorly by the sartorius, posteriorly by the pectineus, and externally by the iliacus and vastus internus. Its internal wall is formed by the femoral fascia and the gracilis. Its upper or abdominal opening (Lacuna vasorum) lies behind and a little internal to the internal inguinal ring and is bounded anteriorly by the inguinal ligament, posteriorly by the anterior border of the pubis, and externally by the tendon of the psoas minor. The canal terminates below at the insertion of the pectineus. It contains the deep inguinal lymph glands, the femoral artery and vein, and the saphenous nerve.

2. =Adductor=[66] (Figs. 179, 186, 451) (Great and small adductors of the thigh).—This fleshy, prismatic muscle lies behind the pectineus and vastus internus. It extends downward and forward from the ventral surface of the pelvis to the internal condyle of the femur.

Origin.—The ventral surface of the pubis and ischium and the tendon of the gracilis.

Insertion.—(1) The posterior surface of the femur from the level of the external trochanter to the groove for the femoral vessels; (2) the internal epicondyle of the femur and the internal lateral ligament of the stifle joint.

Action.—To adduct the limb and assist in extending the hip joint. It may also rotate the femur outward.

Structure.—It is almost entirely fleshy, and is composed of parallel bundles united rather loosely. It is usually possible to separate from the principal mass a small anterior short portion,[67] which is inserted into the femur behind the pectineus. The principal mass[68] is perforated below its middle by the femoral vessels, and is thus divided into two branches. The outer branch is inserted into the back of the femur with the short portion, while the inner branch is attached to the internal epicondyle and lateral ligament. There is often a superficial slip which reaches the internal straight patellar ligament. Some fibers pass under the lateral ligament and end on the tendon of the semimembranosus.

Relations.—Internally, the gracilis, and branches of the femoral artery and of the obturator nerve; externally, the femur, the obturator externus, quadratus femoris, biceps femoris, and gastrocnemius, and the femoral, deep femoral, and obturator arteries; anteriorly, the pectineus, vastus internus, and a large branch of the obturator nerve; posteriorly, the semimembranosus and the great sciatic nerve.

Blood-supply.—Femoral, deep femoral, and obturator arteries.

Nerve-supply.—Obturator nerve.

3. =Semimembranosus.=—Described on p. 280.

THIRD LAYER

1. =Quadratus femoris= (Ischio-femoralis).—This is a narrow, flat muscle, which lies under cover of the upper part of the adductor (Figs. 455, 456).

Origin.—The ventral surface of the ischium, just in front of the semimembranosus.

Insertion.—An oblique line on the posterior surface of the femur, near the lower part of the internal trochanter.

Action.—To extend the hip joint, and to adduct the thigh and rotate it outward.

Structure.—It is composed of parallel bundles of fibers directed downward, forward, and outward.

Relations.—Internally, the adductor, semimembranosus, and the obturator vessels; externally, the obturator externus and biceps femoris, the deep femoral artery, and the great sciatic nerve.

Blood-supply.—Deep femoral and obturator arteries.

Nerve-supply.—Great sciatic nerve.

2. =Obturator Externus= (Fig. 456).—This is a pyramidal muscle which extends across the back of the hip joint from the obturator foramen to the trochanteric fossa.

Origin.—The ventral surface of the pubis and ischium, and the margin of the obturator foramen.

Insertion.—The trochanteric fossa.

Action.—To adduct the thigh and to rotate it outward.

Structure.—It is almost entirely fleshy, the muscle-bundles being rather loosely connected. The insertion is pointed, flattened, and partly tendinous. The origin is perforated by the obturator vessels and nerve.

Relations.—Internally, the adductor and quadratus femoris and the deep femoral vessels; externally, the gemellus, the tendon of the obturator internus, the biceps femoris, and the great sciatic nerve; anteriorly, the hip joint, the pectineus, and the external pudic vein.

Blood-supply.—Deep femoral and obturator arteries.

Nerve-supply.—Obturator nerve.

3. =Obturator internus= (Fig. 451).—This arises by two heads within the pelvic cavity, the tendon emerging through the lesser sciatic foramen.

Origin.—(1) The pelvic surface of the pubis and ischium around the obturator foramen; (2) the pelvic surface of the shaft of the ilium and the wing of the sacrum.

Insertion.—The trochanteric fossa.

Action.—To rotate the femur outward.

Structure.—The ischio-pubic head lies on the pelvic floor and covers the obturator foramen. It is thin and fan-shaped. The iliac head extends along the lateral wall of the pelvis, and is pennate, with a central tendon throughout. Both terminate on a flat tendon which passes outward through the lesser sciatic foramen to be inserted into the trochanteric fossa. A synovial bursa facilitates the play of the tendon over the external border of the ischium.[69]

Relations.—The pelvic surface is covered by the pelvic fascia and the peritoneum. The obturator vessels and nerve lie between the two heads, and the internal pudic vessels and nerve lie along the dorsal edge of the iliac head.

Blood-supply.—Obturator and internal pudic arteries.

Nerve-supply.—Great sciatic nerve.

4. =Gemellus=[70] (Fig. 455).—This is a thin, triangular muscle, which extends from the external border of the ischium to the trochanteric fossa.

Origin.—The external border of the ischium near the ischiatic spine.

Insertion.—The trochanteric fossa.

Action.—To rotate the femur outward.

Structure.—Fleshy, some fibers being inserted into the tendon of the obturator internus.

Relations.—Dorsally, the tendon of the obturator internus and the gluteus profundus; ventrally, the obturator externus.

Blood-supply.—Obturator artery.

Nerve-supply.—Sciatic nerve.

III. ANTERIOR MUSCLES OF THE THIGH

This group consists of the sartorius, quadriceps femoris, and capsularis.

1. =Sartorius.=—This is described on p. 281.

2. =Quadriceps femoris= (Figs. 179, 186, 200) (Crural triceps).—This constitutes the large muscular mass which covers the front and sides of the femur. It has four heads, one of which, the rectus, arises from the ilium; the other three arise from the femur. All are inserted into the patella.

(1) =Rectus femoris.=—This is fusiform and rounded. It arises by two tendons.

Origin.—Two depressions on the shaft of the ilium above and in front of the acetabulum.

Insertion.—The base and anterior surface of the patella.

Action.—To extend the stifle joint and to flex the hip joint.

Structure.—It has two short strong tendons of origin; beneath the outer one is a =bursa=. The belly is rounded and rests in a groove formed by the other portions of the quadriceps. Its sides are covered by a strong tendinous layer which furnishes insertion to fibers of the vasti. The tendon of insertion is formed by the fusion of these tendinous layers on the lower part of the muscle. The lower portion of the muscle is pennate, the fibers on either side converging on the tendon at an acute angle.

Relations.—Internally, the iliacus, sartorius, and vastus internus; externally, the tensor fasciæ latæ, glutei, and vastus externus; posteriorly, the hip joint and the vastus intermedius; anteriorly, the fascia lata and the skin. The anterior femoral artery and branches of the femoral nerve descend into the interspace between the upper part of the rectus and the vastus internus; similarly, the iliaco-femoral artery dips in between the rectus femoris and vastus externus.

Blood-supply.—Femoral and iliaco-femoral arteries.

Nerve-supply.—Femoral nerve.

(2) =Vastus externus= (s. lateralis).—This lies on the outer surface of the thigh, extending from the great trochanter to the patella. It is thick and wide in its upper part, and becomes much thinner and narrow below.

Origin.—The external border and surface of the femur, from the great trochanter to the supracondyloid fossa.

Insertion.—(1) The outer part of the anterior surface of the patella; (2) the tendon of the rectus femoris.

Action.—To extend the stifle joint.

Structure.—The fibers are directed downward and forward, many being inserted into the tendinous sheet which covers the side of the rectus. A =bursa= is often found between the distal end and the patella.

Relations.—Externally, the fascia lata and skin, tensor fasciæ latæ, superficial gluteus, and biceps femoris; internally, the femur and femoro-patellar joint capsule, the rectus femoris, vastus intermedius, and the iliaco-femoral artery.

Blood-supply.—Iliaco-femoral artery.

Nerve-supply.—Femoral nerve.

(3) =Vastus internus= (s. medialis).—This is smaller than the preceding muscle, and lies in a similar position on the inner side of the thigh.

Origin.—The internal surface of the femur, from the neck to the distal third.

Insertion.—(1) The upper part of the inner border of the patella and its cartilage; (2) the tendon of the rectus femoris.

Action.—To extend the stifle joint.

Structure.—This is very similar to that of the vastus externus. It is, however, more difficult to separate from the intermedius, because many fibers of the latter arise on the tendinous sheet which covers the contact surface of the inner vastus. Its insertion into the patella is chiefly by means of a broad strong tendon. From the deep face fleshy fibers are inserted also into the femoro-patellar capsule.

Relations.—Internally, the skin and fascia lata, the iliacus, sartorius, pectineus, and adductor, the femoral vessels and saphenous nerve; externally, the femur, femoro-patellar joint capsule, rectus femoris, and vastus intermedius, the anterior femoral artery, and branches of the femoral nerve.

Blood-supply.—Femoral and anterior femoral arteries.

Nerve-supply.—Femoral nerve.

(4) =Vastus intermedius= (Crureus).—This muscle is deeply situated on the anterior face of the femur, and is entirely covered by the preceding heads.

Origin.—(1) The anterior and external surfaces of the femur, from the proximal to the distal fourth; (2) the aponeurosis of the vastus internus.

Insertion.—(1) The base of the patella; (2) the femoro-patellar joint capsule.

Action.—(1) To extend the stifle joint; (2) to tense (raise) the femoro-patellar capsule during extension of the joint.

Structure.—The muscle is usually quite difficult to isolate from the other vasti, so that many since Günther have declared it an artefact.[71] It is entirely fleshy, and is small at its proximal end, but when traced downward increases in bulk by the accession of fibers arising on the femur and the tendinous covering of the vastus internus. The terminal part is intimately adherent to the femoro-patellar joint capsule, where the latter bulges upward above the level of the patella.

Relations.—Internally, the vastus internus; externally, the vastus externus; anteriorly, the rectus; posteriorly, the femur and femoro-patellar capsule.

Blood-supply.—Iliaco-femoral and anterior femoral arteries.

Nerve-supply.—Femoral nerve.

The straight ligaments of the patella are to be regarded as tendons of the quadriceps which communicate the action of the latter to the tibia, the patella being intercalated as a sesamoid bone.

3. =Capsularis= (Fig. 154) (Rectus parvus).—This is a small muscle (scarcely as large as one’s finger), which arises on the ilium immediately above the outer tendon of the rectus femoris, and passes down between the vastus internus and externus to be inserted into the anterior surface of the femur. It passes over the front of the hip joint, to the capsule of which some fibers are attached. Sometimes the muscle has two distinct heads, in which case the additional head arises between the two tendons of origin of the rectus femoris. Its action may be to raise the capsule during flexion of the joint.

IV. THE MUSCLES OF THE LEG AND FOOT

The muscles of this region cover almost all of the tibia except its internal face, which is largely subcutaneous. As in the forearm, the muscles fall into two groups, an anterior or dorso-lateral, and a posterior or plantar. The muscles of the first group are extensors of the digit and flexors of the hock, those of the second have the opposite action.

A. ANTERIOR GROUP

1. =Anterior= or =long digital extensor= (M. extensor digitalis longus; extensor pedis; anterior extensor of the phalanges).—This muscle is situated superficially on the antero-external aspect of the leg, and is provided with a long tendon which passes down over the front of the tarsus, metatarsus, and digit.

Origin.—The small fossa (Fossa extensoria) between the external condyle and the trochlea of the femur.

Insertion.—(1) The extensor process of the third phalanx; (2) the anterior surface of the proximal extremities of the first and second phalanges.

Action.—To extend the digit and flex the hock. It also assists in fixing the stifle joint.

Structure.—The origin is by means of a strong tendon in common with the peroneus tertius, on which also many fibers arise. The common tendon passes downward in the groove between the outer condyle and the tuberosity of the tibia, where a pouch from the femoro-tibial capsule descends four or five inches (ca. 10 to 12 cm.) beneath the tendon. The belly is fusiform and somewhat flattened. The long tendon of insertion begins in the belly about its middle, and is clear of the fleshy part near the tarsus. It passes downward over the front of the hock, bound down by the three annular ligaments already described (see crural fascia), and enveloped by a =synovial sheath= which begins a little above the level of the external malleolus, and extends nearly to the junction with the lateral extensor tendon. This union occurs usually about a hand’s breadth below the tarsus. In the angle of union the extensor brevis also joins the principal tendon. Beyond this point the arrangement is the same as in the fore limb.

Relations.—Superficially, the skin and fascia; deeply, the femoro-tibial capsule, peroneus tertius, and tibialis anterior; behind, the lateral extensor and the superficial and deep peroneal nerves. In front of the tarsus the anterior tibial artery crosses the deep face of the tendon (Fig. 460).

Blood-supply.—Anterior tibial artery.

Nerve-supply.—Peroneal nerve.

2. =Lateral digital extensor= (M. extensor digitalis lateralis; peroneus; lateral extensor of the phalanges).—This muscle lies on the outer surface of the leg, behind the preceding one.

Origin.—The external lateral ligament of the stifle joint, the fibula, the external border of the tibia, and the interosseus ligament.

Insertion.—The tendon of the anterior extensor, about a third of the way down the metatarsus.

Action.—To assist the anterior extensor.

Structure.—The belly is fusiform, flattened, and pennate. The tendon runs through the entire length of the belly and becomes free from it at the lower fourth of the tibia. It passes downward through the groove on the external malleolus, bound down by an annular ligament, and, inclining forward, blends (usually) with the tendon of the anterior extensor. It is provided with a =synovial sheath=, which begins about one inch (ca. 2 to 3 cm.) above the external malleolus and ends about one and one-half inches (ca. 3 to 4 cm.) above the junction. Sometimes the fusion does not occur, and the tendon then passes down the metatarsus, alongside of that of the long extensor, to lie inserted into the first phalanx like the corresponding muscle of the thoracic limb.

Relations.—Externally, the skin and fascia and the superficial peroneal nerve; internally, the tibia and fibula; anteriorly, the intermuscular septum, the long extensor, and the tibialis anterior; posteriorly, the deep flexor and the soleus.

Blood-supply.—Anterior tibial artery.

Nerve-supply.—Peroneal nerve.

3. =Peroneus tertius= (Tendinous part of the flexor metatarsi; tendo femorotarseus [Schmaltz]).—This consists in the horse of a strong tendon which lies between the anterior extensor and the tibialis anterior.[72]

Origin.—The fossa extensoria (between the external condyle and the trochlea of the femur), in common with the anterior extensor.

Insertion.—(1) The proximal extremities of the large (third) and external small (fourth) metatarsal bones, and the third tarsal bone; (2) the fibular and fourth tarsal bones.

Action.—Mechanically to flex the hock when the stifle joint is flexed.

Structure.—This is entirely tendinous. The proximal end and the underlying prolongation of the synovial membrane of the femoro-tibial joint have been mentioned in the description of the anterior extensor. Fibers of the latter muscle and of the tibialis anterior arise on the tendon as it passes downward to the front of the hock. At the distal end of the tibia the tendon divides into two branches, between which the tendon of the tibialis anterior emerges. The anterior branch is attached to the third tarsal and third and fourth metatarsal bones, while the outer one curves outward, bifurcates, and is inserted into the fibular and fourth tarsal bones (Fig. 460).

Relations.—Superficially, the anterior extensor; deeply, the tibialis anterior. The anterior tibial vessels cross the deep face of the outer branch.

4. =Tibialis anterior= (Muscular portion of the flexor metatarsi).—This lies on the antero-external face of the tibia; it is wide and flattened above, pointed below.

Origin.—The external condyle and border of the tibia and a small area on the external surface of the tuberosity.

Insertion.—(1) The metatarsal tuberosity; (2) the first and second tarsal bones.

Action.—To flex the hock joint.

Structure.—The origin is fleshy, and is divided by the groove in which lie the common tendon of the long extensor and peroneus tertius and a synovial pouch. Passing downward on the tibia, the belly is united by tendinous and fleshy fibers with the peroneus tertius, and terminates close to the tarsus in a point on the tendon of insertion. The latter emerges between the branches of the peroneus tertius and bifurcates, the anterior branch being inserted into the metatarsal tuberosity, the inner one (cunean tendon) into the fused first and second tarsal bones (cuneiform parvum). The tendon is provided with a =synovial sheath= at its emergence, and a =bursa= (cunean bursa) is interposed between the inner branch and the internal lateral ligament.

Relations.—Superficially, the anterior and lateral extensors, the peroneus tertius, and the deep peroneal nerve; deeply, the tibia, the deep flexor, and the anterior tibial vessels.

Blood-supply.—Anterior tibial artery.

Nerve-supply.—Peroneal nerve.

B. POSTERIOR GROUP

1. =Gastrocnemius= (Figs. 179, 201, 203).—This muscle extends from the lower third of the femur to the point of the hock. It arises by two heads.

Origin.—(1) Outer head, from the external rough margin of the supracondyloid fossa (Fossa plantaris); (2) inner head, from the supracondyloid crest.

Insertion.—The posterior part of the tuber calcis.

Action.—To extend the hock.

Structure.—The two bellies are thick, fusiform, and somewhat flattened. They are covered by a strong aponeurosis and contain tendinous intersections. They terminate toward the middle of the leg on a common tendon (Tendo calcaneus s. Achillis), which at first lies posterior to that of the superficial flexor, but, by a twist in both, comes to lie in front of the latter. The deep fascia blends with the tendon throughout its length, and the soleus muscle is inserted into its anterior edge. A small =bursa= (Bursa tendinis calcanei) lies in front of the insertion on the tuber calcis, and a large =bursa= is interposed between the two tendons from the twist downward. The superficial flexor lies between the two heads and is adherent to the outer one (Fig. 459).

Relations.—Anteriorly, the stifle joint, the superficial flexor, popliteus, deep flexor, popliteal vessels, and tibial nerve; internally (above), the semitendinosus, semimembranosus, and adductor, (below) the fascia and skin; externally (above), the biceps femoris and peroneal (anterior tibial) nerve, (below) the fascia and skin.

Blood-supply.—Popliteal artery.

Nerve-supply.—Tibial nerve.

2. =Soleus= (Fig. 201).—This muscle is very small in the horse. It lies immediately under the deep fascia, on the proximal half of the outer surface of the leg, and is directed obliquely downward and backward.

Origin.—The head of the fibula.

Insertion.—The tendon of the gastrocnemius, about the middle of the leg.

Action.—To assist the gastrocnemius.

Structure.—It is a thin, fleshy band, about an inch (ca. 2 to 3 cm.) in width, terminating on a thin tendon which fuses with that of the gastrocnemius.[73]

Relations.—Superficially, the skin, fascia, and peroneal nerve; deeply, the lateral extensor and deep flexor.

Blood-supply.—Posterior tibial artery.

Nerve-supply.—Tibial nerve.

3. =Superficial digital flexor= or =flexor pedis perforatus= (Figs. 199, 201, 203) (M. flexor digitalis pedis sublimis; superficial flexor of the phalanges).—The proximal part of this muscle lies between and under cover of the two heads of the gastrocnemius (Fig. 459). It consists almost entirely of a strong tendon, the belly being very little developed.

Origin.—The supracondyloid fossa (Fossa plantaris) of the femur.

Insertion.—(1) The front and sides of the tuber calcis; (2) the eminences on the proximal extremity of the second phalanx, and the distal extremity of the first phalanx behind the lateral ligament.

Action.—To flex the digit and extend the hock joint. On account of the exceedingly small amount of muscular tissue the action is to be regarded chiefly as a mechanical effect, resulting from the action of other muscles on the stifle joint.

Structure.—The origin is by means of a strong round tendon which is incompletely covered with fleshy fibers as far as the upper third of the leg. It is pretty intimately attached to the gastrocnemius, especially to the outer head. At the distal third of the tibia it winds around the inner surface of the gastrocnemius tendon, and then occupies a position behind the latter. At the point of the hock it widens out, forming a sort of cap over the tuber calcis, detaching on either side a strong band which is inserted into the tuber calcis with the tarsal tendon of the biceps and semitendinosus. It then passes downward over the plantar (Calcaneo-metatarsal) ligament, becomes narrower, and is arranged below as in the thoracic limb. A large synovial bursa lies under the tendon from the distal fourth of the tibia to the middle of the tarsus. A =subcutaneous bursa= is sometimes found on the wide part of the tendon at the point of the hook. (Either or both of these bursæ may be involved in so-called “capped hock.”)

Relations.—Posteriorly, the gastrocnemius, fascia, and skin; anteriorly, the femoro-patellar capsule, the popliteus, the deep flexor, and the popliteal vessels; internally, the tibial nerve.

Blood-supply.—Femoro-popliteal artery.

Nerve-supply.—Tibial nerve.

4. =Deep digital flexor= or =flexor pedis perforans= (M. flexor digitalis pedis profundus; deep and oblique flexors of the phalanges).—The belly of this muscle lies on the posterior surface of the tibia, and is divisible into three parts or heads, which, however, finally unite on a common tendon of insertion.

Origin.—(1) The posterior edge of the external condyle of the tibia; (2) the external border of the external condyle of the tibia, just behind the facet for the fibula; (3) the middle third of the posterior surface and the upper part of the external border of the tibia, the posterior border of the fibula, and the interosseous ligament.[74]

Insertion.—The semilunar crest of the third phalanx and the adjacent surface of the lateral cartilage.

Action.—To flex the digit and to extend the hock joint.

Structure.—(1) The inner head (M. flexor digitalis longus s. flexor accessorius) is easily isolated (Figs. 203, 459). It has a fusiform belly, which crosses the leg obliquely and lies in a groove formed by the other heads and the popliteus. This terminates near the lower third of the tibia on a round tendon which passes downward, partly embedded in the internal lateral ligament of the hock, and joins the common tendon about a third of the way down the metatarsus. In its course over the inner surface of the hock the tendon lies in a canal formed by the strong tarsal fascia, the tibia, and the lateral ligament, and is provided with a =synovial sheath= which extends from the distal fourth of the tibia to the junction with the principal tendon. (2) The superficial head (M. tibialis posterior) is only partially separable. It has a flattened belly, terminating near the lower third of the tibia on a flat tendon which soon fuses with the principal tendon. (3) The deep head (M. flexor hallucis longus) is much the largest. It lies on the posterior surface of the tibia, from the popliteal line outward and downward. The belly contains much tendinous tissue, and terminates behind the distal end of the tibia on a strong round tendon. The latter receives the tendon of the tibialis posterior, passes downward in the tarsal groove, bound down by the strong tarsal fascia (Ligamentum laciniatum) and enveloped in a synovial sheath, receives the tendon of the inner head below the hock, and, a little further down, the so-called check ligament (Caput tendineum [Schmaltz]). The =tarsal sheath= (Vagina tarsea) begins about two to three inches (ca. 5 to 7.5 cm.) above the level of the internal malleolus, and extends about one-fourth of the way down the metatarsus. (Distention of the sheath, as in “thoroughpin,” affects chiefly its proximal end.) The check ligament resembles that of the fore limb, except that it is longer and very much weaker; it may be absent. The remainder of the tendon is arranged like that of the thoracic limb.

Relations.—Anteriorly, the tibia and fibula, the popliteus, lateral extensor, tibialis anterior, and the tibial vessels; posteriorly, the gastrocnemius, superficial flexor, and the tibial nerve; externally, the fascia, skin, and the soleus; internally, the fascia and skin.

Blood-supply.—Posterior tibial artery.

Nerve-supply.—Tibial nerve.

5. =Popliteus= (Fig. 203).—This thick and triangular muscle lies on the posterior surface of the femoro-tibial articulation and the posterior surface of the tibia above the popliteal line.

Origin.—A small depression on the external epicondyle of the femur, close to the articular surface and under the lateral ligament.

Insertion.—A triangular area on the posterior surface of the tibia, above and internal to the popliteal line; also the proximal half of the inner border and a narrow adjacent part of the internal surface of the tibia.

Action.—To flex the femoro-tibial joint and to rotate the leg inward.

Structure.—The strong tendon of origin lies at first under the lateral ligament, and curves backward and inward over the external condyle of the tibia and its semilunar cartilage, being invested by a reflection of the synovial capsule of the joint (Fig. 459). The tendon is succeeded by a thick triangular belly, the fibers of which are directed obliquely downward and inward.

Relations.—Superficially, the fascia and skin, semitendinosus, gastrocnemius, superficial flexor; deeply, the femoro-tibial joint, the tibia, the popliteal vessels and their divisions. The saphenous vessels and nerve lie along the inner border of the muscle, separated from it, however, by the deep fascia.

Blood-supply.—Popliteal and posterior tibial arteries.

Nerve-supply.—Tibial nerve.

MUSCLES OF THE METATARSUS AND DIGIT

=Extensor pedis brevis= (M. extensor digitalis brevis).—This small muscle lies in the angle of union of the long and lateral extensors of the digit. (Shown in Fig. 201, but not marked.)

Origin.—The outer tendon of the peroneus tertius, the middle annular ligament, and the outer lateral ligament of the hock.

Insertion.—The tendon of the anterior or long extensor.

Action.—To assist the anterior or long extensor.

Structure.—It is principally fleshy, having a superficial origin from the annular ligament, and a deep one (by a thin tendon) from the outer tendon of the peroneus tertius. The insertion is by a thin tendon.

Relations.—Superficially, the skin and fascia and the tendons of the anterior and lateral extensors; deeply, the joint capsule, the great metatarsal artery, and the deep peroneal nerve.

Blood-supply.—Great metatarsal artery.

Nerve-supply.—Deep peroneal nerve.

The =interossei= and =lumbricales= are arranged like those of the thoracic limb, the only noticeable difference being the greater development of the lumbricales in the pelvic limb.

THE MUSCLES OF THE OX

MUSCLES OF THE FACE

The =panniculus= is much more developed than in the horse, presenting as a special feature the extensive =frontalis= muscle, which covers the frontal and nasal regions.

The =orbicularis oris= does not form a complete ring, the defect being in the middle of the upper lip.

The =levator nasolabialis= is extensive, thin, and not very distinct from the frontalis; it divides into two layers, between which the levator labii superioris proprius and the lateral dilator of the nostril pass. The superficial layer ends in the nostril and upper lip, the deep layer on the accessory (lateral) nasal cartilages and on the nasal process of the premaxilla.

The =levator labii superioris proprius= arises on and before the facial tuberosity and terminates by several tendons in the muzzle. It passes between the two layers of the preceding muscle, blending in part with the deep layer.

The =zygomaticus= is much stronger than in the horse. It arises on the masseteric fascia, and ends chiefly in the upper lip.

The =depressor labii superioris= does not resemble the muscle of the same name in the horse. It arises just in front of the facial tuberosity, and divides usually into two branches, which terminate in a number of tendons that form a network in the muzzle and upper lip.

The =incisivus inferior= is a small, rounded muscle, which arises on the body of the mandible below the second and third incisors, and ends in the lower lip, blending with the orbicularis.

The =depressor labii inferioris= is thin, and does not extend as far backward as in the horse; only the anterior end is distinct from the buccinator.

The =buccinator= shows no marked variation, but its superficial layer is well developed.

The =dilatator naris lateralis= arises in front of the facial tuberosity, passes forward between the branches of the levator nasolabialis, and terminates in the outer wing of the nostril.

The dilatator naris transversus is replaced by the =dilatator naris apicalis=, which is situated in the muzzle and joins its fellow at a median raphé. It arises on the border and upper surface of the body of the premaxilla, the fibers passing obliquely upward and outward to the inner wing of the nostril.

The =dilatator naris superior= arises from the alar cartilage of the nostril and ends in the inner wing of the nostril.

The =dilatator naris inferior= consists of two layers which arise on the nasal process of the premaxilla and the lateral nasal cartilage and end in the outer wing of the nostril.

The =orbicularis oculi= is well developed.

The corrugator supercilii is not present as a separate muscle, its place and function being taken by the frontalis.

The =malaris= is broad, and spreads out below on the fascia over the buccinator and masseter; it is divided into two parts.

MUSCLES OF MASTICATION

The =masseter= is not so large as in the horse; a considerable part of it arises on the facial tuberosity and is directed obliquely backward and downward, so that it would draw the lower jaw forward as well as upward.

The =temporalis= conforms to the temporal fossa, and is therefore longer and entirely lateral in position.

The =pterygoidei= are not quite clearly separated; their direction is more oblique, and the origin of the pterygoideus internus is nearer the median plane than in the horse.

The =stylo-mandibularis= is absent.

The =digastricus= has a tendinous origin on the paramastoid or styloid process of the occipital bone; its bellies are short and thick. It does not perforate the stylo-hyoideus. The two digastrici are connected beneath the root of the tongue by a layer of transverse muscle-fibers (Transversus mandibulæ).

HYOID MUSCLES

The =mylo-hyoideus= is thicker and more extensive than in the horse.

The =stylo-hyoideus= has a long thin tendinous origin and is not perforated by the digastricus.

The =genio-hyoideus= is more developed.

The =kerato-hyoideus= has an additional attachment on the middle cornu of the hyoid bone.

The =hyoideus transversus= is bifid.

The =sterno-thyro-hyoideus= has no intermediate tendon and is thicker.

The =omo-hyoideus= arises as a thin band from the fascia over the third and fourth cervical vertebræ. It blends here with the rectus capitis anterior major. The =occipito-hyoideus= is thick.

MUSCLES OF THE NECK

A. VENTRAL GROUP

The =sterno-cephalicus= consists of two muscles. They arise from the manubrium sterni and first rib. The superficial muscle (Sterno-mandibularis) is inserted on the anterior border of the masseter, the ramus of the mandible, and the buccal fascia. The deep muscle (Sterno-mastoideus) crosses under the preceding and ends on the mastoid process, the mandible, and, in common with the rectus capitis anterior major, on the basilar process of the occipital bone.

There are two scaleni. The =scalenus ventralis= (s. primæ costæ) arises on the first rib and ends on the transverse processes of the third to the seventh cervical vertebræ. It is traversed by the roots of the brachial plexus, which partially divide it into a small dorsal and a large ventral part. The brachial vessels lie below the latter. The =scalenus dorsalis= (s. supracostalis) arises usually on the second, third, and fourth ribs, and ends on the transverse processes of the third to the sixth cervical vertebræ.

The =rectus capitis anterior major= arises on the third to the sixth cervical transverse processes, and blends at its insertion with the sterno-mastoideus and the mastoid portion of the mastoido-humeralis.

The =rectus capitis anterior minor= is larger than in the horse.

The =rectus capitis lateralis= and =longus colli= resemble those of the horse.

The =intertransversales= are large. From the sixth cervical vertebra forward they form a muscular mass (M. intertransversarius longus) which is inserted into the wing of the atlas.

B. LATERAL GROUP

The =splenius= is thin. It arises directly from the first three or four thoracic spines, and ends by a thin tendon on the occipital bone, the wing of the atlas, and the transverse process of the axis, blending with the mastoido-humeralis, trachelo-mastoideus, and omo-transversarius. The remaining muscles present no very marked differential features.

MUSCLES OF THE THORAX

The =levatores costarum= number ten or eleven pairs.

The =diaphragm= presents several important differential features. Its slope is much steeper and its width is greater than in the horse. The costal attachment extends almost in a straight line from the upper fourth of the last rib to the junction of the eighth rib with its cartilage, and along the latter to the sternum. The mid-line slopes from the twelfth thoracic vertebra obliquely as far as the vena cava, beyond which it is almost vertical. The right crus divides into two branches, which circumscribe the œsophageal opening, unite below, and then spread out in the tendinous center. The left crus is small. The œsophageal opening is situated about four to five inches (10 to 12 cm.) below the eighth thoracic vertebra, a little to the left of the median plane. The foramen venæ cavæ is a little more ventral and almost in the median plane. The other muscles resemble those of the horse.

MUSCLES OF THE BACK AND LOINS

The =serratus anticus= is very thin. It is inserted on the fifth to the eighth ribs. It may be reduced to two or three digitations or may be absent. The =serratus posticus= is usually inserted on the last three or four ribs.

The =transversalis costarum= (Iliocostalis) has a distinct lumbar portion which is attached to the lumbar transverse processes and the external angle of the ilium.

The =longissimus= resembles that of the horse, but it is more fleshy anteriorly, and the spinalis dorsi is clearly distinguishable from the common mass. In the lumbar region the tendons meet across the summits of the spines.

=Intertransversales= are present in the back, and =interspinales= in the back and loins.

MUSCLES OF THE TAIL

These resemble those of the horse; the coccygeus is, however, much more developed.

ABDOMINAL MUSCLES

The =obliquus abdominis externus= is somewhat thinner and has a less extensive origin, which begins at the lower part of the fifth intercostal space and ends on the last rib above its middle. The direction of the fibers in the flank is horizontal, and they do not reach to the external angle of the ilium, nor as high as the lumbar transverse processes. (In this region the abdominal tunic has a strong attachment to the point of the hip and the lumbo-dorsal fascia.) The aponeurosis is intimately united with the abdominal tunic, and does not detach a layer on the inner surface of the thigh.

The =obliquus abdominis internus= is more developed and has an additional origin from the lumbo-dorsal fascia. The aponeurosis blends with that of the external oblique near the linea alba, and detaches a layer which assists in the formation of the internal sheath of the rectus.

The =rectus abdominis= arises on the lateral border of the sternum as far forward as the third costal cartilage. The two muscles are separated, except near the pelvis, by an interval varying from two to four inches (ca. 5 to 10 cm.), so that this part of the abdominal wall is entirely fibrous. (The umbilicus is in a transverse plane through the third lumbar vertebra.) There are five tendinous inscriptions, on the third of which is a foramen for the passage of the subcutaneous abdominal vein (milk vein). The prepubic tendon has, in addition to branches inserted into the ilio-pectineal eminences, a strong attachment to the median common tendon of the adductors of the thigh, so that the abdominal wall is strongly retracted and almost vertical at its junction with the pelvis.

The =transversus abdominis= presents no striking differential features. The transversalis fascia is strong and distinct except over the diaphragm.

MUSCLES OF THE THORACIC LIMB

I. MUSCLES OF THE SHOULDER GIRDLE

The =trapezius= is much thicker and broader than in the horse, and is undivided. It arises on the ligamentum nuchæ and supraspinous ligament, from the atlas to the twelfth thoracic vertebra.

The =omo-transversarius= is a muscle (not present in the horse) which arises on the wing of the atlas, and, inconstantly, the transverse process of the second cervical vertebra, and is inserted into the scapular spine and fascia.

The =rhomboideus= is clearly divided into cervical and dorsal portions, the latter extending further than in the horse.

The =latissimus dorsi= has a broad tendon of origin, which blends with the lumbo-dorsal fascia; it is also attached to the eleventh and twelfth ribs, the fascia over the external intercostal and oblique abdominal muscles, and by a tendinous slip to the deep pectoral muscle.

The =mastoido-humeralis= has two distinct parts. The dorsal division (M. cleido-occipitalis) arises on the occipital bone and the ligamentum nuchæ. The ventral part (M. cleido-mastoideus) is smaller and arises by a round tendon on the mastoid process and the rectus capitis anterior major, and by a thin tendon on the mandible. At the shoulder a small muscle which arises on the first rib blends with the deep face of the mastoido-humeralis.[75]

The =superficial pectoral= muscle does not present any striking difference.

The =deep pectoral= arises as far forward as the second rib and is undivided. However, the scapular portion may be considered to be represented by a small branch extending over the lower part of the supraspinatus. A tendon is detached from the dorsal edge, which blends with the latissimus dorsi and coraco-brachialis.

The =serratus magnus= is clearly divided into cervical and thoracic portions. The former is large and extends from the third (or second) cervical vertebra to the fifth rib, being overlapped behind by the thoracic part. The thoracic part is relatively thin and is covered by a very strong aponeurosis; it is attached to the fourth to the ninth ribs by six digitations, and is inserted by a flat tendon into the inner face of the dorsal angle of the scapula.

II. MUSCLES OF THE SHOULDER

The =deltoid= is clearly divided into acromial and scapular portions. The former arises on the acromion, the latter on the posterior border of the scapula and the aponeurotic covering of the infraspinatus. The scapular part of the muscle is largely inserted into the fascia covering the triceps.

The =supraspinatus=, =infraspinatus=, and =teres minor= do not differ materially from those of the horse.

The =subscapularis= consists of three portions with a common tendon of insertion.

The =teres major= and =coraco-brachialis= resemble those of the horse.

III. MUSCLES OF THE ARM

The =biceps= is smaller and less tendinous, and is situated more internally than in the horse. The tendon of origin is flat, and is bound down in the bicipital groove by a fibrous band. (In the sheep the tendon is round and passes through the shoulder joint.)

The internal head of the =triceps= is more developed than in the horse.

The =tensor fasciæ antibrachii= is a slender muscle.

IV. MUSCLES OF THE FOREARM

A. EXTENSOR DIVISION

The =extensor carpi radialis= is like that of the horse. There is sometimes a small muscle lying along its inner border, which may represent the extensores pollicis.

There are three =digital extensors=: 1. The =common= or =anterior digital extensor= (M. extensor digitalis communis) arises by two heads from the extensor epicondyle and the ulna. The heads fuse about the middle of the forearm, and terminate soon on a tendon which passes over the carpus and metacarpus, gradually inclining forward. At the fetlock joint it divides into two branches, which are inserted into the third phalanges. 2. The =internal digital extensor= (M. extensor digiti tertii proprius) arises on the extensor epicondyle, and is inserted by two branches into the second and third phalanges of the inner digit. The tendon receives two reinforcing slips from the suspensory ligament. 3. The =lateral digital extensor= (M. extensor digitalis lateralis s. digiti quarti proprius) arises from the external lateral ligament of the elbow joint, the external tuberosity of the radius, and the ulna. The tendon terminates like that of the preceding muscle.

The =extensor carpi obliquus= resembles that of the horse.

The extensor tendons are bound down at the carpus by an annular ligament, and are furnished with synovial sheaths (Figs. 215, 216).

B. FLEXOR DIVISION

The three =flexors of the carpus= are like those of the horse.

The =superficial digital flexor= is somewhat blended at its origin with the middle flexor of the carpus. It divides into two bellies, superficial and deep, terminating on tendons at the distal part of the forearm. The superficial tendon passes over the posterior annular ligament (Ligamentum carpi transversum), perforates the metacarpal fascia, and joins the deep tendon about the middle of the metacarpus. The deep belly is connected with the deep flexor by a strong fibrous band. Its tendon passes under the annular ligament in a groove on the deep flexor, from which it receives fibers. The conjoined tendon soon bifurcates, each branch receiving a reinforcing band from the suspensory ligament, and forming near the fetlock a ring for the corresponding branch of the deep flexor tendon. Passing under two digital annular ligaments, they are inserted into the volar surfaces of the second phalanges by three slips.

The =deep digital flexor= has the same heads as in the horse, the humeral head, as before mentioned, being connected with the deep portion of the superficial flexor. The tendon divides near the distal end of the metacarpus into two branches which are inserted into the volar surfaces of the third phalanges.

The synovial sheaths at the carpus present the following special features: One is found in connection with the tendon of the superficial part of the superficial flexor of the digits. There is a common sheath for the tendons of the common extensor and the inner extensor.

Bursæ may occur under the tendons of the proper extensors of the digits at the fetlock; they are constant only in old animals (Schmidtchen). The branches of the tendon of the common extensor are provided with synovial sheaths from their origin to the middle of the second phalanx. There are two digital synovial sheaths for the flexor tendons; they may communicate at their upper part, and extend from the distal third of the metacarpus nearly to the distal sesamoids. Bursæ occur between the latter and the branches of the deep flexor tendon.

METACARPAL MUSCLES

The =lumbricales= are absent, unless we regard as such the muscular bundles which arise on the deep flexor and are inserted into the superficial flexor tendon at the carpus.

The =interosseus medius= or suspensory ligament is somewhat more muscular than in the horse; indeed, in the young subject it may be almost entirely fleshy. Its arrangement is somewhat complex. Single at its origin, it detaches about the middle of the metacarpus a band which joins the tendon of the superficial flexor and concurs near the fetlock in the formation of the ring for the deep flexor tendon. A little lower down it divides into three and then into five branches. The lateral branches (two pairs) are attached to the corresponding sesamoids and tendons of the proper extensors, while the middle branch passes through the sulcus at the distal end of the metacarpus and bifurcates, each division fusing with the tendon of the corresponding proper extensor.

It is to be noted that the fascia on the posterior face of the metacarpus and digit is very thick. It is continuous above with the ligamentum carpi transversum, and is attached on either side to the metacarpal bone. At the fetlock it forms the fibrous basis for the small claws, and below this it detaches two strong bands which diverge to be inserted into the second and third phalanges, blending with the lateral ligaments.

THE MUSCLES OF THE PELVIC LIMB

I. THE SUBLUMBAR MUSCLES

The =psoas minor= begins at the disc between the twelfth and thirteenth thoracic vertebræ.

The =psoas major= has a fleshy origin on the posterior border of the last rib, and a thin tendon attached to the twelfth rib.

The =iliacus= begins under the body of the sixth lumbar vertebra, and is more closely united with the psoas major than in the horse.

The =quadratus lumborum= extends as far forward as the body of the tenth or eleventh thoracic vertebra.

II. EXTERNAL MUSCLES OF THE HIP AND THIGH

The =tensor fasciæ latæ= is large, and the fleshy part extends further down than in the horse.

The gluteus superficialis is not present as such; apparently its anterior part has fused with the tensor fasciæ latæ and its posterior with the biceps femoris.

The =gluteus medius= is small, the lumbar portion being insignificant and extending forward only to the fourth lumbar vertebra. Its deep portion (Gluteus accessorius) is easily separable, and its strong tendon is inserted into the femur below the trochanter major, under cover of the upper part of the vastus externus.

The =gluteus profundus= is thin, but extensive, arising as far forward as the external angle of the ilium, and from the lower part of the sacro-sciatic ligament. The fibers converge on a tendon which passes under the upper part of the vastus externus, and is inserted into a tubercle a short distance below the great trochanter.

The =biceps femoris= is very wide at its upper part, having apparently absorbed the posterior part of the superficial gluteus. It is divided in the thigh into two portions, which end on a wide aponeurosis. There is no femoral attachment. A large bursa occurs between the muscle and the great trochanter in the adult. The part of the tendon which fuses with the external patellar ligament presents a fibro-cartilaginous thickening, and an extensive bursa is interposed between it and the external condyle of the femur.

The =semitendinosus= and =semimembranosus= arise on the ischium only. The latter has a branch attached to the internal condyle of the tibia.

III. ANTERIOR MUSCLES OF THE THIGH

The =quadriceps femoris= resembles that of the horse; the =vastus intermedius= is more clearly separable, and consists of two parts. Bursæ occur under the insertions of the internal and external vasti, and often under the end of the biceps in the adult.

The =articularis genu= or subcrureus is a small muscle which lies under the lower part of the vastus intermedius, and is inserted on the suprapatellar cul-de-sac of the synovial membrane.

The =capsularis= is absent.

IV. INTERNAL MUSCLES OF THE THIGH

The =sartorius= arises by two heads, one from the tendon of the psoas minor and the iliac fascia, the other from the shaft of the ilium. The femoral vessels pass between them.

The =gracilis= is more extensively united with its fellow than in the horse.

The =pectineus= is large, and arises by a single head from the pubic crest and prepubic tendon. It divides into two branches, one of which is inserted as in the horse, while the other extends to the internal epicondyle of the femur.

The =adductor= resembles that of the horse, but does not reach to the internal condyle.

The =quadratus femoris= and =obturator externus= resemble those of the horse.

The =obturator internus= arises from the ischium only, and its tendon passes through the obturator foramen.

The =gemellus= is large.

V. MUSCLES OF THE LEG AND FOOT

There are four =digital extensors=, two of which are fused with each other and the peroneus tertius in the upper third of the leg.

1. The =anterior= or =long digital extensor= (M. extensor digitalis longus) arises by the common tendon in the extensor fossa of the femur, and separates from the other muscles near the middle of the leg. At the distal end of the tibia it terminates on a tendon which passes down over the hock (bound down by two annular ligaments) and ends like that of the thoracic limb.

2. The =internal digital extensor= (M. extensor digiti tertii proprius) arises in common with the preceding muscle and the peroneus tertius, and is covered by them to the distal third of the tibia. Its tendon passes under the annular ligaments between those of its cogeners and ends on the second phalanx of the inner digit.

3. The =lateral digital extensor= (M. extensor digitalis lateralis s. digiti quarti proprius) arises on the external lateral ligament of the stifle joint and the external condyle of the tibia. Its tendon passes over the outer surface of the hock, and terminates on the anterior surface of the second phalanx of the external digit.

The reinforcing bands from the suspensory ligament are arranged as in the fore limb.

4. The =extensor brevis= resembles that of the horse, but is inserted on the tendon of the anterior extensor only.

The =peroneus longus= (not present in the horse) is situated in front of the lateral extensor. It arises on the external condyle of the tibia and the fibrous band which represents the shaft of the fibula. Its tendon passes downward and backward over the outer surface of the hock, crosses over that of the lateral extensor and under the lateral ligament, and ends on the first tarsal bone (Cuneiform parvum). It is enveloped by a synovial sheath. It would apparently act as an inward rotator at the hock joint.

The =peroneus tertius= is a well-developed muscle which arises on the common tendon with the long and internal extensors. It has a large, fusiform belly, which is superficially situated on the front of the leg. Close to the tarsus it ends on a flat tendon, which is perforated by that of the tibialis anterior, and ends on the large metatarsal and second and third (fused) tarsal bones (Cuneiform magnum).

The =tibialis anterior= is smaller, and arises by two heads. The larger head springs from the outer surface of the tuberosity of the tibia; the outer, smaller one (M. extensor hallucis longus), from the upper part of the external border of the tibia and the fibrous band which replaces the shaft of the fibula. The tendon perforates that of the preceding muscle and ends on the metatarsal and second and third tarsal bones.

B. POSTERIOR GROUP

The =gastrocnemius= and =soleus= resemble those of the horse. The =superficial flexor= is more fleshy than in the horse. Its tendon terminates as in the fore limb.

The =deep flexor= has in its upper part a close resemblance to that of the horse, but the superficial head (tibialis posterior) is larger and distinct. The tendon ends like that on the fore limb.

MUSCLES OF THE PIG

MUSCLES OF HEAD

The =facial panniculus= is pale, thin, and difficult to separate from the skin.

The =orbicularis oris= is little developed.

The =levator nasolabialis= is thin and pale, and is undivided.

The =levator labii superioris proprius= may well be termed the =levator rostri=. It has a large penniform belly, which arises in the fossa on the lacrimal bone and maxilla. The tendon ends on the anterior part of the os rostri. A muscular slip connects it with the premaxilla.

The =zygomaticus= arises on the fascia over the masseter and ends at the angle of the mouth.

The =depressor labii inferioris= separates from the buccinator only near the angle of the mouth; it ends by a number of tendinous branches in the lower lip.

The =dilatator naris lateralis= is well developed. It arises under the levator rostri and ends by a tendinous network around the nostril.

The =dilatator naris transversus= is represented only by a few fibers which cross over the os rostri.

The =depressor rostri= arises on the facial crest. It has a long strong tendon which passes below the nostril and turns up and inward to meet the tendon of the opposite side and end in the skin of the snout. It depresses the snout and contracts the nostril.

The malaris is absent, and the other palpebral muscles present no special features.

MUSCLES OF MASTICATION

The =digastricus= has only one belly. It ends on the inner and lower surface of the mandible, in front of the groove for the facial vessels.

The other muscles have no important differential features.

The same is true of the oral hyoid muscles.

MUSCLES OF THE NECK

The =panniculus carnosus= consists of two layers which cross each other obliquely. The fibers of the superficial layer are directed nearly vertically, those of the deep layer toward the face, on which they are continued to form the facial portion.

The =mastoido-humeralis= is described on p. 314.

The =sterno-cephalicus= (Sterno-mastoideus) arises on the sternum and is inserted by a long round tendon on the mastoid process.

The thyroid portion of the =sterno-thyro-hyoideus= has a peculiar arrangement. It arises (separately from the opposite muscle) on the manubrium sterni. About the middle of the neck it has an oblique tendinous intersection, beyond which it divides into two branches; one of these is inserted in the usual fashion, the other ends on the laryngeal prominence. The hyoid portion is well developed.

The =omo-hyoideus= is thin. It arises as in the horse, but has no connection with the mastoido-humeralis nor with the opposite muscle.

The =omo-transversarius= arises on the first or second cervical vertebra (under cover of the mastoido-humeralis), and is inserted into the lower part of the scapular spine.

There are two =scaleni=. The =scalenus ventralis= (s. primæ costæ) resembles that of the ox, is attached to the last four cervical vertebræ, and is perforated by the nerves of the brachial plexus. The =scalenus dorsalis= (s. supracostalis) arises on the transverse processes of the third to the sixth cervical vertebra, and ends on the third rib.

The =straight muscles= of the head present no special features.

The =longus colli= is separated from the opposite muscle, so that part of the bodies of the cervical vertebræ is exposed as in man.

The =intertransversales= resemble those of the ox.

The =splenius= is thick and extensive. It ends in three parts on the occipital, the temporal, and the wing of the atlas (inconstant).

The =trachelo-mastoideus= is small, and its atlantal portion is blended with the longissimus.

The =complexus= is large, and is clearly divided into two portions: the dorsal portion (Biventer cervicis) is marked by several tendinous intersections; the ventral part is the complexus proper.

The =obliquus capitis posterior= is relatively thin.

The =recti capitis= are thick and more or less fused.

MUSCLES OF THE THORAX

The =levatores costarum= and =rectus thoracis= present no special features.

The =external intercostal= muscles are absent under the serratus anticus and posticus and the digitations of the external oblique.

The =internal intercostals= are thick between the cartilages of the sternal ribs.

The =retractor costæ= and the =transversus thoracis= resemble those of the horse; the latter extends back to the eighth cartilage and fuses with the transversus abdominis.

The =diaphragm= has seven costal digitations on each side, the posterior ones being attached to the ribs at some distance (ca. one-third to one-fourth of rib-length) from the costo-chondral junction. The line of attachment reaches the latter at the tenth rib, and passes along the eighth cartilage to the xiphoid process. The tendinous center is more rounded than in the horse. The crura are well developed. The right crus is very large, and is perforated by the extensive slit-like opening for the œsophagus, which is median in position, and lies about two and one-half to three inches (ca. 6 to 8 cm.) below the twelfth thoracic vertebra. The hiatus aorticus is between the crura.

ABDOMINAL MUSCLES

The =abdominal tunic= is little developed.

The =obliquus abdominis externus= has an extensive fleshy portion and a correspondingly narrow aponeurosis; the latter does not detach a femoral lamina, but is reflected in toto to form the inguinal (Poupart’s) ligament.

The =obliquus abdominis internus= resembles that of the ox; a small fusiform muscle, which crosses the inguinal canal obliquely and is attached on the abdominal surface of the inguinal ligament, is apparently a detached slip of the internal oblique.

The =rectus abdominis= is extensive and thick. It has seven to ten intersections. Its tendon of insertion fuses largely with the common tendon of the gracilis, and does not give off an accessory band to the head of the femur.

The fleshy part of the =transversus abdominis= is well developed. It blends in front with the transversus thoracis.

The =cremaster externus= is present in the female as well as in the male.

MUSCLES OF THE BACK AND LOINS

The =serratus anticus= is inserted into the fifth to the eighth ribs inclusive, the serratus posticus into the last four or five ribs. There are usually no digitations attached to the ninth and tenth ribs.

The =transversalis costarum= (Iliocostalis) extends to the wing of the atlas.

The =spinalis et semispinalis= can be separated without much difficulty from the longissimus dorsi, the division from the common mass of the loins beginning about the first lumbar vertebra.

The =multifidus= resembles that of the horse.

=Interspinales= are present, as well as distinct =intertransversales= of the back and loins.

MUSCLES OF THE TAIL

The superior and lateral =sacro-coccygei= arise as far forward as the last lumbar vertebra. Gurlt explains the twist of the tail as being due to the spiral arrangement of the insertions of the tendons.

MUSCLES OF THE THORACIC LIMB

MUSCLES OF THE SHOULDER GIRDLE

The =trapezius= is very wide, its line of origin extending from the occipital bone to the tenth thoracic vertebra. There is no clear division between its two parts, which are both inserted into the scapular spine.

The =omo-transversarius= resembles that of the ox.

The =rhomboideus= consists of three portions. The =cervical portion= (Rhomboideus cervicalis) is greatly developed, its origin extending from the second cervical to the sixth thoracic vertebra. The =cephalic portion= (Rhomboideus capitis) arises with the splenius on the occipital bone, and is inserted with the cervical part. The =dorsal portion= (Rhomboideus thoracalis) extends as far back as the ninth or tenth thoracic vertebra.

The =latissimus dorsi= is attached to the four ribs preceding the last. It is inserted into the internal lip of the bicipital groove.

The =mastoido-humeralis= divides into two parts, the =cleido-mastoideus= and =cleido-occipitalis=, which arise on the mastoid process and occipital crest respectively, and unite at the fibrous vestige of the clavicle.

The =anterior superficial pectoral= is thin. The =posterior superficial pectoral= is divided into parts, one of which ends on the humerus, the other on the fascia of the forearm.

The =deep pectoral= is clearly divided into two parts. The =scapular portion= resembles that of the horse, but its origin does not extend behind the first two chondro-sternal joints. The =posterior deep pectoral= is very long.

The cervical portion of the =serratus magnus= is greatly developed, its origin extending from the wing of the atlas to the upper part of the fifth rib, and passing under the thoracic portion; the latter resembles that of the ox.

MUSCLES OF THE SHOULDER

The =deltoid= is undivided; it arises from the aponeurosis covering the infraspinatus, and it ends largely on the deltoid ridge, but partly on the fascia of the arm.

The =supraspinatus= is large; it ends chiefly on the external tuberosity of the humerus.

The =infraspinatus= is inserted into a depression below the posterior division of the external tuberosity.

The =teres minor= is well developed; it ends on a tubercle between the external and deltoid tuberosities of the humerus.

The =subscapularis= and =teres major= have no remarkable features.

The =coraco-brachialis= is short and undivided.

The =capsularis= is very small and frequently absent.

MUSCLES OF THE ARM

The =biceps brachii= is fusiform and not greatly developed. Its tendon of origin is rounded and passes through the capsule of the shoulder joint. The tendon of insertion is bifid, ending on the radius and ulna.

The =brachialis= is well developed, and also ends on the radius and ulna.

The =tensor fascias antibrachii= resembles that of the horse.

The =long head of the triceps= is divided into two parts, except at its distal end, under which there is a synovial bursa. The =external head= is inserted into a crest on the outer surface of the olecranon by a thin tendon, under which there is a bursa. The =internal head= arises near the head of the humerus, beneath the coraco-brachialis.

There are two =anconei=.

MUSCLES OF THE FOREARM AND DIGITS

The =extensor carpi radialis= is a strong, fleshy muscle, the tendon of which is inserted into the proximal end of the third metacarpal bone. It may be divided into two parts (M. extensor carpi radialis longus, brevis).

The =extensor carpi obliquus= is well developed; it ends on the second metacarpal bone.

There are three (or four) =digital extensors=.

1. The =common= or =anterior digital extensor= (M. extensor digitalis communis) arises on the extensor epicondyle of the humerus and the lateral ligament of the elbow, and divides into three parts. The tendon of the =inner part= divides into two branches which end on the =second= and =third digits=; the tendon to the second digit is small and often absent. The tendon of the =middle part= divides lower down into two branches for the =third= and =fourth= (chief) =digits=; above this bifurcation it detaches a small branch to the =second digit=, which usually unites with the tendon of the extensor indicis. The tendon of the =deep head= divides into two branches, the inner one joining the tendon of the middle head, while the outer one ends on the =fifth digit=.

2. The =extensor of the second digit= (M. extensor indicis proprius) is covered by the preceding muscle, with which it is partially fused. It arises on the ulna. Its delicate tendon usually unites with the tendon of the middle head of the common extensor which goes to the =second digit=.

3. The =lateral digital extensor= (M. extensor digitalis lateralis) consists of two distinct parts: (1) The large =dorsal= (anterior) =muscle= (M. extensor digiti quarti proprius) has a long tendon which ends on the =fourth digit=, and often sends a slip to the fifth digit. (2) The small =volar= (posterior) =muscle= (M. extensor digiti quinti proprius) ends by a long tendon on the lateral aspect of the =fifth digit=.

The =supinator= has been found by Arloing and Lesbre, but is usually absent in the pig.

The =pronator teres= is a small muscle which lies on the internal lateral ligament of the elbow. It arises from the internal epicondyle of the humerus, and reaches about to the middle of the inner border of the radius.

The =flexor carpi internus= is well developed. It arises on the flexor epicondyle of the humerus, and is inserted into the third metacarpal bone.

The =flexor carpi medius= is narrow and has no ulnar head.

The =flexor carpi externus= (M. extensor carpi ulnaris) is covered by a thick, tendinous band, which extends from the extensor epicondyle to the accessory carpal bone and outer aspect of the carpus. The tendon of the muscle perforates this band in the distal part of the forearm and ends on the proximal end of the fifth metacarpal bone.

The =superficial digital flexor= has two bellies. The tendon of the =superficial head= passes down behind the posterior annular ligament of the carpus (bound down by a special annular ligament), forms a ring at the fetlock for a tendon of the deep flexor, and ends by two branches on the second phalanx of the =fourth digit=. The tendon of the =deep head=, after detaching a slip to the tendon of the deep flexor, passes down with the latter (for which it forms a ring), and ends on the =third digit=.

The =deep digital flexor= has three heads—humeral, ulnar, and radial. The common tendon divides into four branches, the larger central pair ending on the third phalanges of the principal digits, the smaller pair on the accessory digits. The latter are bound down by a peculiar spiral band. There is no check ligament.

The =lumbricales= are represented by bundles which extend from the deep flexor tendon to the tendon of the deep head of the superficial flexor.

The third and fourth =interossei= are present. Each sends two slips to the corresponding sesamoid bones and extensor tendon.

There are =flexors=, =adductors=, and =abductors= of the =second= and =fifth digits=.

MUSCLES OF THE PELVIC LIMB

SUBLUMBAR MUSCLES

The =psoas minor= is intimately united with the psoas major in front, and has a long small tendon which ends on the psoas tubercle. It has no thoracic part.

The =psoas major= is large and rounded. It begins at the last rib.

The =quadratus lumborum= is well developed, and extends forward as far as the twelfth rib.

MUSCLES OF THE HIP AND THIGH

The =tensor fasciæ latæ= is broad, and its fleshy part reaches almost to the patella.

The =gluteus superficialis= has a sacral head only; it blends with the biceps femoris.

The =gluteus medius= has a small lumbar part which does not extend so far forward as in the horse. The deep part (Gluteus accessorius) is pretty clearly marked.

The =gluteus profundus= is extensive, reaching nearly to the external angle of the ilium.

The =biceps femoris= has a narrow origin from the sacro-sciatic ligament and tuber ischii. It ends below like that of the ox.

The =semitendinosus= resembles that of the horse.

The =semimembranosus= has two insertions as in the ox.

The =sartorius= has two heads of origin, between which the external iliac vessels are situated. The inner arises from the tendon of the psoas minor, the outer one from the iliac fascia.

The =graciles= are united at their origin even more than in the ox.

The =pectineus= is well developed and is flattened from before backward.

The =adductor= shows no division and is partially fused with the gracilis. It ends on the femur just above the origin of the gastrocnemius.

The =quadratus femoris= is large.

The =obturator externus= resembles that of the horse.

The =obturator internus= is extensive and strong; its tendon emerges through the obturator foramen.

The =gemellus= is fused in part with the obturator internus.

The =quadriceps femoris= is more clearly divided than in the horse, and its action is transmitted by a single patellar ligament.

The =capsularis= is absent.

MUSCLES OF THE LEG AND FOOT

The =peroneus tertius= resembles that of the ox, but its tendon is not perforated by that of the tibialis anterior, and ends on the inner (second) large metatarsal bone, detaching a strong branch to the first and second tarsal bones.

The =tibialis anterior= arises on the tibial crest. Its tendon passes over the hock along the inner border of the tendon of the peroneus tertius (both being bound down by an annular ligament), and is inserted into the inner (second) metatarsal and first tarsal bones.

The =peroneus longus= lies partly on the outer surface of the peroneus tertius. Its tendon crosses those of the lateral extensor, passes along a groove on the plantar surface of the fourth tarsal bone, and ends on the first tarsal and second metatarsal bones.

The =anterior= or =long digital extensor= arises with the peroneus tertius and is largely covered by it and the peroneus longus. It divides into three parts. The tendon of the =inner division= (M. extensor digiti tertii) ends on the =inner chief= (third) =digit=; that of the =middle division= by two branches on the =principal digits=; and that of the =outer division= by three branches on the =second=, =fourth=, and =fifth digits=.

The =lateral digital extensor= divides into proper extensors of the =fourth= and =fifth digits= (Extensores digiti quarti et quinti).

The =extensor hallucis longus= arises on the fibula and ends on the =inner small= (second) =digit=.

The =extensor digitalis brevis= is well developed and consists of three parts. The inner and outer tendons end on the first phalanges of the chief digits, the middle one joins the common extensor tendon of the same.

The =gastrocnemius= presents nothing special.

The =soleus= is wide, and is partly blended with the external head of the gastrocnemius.

The =superficial digital flexor= has a large belly. Its tendon ends on the two chief digits, and is also connected with the ligaments of the accessory digits.

The =inner head= of the =deep digital flexor= (flexor digitalis longus) is relatively small. The common tendon ends as in the thoracic limb.

The =popliteus= presents no special features.

The =plantar muscles= resemble their homologues of the fore limb, except that the adductors of the accessory digits and the lumbricales are absent.

THE MUSCLES OF THE DOG

MUSCLES OF THE FACE

The =panniculus carnosus= is well developed and presents two distinct bands, one of which is inserted into the lower lip, the other into the scutiform cartilage of the ear.

The =orbicularis oris= is poorly developed. In the upper lip it is divided centrally, and in the lower it is distinct only near the angles of the mouth.

The =levator nasolabialis= is wide and undivided, and ends in the upper lip.

The =levator labii superioris proprius= arises behind the infraorbital foramen, runs forward under the preceding muscle, and ends in numerous small tendons around the nostril, some blending with those of the opposite side.

The =zygomaticus=, long and narrow, arises on the scutiform cartilage, and ends at the angle of the mouth.

The =depressor labii inferioris= is absent, unless we recognize as such a thin stratum arising on the body of the mandible and spreading out in the orbicularis.

There are no special =nasal muscles=, the homologue of the lateral dilator ending almost entirely in the upper lip.

The =buccinator= is wide and very thin, and the two planes of fibers cross each other.

The =palpebral muscles= present no very noteworthy special characters.

MUSCLES OF MASTICATION

The =masseter= is large. It arises from the zygomatic arch, and extends beyond the branch of the jaw below and behind. Three strata may be recognized in its structure.

The =temporalis= is very large and strong, and contains much tendinous tissue. It arises from the temporal fossa and the orbital ligament.

The =digastricus= is usually not digastric, but is a strong, round, fleshy muscle, which arises on the paramastoid or styloid process and is inserted into the border and inner surface of the ramus at the level of the last molar teeth.

The stylo-mandibularis is absent.

HYOID MUSCLES

The =mylo-hyoideus= is well developed.

The =stylo-hyoideus= is very slender, and is inserted into the body of the hyoid bone: it is not perforated by the digastricus.

The hyoideus transversus and =omo-hyoideus= are absent.

The =sterno-thyro-hyoideus= is large and arises chiefly on the first costal cartilage.

MUSCLES OF THE THORACIC LIMB

The =trapezius= is thin, and is not clearly divided into cervical and thoracic portions. Its line of origin extends from about the middle of the neck to the ninth or tenth thoracic spine, the right and left muscles meeting on a median fibrous raphé.

The =omo-transversarius= arises by a tendon on the lower part of the spine of the scapula (often partially blended with the trapezius), and is inserted into the wing of the atlas.

The =rhomboideus= consists of three parts. The =dorsal portion= (Rhomboideus thoracalis) is small; it arises from the fourth to the sixth or seventh thoracic spine, and is inserted into the inner surface (chiefly) of the dorsal angle of the scapula. The =cervical portion= (Rhomboideus cervicalis) arises from the ligamentum nuchæ as far forward as the second or third cervical vertebra, and is inserted into the inner surface of the cervical angle of the scapula. The =cephalic portion= (Rhomboideus capitis) is a continuation of the preceding which is inserted into the occipital crest.

The =latissimus dorsi= is extensive, and has a fleshy attachment to the last two ribs. Its lower edge blends near the shoulder with the panniculus.

The =mastoido-humeralis= contains in front of the shoulder a fibrous mass in which the clavicle is embedded. Anterior to this it separates into two diverging portions. The =dorsal portion= (M. cleido-cervicalis) is attached to the median raphé of the neck and to the occipital bone. The =ventral portion= (M. cleido-mastoideus) is attached to the mastoid process. The common mass posterior to the clavicle and the fibrous intersection, which is attached to the humerus, is homologous with the clavicular portion of the deltoid of man.

The =superficial pectoral= muscle is small. It arises on the sternum from the first to the third costal cartilage, and is inserted into the anterior surface of the humerus. A superficial slip detached from it is inserted into the fascia of the forearm. The =deep pectoral= has no prescapular portion. It arises on the sternum and costal cartilages from the second costal to the xiphoid cartilage. It is inserted chiefly into the internal tuberosity of the humerus, but also by small slips into the external tuberosity and the fascia of the arm.

The =serratus magnus= shows no clear division into cervical and thoracic portions. It arises from the last five cervical vertebræ and the first seven or eight ribs, and is inserted into the upper part of the costal surface of the scapula.

The =deltoid= is clearly divided into scapular and acromial parts, the former arising on the spine, the latter on the acromion of the scapula. Both end on the deltoid tuberosity.

The =supraspinatus= ends chiefly on the external tuberosity of the humerus, but has a small attachment to the internal tuberosity also.

The =infraspinatus= is inserted into the outer surface of the external tuberosity of the humerus.

The =teres minor= arises on a tubercle on the posterior border of the scapula, just above the glenoid cavity, and is inserted into the deltoid ridge.

The =subscapularis= is wide and is somewhat multipennate in structure, being intersected by fibrous septa which are attached to the rough lines on the costal surface of the scapula.

The =teres major= is thick. It arises on the upper part of the posterior border of the scapula and on the subscapularis.

The =coraco-brachialis= is short and undivided, and is inserted into the upper part of the postero-internal surface of the humerus.

The capsularis is absent.

The =biceps brachii= lies almost entirely on the inner surface of the humerus. It is long and fusiform. The tendon of origin is round, and passes through the capsule of the shoulder joint. The tendon of insertion is bifid, one branch being attached to the ulnar, the other to the radial tuberosity.

The =brachialis= is very little curved, and is inserted chiefly into the tuberosity and inner border of the ulna.

The =tensor fasciæ antibrachii= is thin and narrow. It arises on the outer surface of the latissimus dorsi, and ends on the olecranon and the fascia of the forearm.

The =triceps= has an additional deep head (Caput accessorium), which arises just below the head of the humerus.

The =brachio-radialis= is a long, narrow, delicate muscle, situated superficially on the anterior surface of the forearm. It arises with the extensor carpi on the crest above the extensor epicondyle of the humerus, and is inserted into the distal part of the inner border of the radius. It is often much reduced, and is sometimes absent. It rotates the forearm and paw outward.

The =extensor carpi= divides into two parts. The larger outer part, the =extensor carpi radialis brevis=, ends on the proximal end of the third metacarpal bone. The inner and more superficial part, the =extensor carpi radialis longus=, ends on the second metacarpal bone. (A tendon to the fourth metacarpal may occur.)

The =extensor carpi obliquus= or =abductor pollicis longus= arises from the outer border and anterior surface of the ulna, the interosseous ligament, and the outer border of the radius. It is inserted into the first metacarpal bone by a tendon which contains a small (sesamoid?) bone. It abducts the first digit.

There are three (or four) =extensors of the digits=.

1. The =common digital extensor= (M. extensor digitalis communis) arises on the extensor epicondyle of the humerus and the lateral ligament of the elbow joint. It has four bellies, each terminating in a tendon. These are inserted into the third phalanges of the second, third, fourth, and fifth digits.

2. The =extensor of the first and second digits= (M. extensor pollicis longus et extensor indicis proprius) is small, and is covered by the common and lateral extensors. It arises on the proximal part of the ulna. Its tendon passes down with that of the common extensor and divides into two branches. The delicate inner branch ends on the first digit, while the other blends with the tendon of the common extensor for the second digit.

3. The =lateral digital extensor= (M. extensor digitalis lateralis) consists of two muscles which are not rarely fused. They arise on the extensor epicondyle of the humerus and the lateral ligament of the elbow joint. The larger, =superficial belly= (M. extensor digiti tertii et quarti) terminates on a tendon which divides at or near the carpus into two branches, which are inserted into the third phalanges of the third and fourth digits, blending with the corresponding tendons of the common extensor. The =posterior belly= (M. extensor digiti quinti) terminates by a tendon which fuses with that of the common extensor for the fifth digit.

The =ulnaris lateralis= or =extensor carpi ulnaris= corresponds to the flexor carpi externus of the horse, but is an extensor of the carpus. It is a large flat muscle which lies on the outer surface of the ulna. It arises on the extensor epicondyle of the humerus, and is inserted into the proximal end of the fifth metacarpal and the accessory carpal bone.

The =ulnaris medialis= or =flexor carpi medius= (s. ulnaris) consists of two quite distinct heads. The larger, =humeral head= arises on the flexor epicondyle, while the smaller, superficial =ulnar head= arises on the posterior border of the ulna. The tendons of the two end together on the accessory carpal bone.

The =radialis volaris= or =flexor carpi internus= (s. radialis) arises on the flexor epicondyle of the humerus and is inserted by a bifid tendon into the second and third metacarpal bones.

The tendons of the foregoing muscles are provided with synovial sheaths at the carpus.

The =pronator teres= is a round muscle, which is situated superficially on the inner border of the proximal part of the radius. It arises on the flexor epicondyle of the humerus, and is inserted into the internal border of the radius. Its action is to flex the elbow and rotate the forearm inward.

The =superficial digital flexor= is situated superficially on the posterior and inner surfaces of the forearm. It arises on the flexor epicondyle of the humerus and terminates near the carpus on a tendon which passes downward outside of the carpal canal and receives below the carpus two reinforcing bands, one from the accessory carpal, the other from the sesamoid bone at the inner side of the carpus. Below this it divides into four branches, which are inserted into the second phalanges of the second, third, fourth, and fifth digits.

The =deep digital flexor= has =humeral=, =ulnar=, and =radial heads=, the latter arising from the inner border of the radius. They unite on a common tendon which passes down through the carpal canal, gives off a branch to the first digit, and divides into four branches. These perforate the tendons of the superficial flexor and are inserted into the third phalanges of the second to the fifth digits. The tendons are provided with synovial sheaths from the middle of the metacarpus downward, and are held in place by three digital annular ligaments.

The =palmaris longus accessorius= (?) is a small muscle which arises from the deep flexor below the middle of the forearm and ends by two tendons which unite with those of the superficial flexor for the third and fourth digits.

The =supinator= is a short, wide muscle which arises on the humerus beneath the external lateral ligament, crosses the anterior surface of the radius, and is inserted into the inner border of the radius. (A pouch of the capsule of the elbow joint lies under the tendon of origin.) Its action is to rotate the forearm outward (supination).

The =pronator quadratus= consists of fibers which cross the inner surface of the interosseous ligament of the forearm, except at the two extremities of the latter. It is attached to the inner border of the ulna and the anterior surface of the radius. It rotates the forearm inward (pronation).

The =palmaris brevis= (?) is a very small muscle, which arises on the tendon of the superficial digital flexor for the fifth digit, and is inserted at the fifth metacarpo-phalangeal joint into the sheath and annular ligament of the deep flexor.

The =lumbricales= are three very delicate muscles, which arise on the tendons of the deep flexor, and are inserted into the first phalanges of the third, fourth, and fifth digits.

The =abductor pollicis brevis et opponens pollicis=, a very small pale muscle, arises on the fibrous band which connects the superficial flexor tendon with the inner carpal sesamoid, and ends on the distal end of the first metacarpal bone and the first phalanx of the first digit. It abducts the first digit.[76]

The =flexor pollicis brevis= arises on the posterior carpal ligament over the second metacarpal bone, and ends on the volar sesamoid of the first digit.

The =adductor pollicis=, situated externally to the preceding, is the largest of the thumb muscles. It arises between the preceding and the second interosseous muscle, and is inserted into the first phalanx of the first digit.

The =adductor digiti secundi= is situated between the second interosseous muscle and the adductor digiti quinti. It arises on the carpal ligament, and ends on the first phalanx of the second digit.

The =adductor digiti quinti= arises close to the preceding muscle, and passes outward to end on the first phalanx of the fifth digit.

The =flexor digiti quinti= arises on the ligament connecting the accessory carpal to the third and fourth metacarpal bones, crosses the corresponding interosseous muscle, and ends on the fifth digit with the next muscle.

The =abductor digiti quinti= is larger than the two preceding muscles; it arises on the accessory carpal bone, and ends on the outer sesamoid of the fifth digit and on the lateral ligament.

There are four =interossei= which lie on the volar (posterior) surface of the metacarpus. They are well developed and fleshy. They arise on the distal row of the carpus and on the proximal ends of the metacarpals. Each divides distally into two branches, which are inserted by small tendons on the corresponding sesamoid bones, and detach slips to the extensor tendons.

MUSCLES OF THE NECK

The =sterno-cephalicus= is well developed. It arises on the manubrium sterni and ends on the mastoid process, blending with the cleido-cervicalis.

The =scalenus ventralis= (s. primæ costæ) arises on the last four cervical transverse processes, and is inserted into the first rib.

The =scalenus dorsalis= (s. supracostalis) is large. Anteriorly it blends with the preceding muscle, while posteriorly it divides into two parts. The upper part is inserted on the third and fourth ribs, the =lower part= by a long, thin tendon on the eighth rib.

The =longus colli= resembles that of the horse.

The =rectus capitis anterior major= arises on the transverse processes of the second to the sixth cervical vertebræ and ends as in the horse. The =rectus minor= and =lateralis= resemble those of the horse.

The =intertransversales= resemble those of the ox.

The =splenius= is strong and extensive. It arises on the first four or five thoracic spines and the median raphé of the neck, and is inserted into the occipital crest and mastoid process.

The =complexus= is composed of two parts—the biventer cervicis and the complexus major proper. The =biventer cervicis= arises from the transverse processes of the fifth and sixth, and the spines of the second to the fifth (or sixth) thoracic vertebræ, from the ligamentum nuchæ, and the median raphé. It has four tendinous intersections. The =complexus major= arises on the transverse processes of the first three or four thoracic vertebræ and the articular processes of the last five cervical. Both end on a strong common tendon which is inserted into the occipital crest and the depression below it.

The =trachelo-mastoideus= consists of two unequal portions. The large =dorsal part= (M. longissimus capitis) arises on the transverse processes of the first four thoracic and the articular process of the last three or four cervical vertebræ, and ends with the splenius on the mastoid process of the temporal bone. The small =ventral part= (M. longissimus atlantis) arises on the articular processes of the third, fourth, and fifth cervical vertebræ, and ends on the wing of the atlas.

The other muscles present no striking differential features, but it may be noted that distinct =interspinales= are present.

MUSCLES OF THE THORAX

There are twelve pairs of =levatores costarum=.

The =external intercostals= do not occupy the spaces between the costal cartilages.

The =diaphragm= has a small tendinous center. The =œsophageal opening= is between the pillars. The fleshy rim is attached at the costo-chondral junctions from the eighth backward, and along the thirteenth rib nearly its entire length.

MUSCLES OF THE BACK AND LOINS

The =serratus anticus= arises from the median raphé of the neck and the first six or seven thoracic spines, and is inserted into the second to the ninth ribs. It is well developed. The =serratus posticus=—much weaker—arises on the lumbo-dorsal fascia, and is inserted into the last three or four ribs. Thus one or two ribs intervene between the two.

The =transversalis costarum= (Iliocostalis) is well developed, and extends from the ilium to the sixth, fifth, or fourth cervical vertebra.

The =longissimus= resembles that of the other animals, but the spinalis et semispinalis separates clearly at the sixth or seventh thoracic vertebra. It is inserted into the articular and spinous processes of the last six cervical vertebræ. It has no depression in the lumbar region for the gluteus medius.

The =intertransversales= are fleshy, as in the ox.

The =interspinales= are distinct, especially in the lumbar region.

MUSCLES OF THE TAIL

These present the same general arrangement as in the horse. The =sacro-coccygei=, however, arise on the lumbar vertebræ also, and the =coccygeus= on the ischiatic spine. There is found a =sacro-coccygeus accessorius=, which arises on the internal border of the ilium, the edge of the sacrum, and the transverse processes of the first coccygeal vertebræ, and is inserted between the superior and lateral sacro-coccygei. It is homologous with the intertransversales.

MUSCLES OF THE ABDOMEN

The abdominal tunic is practically absent.

The =obliquus abdominis externus= has an extensive fleshy portion. It arises from the last eight or nine ribs and the lumbo-dorsal fascia.

The =obliquus abdominis internus= arises from the external angle of the ilium and the lumbo-dorsal fascia. The fibers have an almost vertical direction, and there is a fleshy attachment to the last rib.

The =rectus abdominis= is attached by a long tendon on the first five or six costal cartilages, and by fleshy fibers on the xiphoid cartilage. It has three to six indistinct tendinous inscriptions.

The =transversus abdominis= presents no special features except that the posterior part of its aponeurosis splits into two layers which include the rectus between them.

MUSCLES OF THE PELVIC LIMB

The =psoas minor= arises on the last thoracic and first four or five lumbar vertebræ, and is inserted into the ilio-pectineal line.

The =psoas major= is short, arising from the last four lumbar vertebræ.

The outer head of the =iliacus= is small, while the inner head is large and fuses with the psoas major.

The =quadratus lumborum= is well developed, and extends laterally beyond the outer edge of the ilio-psoas. It arises from the last four ribs and the lumbar transverse processes, and ends on the pelvic surface of the wing of the ilium.

The =tensor fasciæ latæ= consists of two parts. The anterior part is long and rounded; the posterior is shorter and fan-like.

The =gluteus superficialis= is small. It arises on the sacrum, first coccygeal vertebræ, and sacro-sciatic ligament. It is inserted below and behind the trochanter major of the femur, on the outer branch of the linea aspera.

The =gluteus medius= has no lumbar portion. It is inserted into the trochanter major by a strong tendon.

The =gluteus profundus= is broad and fan-shaped. It arises on the superior ischiatic spine and on the ilium as far forward as the gluteal line, and is inserted into the trochanter major below the medius.

The =pyriformis= is not blended with the gluteus medius. It arises from the border of the sacrum and from the sacro-sciatic ligament, and ends on the trochanter major.

The =biceps femoris= has two heads of origin which soon fuse. The larger head arises from the sacro-sciatic ligament and tuber ischii, the smaller one from the tuber ischii. The aponeurosis of insertion ends on the patella, the patellar ligament, and the tibial crest. There is also a tendinous band, which comes from the deep face of the muscle and terminates at the tarsus, as in the horse.

The =abductor cruris posterior= may be regarded as an accessory head of the biceps femoris. It is a thin, muscular band which arises on the sacro-sciatic ligament, passes downward between the biceps and semimembranosus, and ends by blending with the former.

The =semitendinosus= arises from the tuber ischii only. It ends as in the horse.

The =semimembranosus= is large and arises from the ischium only. It divides into two portions: the anterior portion ends on the tendon of the pectineus, on the femur above the internal condyle, and on the inner (Vesalian) sesamoid bone; the posterior portion ends on the internal condyle of the tibia, the tendon passing under the internal lateral ligament of the stifle joint.

The =rectus femoris= has only one tendon of origin. The single patellar ligament acts as the tendon of insertion of the quadriceps.

The =capsularis= is usually present, but is small and pale.

The =sartorius= consists of two portions. It arises from the external angle and border of the ilium. The anterior portion ends on the patella, the posterior on the internal surface of the tibia, its tendon blending with that of the gracilis.

The =graciles= are not so much fused at their origin as in the other animals.

The =pectineus= is long and slender. It arises from the ilio-pectineal eminence and ends on the internal branch of the linea aspera above the distal end of the femur.

There are two distinct =adductors=. The small anterior one ends on the proximal third of the femur, the posterior one on the distal part of the femur and on the inner face of the stifle joint.

The =quadratus femoris= is short and strong. The other external rotators of the thigh present no special features.

The =peroneus tertius= is represented by a tendinous band which arises on the internal surface of the tibia below the crest. It passes downward on the inner surface of the tibialis anterior, blends with the annular ligament above the tarsus, and is attached to the joint capsule and the proximal end of the third metatarsal bone.

Arloing and Lesbre say: “The third peroneus is a proper extensor of the fifth digit; it is a very feeble, fleshy band, situated behind the peroneus brevis, which it partially covers. It is attached to the upper part of the fibula, and is continued by a long, delicate tendon which passes in the same malleolar groove with the muscle mentioned; it then crosses behind the tendon of the peroneus longus and extends to the phalanges of the outer digit, where it joins one of the branches of the common extensor.”

The =tibialis anterior= is large and superficial. It arises on the external condyle and crest of the tibia, and is inserted into the first metatarsal bone, or into the first tarsal and second metatarsal.

There are four =extensors of the digits=.

1. The =anterior= or =long digital extensor= (M. extensor digitalis longus) is fusiform, and lies largely under the preceding muscle. It arises from the extensor fossa of the femur. The tendon is bound down by two annular ligaments, and divides below the tarsus into four branches, which end on the distal phalanges of the digits (second to fifth).

2. The =lateral digital extensor= (M. extensor digitalis lateralis) is feeble and semipennate. It arises on the fibula below the head. The tendon passes under the lateral ligament of the tarsus and joins the branch of the tendon of the long extensor for the fifth digit.

3. The =extensor hallucis longus= is a very thin muscle which arises from the fibula under the long extensor. Its delicate tendon accompanies that of the tibialis anterior to the first metatarsal bone, or becomes lost in the fascia.

4. The =extensor digitalis brevis= has three divisions. It arises on the fibular tarsal bone and the adjacent ligaments. The three tendons are inserted into the second, third, and fourth digits, blending with the interossei. (Sometimes there is found a tendon to the rudimentary first digit, which may represent the extensor hallucis brevis. There may be a fourth belly for the tendon to the second digit.)

The =peroneus longus= arises on the external condyle of the tibia, the head of the fibula, and the lateral ligament. The long tendon passes down the leg parallel to the fibula, crosses the plantar (posterior) surface of the tarsus transversely, and ends on the first metatarsal bone. A short branch is detached to the external metatarsal bone.

The =peroneus brevis= is semipennate and arises from the distal half or more of the outer face of the tibia and the fibula. Its tendon accompanies that of the lateral extensor over the external malleolus of the tibia, and ends on the proximal end of the fifth metatarsal bone.

The soleus is absent. (It is present and large in the cat.)

The =gastrocnemius= arises on the rough lines above the condyles of the femur. The heads of origin each contain a bone about the size of a pea—the sesamoid of Vesal—which articulates with the corresponding condyle of the femur. The tendon comports itself as in the horse.

The =superficial digital flexor= has a large round belly. It arises in common with the outer head of the gastrocnemius from the external rough line and Vesalian sesamoid bone, and from the aponeurosis of the vastus externus. The tendon winds around that of the gastrocnemius, passes over the tuber calcis (where it is arranged as in the horse), and divides below the tarsus into two branches. Each of these divides into two branches which end as in the fore limb. The outer and inner branches detach slips to the suspensory ligaments of the large digital pad. Muscle-fibers often occur in the tendon in the metatarsal region.

The =deep digital flexor= has two heads. The large outer head, the =flexor hallucis longus=, arises from the tibia and fibula, filling the interosseous space. The small inner head, =flexor digitalis pedis longus=, also arises from the tibia and fibula, its tendon joining that of the large head below the tarsus. The common tendon detaches a branch to the large digital pad and terminates as in the fore limb.

The =tibialis posterior= is a small but distinct muscle, which arises on the proximal part of the fibula. The thin tendon accompanies that of the flexor longus and ends on the internal lateral ligament of the tarsus.

The tendon of the =popliteus= contains a small sesamoid bone.

The =adductors of the second and fifth digits=, the =lumbricales=, and the =interossei= are arranged as in the thoracic limb.

The =quadratus plantæ= arises on the outer surface of the distal end of the fibular tarsal bone and on the lateral tarsal ligament, passes downward and inward, and terminates on a thin tendon which fuses with that of the deep flexor.

The =abductor digiti quinti= is a very small muscle which consists of two parts. One of these is a tendinous slip which extends from the posterior surface of the proximal part of the fibular tarsal bone to the head of the fifth metatarsal bone; the other part arises from the inner surface of the fibular tarsal bone (or from the tendinous part) and ends on the first phalanx of the fifth digit.

In case the skeleton of the first digit is well developed, there are found three muscles homologous with those of the same digit in the fore limb. These are the =abductor hallucis=, =adductor hallucis=, and =flexor hallucis brevis=.

THE DIGESTIVE SYSTEM

This system consists of the organs directly concerned in the reception and digestion of the food, its passage through the body, and the expulsion of the unabsorbed portion. These organs are conveniently grouped under two heads, viz.: (1) the =alimentary canal=; (2) the =accessory organs=.

The =alimentary canal= (Tractus alimentarius) is a tube, about 100 feet (ca. 30 m.) in length in the horse, and extends from the lips to the anus. It has a complete lining of mucous membrane, external to which is an almost continuous muscular coat. The abdominal portion of the tube is largely covered with a serous membrane—the visceral peritoneum. The canal consists of the following consecutive segments:

1. Mouth.

2. Pharynx.

3. Œsophagus.

4. Stomach.

5. Small intestine.

6. Large intestine.

The =accessory organs= are the teeth, tongue, salivary glands, liver, and pancreas.

DIGESTIVE SYSTEM OF THE HORSE

THE MOUTH

The =mouth=[77] (Cavum oris) is the first part of the alimentary canal. In the horse it is a long cylindrical cavity, and when closed, it is almost entirely filled up by the contained structures. The entrance to it (Rima oris) is closed by the lips. Laterally it is bounded by the cheeks; above, by the hard palate; below, by the body of the mandible and the mylo-hyoid muscles; behind, by the soft palate.

The =cavity= of the mouth is subdivided into two parts by the teeth and alveolar processes. The space external to these and inclosed externally by the lips and cheeks is termed the vestibule of the mouth (Vestibulum oris). In the resting state of the parts the walls of this cavity are in contact, and the space is practically obliterated. Its existence becomes very evident in facial paralysis, when the food tends to collect in it laterally, pouching out the cheeks. The space within the teeth and alveolar processes is termed the mouth cavity proper (Cavum oris proprium). When the teeth are in contact, it communicates with the vestibule only by the interdental spaces and the intervals behind the last molar teeth. Posteriorly it communicates with the pharynx through the isthmus of the fauces.

The =mucous membrane= lining the mouth (Tunica mucosa oris) is continuous at the margin of the lips with the common integument, and behind with the mucous lining of the pharynx. During life it is chiefly of a pink color.

The =lips= (Labia oris) are two musculo-membranous folds which surround the orifice of the mouth. Their angles of union (Anguli oris s. commissuræ labiorum) are situated near the first cheek tooth and are rounded. Each lip presents two surfaces and two borders. The outer surface is covered by the skin, which presents long tactile hairs in addition to the ordinary fine hair. The upper lip shows a shallow median furrow (Philtrum), the lower a rounded prominence, the chin (Mentum). The inner or oral surface is covered with mucous membrane which is commonly more or less pigmented. The small papillæ on this surface show on their summits the openings of the ducts of the labial glands. Small folds of mucous membrane which pass from the lip to the gum represent the frænula labii (superioris, inferioris). The free border of the lip is dense and presents short, very stiff hairs. The attached border is continuous with the surrounding structures, and is adherent to the alveolar borders of the bones of the jaws.

=Structure.=—The lips are covered externally by the skin, and are lined by mucous membrane; between these are muscular tissue and glands. The skin lies directly on the muscles, many fibers of which are inserted into the former. The muscles have been described (page 214). The =labial glands= (Glandulæ labiales) form a compact mass near the angles; they are numerous in the upper lip, fewer in the lower. The mucous membrane is often pigmented, and is reflected upon the bones of the jaws to form the gums.

=Blood-vessels and Nerves.=—The arteries are derived from the superior and inferior labial and palato-labial arteries. The sensory nerves come from the trigeminus, and the motor from the facial nerve.

The =cheeks= (Buccæ) form the sides of the mouth, and are continuous in front with the lips. They are attached above and below to the alveolar borders of the bones of the jaws.

=Structure.=—This comprises: (1) The skin; (2) the muscular and glandular layer; (3) the mucous membrane. The skin offers nothing in particular to notice. The muscular tissue is formed mainly by the buccinator, but also by parts of the panniculus, zygomaticus, dilatator naris lateralis, levator nasolabialis, and depressor labii inferioris. The =buccal glands= (Glandulæ buccales) are arranged in two rows. The upper row (superior buccal or molar glands) is found on the outer surface of the buccinator muscle, near its upper border. The anterior part of the row consists of scattered lobules; the posterior part, which lies under cover of the masseter muscle, is more developed and compact. The lower row (inferior buccal or molar glands), less voluminous than the upper, is situated in the submucous tissue at the lower border of the buccinator muscle. The mucous membrane is reflected above and below upon the gums, and is continuous behind with that of the soft palate. It is reddish in color and frequently shows pigmented areas. Opposite the third upper cheek tooth is the opening of the parotid (Stenson’s) duct, surrounded by a circular fold of the mucous membrane. A linear series of small papillæ above and below indicates the orifices of the small ducts from the buccal glands.

=Blood-vessels and Nerves.=—The blood-supply is derived from the facial and buccinator arteries. The sensory nerves come from the trigeminus and the motor from the facial nerve.

The =gums= (Gingivæ) are composed of a dense fibrous tissue intimately united with the periosteum of the alveolar processes, and blending at the edges of the alveoli with the alveolar periosteum, which fixes the teeth in their cavities. They are covered by a smooth mucous membrane, destitute of glands, and of a low degree of sensibility.

The =hard palate= (Palatum durum) is bounded in front and on the sides by the alveolar arches; behind it is continuous with the soft palate. Its osseous basis is formed by the premaxilla, maxilla, and palate bones. The mucous membrane is smooth, and is attached to the bones by a submucosa which contains in its anterior part an exceedingly rich venous plexus, constituting an erectile tissue. A central raphé (Raphé palati) divides the surface into two equal portions. Each of these presents about eighteen transverse curved ridges (Rugæ palatini) which have their concavity and their free edges directed backward. They are further apart and more prominent anteriorly. There are no glands in the submucosa.

=Vessels and Nerves.=—The blood-supply is derived chiefly from the palatine arteries and the nerves from the trigeminus.

The =soft palate= (Palatum molle) is a musculo-membranous curtain which separates the cavity of the mouth from that of the pharynx. It slopes downward and backward from its junction with the hard palate. The =oral surface= faces downward and forward, and is covered with a mucous membrane continuous with that of the hard palate. It is corrugated and presents numerous small orifices (of gland-ducts) and two sagittal ridges. On each side a short, thick fold passes to join the base of the tongue; this is the =anterior pillar= of the soft palate (Arcus glossopalatinus). The =pharyngeal surface= looks upward and backward and is covered by a mucous membrane continuous with that of the nasal cavity. The =free border= (Arcus palatinus) is concave and thin; it is in contact (except during deglutition) with the epiglottis. It is continuous with a fold of the mucous membrane, which passes on each side along the lateral wall of the pharynx and unites with its fellow over the beginning of the œsophagus; this fold is termed the =posterior pillar= of the soft palate (Arcus pharyngopalatinus). The space between the diverging anterior and posterior pillars (Sinus tonsillaris) is occupied by the faucial =tonsil=. In the horse, however, there is not a compact tonsil, as in man, dog, etc., but a somewhat extended group of mucous glands and masses of lymphoid tissue. These cause elevations of the surface, on which are seen numerous depressions (crypts) in which the gland-ducts open. The soft palate is greatly developed in equidæ, its length being five to six inches (12 to 15 cm.). Its length and contact with the epiglottis account for the fact that in these animals mouth-breathing does not occur under normal conditions, and that in vomiting the ejected matter escapes usually through the nasal cavity.[78]

=Structure.=—The soft palate consists of: (1) The oral mucous membrane, continuous with that of the hard palate; (2) the =palatine glands= (Glandulæ palatinæ), which form a layer about half an inch in thickness; (3) the aponeurotic and muscular layer; (4) the pharyngeal mucous membrane, continuous with that of the nasal cavity.

The =muscles= proper to the soft palate are the azygos uvulæ, the levator palati, and the tensor palati.

The =azygos uvulæ= (M. palatinus) consists of two small muscular bundles which lie together at the median line. It is attached through the medium of the palatine aponeurosis to the palatine arch, and terminates near the free edge of the soft palate. Its action is to shorten and raise the soft palate.

The =levator palati= (M. levator veli palatini) arises from the muscular process of the petrous temporal bone and from the Eustachian tube, and passes forward and downward externally to the latter, to spread out on the pharyngeal surface of the soft palate. It raises the soft palate, thus closing the posterior nares during deglutition.

The =tensor palati= (M. tensor veli palatini) is larger than the levator, and is fusiform and flattened. It arises from the muscular process of the petrous temporal bone and the Eustachian tube, and passes forward external to the levator. Its tendon is then reflected around the hamulus of the pterygoid bone, being held in position by a fibrous band and lubricated by a bursa. It turns inward and expands in the aponeurosis of the soft palate. It tenses the soft palate.

=Vessels and Nerves.=—The blood-supply of the soft palate is derived from the internal and external maxillary arteries. The nerves come from the trigeminus, vagus, and glosso-pharyngeal nerves.

The =floor= of the mouth in its anterior part is free and is formed by the body of the mandible, covered by mucous membrane. The remainder is concealed by the attached portion of the tongue, with the exception of a narrow space on each side of the latter. About opposite the canine tooth on each side is a papilla, the =caruncula sublingualis=, through which the duct of the submaxillary gland opens. Just behind these papillæ is a median fold of mucous membrane which passes to the under surface of the tongue, constituting the =frenum linguæ=. On either side are the sublingual crests, which extend from the frenum to the level of the fourth cheek tooth. The crest presents numerous small papillæ, through which open the ducts from the subjacent sublingual gland.[79] Behind the last tooth a vertical fold of the mucous membrane passes from upper to lower jaw. This is termed the plica pterygomandibularis: it contains a ligament of like name.

The =isthmus faucium= is the orifice of communication between the mouth and the pharynx. It is bounded above by the soft palate, below by the root of the tongue, and laterally by the anterior pillars of the soft palate. It is long, relatively small, and not very dilatable in the horse, and is closed by the soft palate under normal conditions, except during deglutition.

THE TONGUE

The =tongue= (Lingua) is situated on the floor of the mouth, between the rami of the mandible, and is supported mainly in a sort of sling formed by the mylo-hyoid muscles. Its posterior portion, the =root= (Radix linguæ), is attached to the hyoid bone, soft palate, and pharynx. Only the upper surface of this part is free, and slopes downward and backward. The middle portion, the =body= (Corpus linguæ), has three free surfaces. The upper surface or =dorsum= (Dorsum linguæ) is rounded. The lateral surfaces are nearly flat for the most part, but anteriorly become rounded and narrower. The lower surface is attached to the mandible. The =apex= or tip (Apex linguæ) is free, spatula-shaped, and presents superior and inferior surfaces and a rounded border.

=Structure.=—The tongue consists of: (1) The mucous membrane; (2) the glands; (3) the muscles.

The =mucous membrane= (Tunica mucosa linguæ) adheres intimately to the subjacent tissue, except on the lower part of the lateral surfaces of the body and the under surface of the tip. It varies considerably in thickness. On the dorsum it is very thick and dense. Underneath this portion there is a dense fibrous cord, which extends medially a distance of five or six inches forward from the vallate papillæ. On the sides and under surface of the tongue the membrane is much thinner and smooth, and can more readily be dissected off the muscular tissue. From the under surface of the free part of the tongue a fold of the mucous membrane passes to the floor of the mouth, forming the =frenum linguæ=. This contains the anterior edges of the genio-glossi muscles. Posteriorly a fold passes on each side from the edge of the dorsum to join the soft palate, forming the anterior pillars of the latter. A central glosso-epiglottic fold (Plica glossoepiglottica) passes from the root to the base of the epiglottis. The mucous membrane presents numerous =papillæ=, which are of four kinds—filiform, fungiform, vallate, and foliate. The =filiform papillæ= (Papillæ filiformes) are fine, pointed projections. They cover the upper surface of the body and tip, to which they give a distinct pile. The =fungiform papillæ= (Papillæ fungiformes) are rounded at the free end, which is supported by a neck. They occur principally on the lateral part of the tongue, but are also found scattered over the dorsum and upper surface of the free portion. The =vallate papillæ= (Papillæ vallatæ) are usually two or three in number. The two constant ones have a diameter of about 7 mm., and are found on the posterior part of the dorsum, one on each side of the median plane, about an inch (ca. 3 cm.) apart. The third, when present, is behind these, is centrally situated, and is always smaller. Rarely a fourth may be seen. They are rounded, broader at their exposed than at their attached surfaces, and are situated in a cup-shaped cavity. The =foliate papillæ= (Papillæ foliatæ) are situated just in front of the anterior pillars of the soft palate, where they form a rounded eminence about an inch (ca. 2 to 3 cm.) in length, marked by transverse fissures. The last three varieties are covered with microscopic secondary papillæ and are furnished with taste buds. The mucous membrane of the root of the tongue presents numerous folds and depressions. Into the latter open the ducts from the =lingual glands= (Glandulæ linguales), which constitute a thick layer in the loose submucous tissue. Mucous glands are found also on the dorsum and sides of the tongue. In the submucosa of the root is found also a large quantity of lymph follicles (Folliculi tonsillares) and diffuse lymphoid tissue.

The =muscular tissue= may be divided into intrinsic and extrinsic. The intrinsic musculature consists, not of distinct muscles, but rather of systems of fibers which run longitudinally, vertically, and transversely, blending with the extrinsic muscles, which are now to be described.

1. =Stylo-glossus.=—This is a long, thin muscle, which lies on the lateral part of the tongue. It arises by a thin tendon from the outer surface of the great cornu of the hyoid bone, near the articulation with the small cornu. It terminates near the tip of the tongue by blending with its fellow of the opposite side and with the intrinsic musculature. The action is to retract the tongue. Unilateral contraction would also draw the tongue toward the side of the muscle acting.

2. =Hyo-glossus.=—This is a wide, flat muscle, somewhat thicker than the preceding. It lies on the lateral part of the root and body of the tongue, partly under cover of the preceding muscle. Its deep face is related to the genio-glossus. It arises from the lateral aspect of the hyoid bone, from the lingual process to the oral extremity of the great cornu, and from the thyroid cornu. The fibers pass obliquely forward and upward, and for the most part turn toward the median plane of the dorsum of the tongue. Its action is to retract and depress the tongue.

(It is usually possible to recognize in this muscle three portions, which would correspond to the baseo-, kerato-, and chondro-glossus of human anatomy.)

3. =Genio-glossus.=—This is a fan-shaped muscle, which lies parallel to the median plane of the tongue. It is separated from the muscle of the opposite side by a quantity of fat and connective tissue. It arises from the inner surface of the ramus of the mandible near the symphysis. From the tendon the fibers pass in a radiating manner, some toward the tip, others toward the dorsum, and others toward the root of the tongue; some fibers pass from the posterior end of the tendon to the body and small cornu of the hyoid bone. The muscle as a whole is a depressor of the tongue, and especially of its middle portion; when both muscles act, a median groove is formed on the dorsum. The posterior fibers protrude the tongue, the middle fibers depress the tongue, and the anterior fibers retract the tip of the tongue.

=Vessels and Nerves.=—The arteries of the tongue are the lingual and sublingual branches of the external maxillary artery. The sensory nerves are the lingual and glosso-pharyngeal, and the muscles are innervated by the hypoglossal nerve.

THE TEETH

The teeth are hard white or yellowish-white structures, implanted in the alveoli of the bones of the jaws—premaxilla, maxilla, and mandible. Morphologically they are large calcified papillæ. Functionally they are organs of prehension and mastication, and may serve as weapons of offense and defense. They are classified according to form and position as follows:

1. =The incisor teeth= (Dentes incisivi) are situated in front and are implanted in the premaxilla and mandible.

2. =The canine teeth= (Dentes canini) are situated a little further back, at or near the premaxillary suture in the upper jaw; in the lower jaw they are nearer the incisors.

3. =The cheek teeth= (Dentes premolares et molares), the remaining teeth, occupy the sides of the dental arch. The space between the incisors and cheek teeth is termed the interdental or interalveolar space. It is customary to divide the cheek teeth into an anterior series, termed =premolars=, which appear as temporary, deciduous, or milk teeth, and are replaced by permanent successors, and a posterior series, the =molars=, which appear only as permanent teeth without deciduous predecessors.

As the teeth of the two sides of the jaws are alike in number and character (in normal cases), the complete dentition may be briefly indicated by a =dental formula= such as the following:

( 2 1 2 3 ) 2 ( I - C - P - M - ) = 32. ( 2 1 2 3 )

In this formula the letters indicate the kinds of teeth, and the figures above and below the lines give the number of teeth of one side in the upper and lower jaw respectively in man.

The individual teeth of each group are designated numerically, the starting-point being the middle line; thus the incisor on either side of the middle line is the first incisor, and may be conveniently indicated by the notation I^1. The temporary or deciduous teeth may be designated in a similar manner, prefixing D (for deciduous) to the letter indicating the kind of tooth. In addition to the above systematic method of notation other terms have received the sanction of popular usage. Thus the first incisors are commonly called “pinchers,” or “nippers”; the second, intermediate; and the third, corner teeth. The canines, when highly developed, may be termed tusks or fangs. The vestigial and inconstant first premolar of the horse is popularly termed the “wolf tooth.”

Each tooth presents for description a portion coated with enamel, termed the =crown= (Corona dentis), and a portion covered with cement, termed the =root= (Radix dentis). The line of union of these parts is the neck (Collum dentis).[80] In many teeth the neck is distinct and is embraced by the gum, e. g., the teeth of the dog and the temporary incisors of the horse. In other teeth no constriction is seen, as in the permanent incisors of the horse. Between these extremes may be noted the molars of the horse, in which the neck is seen only in advanced age.

The =surface= of a tooth directed toward the lips is termed =labial=; toward the cheek, =buccal=; and toward the tongue, =lingual= (Facies labialis, buccalis, lingualis). The surface opposed to a neighboring tooth of the same dental arch is termed the =contact surface= (Facies contactus). The grinding or =masticatory= or “table” surface (Facies masticatoria) is that which comes in contact with a tooth or teeth of the opposite jaw.

=Structure.=—Teeth are composed of four tissues, which are considered here from within outward. The =pulp= (Pulpa dentis) is a soft gelatinous tissue, which occupies a space in the central part of the tooth termed the =pulp cavity= (Cavum dentis). The pulp is well supplied with blood-vessels and nerves. It occupies a relatively large space in young growing teeth, but later the dentine deposited on its surface gradually encroaches on it until, in advanced age, the cavity is obliterated or much reduced. The =dentine= (Substantia eburnea) forms the bulk of most teeth, covering the surface of the pulp. It is very hard, and is yellowish-white in color. The =enamel= (Substantia adamantina) constitutes a layer of varying thickness covering the dentine of the crown of the tooth. It is easily distinguished by its clear bluish-white appearance and its extreme density. The =cement= (Substantia ossea) is always the outermost tooth substance. In simple teeth it forms usually a thin layer on the surface of the dentine of the root only, but in complex teeth it exists in considerable quantity, tending to fill in the spaces between the enamel folds of the crown also. Its structure is practically the same as that of bone without Haversian canals, and even these occur where the cement forms a very thick layer. The embedded part of the tooth is attached to the alveolus by a vascular layer of connective tissue, the =alveolar periosteum= (Periosteum alveolare), which constitutes the periosteum at once of tooth and alveolus.

The =blood-supply= to the pulp is derived from the alveolar or dental branches of the internal artery; the =nerve-supply= comes from branches of the trigeminus.

THE TEETH OF THE HORSE

THE PERMANENT TEETH

The formula of the permanent teeth of the horse is:

( 3 1 3 or 4 3 ) 2 ( I - C - P —————— M - ) = 40 or 42 ( 3 1 3 3 )

In the mare the canines usually are very small and do not erupt, reducing the number to 36 or 38.[81]

=Incisor Teeth.=—These are twelve in number. The six in each jaw are placed close together, so that their labial edges form almost a semicircle. They have the peculiarity (not found in existing mammals other than the equidæ) of presenting, instead of the simple cap of enamel on the crown, a deep invagination, the =infundibulum=, which becomes partly filled up with cement. Hence as the tooth wears the table surface has a central ring of enamel surrounding this cavity in addition to the peripheral enamel. The cavity becomes darkened by deposits from the food, and is commonly termed the “cup” or “mark.” Each tooth is curved so that the labial surface is convex and the roots converge. The average length of the incisors at five or six years of age is about two and a half to three inches (ca. 7 cm.). They taper regularly from crown to root, without any constriction, and in such a manner that in young horses the exposed crown is broad transversely; toward the middle, the two diameters of a cross-section are about equal; near the root the antero-posterior diameter is considerably greater than the transverse.

This fact is of value in the determination of age by the teeth, since the table surface at different ages represents a series of such cross-sections. As the exposed crown wears down the embedded part (reserve crown) pushes up out of the alveolus, so that the tables of the first and second lower incisors are at first oval, with the long diameter transverse; later—at about fourteen years usually for the first lower incisors—the tables are triangular, with the base at the labial edge. At the same time the infundibulum or cup becomes smaller, approaches the lingual border, and finally disappears; it remains longer on the upper incisors, as it is deeper in them. Another marked feature in old age is the progressive approach to a horizontal direction as seen in profile; at the same time the teeth become parallel and finally convergent.

=Canine Teeth.=—These are four in number in the male; in the mare they are usually absent or rudimentary.[82] They interrupt the interdental space, dividing it into two unequal parts. The upper canine is situated at the junction of the premaxilla and the maxilla; the lower canine is placed nearer the corner incisor. The canines are simple teeth, smaller than the incisors, and are curved with the concavity directed backward. The crown is compressed, convex, and smooth externally; concave with a median ridge internally; its edges are sharp, and the apex is pointed in the unworn tooth. The root is round and the pulp cavity is large, persisting to advanced age.

=Cheek Teeth= (Premolars and Molars).[83]—The constant number of these is twenty-four—twelve in each jaw. Quite commonly, however, the number is increased by the presence in the upper jaw of the so-called wolf tooth. This tooth is usually situated just in front of the first well-developed tooth; it is a much reduced vestige, not often more than one-half or three-fourths of an inch (ca. 1 to 2 cm.) in length. (It is interesting as being the remnant of a tooth which was well developed in the Eocene ancestors of the horse.) It may erupt during the first six months, and is often shed about the same time as the milk-tooth behind it, but may remain indefinitely. The occurrence of a similar tooth in the lower jaw—which rarely erupts—increases the dental formula to 44, which is considered the typical number for mammals. They may be regarded as belonging to the permanent set, having no predecessors. The cheek teeth are very large, prismatic in form, and quadrilateral in cross-section, except the first and last of the series, which are triangular. The crown is remarkably long, most of it being embedded in the bone or situated in the maxillary sinus in the young horse. As the exposed part wears down the embedded part pushes up to replace it, so that a functional crown of about four-fifths of an inch (ca. 2 cm.) is maintained. The root begins to grow at four or five years of age, and is complete at twelve to fourteen, although the deposition of cement may continue indefinitely.

The =upper= or =maxillary teeth= are embedded in the alveolar processes of the maxilla. The exposed parts of the crowns are normally in close contact, forming a continuous row which is slightly curved, with the convexity toward the cheek. The embedded parts diverge in the manner shown in the annexed figures (Figs. 251, 253). Thus the long axis of the first is directed upward and a little forward, that of the second is almost vertical, while in the remainder it is curved backward in an increasing degree. The average =length= at six years of age is about three to three and a half inches (ca. 7 to 9 cm.). The =buccal= (or outer) =surface= presents a central ridge running lengthwise, and separating two grooves; the first tooth has, in addition, a less prominent ridge in front of the primary one. The =lingual= (or inner) =surface= is marked by a wide, rounded ridge, the accessory pillar or column, which separates two very shallow grooves. The =masticatory= or =table surface= presents two infundibula, an anterior and a posterior. It slopes obliquely downward and outward, so that the outer edge is prominent and sharp. The first and last teeth have three roots, the remainder four or three.

The position of the embedded crowns and roots of the last four varies at different ages and in different subjects. Two factors in this variation may be noted. All these teeth are developed in the maxillary sinus close to the orbital fossa. As growth proceeds the teeth move forward, so that commonly only the last three, but sometimes also the third, remain (except as to their free crowns) in the sinus. The second cause of variation is the fact that the anterior limit of the maxillary sinus may be at the extremity of the facial ridge, or more than an inch beyond it. In the latter case the third tooth projects into the sinus.[84]

The =structure= is quite complex. Two infundibula run vertically through the entire length of the crown; these become filled with cement. There are five main divisions of the pulp-cavity and five enamel folds, four of which are arranged symmetrically, while the fifth is an outgrowth from the inner side of the antero-internal fold. On the exposed crown of the unworn tooth the enamel folds form rounded ridges covered with a thin layer of cement. After the tooth comes into wear the enamel on the masticatory surface stands out in the form of sharp prominent ridges. Progressive cementation of the periphery of the tooth takes place, thus leveling up the irregularities of surface to a considerable extent.

The =lower= or =mandibular cheek teeth= are implanted in the rami of the mandible, forming two straight rows which diverge behind. The space between the rows is considerably less than that separating the upper teeth, especially in the middle of the series. The length of the lower teeth is about the same as that of the upper set. Their direction is also similar, but the embedded portions diverge even more, with the exception of the first and second. The long axis of the first is vertical; the remainder project downward and backward in a gradually increasing obliquity. The =buccal= (outer) =surface= has a longitudinal furrow; the last molar has a secondary, shallower furrow in addition. The =lingual= (inner) =surface= is uneven, but the grooves are not regular; there are usually three on the first and last tooth. The =masticatory= or =table surface= is oblique, sloping upward and inward in correspondence with the opposing tooth; thus the inner edge is prominent. The first five have two roots, while the sixth commonly has three. The width of the lower molars is a little more than half that of the upper. There are two infundibula, which are open along the inner face of the tooth until closed by cement. The pulp-cavity has two principal divisions, and four or three secondary diverticula. The enamel folds correspond, forming a pattern even more complicated than on the upper teeth.

THE TEMPORARY TEETH

The =deciduous=, temporary, or “milk” =teeth= (Dentes decidui) are smaller and fewer than those of the permanent set. The formula is:

( 3 0 3 ) 2 ( Di - Dc - Dp - ) = 24. ( 3 0 3 )

The =deciduous incisors= are much smaller than the permanent ones. They have a distinct =neck= at the junction of the crown and root. The =crown= is short, white in color, and its labial surface is smooth. The =infundibulum= is shallow. The =root= is flattened; it undergoes absorption as the permanent tooth develops behind it.

The =deciduous canines= are quite vestigial. They occur in both sexes as slender spiculæ about a quarter of an inch in length, but do not erupt. The lower one develops close to the corner incisor. They are not usually included in the formula, as they are never functional.

The =deciduous cheek teeth= differ from the permanent set chiefly in that they have much shorter crowns than the latter. The roots form early, so that a distinct neck occurs.

The subjoined table indicates the average periods of the eruption of the teeth:

TEETH ERUPTION

A. Deciduous: 1st incisor (Di 1) Birth or first week. 2nd „ (Di 2) 4–6 weeks. 3rd „ (Di 3) 6–9 months. Canine (Dc ) 1st premolar (Dp 2) Birth or first two weeks. 2nd „ (Dp 3) „ 3rd „ (Dp 4) „

B. Permanent:

1st incisor (I 1) 2½ years. 2nd „ (I 2) 3½ years. 3rd „ (I 3) 4½ years. Canine. (C ) 4–5 years. 1st premolar (or wolf tooth) (P 1) 5–6 months. 2nd „ (P 2) 2½ years. 3rd „ (P 3) 3 years. 4th „ (P 4) 4 years. 1st molar (M 1) 10–12 months. 2nd „ (M 2) 2 years. 3rd „ (M 3) 3½–4 years.

(The periods given for P 3 and 4 refer to the upper teeth; the lower ones may erupt about six months earlier.)

THE SALIVARY GLANDS

This term is usually restricted to the three pairs of large glands situated on the sides of the face and the adjacent part of the neck—the parotid, submaxillary, and sublingual. Their ducts open into the mouth.

The =parotid gland= (Glandula parotis) (Fig. 172)—so named from its proximity to the ear—is the largest of the salivary glands in the horse. It is situated chiefly in the space between the ramus of the mandible and the wing of the atlas. It is somewhat triangular in shape, the apex partially embracing the base of the external ear. Its length is about eight to ten inches (ca. 20 to 25 cm.), and its average thickness nearly an inch (ca. 2 cm.). Its average weight is about seven ounces (ca. 200 to 225 g.).

It presents for description two surfaces, two borders, a base, and an apex. The =external= (or superficial) =surface= is covered by the parotid fascia, the panniculus, and the parotido-auricularis muscle. It is crossed obliquely by the jugular vein, which is embedded in the gland tissue to a varying extent. It is also related to the great auricular vein, the cervical branch of the facial nerve, and branches of the second cervical nerve. The =internal= (or deep) =surface= is very uneven, and has numerous important relations. Some of these are: the guttural pouch, and the great cornu of the hyoid bone; the masseter, stylo-maxillaris, digastricus, and occipito-hyoideus muscles; the tendons of the mastoido-humeralis and sterno-cephalicus (which separate the parotid from the underlying submaxillary gland); the external carotid artery and some of its branches; the facial nerve; the pharyngeal lymph glands. The =anterior= (or facial) =border= is closely attached to the ramus of the mandible and the masseter muscle; it overlaps the latter to a varying extent. (In some cases there is a well-marked triangular facial process, which covers the temporo-mandibular joint, the facial nerve, and the transverse facial vessels.) The =posterior= (or cervical) =border= is somewhat concave, and is loosely attached to the underlying muscles. The =base= or =ventral border= is related to the external maxillary vein. The =apex= is attached to the base of the external ear, which it partially embraces.

The gland has a yellowish-gray color and is distinctly lobulated. It is inclosed in a capsule formed by the parotid fascia. The =parotid duct= (Ductus parotideus Stenonis) is formed at the lower part of the gland, near the facial edge, by the union of three or four radicles. It leaves the gland about an inch (ca. 2 to 3 cm.) above the external maxillary vein, crosses the tendon of the sterno-cephalicus, and gains the inner face of the pterygoideus internus. It then runs forward in the submaxillary space below the external maxillary vein and winds around the lower border of the mandible behind the vein, passes upward between the vein and the masseter muscle for about two inches (ca. 5 cm.), turns forward underneath the facial vessels, and perforates the cheek obliquely opposite the third upper cheek tooth. Before piercing the cheek it is somewhat dilated, but its termination is small, and is surrounded by a circular mucous fold (Papilla salivalis).

=Blood-supply.=—Branches of the carotid and maxillary arteries.

=Nerve-supply.=—Trigeminal, facial, and sympathetic nerves.

The =submaxillary gland= (Glandula submaxillaris) is much smaller than the parotid. It is long, narrow, and curved, the dorsal edge being concave. It extends from the fossa below the wing of the atlas to the body of the hyoid bone, so that it is covered partly by the parotid gland, partly by the lower jaw. Its length is about eight to ten inches (ca. 20 to 25 cm.), its width an inch to an inch and a half (ca. 2.5 to 3 cm.), and its thickness about half an inch (ca. 1 cm.). It weighs about one and a half to two ounces (ca. 45 to 60 g.). It is often divisible into two parts.

It presents for description two surfaces, two borders, and two extremities. The =external surface= is covered by the parotid gland, the stylo-maxillaris, digastricus, and pterygoideus internus muscles. The tendon of the sterno-cephalicus crosses this surface, and is a useful guide in separating the parotid gland from it. The =internal surface= is related chiefly to the flexor muscles of the head; the guttural pouch; the larynx; the division of the carotid artery; and the tenth, eleventh, and sympathetic nerves. The =superior border= is concave and thin. It is related to the guttural pouch and the duct of the gland. The =inferior border= is convex and thicker. It is related to the thyroid gland and the external maxillary vein. The =posterior extremity= is loosely attached in the fossa atlantis. The =anterior extremity= lies at the side of the root of the tongue, and is crossed externally by the external maxillary artery.

The =submaxillary duct= (Ductus submaxillaris Whartoni) is formed by the union of small radicles which emerge along the concave edge. It runs forward along this border, and, after leaving the anterior extremity, crosses the intermediate tendon of the digastricus, passes between the hyo-glossus and mylo-hyoideus, and gains the inner surface of the sublingual gland. Its terminal part lies on the body of the mandible, under the mucous membrane, which it pierces opposite the canine tooth. The orifice is at the end of a flattened papilla (Caruncula sublingualis).

=Blood-supply.=—Occipital, external carotid, and external maxillary arteries.

=Nerve-supply.=—Chorda tympani and sympathetic nerves.

The =sublingual gland= (Glandula sublingualis) is situated beneath the mucous membrane of the mouth, between the body of the tongue and the ramus of the mandible. It extends from the symphysis to the third or fourth lower cheek tooth. Its length is about five or six inches (ca. 12 to 15 cm.) and its weight about half an ounce (ca. 15 to 16 g.).

It is flattened laterally, and has a thin =upper border= which causes an elevation of the mucous membrane of the floor of the mouth, termed the =sublingual crest= (Plica sublingualis). The =external surface= is related to the mylo-hyoideus muscle, and the =internal surface= to the genio-glossus and stylo-glossus, the submaxillary duct, and branches of the lingual nerve. The =inferior border= is related to the genio-hyoid muscle.

The =sublingual ducts= (Ductus sublinguales minores), about thirty in number, are small, short, and twisted; they open on small papillæ on the sublingual crest.

=Blood-supply.=—Sublingual artery.

=Nerve-supply.=—Trigeminal and sympathetic nerves.

THE PHARYNX

The pharynx is a musculo-membranous sac which belongs to the digestive and respiratory tracts in common. It is funnel-shaped, the base joining the mouth and nasal cavity, while the apex is continued by the œsophagus. Its long axis is directed obliquely downward and backward, and has a length of about six inches (ca. 15 cm.).

The pharynx is attached by its muscles to the palate, pterygoid, and hyoid bones, and to the cricoid and thyroid cartilages of the larynx.

Its principal =relations= are: dorsally, the base of the cranium and the guttural pouches; ventrally, the larynx; laterally, the internal pterygoid muscle, the great cornu of the hyoid bone, the internal and external maxillary arteries, the glosso-pharyngeal, superior laryngeal, and hypoglossal nerves, the submaxillary salivary gland, and the pharyngeal lymph glands.

It presents =seven openings=. The =posterior nares= (Choanæ) communicate dorsally with the nasal chambers. The pharyngeal orifices (Ostia pharyngea) of the two =Eustachian tubes= are situated on the lateral wall behind the nares and a little below the level of the inferior nasal meatus. They are slit-like openings, slightly oblique downward and backward, and are a little more than an inch (ca. 3 cm.) in length. They are bounded internally by a valvular flap formed by the expanded extremity of the cartilaginous Eustachian tube. The =isthmus faucium= is situated below and in front. It is closed by the soft palate except during swallowing. The =laryngeal orifice= (Aditus laryngis) occupies the greater part of the ventral wall or floor of the pharynx. Behind this is the =œsophageal opening=.

The wall of the pharynx comprises from without inward: the muscles, the pharyngeal aponeurosis, and the mucous membrane.

The =muscles= (Figs. 243, 244) are covered by the pharyngeal fascia, which is attached to the base of the skull, the great cornu of the hyoid bone, and the thyroid cartilage of the larynx. They are as follows:

1. The =stylo-pharyngeus= arises from the inner surface of the dorsal third of the great cornu of the hyoid bone, passes downward and inward, and enters the wall of the pharynx by passing between the pterygo-pharyngeus and palato-pharyngeus. Its fibers radiate, many passing forward, others inward beneath the hyo-pharyngeus. It raises and dilates the pharynx to receive the bolus in swallowing.

2. The =palato-pharyngeus= arises by means of the aponeurosis of the soft palate from the palate and pterygoid bones. Its fibers pass backward on the lateral wall of the pharynx, and are inserted in part into the upper edge of the thyroid cartilage, in part turn inward to end at the median fibrous raphé. Its action is to shorten the pharynx, and to draw the larynx and œsophagus toward the root of the tongue in swallowing.

3. The =pterygo-pharyngeus= is flat and triangular. It lies on the anterior part of the lateral wall of the pharynx. It arises from the pterygoid bone above the preceding muscle—from which it is not distinctly separated—crosses the levator palati, and is inserted into the median raphé. Its action is similar to the preceding.

4. The =hyo-pharyngeus= may consist of two portions:

(a) The =kerato-pharyngeus= is a small and inconstant muscle which arises from the inner surface of the great cornu of the hyoid bone near its lower end. It passes upward and backward, turns inward toward the raphé, and spreads out under the next muscle.

(b) The =chondro-pharyngeus=, broad and fleshy, arises from the thyroid cornu of the hyoid bone and by a thin fasciculus from the wing of the thyroid cartilage and ends at the median raphé.

5. The =thyro-pharyngeus= arises from the lateral surface of the wing of the thyroid cartilage. Its fibers pass forward and inward to the median raphé.

6. The =crico-pharyngeus= arises from the cricoid cartilage and ends at the raphé. The fibers are directed upward, forward, and inward; they blend behind with the longitudinal fibers of the œsophagus.

The last three muscles are constrictors of the pharynx.

The =pharyngeal aponeurosis= is attached to the base of the cranium. It is well developed on the inner face of the palato-pharyngeus muscle and forms a median raphé (Raphé pharyngis) dorsally, which is wide in its posterior part.

The =mucous membrane= of the pharynx is continuous with that of the several cavities which open into it. It is thin and closely adherent to the base of the skull in the vicinity of the posterior nares, where the muscular wall is absent. Behind the Eustachian openings is a median cul-de-sac, the =pharyngeal recess=. The recess is somewhat variable, but is usually about an inch in depth and will admit the end of the finger. In the ass and mule it is much deeper. Here also the muscular wall is absent and the mucous membrane lies against the guttural pouches. From the Eustachian opening a fold of the mucous membrane (Plica salpingo-pharyngea) passes toward but does not reach the laryngeal opening. Below, a horizontal fold, the =posterior pillar of the soft palate= (Arcus pharyngopalatinus), passes along the lateral wall and unites with its fellow over the entrance to the œsophagus. The upper part of the cavity (the naso-pharynx) is lined with a ciliated epithelium, while the lower part (oro-pharynx) has a stratified squamous epithelium. The communication between the two is oval and is bounded by the free edge of the soft palate and its posterior pillars; it is termed the =pharyngeal isthmus=. On either side of the laryngeal opening is a narrow deep depression, the =pyriform sinus= (Recessus piriformis).

The submucous tissue contains numerous mucous glands (Glandulæ pharyngeæ). In the young subject the lymph follicles are numerous and form a collection dorsally and between the Eustachian openings, known as the pharyngeal tonsil.

=Blood-supply.=—External carotid, external maxillary, and thyro-laryngeal arteries.

=Nerve-supply.=—Glosso-pharyngeal, vagus, and sympathetic nerves.

THE ŒSOPHAGUS

The œsophagus is a musculo-membranous tube, about 50 to 60 inches (ca. 125 to 150 cm.) in length, which extends from the pharynx to the stomach. It begins in the median plane above the cricoid cartilage of the larynx. In its course it shows several changes of direction. At the level of the fourth cervical vertebra it inclines to the left side of the trachea, and continues this relation to the level of the third thoracic vertebra. Here it again gains the dorsal surface of the trachea, and passing backward, crosses the left bronchus, being here almost in the median plane. It continues in the mediastinum between the lungs backward, upward, and a little to the left, to reach the hiatus œsophageus of the diaphragm. Passing through this it terminates at once at the cardiac orifice of the stomach, at the level of the fourteenth thoracic vertebra, a little to the left of the median plane, and about four or five inches (ca. 10 to 12 cm.) ventral to the vertebral column.

Viewed with reference to the frontal plane, its course is downward and backward till it enters the thorax and passes upward to gain the dorsal face of the trachea. For a short distance (i. e., to the root of the lung) its direction is almost horizontal; behind this it passes somewhat upward to its termination. The =cervical part= of the tube is about four to six inches (10 to 15 cm.) longer than the =thoracic part=, while the so-called abdominal part is about an inch (2 to 3 cm.) long.[85]

The principal =relations= of the œsophagus at its origin are: to the cricoid cartilage below; to the guttural pouches and the ventral straight muscles above; and to the carotid arteries laterally. In the middle of the neck the relations are: to the left longus colli muscle above; to the trachea internally; to the left carotid artery, vagus, sympathetic, and recurrent nerves externally. At its entrance into the thorax it has the trachea on its inner side; the first rib, the roots of the brachial plexus of nerves and the inferior cervical ganglion externally. After gaining the upper surface of the trachea, it has the aorta on its left and the vena azygos and right vagus nerve on its right side. In its course through the posterior mediastinum the œsophageal trunks of the vagus nerves lie above and below it, and the œsophageal artery is dorsal to it.

=Structure.=—The wall is composed of four coats: (1) A fibrous sheath; (2) the muscular coat; (3) a submucous layer; (4) the mucous membrane. The =muscular coat= is of the striped variety as far as the base of the heart, where it rapidly changes to the unstriped type. In addition to this change, the muscular coat becomes much thicker and firmer, while the lumen is diminished.[86] The outer fibers are arranged longitudinally, beginning in two bundles attached in the interval between the arytenoid and cricoid cartilages. The inner fibers run in two spiral strata to the terminal part of the tube, where the arrangement is an outer longitudinal and an inner circular layer.[87] The =mucous membrane= is pale, and is covered with squamous stratified epithelium. It is loosely attached to the muscular coat by an abundant submucosa, and lies in longitudinal folds which obliterate the lumen except during deglutition.

=Blood-supply.=—Carotid, broncho-œsophageal, and gastric arteries.

=Nerve-supply.=—Vagus, glosso-pharyngeal, and sympathetic nerves.

THE ABDOMINAL CAVITY

The abdominal cavity (Cavum abdominis) is the largest of the body cavities. It is separated from the thoracic cavity by the diaphragm and is continuous behind with the pelvic cavity.

It is ovoid in form but somewhat compressed laterally. Its long axis extends obliquely from the center of the pelvic inlet to the sternal part of the diaphragm. Its dorso-ventral diameter is greatest at the first lumbar vertebra, while its greatest transverse diameter is a little nearer the pelvis.

The =dorsal wall= or roof is formed by the lumbar vertebræ, the lumbar muscles, and the lumbar part of the diaphragm.

The =lateral walls= are formed by the oblique and transverse abdominal muscles, the abdominal tunic, the anterior parts of the ilia, the cartilages of the asternal ribs, and the parts of the posterior ribs which are below the attachment of the diaphragm.

The =ventral wall= or floor consists of the two recti, the aponeuroses of the oblique and transverse muscles, the abdominal tunic, and the xiphoid cartilage.

The =anterior wall= is formed by the diaphragm, which is very deeply concave, thus greatly increasing the size of the abdomen at the expense of the thorax.

It should be noted that the diaphragm also concurs practically in the formation of a considerable part of the lateral walls, since its costal portion even during ordinary inspiration lies directly on the ribs over a width of four or five inches (ca. 10 to 12 cm.); in expiration this area of contact would be about twice as wide, including about all of the fleshy rim. This fact is of clinical importance, with reference to auscultation and percussion, and penetrating wounds. The cupola of the diaphragm extends as far forward as a plane through the sixth intercostal space to the right of the heart.

There is no wall between the abdominal and pelvic cavities. The line of demarcation here is the =terminal line= (Linea terminalis) or brim of the pelvis; it is formed by the base of the sacrum, the ilio-pectineal lines, and the anterior borders of the pubic bones.

The muscular walls are lined by a layer of fascia, distinguished in different parts as: (1) the diaphragmatic fascia; (2) the transversalis fascia; (3) the iliac fascia; (4) the deep layer of the lumbo-dorsal fascia.

The subperitoneal or extraperitoneal connective tissue (Tela subserosa) unites the fascia and peritoneum. It is composed of areolar tissue, more or less loaded with fat according to the condition of the subject, except over the diaphragm. It sends laminæ into the various peritoneal folds.

The peritoneum, the serous membrane which lines the cavity, will be described later.

The abdominal walls are pierced in the adult by =five apertures=. These are: the three openings in the diaphragm which transmit the aorta, posterior vena cava, and the œsophagus; the inguinal canals, which contain the spermatic cord or the round ligament (in female carnivora). In the fœtus there is the umbilical opening also.

The cavity contains the greater part of the digestive and urinary organs, part of the internal generative organs, numerous nerves, blood-vessels, lymph vessels and glands, ductless glands (spleen and adrenal bodies), and certain fœtal remains.

For topographic purposes the abdomen is divided into nine regions by imaginary planes.[88] Two of these planes are sagittal, and two are transverse. The sagittal planes cut the middles of the inguinal (Poupart’s) ligaments; the transverse planes pass through the last thoracic and fifth lumbar vertebræ, or the lower end of the fifteenth rib and the external angle of the ilium respectively. The transverse planes divide the abdomen into three zones, one behind the other, viz., =epigastric=, =mesogastric=, and =hypogastric=: these are subdivided by the sagittal planes as indicated in the subjoined table.

Left parachondriac Xiphoid Right parachondriac Left lumbar Umbilical Right lumbar Left iliac Prepubic Right iliac.

Other useful regional terms are: sublumbar, diaphragmatic, inguinal. The first two require no explanation. The inguinal regions (right and left) lie in front of the inguinal (Poupart’s) ligament. The flank is that part of the lateral wall which is formed only of soft structures. The depression on its upper part is termed the paralumbar fossa.

THE PERITONEUM[89]

The peritoneum is the thin serous membrane which lines the abdominal and (in part) the pelvic cavity, and covers to a greater or less extent the viscera contained therein. In the male it is a completely closed sac, but in the female there are two small openings in it; these are the abdominal orifices of the Fallopian tubes, which at their other ends communicate with the uterus, and so indirectly with the exterior. The peritoneal cavity is only a potential one, since its opposing walls are normally separated only by the thin film of serous fluid (secreted by the membrane) which acts as a lubricant.

The free surface of the membrane has a glistening appearance and is very smooth. This is due to the fact that this surface is formed by a layer of flat endothelial cells, and is moistened by the peritoneal fluid. Friction is thus reduced to a minimum during the movements of the viscera. The outer surface of the peritoneum is related to the subperitoneal tissue, which attaches it to the abdominal wall or the viscera.

In order to understand the general disposition of the peritoneum, we may imagine the abdominal cavity to be empty and lined by a simple layer of peritoneum, termed the =parietal layer= (Lamina parietalis). We may regard the organs as beginning to develop in the subperitoneal tissue, enlarging, and migrating into the cavity to a varying extent. In doing so they carry the peritoneum before them, producing introversion of the simple sac, and forming folds which connect them with the wall or with each other. The viscera thus receive a more or less complete covering of peritoneum, termed the =visceral layer= (Lamina visceralis). The connecting folds are termed =omenta=, =mesenteries=, =ligaments=, etc. They contain a varying quantity of connective tissue, fat and lymph glands, and furnish a path for the vessels and nerves of the viscera. Some contain unstriped muscular tissue. An omentum is a fold which passes from the stomach to other viscera. There are three of these, namely: (1) the =small= or =gastro-hepatic omentum= (Omentum minus), which passes from the lesser curvature of the stomach to the liver; (2) the =gastro-splenic omentum= (Ligamentum gastrolienale), which extends from the greater curvature of the stomach to the spleen; (3) the =great omentum= (Omentum majus), which passes from the greater curvature of the stomach and from the spleen to the terminal part of the great colon and the origin of the small colon. It does not pass directly from one organ to the other, but forms an extensive loose sac (Figs. 278, 279). A =mesentery= (Mesenterium) is a fold which attaches the intestine to the dorsal wall of the abdomen. There are two mesenteries, namely: (1) the =great mesentery= which connects the greater part of the small intestine with the dorsal abdominal wall; (2) the =colic mesentery=, which attaches the small colon to the dorsal abdominal wall. =Ligaments= are folds which pass between viscera other than parts of the digestive tube, or connect them with the abdominal wall. The term is also applied to folds which attach parts of the digestive tract to the abdominal wall, but do not contain their blood-vessels and nerves. In some cases (e. g., the lateral and coronary ligaments of the liver) they are strengthened by fibrous tissue; in other cases (e. g., the broad ligaments of the uterus) they contain also unstriped muscular tissue.

THE PELVIC CAVITY

The pelvis is the posterior part of the trunk. It incloses the pelvic cavity (Cavum pelvis), which communicates in front with the abdominal cavity, the line of demarcation being the pelvic brim or terminal line.

The =dorsal wall= or roof is formed by the sacrum and first three coccygeal vertebræ. The =lateral walls= are formed by the parts of the ilia behind the ilio-pectineal lines and the sacro-sciatic ligaments. The =ventral wall= or floor is formed by the pubic and ischial bones. The boundary of the =outlet= is formed by the third coccygeal vertebra dorsally, the ischial arch ventrally, and the posterior edges of the sacro-sciatic ligaments and the semimembranosus muscles laterally. The outlet is closed by the perineal fascia; this consists of superficial and deep layers, which are attached around the margin of the outlet and centrally to the organs at the outlet—the anus and its muscles, the vulva (in the female), and the root of the penis (in the male).

The cavity contains the rectum, parts of the internal generative and urinary organs, some fœtal remnants, muscles, vessels, and nerves. It is lined by the fascia pelvis, and in part by the peritoneum.

The =pelvic peritoneum= is continuous in front with that of the abdomen. It lines the cavity as far back as the third or fourth sacral vertebra in the horse, where it is reflected on to the viscera, and from one organ to another. We may therefore distinguish an anterior, peritoneal, and a posterior, retroperitoneal part of the cavity. Along the mid-dorsal line it forms a continuation of the colic mesentery, the =mesorectum=, which attaches the first or peritoneal part of the rectum to the roof. In animals in fair condition a considerable quantity of subperitoneal and retroperitoneal fat is found on the walls and in the various interstices.

In the male the general disposition of the peritoneum here is as follows. If traced along the dorsal wall, it is reflected at the third or fourth sacral vertebra on to the rectum, forming the visceral peritoneum for the first part of that tube. Laterally it is reflected in a similar fashion. If the rectum be raised, it will be seen that the peritoneum passes from its ventral surface and forms a transverse fold which lies on the dorsal surface of the bladder (Fig. 272). This is the =urogenital fold= (Plica urogenitalis). Its concave free edge passes on either side into the inguinal canal. The ventral layer of this fold is reflected on to the dorsal surface of the bladder near its neck. Thus there is formed a pouch between the rectum and bladder—the =recto-vesical pouch= (Excavatio recto-vesicalis), which is partially subdivided by the urogenital fold into recto-genital and vesico-genital cavities. The fold contains the vasa deferentia, part of the vesiculæ seminales, and the uterus masculinus (a fœtal remnant). The space on either side of the rectum is occupied by coils of the small colon and the pelvic flexure of the great colon usually. If the bladder is now raised, it is seen that the peritoneum passes from its ventral surface on to the pelvic floor, forming a median fold, the so-called =middle ligament= (Plica umbilicalis media). It also forms on each side a lateral fold, the =lateral ligament= (Plica umbilicalis lateralis), which contains in its edge the so-called =round ligament= (Ligamentum teres)—the partially occluded umbilical artery, which is a large vessel in the fœtus.

In the female the arrangement is modified by the presence of the uterus; the urogenital fold is very large, so as to inclose the uterus and a small part of the vagina. It forms two extensive folds, the =broad ligaments of the uterus= (Ligamenta lata uteri), which attach that organ to the sides of the pelvic cavity and the lumbar part of the abdominal wall (Fig. 271). It thus divides the recto-vesical pouch completely into dorsal and ventral compartments—the =recto-genital pouch= (Excavatio recto-uterina), and the =vesico-genital= pouch (Excavatio vesico-uterina).

Further details will be given in the description of the pelvic viscera.

THE STOMACH

The stomach (Ventriculus) is the large dilatation of the alimentary canal between the œsophagus and the small intestine. It is a sharply curved, U-shaped sac, the right branch being, however, much shorter than the left one. The convexity is directed ventrally. When moderately distended, there is often a slight constriction which indicates the division into right and left sacs. It is relatively small, and is situated in the dorsal part of the abdominal cavity behind the diaphragm and liver, mainly to the left of the median plane.

It presents for description two surfaces, two curvatures, and two extremities. The =parietal surface= (Facies parietalis) is convex and is directed forward, upward, and toward the left; it lies against the diaphragm and liver. The =visceral surface= (Facies visceralis), also convex, faces in the opposite direction; it is related to the terminal part of the large colon, the pancreas, the small colon, and the small intestine. The =lesser curvature= (Curvatura minor) is very short, extending from the termination of the œsophagus to the junction with the small intestine. When the stomach is in situ, its walls are here in contact, and the cardia and pylorus close together. The =greater curvature= (Curvatura major) is very extensive. From the cardia it is first directed upward and curves over the left extremity; it then descends, passes to the right, crosses the median plane, and curves upward to end at the pylorus. Its left portion is related to the spleen, while its ventral portion rests on the left divisions of the great colon. The =left extremity= or =saccus cæcus= is a rounded cul-de-sac which lies under the upper ends of the fourteenth, fifteenth, and sixteenth ribs and the diaphragm.[90] It is related to the pancreas behind and the base of the spleen externally. The =right= or =pyloric extremity= is much smaller and is continuous with the duodenum, the junction being indicated by a marked constriction. It lies on the liver, a little to the right of the median plane, and a little lower than the cardiac opening. About two or three inches (ca. 5 to 8 cm.) from the pylorus there is a constriction which marks off the =antrum pylori= from the rest of the right sac. The =œsophageal orifice= or =cardia= is situated at the left extremity of the lesser curvature, but about eight to ten inches (ca. 20 to 25 cm.) from the left extremity. The œsophagus joins the stomach very obliquely. The opening is closed by the sphincter cardiæ and numerous folds of mucous membrane. The =pyloric orifice= communicates with the duodenum. Its position is indicated externally by a distinct constriction. Internally it presents a circular ridge produced by a ring of muscular tissue—the =sphincter pylori=.

The stomach is held in position mainly by the pressure of the surrounding viscera and by the œsophagus. The following peritoneal folds connect it with the adjacent parts:

1. The =gastro-phrenic ligament= (Lig. gastrophrenicum) connects the great curvature, from the cardia to the left extremity, with the crura of the diaphragm. This leaves a narrow area uncovered with peritoneum, and here the stomach is attached to the diaphragm by loose connective tissue.

2. The =small= or =gastro-hepatic omentum= (Omentum minus) connects the lesser curvature and the first part of the duodenum with the liver below the œsophageal notch and the portal fissure.

3. The =gastro-splenic omentum= (Lig. gastrolienale) passes from the left part of the great curvature to the hilus of the spleen.

4. The =great= or =gastro-colic omentum= (Omentum majus) connects the ventral part of the great curvature and the first curve of the duodenum with the terminal part of the great colon and the initial part of the small colon.

5. The =gastro-pancreatic fold= (Plica gastro-pancreatica) extends from the left sac above the cardia to the duodenum. It is attached dorsally to the liver and vena cava, ventrally to the pancreas.

The stomach of the equidæ is relatively small, its capacity varying from two to four gallons (ca. 8 to 15 liters).

The size, form, and position of the stomach are subject to considerable variation. When the stomach is nearly empty the saccus cæcus contains only gas and is strongly contracted; the middle portion (physiological fundus) contains the ingesta and preserves its rounded character, while the pyloric portion is contracted. When distended the middle portion settles down some four or five inches, pushing back coils of the small colon and small intestine which may lie between the great curvature and the large colon, and also pushing to the left or right the left dorsal part of the great colon; the spleen, small colon, and small intestines are pushed back by the distention of the left sac.

=Structure.=—The wall is composed of four coats—serous, muscular, submucous, and mucous.

The =serous coat= (Tunica serosa) covers the greater part of the organ and is closely adherent to the muscular coat except at the curvatures. It partially bridges over the lesser curvature, and covers here elastic tissue which assists in retaining the bent form of the stomach. The peritoneal folds have been described.

The =muscular coat= consists of three incomplete layers, an external of longitudinal, a middle of circular, and an internal of oblique fibers. The layer of =longitudinal fibers= (Stratum longitudinale) is very thin and exists only along the curvatures and at the antrum. At the lesser curvature it is continuous with the longitudinal fibers of the œsophagus. On the antrum pylori it forms a well developed complete layer. The layer of =circular fibers= (Stratum circulare) exists only on the right sac. At the pyloric orifice it forms a thick ring—the =pyloric sphincter=. Another ring, the antral sphincter, is found at the left end of the antrum pylori. The =oblique fibers= (Fibræ obliquæ) are arranged in two layers; the external stratum covers the left sac and is a continuation (in part) of the longitudinal fibers of the œsophagus; the internal stratum is found also on the left sac, and exchanges fibers with the circular and external oblique layers. It forms a remarkable loop around the cardiac orifice, constituting a powerful =cardiac sphincter= (Sphincter cardiæ).

The =submucous coat= is a layer of loose connective tissue which connects the muscular and mucous coats; in it the vessels and nerves ramify before entering the mucosa.

The =mucous coat= is clearly divided into two parts. That which lines the left sac resembles the œsophageal mucous membrane, and is termed =œsophageal= or =cuticular=. It is white in color, destitute of glands, and covered with a thick, squamous, stratified epithelium. At the cardiac orifice it presents numerous folds which occlude the opening.[91] It terminates abruptly at an elevated, denticulated, sinuous line, termed the =cuticular ridge= (Margo plicatus). Below and to the right of this line the mucous membrane has a totally different character, being soft and velvety to the touch, and covered by a mucoid secretion. It is glandular, and three zones may be recognized, although no sharp line of demarcation exists. A narrow zone next to the cuticular ridge has a yellowish-gray color, and contains short tubular =cardiac glands= (Cardiac gland region). Next to this is a large area which has a mottled reddish-brown color, and contains =fundus glands= (fundus gland region). This part of the mucous membrane is thick and very vascular, and corresponds to the fundus of the stomach in man and the dog. The remainder of the mucous membrane has a reddish-gray color and contains branched, tubular, =pyloric glands= (pyloric gland region); it corresponds to the pyloric portion of man and the dog.

The folding of the stomach wall at the lesser curvature produces a prominent ridge which projects into the cavity of the stomach. Circular ridges occur at the antral and pyloric sphincters.

=Blood-vessels and Nerves.=—The stomach receives blood from all the branches of the cœliac artery. The gastric veins drain into the portal vein. The nerves are derived from the vagus and sympathetic nerves.

THE SMALL INTESTINE

The =small intestine= (Intestinum tenue) is the tube which connects the stomach with the large intestine. It begins at the pylorus and terminates at the lesser curvature of the cæcum. Its average length is about seventy feet (ca. 22 meters). When distended its diameter varies from two to four inches (5 to 10 cm.). Its capacity is about twelve gallons (40 to 50 liters).

It is clearly divisible into a =fixed= and a =mesenteric= or floating portion. The fixed portion is termed the duodenum, while the mesenteric portion (Intestinum tenue mesenteriale) is arbitrarily divided into parts termed the jejunum and ileum.[92]

The =duodenum= is about three to four feet (ca. 1 to 1.25 m.) long. Its shape is somewhat like a horseshoe, the convexity being directed toward the right. The =first part= is directed to the right and forms an ᔕ-shaped curve. The convexity of the first part of the curve is dorsal, of the second ventral. It lies on the middle and right lobes of the liver, and presents two dilatations (Ampullæ) with a constriction between them. The duodenal angle or head of the pancreas lies in the concavity of the second curve, and here, five to six inches (ca. 12 to 15 cm.) from the pylorus, the pancreatic duct and the bile-duct pierce the bowel wall. The =second part= passes upward and backward on the right lobe of the liver and, on reaching the right kidney and the base of the cæcum, it curves toward the median plane, opposite the last rib. The =third part= passes almost transversely from right to left behind the base of the cæcum, crosses the median plane under the third and fourth lumbar vertebræ, and turns forward to become continuous with the jejunum under the left kidney. The sacculations of the first part have a diameter of three to four inches (ca. 7.5 to 10 cm.).

It is attached by a short peritoneal fold termed the =mesoduodenum=. This fixes the first part of the duodenum closely to the liver and the right dorsal part of the colon; the remainder is somewhat less closely attached by it to the cæcum and right kidney, the sublumbar muscles, and (more closely) to the terminal part of the great colon and the first part of the small colon.[93]

The =jejunum= and =ileum= together (Jejuno-ileum) constitute the mesenteric or floating portion of the small intestine. No distinct point exists at which to make the demarcation. With the exception of the last two or three feet, the mesenteric part of the intestine varies so much in position that only a general statement can be made. It lies in numerous coils (Ansæ) mingled with those of the small colon, chiefly in the dorsal part of the left half of the abdomen, from the visceral surface of the stomach to the pelvis. It may insinuate itself between the left portions of the colon and the abdominal wall; also between the ventral portions of the colon, reaching the floor of the abdomen. The terminal part of the intestine (ileum) ascends almost vertically, a little to the right of the median plane, to open into the cæcum at the lesser curvature of its base. The average diameter of the jejuno-ileum is about two and a half to three inches (ca. 6 to 7 cm.). In the cadaver one finds most of the tube presenting irregular constricted and dilated parts. The last three or four feet (ca. 1 meter) are usually tightly contracted, resembling somewhat the terminal part of the œsophagus. This part may be termed the ileum.

The mesenteric part is connected with the dorsal abdominal wall by the =great mesentery=. This is a wide fan-shaped fold, consisting of two layers of peritoneum, between which the vessels and nerves reach the bowel; it also contains the mesenteric lymph glands and some fat. The visceral border of the mesentery contains the intestine, while the parietal border or =root= (Radix mesenterii) is attached to a small area around the great mesenteric trunk under the first and second lumbar vertebræ. The root is thick, as it contains a large number of vessels and nerves placed close together. The mesentery is short at first, but soon reaches a length of one and a half to two feet (ca. 50 cm.)—sufficient to allow coils of the intestine to reach the abdominal floor, the pelvic cavity, or even to descend into the scrotum through the inguinal canal. Near its termination the intestine (ileum) leaves the border of the mesentery, so that the latter has a free edge which passes to the cæcum.

=Structure.=—The wall consists of four coats—serous, muscular, submucous, and mucous, enumerated from without inward.

The =serous coat= is complete except at the mesenteric edge, where the vessels and nerves reach the bowel.

The =muscular coat= consists of an outer longitudinal and an inner circular layer, the latter being the thicker. In the last few feet of the intestine the muscular coat is very thick, and being usually firmly contracted in the dead subject, gives the impression that this part of the bowel is of smaller caliber; such, however, is not the case during life.

The =submucous coat= is a layer of areolar tissue in which the vessels and nerves ramify. It contains also the duodenal glands and the bases of the solitary glands and Peyer’s patches.

The =mucous membrane= is soft and velvety. It has a grayish or yellowish-red color and is very vascular. About five or six inches from the pylorus it forms a pouch, the =diverticulum duodeni=, in which the pancreatic and hepatic ducts open. On a small papilla opposite this is the termination of the accessory pancreatic duct. At the ileo-cæcal opening the mucous membrane projects slightly into the cavity of the cæcum, forming the =ileo-cæcal valve=. The free surface is thickly beset with villi, small projections of the mucous membrane which can be seen well by placing a piece of the membrane in water. They are relatively short and thick and have a distinct neck in the horse. Each contains a central lymph vessel (lacteal), and around this a plexus of capillaries, lymphoid tissue, and unstriped muscle-fibers. They are important agents in absorption from the contents of the intestine. The epithelium is columnar, with many goblet cells. Underneath the basement membrane is a layer of unstriped muscle-fibers, the =muscularis mucosæ=.

The glands of the small intestine are of three kinds:

1. The =intestinal glands= (Glandulæ intestinales Lieberkuehni) are found throughout. They are simple tubular glands which open between the villi.

2. The =duodenal glands= (Glandulæ duodenales Brunneri) are found in the first twenty feet (ca. 6 meters) of the bowel. They are racemose glands, and are situated in the submucosa, so that their ducts perforate the muscularis mucosæ and the mucous membrane.

3. The =lymph follicles= (Noduli lymphatici) are found either scattered or in groups. In the former case they are termed =solitary glands= (Noduli lymphatici solitarii), in the latter =Peyer’s patches= (Noduli lymphatici aggregati). The solitary glands are about the size of a millet-seed or a small sago grain. Peyer’s patches are situated chiefly along the surface opposite to the mesenteric attachment and begin about three or four feet from the pylorus. They number one to two hundred, and are usually one to two inches (2 to 5 cm.) long and a quarter of an inch to one-half inch (ca. 2 to 14 mm.) wide. Larger ones occur in the terminal part, where one patch may have a length of seven to fifteen inches (ca. 17 to 38 cm.) and a width of half an inch to one inch (ca. 5 to 25 mm.) in young horses (Ellenberger). They undergo atrophy in old subjects.

=Vessels and Nerves.=—The =arteries= of the small intestine come from the cœliac and anterior mesenteric arteries. The =veins= go to the portal vein. The =lymph vessels= are numerous and go to the mesenteric lymph glands. The =nerves= are derived from the vagus and sympathetic through the solar plexus.

THE LARGE INTESTINE

The large intestine (Intestinum crassum) extends from the termination of the ileum to the anus. It is about twenty-five feet (ca. 7.5 to 8 in.) in length. It differs from the small intestine in its greater size, in being sacculated, for the most part, possessing longitudinal bands, and having a more fixed position. It is divided into cæcum, great colon, small colon, and rectum.

THE CÆCUM

The =cæcum= (Intestinum cæcum) is a great cul-de-sac intercalated between the small intestine and the colon. It has a remarkable size, shape, and position in the horse. Its length is three to four feet (ca. 1 to 1.25 m.), and its capacity about seven to eight gallons (ca. 25 to 30 liters). It is conical in form, and is curved somewhat like a reversed comma. It is situated almost entirely to the right of the median plane, extending from the right iliac and sublumbar regions to the abdominal floor behind the xiphoid cartilage. Both extremities are blind, and the two orifices are placed close together on the concave curvature. It presents for description a base, a body, and an apex.

The =base= (Saccus cæcus) extends from about the thirteenth intercostal space backward almost to the pelvic inlet. Its greater curvature is dorsal, its lesser ventral; connected with the latter are the termination of the ileum and the origin of the colon. The =body= (Corpus cæci) extends downward and forward from the base and rests largely on the ventral wall of the abdomen. Its lesser curvature is about parallel with the costal arch and about five to six inches (10 to 15 cm.) below it. The =apex= (Apex cæci) lies usually on the abdominal floor about a hand’s length behind the xiphoid cartilage.

The base is attached dorsally by connective tissue and peritoneum on the ventral surface of the pancreas and right kidney, the psoas muscles, and the iliac fascia; internally, it is attached to the terminal part of the great colon, and ventrally to the origin of the great colon. The body is attached dorsally to the first part of the colon by the cæco-colic fold. The apex is free, and consequently may vary in position.

The cæcum has four =longitudinal bands= (Tæniæ), situated on the dorsal, ventral, right, and left surfaces; these cause four rows of =sacculations= (Haustra). The ventral band is entirely exposed or free (Tænia libera); the dorsal band is free on the apex. The cæcal arteries are placed on the other two. The right or parietal surface of the cæcum is related chiefly to the right abdominal wall, the diaphragm, duodenum, and liver. The left or visceral surface lies against the left divisions of the colon, the root of the great mesentery, and the small intestine.

The =ileo-cæcal orifice= (Ostium ileocæcale) is situated in the lesser curvature of the base, about four or five inches (ca. 10 to 12 cm.) to the right of the median plane and about opposite the lower end of the last rib. The end of the ileum is partially telescoped into the cæcum, so that the orifice is surrounded by a fold of mucous membrane, forming the =ileo-cæcal valve= (Valvula ileocæcalis). The peritoneum and longitudinal muscle-fibers do not take part in its formation.

The =cæco-colic orifice= (Ostium cæcocolicum) is placed above and external to the preceding one; the interval between them is only about two inches (ca. 5 cm.), and they are separated by a distinct ridge which projects into the interior of the cæcum. The orifice is slit-like and is small in relation to the size of the cæcum and colon. It has a =valvular fold= (Valvula cæcocolica) at its lower margin and a =muscular ring= (Sphincter cæci). Large crescentic or semilunar folds (Plicæ cæci) project into the cavity of the bowel, and between these are large pouches (Cellulæ cæci).

=Vessels and Nerves.=—The cæcal arteries come from the great mesenteric artery. The veins go to the portal vein. The nerves are derived from the great mesenteric plexus of the sympathetic.

THE GREAT COLON

The =great colon= (Colon crassum) begins at the cæco-colic orifice, and terminates by joining the small colon behind the saccus cæcus of the stomach. It is ten to twelve feet (ca. 3 to 3.7 m.) long, and its average diameter (exclusive of its narrowest part) is about eight to ten inches (ca. 20 to 25 cm.). Its capacity is more than double that of the cæcum. When removed from the abdomen, it consists of two parallel portions, which are connected by peritoneum and partially by areolar tissue also. In situ, it is folded so that it consists of four parts, which are designated according to their position or numerically. The three bent connecting parts are termed the flexures. The first, =right ventral part= (Colon ventrale dextrum), begins at the cæco-colic orifice about opposite the middle of the last rib, passes downward and forward along the right costal arch, and reaches the floor of the abdomen. Over the xiphoid cartilage it bends sharply to the left and backward, forming the =sternal flexure= (Flexura diaphragmatica ventralis). The second, =left ventral part= (Colon ventrale sinistrum), passes backward on the left part of the abdominal floor, and, on reaching the pelvic inlet, bends sharply dorsally and forward, forming the =pelvic flexure= (Flexura pelvina). This is continued by the third, =left dorsal part= (Colon dorsale sinistrum), which passes forward above the left ventral portion. On reaching the stomach, diaphragm, and left lobe of the liver, it turns to the right, forming the =diaphragmatic= or =gastro-hepatic flexure= (Flexura diaphragmatica dorsalis). The fourth or =right dorsal part= (Colon dorsale dextrum) passes backward above the first portion, and on reaching the inner or left surface of the base of the cæcum it turns upward and to the left behind the left sac of the stomach; here it becomes constricted, and joins the small colon below the left kidney.

The =caliber= of the great colon varies greatly at different points. At its origin it is only about two to three inches (ca. 5 to 7.5 cm.) in diameter. This soon increases to about eight to ten inches (ca. 20 to 25 cm.) for the ventral portions. Beyond the pelvic flexure the diameter is reduced to about three or four inches (ca. 8 to 9 cm.). Near the diaphragmatic flexure the caliber rapidly increases, and reaches its maximum in the last division, where it may be about twenty inches (50 cm.) in its widest part. This is succeeded by a somewhat funnel-shaped terminal contraction.

The first part of the great colon is attached to the lesser curvature of the cæcum by two layers of peritoneum which form the =cæco-colic fold=. The ventral parts of the colon are similarly connected to the dorsal parts of the same side by the mesocolon. The right portions are united also by areolar tissue and muscular fibers, the surface of contact being about four or five inches (ca. 10 to 12 cm.) wide; the left portions are attached to each other by a peritoneal fold wide enough to allow them to be drawn apart five or six inches (ca. 12 to 15 cm.) near the pelvic flexure. The terminal part of the colon is attached by peritoneum and areolar tissue to the ventral surface of the pancreas dorsally and to the base of the cæcum externally. It is connected indirectly with the diaphragm and liver by means of a fold derived from the right lateral ligament of the liver.

The =relations= are complex, but the more important facts are as follows: The ventral portions (first and second) have extensive contact with the abdominal wall ventrally and laterally. On the right side the colon is almost entirely excluded from contact with the flank by the cæcum; on the left side it has no contact with the upper part of the flank, being excluded here by coils of the small colon and small intestine. Dorsally the chief relations are to the stomach, duodenum, liver, pancreas, small colon, small intestine, aorta, vena cava, and portal vein. Since there are no transverse attachments of the right and left portions, and the latter have no attachment to the wall, they are subject to considerable displacement.[94] The pelvic flexure is variable in position, but usually it is directed to the right across the pelvic inlet.

The ventral portions of the colon have four longitudinal =muscular bands= (Tæniæ) which produce four rows of sacculations (Haustra). On the first part the external and internal bands are free; the dorsal band is covered by the adhesion to the overlying dorsal division, while the ventral band is largely covered by the adhesion to the cæcum. On the left ventral part the external and ventral bands are free; of the two dorsal bands, the inner one is concealed by the mesocolon, the outer one is chiefly free. The pelvic flexure has a band on its lesser curvature, which is continued on to the third part, concealed by the peritoneal attachment; these parts are practically non-sacculated. Near the diaphragmatic flexure two other bands appear, so that the flexure has three bands; two of these are dorsal and free, the third is ventral and concealed. The last part has three bands, of which the inner and outer ones are free, the ventral one covered.

THE SMALL COLON

The =small colon= (Colon tenue) begins at the termination of the great colon, behind the saccus cæcus of the stomach and below the left kidney, and is continued by the rectum at the pelvic inlet. Its length is about ten to twelve feet (ca. 3.5 m.), and its diameter about three to four inches (ca. 7.5 to 10 cm.).

Its coils lie in the space between the stomach and the pelvic inlet, dorsal to the left portions of the great colon. They are mingled with those of the small intestine, from which they are easily distinguished by the tæniæ and sacculation.

It is attached to the sublumbar region by the =colic mesentery=, and to the termination of the duodenum by a short peritoneal fold (Lig. duodeno-colicum). The great omentum is also attached to the origin of the bowel. The colic mesentery is narrow at its origin, but soon reaches a width of about three feet (ca. 80 to 90 cm.). Its parietal border is attached along a line extending from the ventral surface of the left kidney to the sacral promontory; it is continuous in front with the root of the great mesentery, and behind with the mesorectum.

There are two =tæniæ= and two rows of sacculations. Of the tæniæ, one is free, the other concealed by the mesentery. When the bowel is hardened in situ its lumen between the pouches is reduced to a narrow slit.

THE RECTUM

The =rectum= (Intestinum rectum) is the terminal part of the bowel; it extends from the pelvic inlet to the anus.[95] Its length is about one foot (ca. 30 cm.). Its direction may be straight or oblique. The first or =peritoneal part= of the rectum is like the small colon, and is attached by a continuation of the colic mesentery termed the =mesorectum=. The second or =retroperitoneal part= forms a flask-shaped dilatation termed the =ampulla recti=; it is attached to the surrounding structures by connective tissue and muscular bands.

The first part of the rectum is related to the small colon and the pelvic flexure of the great colon. It is frequently deflected to the left by the latter. The second part of the rectum is related dorsally and laterally to the pelvic wall. Ventrally the relations differ in the two sexes. In the male they are the bladder, the terminal parts of the vasa deferentia, the vesiculæ seminales, the prostate, the bulbo-urethral (Cowper’s) glands, and the urethra. In the female they are the uterus, vagina, and vulva.[96]

=Structure of the Large Intestine.=—The =serous coat= covers the different parts in varying degrees. It does not cover (a) the opposed surfaces of the cæcum and colon which are between the layers of the cæco-colic fold and mesocolon; (b) the areas of attachment of the cæcum and colon to the pancreas, right kidney, and sublumbar region; (c) the second part of the rectum.

The =muscular coat= consists of longitudinal and circular fibers. The bulk of the former is in the bands already described. Some of the circular fibers pass from one part of the colon to another, where they are attached to each other, forming the fibræ transversæ coli. The muscular coat of the second part of the rectum presents special features. The longitudinal layer of fibers is very thick and consists of large bundles, loosely united. A large band, the =recto-coccygeus=, is detached from it on either side, and passes upward and backward to be inserted into the fourth and fifth coccygeal vertebræ.

The =submucous tissue= is abundant in the wall of the rectum, so that the mucous membrane is loosely attached to the muscular coat, and forms numerous folds when the bowel is empty.

The =mucous membrane= of the large intestine is thicker and darker in color than that of the small intestine. It forms large crescentic or semilunar folds corresponding to the external constrictions. It has no villi, Brunner’s glands, or Peyer’s patches. The =intestinal glands= (of Lieberkühn) are large and numerous. =Solitary glands= are also numerous, especially at the apex of the cæcum and in the left dorsal part of the colon.

=Blood-supply.=—Greater and lesser mesenteric and internal pudic arteries. The veins go to the portal vein.

=Nerve-supply.=—Mesenteric plexus of the sympathetic nerves.

The =anus= is the terminal orifice of the alimentary canal. It is situated below the root of the tail, where it forms a round projection, with a central depression when contracted. It is covered externally by an integument which is thin, hairless, and provided with numerous sebaceous and sweat glands. The mucous lining is pale, glandless, and covered with a thick, squamous, stratified epithelium.

There are three muscles of the anus.

1. The =sphincter ani internus= is merely a terminal thickening of the circular coat of the bowel.

2. The =sphincter ani externus= is a broad band of striped muscle-fibers outside the internal sphincter. Some fibers are attached to the coccygeal fascia above, others to the perineal fascia below. Its action is to close the anus.

3. The =retractor ani= (M. levator ani) is a flat muscle which lies between the rectum and the sacro-sciatic ligament. It arises from the superior ischiatic spine and the sacro-sciatic ligament, and ends under the external sphincter. Its action is to reduce the partial prolapse which the anus undergoes during defecation.

The suspensory ligament of the anus is a band of unstriped muscle which arises from the first coccygeal vertebra, passes downward over the retractor, and unites with its fellow below the anus. In the male it is largely continued by the retractor penis muscle; in the female it blends with the constrictor vulvæ. It may act as an accessory sphincter of the anus.

=Blood-supply.=—Internal pudic artery.

=Nerve-supply.=—Hæmorrhoidal and perineal nerves (for the sphincter ani externus and retractor ani).

THE PANCREAS

The =pancreas= is situated transversely on the dorsal wall of the abdomen, the greater part being to the right of the median plane. Its central part lies under the sixteenth and seventeenth thoracic vertebræ.

When fresh it has a reddish cream color, but if left in the unpreserved cadaver it rapidly decomposes and becomes dark. It resembles the salivary glands in appearance, but is softer, and its lobules are more loosely united. Its average weight is about twelve ounces (ca. 350 g.).

When hardened in situ its shape is very irregular. It is triangular in outline, and presents for description two surfaces, three borders, and three angles.[97]

The =dorsal surface= faces upward and forward. It is partially covered by peritoneum. It is related chiefly to the ventral surface of the right kidney and adrenal, the posterior vena cava, the portal vein, the cœliac artery and its divisions, the gastro-phrenic ligament and the saccus cæcus of the stomach, the right and caudate lobes of the liver, and the gastro-pancreatic fold. There are grooves for the divisions of the cœliac artery, and a large one for the splenic vein.

The =ventral surface= looks downward and backward; it is in general concave. It presents two impressions, separated by an oblique ridge. The smaller of these (Impressio cæcalis) lies to the right, and is caused by the pressure of the base of the cæcum; the larger one (Impressio colica) indicates the area of contact with the terminal part of the great colon and its junction with the small colon. It has usually no peritoneal covering except over a small area at the anterior angle.

The =right border= is nearly straight; it is related to the second part of the duodenum and the caudate lobe of the liver.

The =left border= is slightly concave, and is related to the first part of the duodenum, the left sac of the stomach, and the splenic vessels.

The =posterior border= presents a deep notch to the right of the median plane for the portal vein, which passes through the gland very obliquely. There is a thin bridge of gland tissue dorsal to the vein, thus forming the =portal ring= (Annulus portarum). The root of the great mesentery is in contact with the border just to the left of the median plane.

The =anterior= or =duodenal angle= (Caput pancreatis) is attached to the concavity of the second curve of the duodenum, and the adjacent part of the right lobe of the liver. The ducts leave at this extremity.

The =left= or =splenic angle= (Cauda pancreatis) corresponds to the tail of the pancreas in man. It is in contact with the base of the spleen, the left kidney and adrenal body, and the saccus cæcus of the stomach.

The =right angle= is rounded, and lies on the ventral surface of the right kidney and adrenal body.

The pancreas is attached dorsally by connective tissue to the kidneys and adrenal bodies, the gastro-phrenic ligament and the suspensory ligament of the spleen, the posterior vena cava, the portal fissure, and the gastro-pancreatic fold. The ventral surface is mainly attached by areolar tissue to the base of the cæcum and the terminal part of the great colon.

There are almost invariably two =ducts=. The large one is termed the =pancreatic duct= (Ductus pancreaticus [Wirsungi]). It is formed by the union of two radicles which come from the right and left extremities, and passes through the duodenal angle to end at the duodenal diverticulum alongside of the bile-duct. The duct is nearly half an inch (ca. 1 cm.) wide, and is very thin-walled. It is situated in the substance of the gland near its dorsal surface; none of it is free. The =accessory pancreatic duct= (Ductus pancreaticus accessorius [Santorini]) arises either from the chief duct or its left radicle, and ends on a papilla in the duodenum opposite the chief duct.

=Structure.=—The pancreas belongs to the class of tubulo-alveolar glands, the alveoli being long, like those of the duodenal glands; in other respects it resembles the serous salivary glands very closely. It has no proper capsule and the lobules are rather loosely united.

=Vessels and Nerves.=—The arteries of the pancreas come from the branches of the cœliac and anterior mesenteric arteries. The nerves are derived from the cœliac and mesenteric plexuses of the sympathetic.

THE LIVER

The =liver= (Hepar) is the largest gland in the body. It is situated obliquely on the abdominal surface of the diaphragm. Its highest point is at the level of the right kidney, its lowest on the left side, usually about three or four inches (ca. 8 to 10 cm.) from the abdominal floor, opposite the lower end of the seventh or eighth rib. The greater part of it lies to the right of the median plane.

It is red-brown in color and is rather friable. Its average weight is about ten to twelve pounds (ca. 5 kg.). When in the body, or if hardened in situ, it is strongly curved and accurately adapted to the abdominal surface of the diaphragm. When removed in the soft state, it flattens out into a cake-like form quite different from its natural configuration. It presents for description two surfaces and a circumference, which may be divided into four borders.

The =parietal surface= (Facies diaphragmatica) is strongly convex, and lies against the diaphragm. It faces chiefly upward and forward. It presents, just to the right of the median plane, a sagittal groove for the posterior vena cava (Fossa venæ cavæ). The vein is partially embedded in the substance of the gland, and receives the hepatic veins.

The =visceral surface= (Facies visceralis) faces in general downward and backward; it is concave and irregular, being moulded on the organs which lie against it. It presents, a little to the right of the median plane, the =portal fissure= (Porta hepatis). Through this the portal vein, hepatic artery, and hepatic plexus of nerves enter, and the hepatic duct and lymph vessels leave the liver. The portal or hepatic lymph glands are also found here. The pancreas is attached at and to the right of the fissure, and the gastro-hepatic omentum to the left of it. Above the fissure is a ridge which represents the caudate lobe (Lobus caudatus Spigelii), and is continued to the right by the pointed caudate process (Processus caudatus). Further to the left there is a large depression (Impressio gastrica) for the stomach. To the right of this may be seen a groove passing to the right and dorsally; this is the duodenal impression (Impressio duodenalis). Ventral to these is a large depression for the great colon (Impressio colica). Dorsal to this is a smaller depression for the blind end of the base of the cæcum.[98] Coils of the small intestine may also lie on this surface, and the apex of the spleen may reach to it when the stomach is empty.

The =dorsal border= (Margo obtusus) is thick for the most part. It presents from right to left: (1) a depression for the right kidney (Impressio renalis); (2) a notch, which is the dorsal end of the fossa venæ cavæ; (3) a deep notch (Impressio œsophagea) which is mainly occupied by the thick margin of the œsophageal opening of the diaphragm.

The =ventral border= is thin, and is marked by two deep =interlobar fissures= or =incisures= (Incisuræ interlobares), which partially divide the organ into three principal =lobes=—=right=, =middle=, and =left=. The right lobe is the largest, except in old subjects, in which it is frequently much atrophied. The middle lobe is the smallest. It is marked by several small fissures, and by the =umbilical fissure= (Incisura umbilicalis); the latter contains the umbilical vein in the fœtus, which is transformed into the round ligament after birth.

The =right border= is thin and long. It is nearly vertical, and extends backward to about the middle of the sixteenth rib.

The =left border= is thin and short. It extends backward to a point opposite the lower part of the ninth or tenth rib. The ventral and lateral borders together constitute the margo acutus.

The liver is held in position largely by the pressure of the other viscera and by its close application to the diaphragm. It has six ligaments.

1. The =coronary ligament= (Lig. coronarium hepatis) attaches it closely to the diaphragm. It consists of two laminæ. The right one is attached to the right of the fossa venæ cavæ; the left one begins to the left of the vena cava and passes upward and outward, becoming continuous with the left lateral ligament at the left margin of the œsophageal notch; it detaches a middle fold which extends to the notch and is continuous with the small omentum. The two laminæ unite below the vena cava to form the next ligament.

2. The =falciform ligament= (Lig. falciforme hepatis) is a crescentic fold which attaches the middle lobe to the sternal part of the diaphragm and to the abdominal floor for a variable distance. In its concave free edge is found.

3. The =round ligament= (Lig. teres hepatis), a fibrous cord which extends from the umbilical fissure to the umbilicus; it is the vestige of the umbilical vein, which in the fœtus carries the blood from the placenta to the liver.

4. The =right lateral ligament= (Lig. triangulare dextrum) is a wide fold which attaches the dorsal border of the right lobe to the costal part of the diaphragm.

5. The =left lateral ligament= (Lig. triangulare sinistrum) attaches the dorsal edge of the left lobe to the tendinous center of the diaphragm.

6. The =hepato-renal= or =caudate ligament= (Lig. hepatorenale) attaches the caudate process to the right kidney and the base of the cæcum. The gastro-hepatic omentum and the mesoduodenum have been described.

As stated above, the liver is divided by fissures into three principal lobes—right, middle, and left. The right lobe is the largest in the young subject and is irregularly quadrilateral in form. On its dorsal part is the caudate lobe, which ends in a pointed process directed outward, and assists in forming the cavity for the right kidney. The middle lobe is normally much the smallest. The left lobe is oval in outline and thickest centrally. In old or middle-aged subjects it often exceeds the right one in size. In some cases the atrophy of the right lobe is so extreme that the middle lobe may exceed it in size.[99]

The =hepatic duct= (Ductus hepaticus) is formed at the ventral part of the portal fissure by the union of right and left chief lobar ducts. It is two or three inches (ca. 5 to 8 cm.) long and about half an inch (ca. 1 to 1.5 cm.) wide. It passes between the two layers of the lesser omentum, and pierces the wall of the duodenum about five or six inches (ca. 12 to 15 cm.) from the pylorus, alongside of the pancreatic duct. The ducts pass obliquely through the wall of the duodenum for about half an inch (ca. 1 cm.) before opening into the diverticulum duodeni. The arrangement forms an effective valve, which prevents regurgitation from the intestine. There is no gall-bladder.

=Structure.=—The liver is covered by an outer serous, and an inner fibrous coat. The =serous coat= covers the gland except at the attachment of the pancreas and at the portal fossa; it is reflected from it to form the ligaments and the lesser omentum. The =fibrous capsule= is in general thin; it sends laminæ into the ligaments, and also delicate strands into the gland substance. At the portal fissure it is abundant and surrounds the vessels and ducts, which it accompanies in the portal canals of the gland substance.

The gland substance is composed of the parenchyma and the interstitial tissue. The =parenchyma= is made up of lobules, 1 to 2 mm. in diameter, which are held together by a small amount of interlobular connective tissue. On account of the very small amount of the latter, the lobulation of the horse’s liver is not usually at all distinct to the naked eye; for the same reason the organ is also quite friable.[100]

=Vessels and Nerves.=—The =portal vein= enters at the portal fissure. It conveys blood from the digestive tract and the spleen, which contains various products of digestion and numerous white blood-cells. The =hepatic artery= also enters at the portal fissure; it may be termed the nutrient vessel. All the blood is returned from the liver to the posterior vena cava by the hepatic veins. The portal vein and the hepatic artery both divide into interlobular branches, which run together in the portal canals of the interlobular tissue. The branches of the portal vein (Venæ interlobulares) give off intralobular branches which form plexuses of capillaries in the lobules and give rise to a central vein (Vena centralis). The branches of the hepatic artery (Rami arteriosi interlobulares) are of relatively small size. They supply mainly (if not exclusively) the interlobular tissue, the capsule, and the walls of the vessels and ducts. The =hepatic veins=[101] (Venæ hepaticæ) empty into the vena cava as it lies in the fossa of the gland. Their ultimate radicles are the central lobular veins, which emerge from the bases of the lobules and join the sublobular veins (Venæ sublobulares); the latter unite to form the hepatic veins. The largest hepatic veins, three or four in number, join the posterior vena cava just before it leaves the liver to pass through the diaphragm.

The nerve-supply comes from the =hepatic plexus=, composed of branches from the vagus and sympathetic nerves.

THE SPLEEN

The =spleen= (Lien) is the largest of the ductless glands.[102] It is situated chiefly in the left parachondrium, in close relation to the left part of the great curvature of the stomach, to which its long axis corresponds. Its size and weight vary greatly in different subjects, and also in the same subject under different conditions, depending chiefly on the great variability of the amount of blood contained in it. The average weight is about 35 ounces (ca. 1 kg.), its length about 20 inches (ca. 50 cm.), and its greatest width about 8 to 10 inches (ca. 20 to 25 cm.). It is usually bluish-red or somewhat purple in color. In the natural state it is soft and yielding, but not friable.

The weight appears to vary ordinarily from about one to eight pounds, although in large horses the latter figure even may be exceeded without any apparent evidence of disease. There does not seem to be any constant relation to the body-weight. For example, the spleen of a colt about ten months old weighed three and a half pounds, while it often weighs less than two pounds in horses weighing 1000 to 1200 pounds. The chief variation in outline consists of increase of width, especially of the dorsal part.

It extends obliquely in a curved direction from the left crus of the diaphragm and the saccus cæcus of the stomach to the ventral third of the ninth or tenth rib. It presents for description two surfaces, two borders, and two extremities.

The =parietal= or =external surface= (Facies parietalis) is convex, and lies chiefly against the diaphragm, but is in direct contact with the upper parts of the last two ribs and to a small extent with the flank at the lumbo-costal angle.

The =visceral= or =internal surface= (Facies visceralis) is in general concave. It is divided into two unequal portions by a longitudinal ridge; on this is a groove, the =hilus=, in which the vessels and nerves are situated. The area in front of the ridge (Facies gastrica) is moulded on the great curvature of the stomach; it is about two inches (ca. 5 cm.) wide. The area behind the ridge (Facies intestinalis) is much more extensive; it is related chiefly to the small colon, the small intestine, and the great omentum. It may be marked by one or two fissures.

The =anterior border= (Margo anterior) is concave and thin.

The =posterior border= (Margo posterior) is convex and thin.

The =base= or =dorsal extremity= is beveled, and fits into the interval between the left kidney and the left crus of the diaphragm and the psoas major. When hardened in situ, it shows an impression (Facies renalis) where it lies against the kidney. The left extremity of the pancreas touches it also. The anterior basal angle fits in between the saccus cæcus of the stomach and the left kidney; the posterior basal angle usually lies against the flank just behind the last rib.

The =apex= or =ventral extremity= is small; it lies—when the stomach is not full—between the left lobe of the liver and the left dorsal portion of the colon; when the stomach is full, the spleen is pushed further back and loses contact with the liver.

The spleen is attached by two peritoneal folds, the suspensory ligament and the gastro-splenic omentum. The =suspensory ligament= (Lig. suspensorium lienis) attaches the base to the left crus of the diaphragm and the left kidney; it contains a quantity of elastic tissue. The part which passes to the diaphragm is the ligamentum phrenico-lienale, and blends with the gastro-phrenic ligament; the part which goes to the kidney is termed the ligamentum renolienale. The =gastro-splenic omentum= (Lig. gastrolienale) passes from the hilus to the left part of the great curvature of the stomach. It is narrow above, where it joins the suspensory ligament; below it becomes much wider and is continuous with the great omentum.

Small globular or lenticular masses of splenic tissue may be found in the gastro-splenic omentum. They are termed =accessory spleens= (Lienes accessoriæ).

=Structure.=—The spleen has an almost complete =serous coat=. Subjacent to this and intimately united with it is a =capsule= of fibrous tissue (Tunica albuginea), which contains many elastic fibers and some unstriped muscular tissue. Numerous =trabeculæ= (Trabeculæ lienis) are given off from the deep face of the capsule and ramify in the substance of the organ to form a supporting network. In the interstices of this framework is the =spleen pulp= (Pulpa lienis), a dark red, soft, grumous material. This is supported by a delicate adenoid =reticulum=, and contains numerous leukocytes, the large splenic cells, red blood-corpuscles, and pigment. The pulp is richly supplied with blood. The branches of the splenic artery enter at the hilus and pass along the trabeculæ. The arteries which enter the pulp have a sheath of lymphoid tissue, which collects on the vessel wall at certain points, forming small =lymph nodules=, the so-called Malpighian corpuscles (Noduli lymphatici lienales). These are visible to the naked eye as white spots, about as large as the head of a pin. The blood passes into cavernous spaces lined by endothelium which is continuous with the cells of the reticulum of the pulp. From these the veins arise. The splenic vein runs in the hilus in company with the artery and nerves, and joins the posterior gastric vein to form a large radicle of the portal vein.

=Blood-supply.=—Splenic artery.

=Nerve-supply.=—Splenic plexus.

THE PERITONEUM

The general disposition of the peritoneum has been described, and other facts in regard to it were mentioned in the description of the viscera. It is now desirable to study it as a continuous whole.[103] (Figs. 256, 257, 278, 279.)

We may consider the peritoneum as consisting of two sacs—a greater and a lesser. The =greater sac= lines the greater part of the abdominal cavity, and covers most of the viscera which have a peritoneal investment. The =lesser sac= is an introversion or recess of the greater sac, formed during the development of the viscera. The two sacs communicate by a relatively narrow passage, termed the =epiploic foramen= of Winslow (Foramen epiploicum). This opening is situated on the visceral surface of the liver above the portal fissure. It can be entered by passing the finger along the caudate lobe of the liver toward its root. Its dorsal (or anterior) wall is formed by the caudate lobe and the posterior vena cava. Its ventral (or posterior) wall consists of the pancreas, the gastro-pancreatic fold, and the portal vein. The walls are normally in contact, and the passage merely a potential one. It is usually about four inches (ca. 10 cm.) in length. It is narrowest at the right extremity, where it is about an inch (ca. 2.5 to 3 cm.) wide.[104] If the finger is passed into the foramen from right to left, it enters the cavity of the lesser sac. If now an opening is made in the great omentum and the other hand introduced through it, the fingers of the two hands touch each other over the lesser curvature of the stomach. The formation and boundaries of the lesser sac should now be examined by spreading out the great omentum. It will be found that the latter now incloses a considerable cavity behind the stomach; this is termed the =omental cavity= (Bursa omentalis). Passing forward over the lesser curvature of the stomach, we enter another space, the =vestibule= of the omental cavity (Vestibulum bursæ omentalis). This space is closed on the left by the gastro-phrenic ligament, below and on the right by the lesser omentum, and dorsally by the gastro-pancreatic fold, which is attached to the dorsal border of the liver and to the posterior vena cava. Above the œsophageal notch the fingers can be passed around the border of the liver and the vena cava till the coronary ligament is encountered. Thus the vestibule is closed except (1) on the right, where it communicates with the cavity of the greater sac by the epiploic foramen; and (2) behind, where it communicates with the cavity of the omentum.

The general arrangement of the great omentum has already been indicated. We may now trace its line of attachment, which would correspond to the mouth of the sac. Beginning at the ventral part of the great curvature of the stomach, the line passes to the ventral face of the pylorus, then crosses obliquely the first part of the duodenum to the point where the pancreas is adherent to it. Here it passes to the anterior face of the terminal transverse part of the great colon, runs along this transversely (from right to left), and continues for some ten or twelve inches (ca. 25 to 30 cm.) on the small colon. It then forms an acute angle, passes inward and forward along the small colon to the dorsal part of the hilus of the spleen, where it blends with the suspensory ligament of the latter, and forms a recess (Recessus lienalis) behind the saccus cæcus of the stomach. It now passes along the hilus of the spleen, and is continued to the great curvature of the stomach by the gastro-splenic omentum. It is convenient to regard the spleen as being intercalated in the left part of the great omentum; on this basis the gastro-splenic omentum would be that part of the great omentum which connects the hilus of the spleen with the great curvature of the stomach. The great omentum is relatively small in the horse, and is usually not visible when the abdomen is opened. It is generally folded up in the space between the visceral surface of the stomach and the intestine.[105]

The lesser sac furnishes the peritoneal covering for: (1) the visceral surface of the stomach and a small area of the first curve of the duodenum; (2) a large part of the dorsal surface of the pancreas and portal vein; (3) a small part of the visceral surface of the liver above the attachment of the lesser omentum and the portal fossa; (4) the posterior vena cava, from the level of the epiploic foramen [of Winslow] to its passage through the diaphragm (in so far as it is not embedded); (5) the part of the parietal surface of the liver between the right and middle divisions of the coronary ligament; (6) the corresponding part of the diaphragm, and the right part of the right crus of the same; (7) part of the anterior surface of the terminal part of the great colon, and the origin of the small colon; (8) the left extremity of the pancreas (inconstant); (9) the spleen.

We may now trace the peritoneum in a longitudinal direction, beginning in front. It is reflected from the ventral abdominal wall and the diaphragm upon the liver, forming the ligaments and serous coat of the gland. It leaves the visceral surface of the liver as lesser omentum, and the crura of the diaphragm as the gastro-phrenic ligament, reaches the saccus cæcus and lesser curvature of the stomach and the first curve of the duodenum, covers these organs, and is continued by the great omentum.

On the left it passes from the left crus of the diaphragm and the left kidney to form the suspensory ligament of the spleen, clothes that organ, and leaves it to be continued by the great omentum.

On the right it passes from the right crus of the diaphragm and the dorsal border of the liver to the concave border of the duodenum, forming the gastro-pancreatic fold (second part of the mesoduodenum), and covering part of the dorsal surface of the pancreas. From the margin of the pancreas, the right kidney, and a small area of the sublumbar region behind the latter, it passes on to the base of the cæcum and the terminal part of the great colon. From these it passes on the right to the duodenum, forming the third part of the mesoduodenum. On the left it clothes part of the ventral surface and the outer border of the left kidney, from which it passes to the base of the spleen, forming the ventral layer of the suspensory ligament of the latter. Behind the terminal part of the great colon it is reflected around the great mesenteric artery to form the great mesentery. Behind this it is reflected almost transversely from the roof of the cavity and from the origin of the small colon on to the duodenum, forming the terminal part of the mesoduodenum. The line of origin of the colic mesentery begins on the inner part of the ventral surface of the left kidney, and extends to the sacral promontory, where the mesorectum begins. At the termination of the latter the peritoneum is reflected from the rectum on the dorsal and lateral walls of the pelvic cavity. Below the rectum it forms the urogenital fold, and passes on to the dorsal surface of the bladder, covers its anterior part, and is reflected on to the body-wall laterally and ventrally, forming the lateral and middle ligaments of the bladder. In the female the broad ligaments of the uterus replace the urogenital fold, with which they are homologous.

In the new-born foal certain folds are specially large. The falciform ligament of the liver extends to the umbilical opening, and contains in its free edge the large umbilical vein. The bladder—at this time an abdominal organ—has a ventral median fold, which connects it and the urachus with the abdominal floor. This is flanked on either side by a fold which also extends to the umbilicus, and contains the large umbilical artery.

DIGESTIVE SYSTEM OF THE OX

THE MOUTH

The cavity of the mouth is shorter and wider than that of the horse, and the vestibule is more capacious.

The =lips= are thick, wide, and comparatively immobile. The middle part of the upper lip and the surface between the nostrils is bare, and is termed the muzzle (Planum nasolabiale). It is smooth, and (in health) is kept cool and moist by a clear fluid secreted by a layer of subcutaneous glands (Glandulæ nasolabiales) about half an inch (ca. 1.5 cm.) thick. It shows irregular lines, mapping out small polygonal areas on which the orifices of the gland-ducts are visible. A narrow bare strip also exists along the edge of the lower lip. The remainder of the integument is provided with ordinary and tactile hairs. The free edge and the lining membrane present verrucose, horny papillæ. The labial glands form compact masses near the angles of the mouth.

The =cheeks= are more capacious than in the horse. The mucous membrane presents large conical pointed =papillæ=, which are directed toward the fauces and are covered with a horny epithelium. The largest of these have a length of about half an inch (ca. 1 to 1.5 cm.) and are situated around the angle of the mouth and parallel with the cheek teeth. The orifice of the parotid duct is opposite the fifth upper cheek tooth. (In the sheep and goat it is opposite the fourth tooth.) The =buccal= or =molar glands= are very well developed, and are arranged in three parts. The dorsal row extends from the maxillary tuberosity to the angle of the mouth. Its lobules are of a light yellow color. The ventral part consists of a compact brownish mass which reaches from the angle of the mouth a short distance under the masseter muscle. The middle part consists of loosely arranged yellow lobules.

A linear series of large papillæ exists on the floor of the mouth on each side of the frenum linguæ. Near these are found the openings of the small ducts of the sublingual gland. The papilla on which the submaxillary duct opens is wide, hard, and has a serrated edge.

The =hard palate= is wide, and is usually more or less pigmented. The body of the premaxilla is covered with a thick layer of dense connective tissue, which has a thick, horny epithelial covering—forming the so-called dental plate or pad. The palatine ridges extend from this backward about two-thirds of the length of the hard palate. They are nearly straight, and, for the most part, are serrated on the free edge. A median furrow extends between the ridges. The posterior third of the palate is smooth. Between the dental plate and the first ridge is the triangular papilla incisiva; on either side of this is a deep furrow, in which is the oral opening of the =naso-palatine canal=. This canal is two inches or more (ca. 5 to 6 cm.) in length and opens on the floor of the nasal cavity; it also communicates by a slit-like opening with the organ of Jacobson.

The =soft palate= is somewhat shorter than that of the horse, but is long enough to close the isthmus of the fauces. The posterior pillars do not extend to the entrance of the œsophagus. The azygos muscle is much better developed than in the horse. The fibrous aponeurosis is for the most part replaced by muscular tissue.

The =isthmus faucium= is wide and dilatable. On either side, behind the anterior pillar of the soft palate, is a deep depression, the =sinus tonsillaris=; external to this is the compact bean-shaped tonsil, which is about one to one and a half inches (ca. 3 to 4 cm.) in length. The tonsil does not project into the fauces, but outward instead: hence it does not occupy the tonsillar sinus, and is not visible internally, as is the case in most animals.

THE TONGUE

The tongue of the ox is often variably pigmented. The root and body are wider than that of the horse, but the free part is more pointed. The posterior part of the dorsum forms a remarkable elliptical prominence, which is sharply defined in front. The filiform papillæ in front of this prominence are large and horny, with sharp points directed backward. They impart to the tongue its rasp-like roughness. The papillæ on the prominence are large, broad and horny; some have a blunt conical form, others are rounded or flattened. Behind the prominence, the papillæ are long and soft, i. e., not horny. The fungiform papillæ are numerous and distinct; they are scattered more generally over the dorsum and edges of the free part. The vallate papillæ are about twenty or thirty in number; they are smaller than those of the horse, and are irregularly distributed on either side of the posterior part of the prominence of the dorsum. The foliate papillæ and the lingual fibrous cord are absent. The muscles are well developed; the hyo-glossus arises by additional portions from the great and middle cornua of the hyoid bone. The tongue is highly protractile and is the chief organ of prehension.

THE TEETH

The dental formula of the ox is:

( 0 0 3 3 ) 2 ( I - C - P - M - ) = 32 ( 4 0 3 3 )

The =incisors= are absent from the upper jaw. There are eight incisors in the lower jaw, arranged in a somewhat fan-like manner. They are simple teeth, without infundibulum. The crown is white, short, and shovel-shaped. The root is rounded, and is embedded in the jaw in such a manner as to allow a considerable degree of movement. There is a distinct =neck=. In addition to the simple numerical designation, the following terms are commonly applied to the individual teeth: pinchers or central, first intermediate, second intermediate, and corner incisors. It is probable that the latter are much modified canines. The incisors of the ox do not advance out of the alveoli, as is the case in the horse; in old age, however, the gum retracts so that the roots are partly exposed and may come into wear.[106]

The =deciduous incisors= differ from the permanent set chiefly in being much smaller. The crowns are narrower and diverge more.

The =canines= are absent (unless the fourth incisors be considered to represent them).

The =cheek teeth= (Fig. 286) resemble those of the horse in number and general arrangement. They are, however, smaller, and also differ in the fact that they progressively increase in size from before backward. This feature is so marked that the first tooth is quite small, and the space occupied by the first three (i. e., the premolars) is only about one-half of that required for the posterior three (i. e., the true molars). The enamel folds stand out even more prominently in relief on the masticatory surface than in the horse. The occurrence of wolf-teeth is rare.

The formula of the deciduous teeth is:

( 0 0 3 ) 2 ( Di — Dc — Dp — ) = 20 ( 4 0 3 )

TABLE OF AVERAGE PERIODS OF ERUPTION OF THE TEETH IN THE OX │ TEETH │ ERUPTION │ A. Temporary: │ First incisor (Di 1) │Birth to 3 weeks. Second incisor (Di 2) │ „ Third incisor (Di 3) │ „ Fourth incisor (Di 4) │ „ ──────────────────────────────┼────────────────── First cheek tooth (Dp 1) │Birth to 3 weeks. ──────────────────────────────┼────────────────── Second cheek tooth (Dp 2)│Birth to few days. Third cheek tooth (Dp 3) │ „ ──────────────────────────────┼────────────────── │ B. Permanent: │ First incisor (I1) │1½ to 2 years. Second incisor (I2) │2 to 2½ years. Third incisor (I3) │3 years. Fourth incisor (I4) │3½ to 4 years. First cheek tooth (P1) │2 to 2½ years. Second cheek tooth (P2) │1½ to 2½ years. Third cheek tooth (P3) │2½ to 3 years. Fourth cheek tooth (M1) │5 to 6 months. Fifth cheek tooth (M2) │1 to 1½ years. Sixth cheek tooth (M3). │2 to 2½ years.

The eruption of the permanent teeth is subject to great variation. The above figures are the average of observations of improved breeds under favorable conditions.

THE SALIVARY GLANDS

The =parotid gland= is smaller than that of the horse, and is light red-brown in color. It has somewhat the form of a very narrow long triangle, and lies chiefly on the posterior part of the masseter muscle. The thick wide upper end partly covers a large subparotid lymph gland. The small lower end is bent forward and fits into the angle of union of the jugular and external maxillary veins; it lies on the submaxillary gland. The parotid duct leaves the lower part of the deep face; in its course it resembles that of the horse, but it pierces the cheek opposite the fifth upper cheek tooth.

The =submaxillary gland= is very large, and is pale yellow in color. It is covered to a small extent by the parotid. Its general form resembles that of the horse, but its lower (or anterior) end is large and rounded, and is separated by a small interval only from the gland of the other side. This part can be distinctly felt in the living animal, and is related externally to a large submaxillary lymph gland. The duct leaves the middle of the superficial face of the gland, crosses the stylo-hyoideus and the intermediate tendon of the digastricus, and is then disposed as in the horse.

The =sublingual gland= consists of two parts. The dorsal part (Glandula sublingualis parvicanalaris) is long, thin, and pale yellow in color. It extends from the anterior pillar of the soft palate about to the symphysis of the jaw. It has numerous small tortuous ducts (Ductus sublinguales minores), which open between the papillæ under the side of the tongue. The ventral part is shorter and thicker, and is salmon pink in color. It has a single duct (Ductus sublingualis major), which either opens alongside of or joins the submaxillary duct.

THE PHARYNX

The pharynx is short and wide. The vault (Fornix pharyngis) is divided into two cul-de-sacs by a median fold of mucous membrane (Septum nasi membranaceum), which is a continuation of that of the septum nasi; on the outer wall of each is the relatively small opening of the Eustachian tube, which is covered by a simple fold of mucous membrane. The posterior nares are small. The entrance to the œsophagus is large.

On the dorsal wall of the pharynx are two large pharyngeal lymph glands, which, when enlarged, cause difficulty in swallowing and breathing (Fig. 281).

THE ŒSOPHAGUS

This is much shorter, wider, and more dilatable than that of the horse. Its average diameter (when moderately inflated) is about two inches (ca. 5 cm.), and its length in a large animal is about three to three and a half feet (ca. 90 to 100 cm.). The wall is relatively thin, and the muscular tissue is striped throughout. The latter consists of two strata of spiral fibers, except near the stomach, where they are longitudinal and circular. Fibers are continued into the wall of the stomach for some distance. There are submucous glands in the first part of the tube. There is no terminal dilatation, and no part in the abdominal cavity.

A very large mediastinal lymph gland lies above the posterior part of the œsophagus and may, if enlarged, obstruct it (Fig. 289).

THE ABDOMINAL CAVITY

The abdominal cavity of the ox is very capacious, both absolutely and relatively, as compared with that of the horse. This is due to several factors. The lumbar portion of the spine is about one-fourth longer than that of the horse. The transverse diameter between the last ribs is greater. The costal attachment of the diaphragm is almost vertical in direction from the lower part of the ninth rib to the extreme upper part of the thirteenth; in fact, the attachment to the last rib is not constant. Thus the abdomen is increased at the expense of the thorax, and the last three or four ribs enter more largely into the formation of the abdominal wall than in the horse. The flank is also much more extensive. The ilia, on the other hand, do not extend forward beyond a transverse plane through the middle of the last lumbar vertebra. The epigastric and mesogastric regions would be separated by a plane through the lower end of the tenth pair of ribs, or through the last thoracic vertebra. The chief differential features in the arrangement of the peritoneum will be described with the viscera.

THE PELVIC CAVITY

The pelvic cavity is relatively long and narrow. The inlet is more oblique than that of the horse; it is elliptical in outline, and the transverse diameter is smaller than that of the horse. The anterior (pubic) part of the floor is about horizontal, but the posterior (ischial) part slopes upward and backward to a marked degree; this part is also deeply concave transversely. The roof is concave in both directions. The peritoneum extends backward as far as the first coccygeal vertebra, so that the retroperitoneal part of the cavity is short.

THE STOMACH

=General Arrangement.=—The stomach of the ox is very large, and occupies nearly three-fourths of the abdominal cavity. It fills all of the left half of the cavity (with the exception of the small space occupied by the spleen) and extends considerably over the median plane into the right half.

It is compound, being composed of four divisions, viz., =rumen=, =reticulum=, =omasum=, and =abomasum=.[107] The division is clearly indicated externally by furrows or constrictions. The first three divisions may be regarded as proventriculi or œsophageal sacculations, the fourth being the stomach proper (in the narrower sense of the term). The œsophagus opens into the stomach on a sort of dome formed by the rumen and reticulum, and is continued through the latter by the œsophageal groove. From the ventral end of the latter a groove traverses the ventral wall of the omasum, thus giving a direct path to the abomasum for finely divided or fluid food. The abomasum joins the small intestine.

=Capacity.=—The capacity of the stomach varies greatly, depending on the age and size of the animal. In cattle of medium size it holds 30 to 40 gallons, in large animals 40 to 60, in small 25 to 35. The relative sizes of the four parts vary with age. In the new-born calf the rumen and reticulum together are about half as large as the abomasum; in ten or twelve weeks this ratio is reversed. During this period the omasum appears to be contracted and functionless. At four months the rumen and reticulum together are about four times as large as the omasum and abomasum together. At about one and one-half years the omasum equals (or nearly so) the abomasum in capacity. The four divisions have now reached their definitive relative capacities, the rumen constituting about 80 per cent., the reticulum 5 per cent., the omasum 7 or 8 per cent., and the abomasum 8 or 7 per cent. of the total amount.

EXTERIOR AND RELATIONS

The =rumen= occupies almost all of the left half of the abdominal cavity, and extends considerably over the median plane ventrally and in its middle. It is somewhat compressed laterally, and may be described as having two surfaces, two curvatures or borders, and two extremities. The =parietal= (or left) =surface= (Facies parietalis) is convex and is related to the diaphragm, spleen, and the left wall of the abdomen. It extends from the lower part of the seventh intercostal space almost to the pelvis. The =visceral= (or right) =surface= (Facies visceralis) is somewhat irregular, and is related chiefly to the omasum and abomasum, the intestine, the liver, pancreas, kidneys, the uterus in the female, and the posterior aorta and vena cava. The =dorsal curvature= (Curvatura dorsalis) is convex, following the curve formed by the diaphragm and sublumbar muscles. It is firmly attached to the left portion of the crura of the diaphragm and the sublumbar muscles by peritoneum and connective tissue. The =ventral curvature= is also convex and lies on the floor of the abdomen. The surfaces are marked by the right and left =longitudinal furrows= (Sulci longitudinales), which indicate externally the division of the rumen into dorsal and ventral sacs.[108] The =reticular= (or anterior) =extremity= (Extremitas reticularis) is divided ventrally by a transverse groove (Sulcus ruminis cranialis) into two sacs. The dorsal sac is the longer of the two, and curves ventrally over the round, blind end of the ventral sac. The former only is continuous with the reticulum, a ventral groove (Sulcus rumino-reticularis) being the only external line of demarcation. Dorsally no natural separation exists, the rumen and reticulum together forming a dome-like vestibule (Atrium ventriculi) on which the œsophagus terminates. The =pelvic= (or posterior) =extremity= (Extremitas pelvina) extends nearly to the pubis, and is related to the intestine and bladder, and the uterus in the cow. It is divided into =dorsal= and =ventral blind sacs= (Saccus cæcus caudalis dorsalis, ventralis) by a deep transverse groove (Sulcus ruminis caudalis) in which the longitudinal furrows terminate. The blind sacs are marked off from the remainder of the rumen by the =dorsal= and =ventral coronary grooves= (Sulcus coronarius dorsalis, ventralis).

The left longitudinal groove (Sulcus longitudinalis sinister) begins at the anterior transverse groove, passes somewhat dorsally and backward and divides into two grooves. The ventral one is the chief groove and passes backward to terminate in the angle of union of the posterior blind sacs. The dorsal (accessory) groove curves dorsally and then backward to terminate in the dorsal coronary sulcus. On the right surface there are two longitudinal grooves. The dorsal one is the chief sulcus (Sulcus longitudinalis dexter). It extends in a curved direction (the convexity being dorsal) and is continuous with the left longitudinal groove by means of the grooves between the dorsal and ventral blind sacs at each end. The ventral groove (Sulcus accessorius) curves somewhat ventrally and backward to rejoin the chief sulcus between the posterior blind sacs.

The =reticulum= is the smallest of the four divisions in the ox. It is also the most anterior, and lies on the concave surface of the diaphragm, so that the median plane divides it into two nearly equal portions. It is somewhat pyriform, but much compressed from before backward. The =parietal= or =phrenic surface= (Facies diaphragmatica) is convex and lies against the diaphragm and liver.[109] The =ruminal= (or posterior) =surface= (Facies ruminalis) is flattened by the pressure of the other three compartments; it ends dorsally by joining the wall of the rumen, the concave line of junction forming, in the interior of the stomach, the lower margin of the large rumino-reticular orifice. The =omasal= (or right) =surface= is narrow and concave, and is connected with the omasum. The =dorsal curvature= by its left portion joins the rumen to form the atrium, and by its right portion touches the liver. The =ventral curvature= lies on the sternal portion of the diaphragm, opposite the sixth and seventh ribs. The =right extremity= forms a rounded cul-de-sac, which is in contact with the liver, omasum, and abomasum.

The =omasum= (Fig. 294) is ellipsoidal in form and somewhat compressed laterally. It is very clearly marked off from the other divisions. It is situated almost entirely to the right of the median plane, opposite the seventh to the eleventh ribs. The =parietal= (right) =surface= (Facies dextra) lies against the diaphragm, liver, and lateral wall of the abdomen. The contact with the latter extends over a small area only (ventral part of seventh to ninth intercostal spaces). The =visceral= (left) =surface= (Facies sinistra) is in contact with the rumen and reticulum. The =dorsal= (greater) =curvature= is convex, and is related to the diaphragm, liver, and vena cava. The =ventral= (lesser) =curvature= is concave, and rests on the abomasum chiefly, the junction with the latter appearing as a constriction near the middle of the curvature; at its anterior part there is a =neck= (Collum omasi) which is the connection with the reticulum.

The =abomasum= (Fig. 294) is an elongated pyriform or flask-shaped sac, which lies for the most part on the abdominal floor, to the right of the ventral sac of the rumen. Its narrow posterior part is strongly curved, the concavity being dorsal. The =parietal= (right) =surface= lies against the abdominal wall from the seventh to the twelfth intercostal space. The =visceral= (left) =surface= is related chiefly to the ventral sac of the rumen. The =dorsal= (lesser) =curvature= is concave, and is in contact with the omasum; the junction of the two sacs is indicated by a constriction about four or five inches (ca. 10 to 12 cm.) from the anterior extremity of the abomasum. The =ventral= (greater) =curvature= is convex, and rests on the abdominal wall from the xiphoid cartilage to the ventral part of the last (right) intercostal space. The =anterior extremity= forms a rounded blind sac which lies against the reticulum. The =posterior= (or pyloric) =extremity= is much smaller and bends upward and forward to join the duodenum. About six to eight inches (ca. 15 to 20 cm.) from the pylorus is a constriction which marks off the pyloric portion from the fundus.

The rumen is attached by peritoneum and connective tissue to the crura of the diaphragm and left psoas muscles, from the hiatus œsophageus backward to about the third or fourth lumbar vertebra.

The lesser omentum attaches the right face of the omasum and the pyloric portion of the abomasum to the visceral surface of the liver.

A small area of the anterior part of the right face of the rumen is adherent to the adjacent surface of the abomasum. The greater part of the ventral curvature of the omasum is attached by connective tissue to the dorsal face of the abomasum.

(Adhesion of the reticulum to the diaphragm is frequently present, but is pathological; the same is true in regard to adhesion to the liver.)

INTERIOR

The cavity of the rumen is partially divided into dorsal and ventral sacs by the =pillars=; these are folds of the wall, strengthened by additional muscular fibers, and correspond with the grooves on the outside. The two most extensive ones project like shelves into the anterior and posterior ends of the cavity forming the blind sacs at either extremity; they are termed the =chief pillars=. The =anterior chief pillar= (Pila cranialis) projects obliquely backward and upward from the ventral wall, and has a thick concave free edge which is opposite to the tenth and eleventh ribs. Its width from the middle of the free edge to its anterior border is about eight to ten inches (ca. 20 to 25 cm.). It is continued on either side by the relatively narrow =longitudinal pillars=, which connect it with the posterior pillar. The =posterior pillar= (Pila caudalis) is more nearly horizontal than the anterior one, and separates the large dorsal and ventral posterior blind sacs. Its concave free border is about a hand’s breadth in front of a transverse plane through the external angles of the ilia. From it are detached three =accessory pillars= on either side; of these, two pass around the blind sacs to meet the corresponding pillars of the opposite side. They thus mark off the posterior blind sacs from the general cavity, and are termed the =coronary pillars=. It will be noticed that the ventral coronary pillar is complete, while the dorsal one is not. The other accessory pillars join the right and left longitudinal pillars. The =right longitudinal pillar= is in part double; its ventral division fades out about the middle of the surface, while the dorsal one joins the posterior chief pillar. The distance between the middles of the anterior and posterior pillars is =only= about sixteen to eighteen inches (ca. 40 to 45 cm.) in a cow of medium size. In this space the dorsal and ventral sacs communicate freely.

The anterior =dorsal blind sac= of the rumen is separated from the reticulum by an almost vertical fold formed by the apposition of the walls of the two compartments. This =rumino-reticular fold= (Pila rumino-reticularis) is opposite to the seventh rib and intercostal space. Its free upper edge is concave and forms the lower and outer margin of the large, oval =rumino-reticular orifice= (Ostium ruminoreticulare). The inner margin of the fold if continued would end about at the cardia, but there is in this region no demarcation between rumen and reticulum; hence it is termed the =atrium ventriculi= or =vestibule=.

If the rumino-reticular fold and an imaginary completion of it be taken as the boundary between the rumen and reticulum, it will be seen that the cardia belongs to the latter. The outer border of the rumino-reticular fold ends above and behind the cardia, the inner one just behind the œsophageal groove and about four to five inches lower down. Undoubtedly food or water swallowed with a moderate degree of force passes first into the rumen. Foreign bodies (which are commonly swallowed by cattle) are, however, found in the reticulum.

The =cardiac orifice= is opposite the seventh rib, a little to the left of the median plane and about four inches (ca. 10 cm.) below the spine; it is not funnel-shaped.

The =mucous membrane= of the rumen is brown in color, except on the margins of the pillars, where it is pale. It is for the most part thickly studded with large papillæ, many of which are nearly half an inch (ca. 1 cm.) long. The edges of the chief pillars and a large part of the wall of the middle of the dorsal sac are, however, not papillated. The papillary arrangement is most marked in the blind sacs. The papillæ vary much in size and form; the largest are foliate, many are narrow or filiform, and others are conical or club-shaped. The mucous membrane on the inner wall of the atrium is finely wrinkled and non-papillated, while above and externally it is papillated. The papillary arrangement also extends over the edge of the rumino-reticular fold an inch or two (ca. 2 to 5 cm.).

The =œsophageal groove= (Sulcus œsophageus) is a semicanal which begins at the cardia and passes downward (ventrally) on the right wall of the reticulum to end at the reticulo-omasal orifice. It is about six or seven inches (ca. 15 to 18 cm.) in length. Its axial direction is practically vertical, but it is twisted in a spiral fashion; thus its thickened edges or lips project first backward, then to the left, and finally forward. The twist involves chiefly the left lip, and the relative positions of the lips is reversed at the ventral end.

The mucous membrane on the lips of the œsophageal groove is brown and wrinkled, like that of the atrium; in the bottom of the groove it is pale, like that of the œsophagus, marked by longitudinal folds, and presents pointed horny papillæ on the lower part.

The =reticulo-omasal orifice= (Ostium reticulo-omasicum) is situated in the right wall of the reticulum about five or six inches above the bottom of the latter. It is rounded, and is limited below and laterally by the junction of the lips of the œsophageal groove.

In the reticulum the mucous membrane is raised into folds about half an inch high, which inclose four-, five-, or six-sided spaces (Cellulæ reticuli); this peculiar arrangement suggested the scientific name, and also the popular term “honeycomb.” These cells are subdivided by smaller folds, and the bottoms are studded with pointed horny papillæ. The cells grow smaller and gradually disappear near the œsophageal groove and the edge of the rumino-reticular fold; an inch or two from the latter the mucous membrane has the papillary arrangement of the rumen. At the reticulo-omasal orifice there are peculiar horny papillæ, which are curved and resemble the claws of a small bird.

The cavity of the omasum is occupied to a considerable extent by about a hundred longitudinal folds, the =laminæ= (Laminæ omasi), which spring from the dorsal and lateral walls. The largest of these—a dozen or more in number—have a convex attached edge, and a ventral, free, slightly concave edge which reaches to within a short distance of the ventral wall of the sac. If these are drawn apart or a cross-section is made, it will be seen that there is a second order of shorter laminæ, and a third order still shorter; finally, there is a series of very low folds or lines. The food is pressed into thin layers in the narrow spaces between the laminæ (Recessus interlaminares), and reduced to a fine state of division by being ground down by the numerous rounded horny papillæ which stud the surfaces of the folds. The ventral wall of the omasum forms a groove (Sulcus omasi), which connects the reticulo-omasal opening with the omaso-abomasal opening; it is about three or four inches (ca. 8 to 10 cm.) long, and is directed backward and a little downward and to the right. It is free from leaves, but marked usually by slight folds and small papillæ; it forms a direct path from the reticulum to the abomasum for fluid and finely divided food. In the neck of the omasum there are thick folds, and a number of the peculiar papillæ already mentioned as occurring in the lower part of the œsophageal groove. The =omaso-abomasal orifice= (Ostium omaso-abomasicum) is oval, and is about four inches (ca. 10 cm.) long. It is bounded in front by a thick muscular pillar which fades out above on the lateral walls of the omasum. Laterally there are two folds (Valvulæ terminales), which probably prevent regurgitation of the contents of the abomasum.

The cavity of the abomasum is divided by a constriction into two areas. The first of these (fundus gland region) is lined with a soft glandular mucous membrane, which forms about a dozen large, slightly spiral folds (Plicæ spirales). The second part (pyloric region) is much narrower and resembles in appearance the corresponding region of the horse’s stomach. A small cardiac gland zone surrounds the omaso-abomasal orifice. The pyloric orifice is small and round.

=Structure.=—The =serous coat= invests all of the free surface of the stomach. The surface of the rumen which is attached to the dorsal abdominal wall is of course uncovered, as well as the adjacent area to which the spleen is attached. The furrows are bridged over by the peritoneum and superficial muscle-fibers, and contain fat and (in most cases) branches of the gastric arteries. The =lesser omentum= passes from the visceral surface of the liver to the right (parietal) face of the omasum and the pyloric portion of the abomasum. There is a fold between the ventral curvature of the omasum and the dorsal curvature of the abomasum. The =great omentum= is formed by the peritoneum leaving the stomach along the left longitudinal and posterior transverse furrows and the right surface of the rumen below the longitudinal furrow, also the ventral curvature of the abomasum. Its superficial (ventral) part is attached on the right to the duodenum, while the deep (dorsal) part blends with the mesentery. It conceals the greater part of the intestine on the right side, with the exception of the duodenum, and covers the ventral sac of the rumen almost entirely. It is not lace-like, as in the horse, and contains a large amount of fat in animals in good condition. The =epiploic foramen= (of Winslow) is almost sagittal in direction.

The =muscular coat of the rumen= consists of an external longitudinal, and a thicker internal circular layer. The latter forms the bulk of the chief pillars, where it is about one-half to one inch (ca. 1 to 2 cm.) thick. Scattered bundles of striped muscle-fibers radiate from the cardia in the wall of the atrium.

The =muscular coat of the reticulum= consists of two chief layers which begin and end at the œsophageal groove; they pass in a circular or oblique fashion around the sac, the fibers of the two layers crossing each other at an acute angle. The walls of the cells contain a central muscular layer. The lips of the œsophageal groove contain a thick layer of longitudinal fibers, which are largely continuous at or cross each other below the reticulo-omasal orifice. The bottom of the groove has two layers of oblique, unstriped muscle-fibers, with a variable outer layer of striped muscle continuous with that of the œsophagus.

The =muscular coat of the omasum= consists of a thin external longitudinal layer and a thick internal circular layer. At the omasal groove there is an additional inner layer of longitudinal fibers. The laminæ contain three muscular strata—a central layer of vertical fibers, flanked on either side by a layer of longitudinal fibers, which are continuous at the attached border with the muscularis mucosæ.

The =muscular coat of the abomasum= consists of longitudinal and circular layers; the latter forms a well-developed pyloric sphincter.

The =mucous membrane= of the first three divisions is destitute of glands, and is covered with a thick, stratified, squamous epithelium; the superficial part of the latter is horny, and is shed in large patches in the rumen and omasum. The =corium= (Tunica propria) is papillated. The mucous membrane of the abomasum is glandular, and corresponds to that of the right sac of the stomach of the horse. The short fundus glands occur in that part which presents the large folds, while the long pyloric glands are found in the remainder, except about the omaso-abomasal orifice, where cardiac glands occur. The mucosa of the fundus gland region is very thin as compared with that of the horse. There is a round prominence on the pyloric valve.

THE INTESTINE

The intestine of the ox lies almost entirely to the right of the median plane, chiefly in contact with the right face of the rumen. It is attached to the sublumbar region by a common mesentery.

The =small intestine= has an average length of about 130 feet (ca. 40 m.) and a diameter of about two inches (ca. 5 to 6 cm.).

The =duodenum= is about three or four feet (ca. 1 m.) in length. It passes forward to the visceral surface of the liver; here it forms, ventral to the right kidney, an =S=-shaped curve. Thence it runs backward almost to the external angle of the ilium, where it turns on itself, passes forward alongside of the terminal part of the colon, and joins the mesenteric part (jejunum) under the right kidney. It is attached to the liver by the lesser omentum, to the abomasum by the great omentum, and to the terminal part of the colon. The =bile-duct= opens in the ventral part of the =S=-shaped curve, about two feet (ca. 60 cm.) from the pylorus. The =pancreatic duct= opens about a foot (ca. 30 cm.) further back.

The remainder of the small intestine is arranged in numerous very close coils, in contact chiefly with the right face of the ventral sac of the rumen, forming a sort of festoon at the edge of the mesentery. It is not subject to much variation in position, but a few coils may find their way behind the blind sacs of the rumen to the left side.

The orifices of the pancreatic duct and the bile-duct are on papillæ, no diverticulum being present. There are permanent transverse folds of the mucous membrane (Plicæ circulares). Duodenal glands (of Brunner) occur in the first fifteen feet or more (4 to 5 m.), intestinal glands throughout. Peyer’s patches are larger and more distinct than in the horse, and vary greatly in size and number; in adult cattle there are eighteen to forty; in calves, twenty to fifty-eight. They usually have the form of narrow bands. Close to the ileo-cæcal valve there is always a patch, and in most cases there is also a patch on the cæcal side of the valve.

The =large intestine= is much smaller in caliber than that of the horse, has no bands, and is not sacculated. Most of it is situated between the layers of the common mesentery.

The =cæcum= is about 20 to 30 inches (ca. 50 to 60 cm.) long, 4 to 5 inches (ca. 10 to 12 cm.) wide, and has a capacity of about 2½ gallons (ca. 9 to 10 liters). It begins opposite the lower part of the eleventh rib, and extends upward and backward against the right flank; its rounded blind end is free, and lies near or in the pelvic inlet.

The =colon= is about 35 feet (ca. 10 m.) long; its diameter is at first about the same as that of the cæcum, but diminishes to about two inches (ca. 5 cm.). Its capacity is about 6 to 8 gallons (ca. 25 to 30 liters). With the exception of its first and terminal parts, it is arranged in double elliptical coils between the layers of the mesentery; the adjacent parts are therefore alternately centripetal and centrifugal (Gyri centripetales et centrifugales). The first part (Ansa proximalis) is marked off from the cæcum only by the ileo-cæcal opening; it forms an =S=-shaped curve and is continued by the spiral part (Ansa spiralis). The coils of this are attached to each other by connective tissue, and are best seen from the inner side. The bowel gradually diminishes in caliber, and the terminal part (Ansa distalis) leaves the spiral mass, passes forward to the great mesenteric artery and turns backward between the duodenum and the initial sigmoid part. From the ventral surface of the right kidney it passes backward, forms an =S=-shaped curve near the pelvic inlet, and joins the rectum; this part is attached to the sublumbar muscles by a short mesentery, and is also attached to the dorsal part of the duodenum.

The =rectum= is somewhat shorter than that of the horse, and is usually covered with peritoneum as far back as the first coccygeal vertebra. The retroperitoneal part is surrounded by a quantity of fat. The anus is not prominent.

The =serous coat= is of course absent on the adherent surfaces of the spiral part of the colon. There is a large amount of fat in the mesentery. The =longitudinal muscular coat= is evenly distributed, consequently there is no sacculation of the bowel. There is a valvular mucous fold at the ileo-cæcal orifice. A Peyer’s patch occurs in the beginning of the cæcum and one in the first part of the colon.

THE LIVER

The liver lies almost entirely to the right of the median plane. Its long axis is directed obliquely downward and forward, about parallel with the median plane, and corresponds to the curvature of the right portion of the diaphragm. It is less extensive, but thicker than that of the horse. Its average weight is about 10 to 12 pounds (ca. 4.5 to 5.5 kg.).

In small subjects the weight (according to Schmaltz) varies from 6½ to 10 pounds (ca. 3 to 4.5 kg.), in large subjects from 11 to 13 pounds (ca. 5 to 6 kg.). According to Schneider, the average weight is about 13 pounds (ca. 6 kg.)—a little over 1 per cent. of the live weight and about 2 per cent. of the dressed carcass.

When hardened in situ, its configuration adapts it accurately to the structures with which it is in contact.

The =parietal surface= is convex and is for the most part applied to the right part of the diaphragm, but a small part of it is in direct contact with the last two ribs and with the flank at the lumbo-costal angle. It faces upward, forward, and to the right.

The curvature of the parietal surface is not regular. It is marked, except in its ventral part, by a blunt oblique ridge which corresponds to the basal border of the right lung. This prominence divides the surface into two areas. Of these, the external one is directed outward, is only slightly convex, and often shows impressions of the last three ribs. The inner area presents a depression produced by the right crus of the diaphragm, and otherwise is regularly convex and adapted to the tendinous center and sternal portion of the diaphragm. The falciform ligament is attached to the surface from the œsophageal notch to the umbilical fissure.

The =visceral surface= is concave and very irregular; it presents impressions of the chief organs which are in contact with it—the omasum and reticulum. It is also related to the pancreas and duodenum.

The following markings are quite distinct on the visceral surface of well hardened specimens: (1) The omasal impression (Impressio omasica) is a deep central cavity below the portal fissure. (2) The reticular impression (Impressio reticularis) is a smaller marginal depression below the œsophageal notch and the inner part of the preceding, from which it is separated by a rounded ridge. (3) The abomasal impression (Impressio abomasica), present only in the calf, is produced by the anterior part of the abomasum. It lies along the ventral part of the surface, and is separated by ridges from the preceding impressions. It disappears as the omasum and reticulum increase in size and displace the abomasum from contact with the liver. (4) Shallow grooves for the =S=-shaped portion of the duodenum (Impressio duodenalis) may be found above and external to the portal fissure. (5) The fossa of the gall-bladder (Fossa vesicæ felleæ) is distinct when that organ is full. In the calf the omasal impression is small, and the abomasal one large in correspondence with the relative sizes of these two sacs.

The =portal fissure= is a well defined rounded depression, situated dorsal to the omasal impression. It contains, besides the vessels and duct, several large hepatic lymph glands. Above and external to it a part of the pancreas is attached.

The =dorsal border= is short and thick; it extends backward a short distance beyond the upper part of the last rib. It presents the large, thick, quadrilateral =caudate lobe=, and a deep depression for the right kidney and adrenal.

The =ventral border= is short and thin and has no interlobar incisures.

The =right= (or external) =border= is marked by a small umbilical fissure, in which the ligamentum teres is attached in the young subject.

The =left border= presents the œsophageal notch below its middle. Above this it is practically median in position, and lodges the posterior vena cava, which is partially embedded in the gland. Below the notch the border extends an inch or two (ca. 2.5 to 5 cm.) to the left of the median plane opposite the ventral third of the sixth rib.

There is no left lateral ligament. The falciform ligament is usually present, but the ligamentum teres is found only in young subjects. The lesser omentum leaves the liver along a line extending from the œsophageal notch to the portal fissure. The only distinct lobe in the adult is the caudate (Processus caudatus).

A =gall-bladder= (Vesica fellea) is present. This is a pear-shaped sac, four to six inches (ca. 10 to 15 cm.) long, which lies partly on the visceral surface of the liver (to which it is attached), but largely on the abdominal wall at the lower part of the eleventh or twelfth intercostal space. It may be regarded as a diverticulum of the bile-duct, enlarged to form a reservoir for the bile. Its neck is continued by the =cystic duct= (Ductus cysticus), which joins the hepatic duct at an acute angle just outside of the portal fissure, to form with it the bile-duct (Ductus choledochus). The latter is short and enters the second bend of the =S=-shaped curve of the duodenum, i. e., about two feet (ca. 60 cm.) from the pylorus. Several small ducts (Ductus hepato-cystici) open directly into the gall-bladder.

In the new-born calf the liver is relatively much larger than in the adult. The visceral surface presents, below the portal fissure, a large rounded eminence, which is caused by the presence in the underlying gland substance of a large venous sinus into which the umbilical and portal veins empty. A large vessel, the =ductus venosus=, leads from this sinus directly to the posterior vena cava. Just external to this elevation is a small but distinct lobe about two inches long (Processus papillaris?). The umbilical fissure is deep and partially divides the gland into two chief lobes.

THE PANCREAS

The pancreas of the ox is irregularly quadrilateral in form, and lies almost entirely to the right of the median plane. Its weight is about the same as that of the horse. Its dorsal surface is related to the crura of the diaphragm, the cœliac and anterior mesenteric arteries, and the liver. It is attached to the liver at and external to the portal fissure, and to the crura of the diaphragm. Between these adhesions it is free and forms the ventral wall of the epiploic foramen (of Winslow). On the right side it extends backward beyond the caudate lobe of the liver between the layers of the mesoduodenum; here it is in contact with the upper part of the flank at the lumbo-costal angle, and is related above to the right kidney and by its ventro-lateral border to the ventral part of the duodenum. The gastro-intestinal or ventral surface is in contact with the dorsal curvature of the rumen and the intestine. The portal vein passes through the gland. The left extremity is small; the right or posterior is wide and thin and is often divided into two branches. The duct leaves the posterior part of the ventro-lateral (or right) border and enters the duodenum about twelve inches (ca. 30 cm.) further back than the bile-duct.

THE SPLEEN

The spleen has an elongated, elliptical outline, both extremities being thin, rounded, and similar in size.

Its average weight is about 30 ounces (ca. 800 to 900 gm.), or about ⅙ per cent. of the body-weight. Its length is about 16 to 20 inches (ca. 40 to 50 cm.), its width about 5 to 6 inches (ca. 12 to 15 cm.), and in the middle its thickness is about an inch (ca. 2 to 3 cm.).

The =dorsal extremity= lies under the upper ends of the last two ribs; the =ventral extremity= is usually opposite the seventh or eighth rib, about a handbreadth above its junction with the cartilage. The =parietal surface= is convex, and is related to the diaphragm. The =visceral surface= is concave, and is related chiefly to the left face of the rumen, but also usually to a narrow adjacent area of the reticulum. The dorsal part is attached to the left crus of the diaphragm and the left surface of the rumen by peritoneum and connective tissue; the ventral part is free. The hilus is situated on the dorsal third of the visceral surface, near the anterior border.

About one-half of the visceral surface of the spleen is attached directly to the rumen and is not covered by peritoneum; the line of reflection of the latter crosses the surface obliquely, from the upper part of the posterior border to the anterior border a little below the middle. Similarly there is a narrow uncovered area on the upper part of the parietal surface along the anterior border. The hilus is not a groove, but a simple depression. When hardened in situ, the organ is seen to be somewhat twisted, so that the upper part of the parietal surface faces upward and forward, while below it is directed outward.

DIGESTIVE SYSTEM OF THE SHEEP

The =lips= are thin and mobile; the upper one is marked by a very distinct philtrum, and otherwise is not bare.

The anterior part of the =hard palate= is prominent and smooth; on the posterior part of this area there are two narrow depressions in the form of a =V=, at the deep anterior ends of which the naso-palatine ducts open. The ridges of the palate are irregular and their edges are smooth. The posterior third or rather more of the palate is not ridged and presents numerous orifices of ducts of the palatine glands. The mucous membrane is often more or less pigmented.

The mucous membrane of the =cheeks= is covered with large papillæ, many of which are long and sharp-pointed, while others are short and blunt. There is also a series of conical papillæ on the floor of the mouth under the lateral part of the tongue.

The =tongue= resembles that of the ox in form, but the tip is comparatively smooth; the papillæ here are very numerous, but short and blunt. This difference is in conformity with the dissimilarity in the mode of prehension. The prominence of the dorsum is commonly not so pronounced nor so sharply marked off in front as in the ox. The root is smooth. The mucous membrane of the dorsum is often pigmented in spots.

The dental formula is the same as that of the ox. The =incisor teeth= form a narrow and strongly curved arch. The crowns are long and narrow; their labial surfaces are strongly convex and end at a sharp edge which is used in cropping the grass. The roots are more firmly embedded than in the ox.

The =cheek teeth= resemble those of the ox. There is a thinner layer of cement, which is usually black.

The average periods of eruption of the teeth are indicated in the subjoined table.

TEETH TEMPORARY PERMANENT I_{1}. At birth or first 1 to 1½ years week I_{2} First or second week 1½ to 2 years I_{3} Second or third week 2½ to 3 years I_{4} Third or fourth week 3½ to 4 years ────────────────────────────────────────────────────────────── P_{1} 2 to 6 weeks. 1½ to 2 years P_{2} „ „ P_{3} „ „ ────────────────────────────────────────────────────────────── M_{1} 3 months (lower), 5 months (upper) M_{2} 9 to 12 months M_{3} 1½ to 2 years

The =salivary glands= resemble those of the ox in general, but the parotid duct crosses the surface of the masseter muscle, and opens opposite the third or fourth cheek tooth.

The =tonsil= is bean-shaped and about half an inch (ca. 12 mm.) in length. It does not project into the fauces. The mucous membrane of the latter presents two deep and very narrow tonsillar sinuses on either side.

The fornix of the =pharynx= presents a median plicated fold which is a continuation of the septum nasi. The pharyngeal orifice of the Eustachian tube has the form of a crescentic slit, placed about on a level with the inferior nasal meatus. There are two large lymph glands and a number of hæmolymph glands above the pharynx.

The =œsophagus= has a lumen of about an inch when moderately distended; otherwise it resembles that of the ox.

The =stomach= is like that of the ox in its general arrangement. Its average capacity is about four gallons (ca. 15 liters). The =cardiac opening= is opposite the middle of the eighth intercostal space; it is just to the left of the median plane and about two to three inches (ca. 5 to 7 cm.) below the vertebral column. The ventral sac of the =rumen= is relatively larger and extends further to the right of the median plane than in the ox. There are no coronary pillars in the dorsal sac, but its upper part is papillated. The papillæ of the rumen are relatively =large= and somewhat tongue-like; the largest are about a fourth of an inch (ca. 6 mm.) in length. The =reticulum= is relatively larger than in the ox. Its ventral part curves more backward and less to the right than in the ox. The parietal surface extends forward as far as the sixth rib or intercostal space, and is related to the diaphragm and liver. The fundus lies on the sternal part of the diaphragm and in contact with the abomasum. The =œsophageal groove= is disposed much like that of the ox, and is about four to five inches (ca. 8 to 10 cm.) long. The =omasum= is much smaller than the reticulum, its capacity being only about one pint. It is oval and compressed laterally. It is situated almost entirely to the right of the median plane, opposite to the ninth and tenth ribs, higher than in the ox, and has no contact with the abdominal wall. It is related on the right to the liver and gall-bladder, on the left to the rumen, and below to the abomasum. The laminæ are less numerous than in the ox; in the neck which connects the omasum with the reticulum they have the form of low thick ridges, and bear long, pointed, horny papillæ. The =abomasum= is relatively larger and longer than in the ox. Its capacity is about twice that of the reticulum and averages about two quarts (ca. 1.75 to 2 liters). The blind anterior end lies almost centrally in the xiphoid region, in relation to the liver and reticulum. Its body extends backward a little below and almost parallel with the right costal arch. The pylorus is opposite the ventral end of the eleventh or twelfth intercostal space. The cells of the reticulum are much shallower than in the ox. At the reticulo-omasal opening and on the adjacent part of the laminæ of the omasum are large, pointed, horny papillæ. The number of laminæ is less than in the ox.

The general arrangement of the =intestine= is like that of the ox. The =small intestine= is about 80 feet (ca. 24 to 25 m.) long; its average diameter is about an inch (ca. 2 to 3 cm.), the caliber increasing in its terminal part, where a very extensive Peyer’s patch is found. The =cæcum= is about 10 to 12 inches (ca. 25 to 30 cm.) long, 2 inches (ca. 5 cm.) wide, and has a capacity of about a quart (ca. 1 liter). The =colon= is about 15 feet (ca. 4 to 5 m.) long. Its caliber is at first about the same as that of the cæcum, but diminishes to about the width of the small intestine.

The =liver= weighs about 20 to 25 ounces (ca. 550 to 700 gm.). It lies entirely to the right of the median plane. The parietal surface is related almost exclusively to the right part of the diaphragm. The visceral surface presents extensive reticular and abomasal impressions, and a small omasal impression internal to the portal fissure. The umbilical fissure is deep and partially divides the gland into two chief lobes. The caudate lobe is prismatic and blunt-pointed. The neck of the gall-bladder is long. The bile-duct joins the pancreatic duct to form a =common duct= (Ductus choledochus communis) which opens into the duodenum about one foot (ca. 30 cm.) from the pylorus.

The =pancreas= is arranged as in the ox. Its duct unites with the bile-duct.

The =spleen= (Fig. 301) is somewhat triangular, with the angles rounded off. It weighs about three or four ounces (ca. 100 grams). Its length is about five inches (ca. 12 to 13 cm.) and its greatest width about four inches (ca. 10 cm.). The parietal surface is convex and is related to the diaphragm, to which rather more than the anterior third is adherent. The visceral surface is concave and its anterior half is attached to the dorsal curvature of the rumen. The dorsal end or base is attached to the left crus of the diaphragm under the last two or three ribs. The hilus is at the posterior basal angle. The ventral end is narrower and thinner than the base; it is situated opposite the ninth and tenth ribs above their middles.

DIGESTIVE SYSTEM OF THE PIG

THE MOUTH

The =rima oris= is extensive, the angles of the mouth being situated far back. The =upper lip= is thick and short, and is blended with the snout; the =lower lip= is small and pointed. The labial glands are few and small.

The mucous membrane of the =cheeks= is smooth. The =buccal glands= are compactly arranged in two rows opposite the cheek teeth. The =parotid duct= opens opposite the fourth or fifth cheek tooth.

The =hard palate= is long and narrow; it is marked by a median furrow, on each side of which are twenty or more ridges. On its anterior part there is a triangular incisive papilla, at which the naso-palatine ducts open.

The =soft palate= is very thick. Its direction almost continues that of the hard palate, i. e., it is nearly horizontal. It extends to the middle of the oral surface of the epiglottis.[110] It has a median prolongation termed the =uvula=. The oral surface presents a median furrow, on either side of which is an oval raised area, marked by numerous crypts; these elevations are the =tonsils=. Tonsillar tissue also occurs in the lateral walls of the fauces.

The =tongue= is long and narrow and the apex is thin. Two or three vallate papillæ are present. The fungiform papillæ are small and are most numerous laterally. The filiform papillæ are soft and very small. On the root there are soft, long, pointed papillæ, directed backward. Foliate papillæ are also present.

There is a well-marked median glosso-epiglottic fold, on either side of which is a depression (Vallecula epiglottica). The frenum linguæ is double.

The =dental formula= of the pig is:

( 3 1 4 3 ) 2 ( I - C - P - M - ) = 44 ( 3 1 4 3 )

The formula for the temporary teeth is:

( 3 1 4 ) 2 ( Di - Dc - Dp - ) = 32 ( 3 1 4 )

The =upper incisors= are small; they are separated from each other by spaces, and from the canines by a larger interval. The first incisors are the largest, are convergent, and have no distinct neck. The second are narrower and somewhat smaller. The third (corner) incisors are much smaller, are flattened laterally, and have three small eminences on the crown.

The =lower incisors= are almost horizontal and are convergent and close together. The first and second are about equal in size, prismatic, and deeply implanted in the jaw. The labial surface is slightly convex, the lingual concave and marked near its extremity by a ridge. The third incisor is much smaller and has a short narrow crown and a distinct neck.

The =canine teeth= or tusks of the male are greatly developed and project out of the mouth. The upper canine of the boar may be about three or four inches (ca. 8 to 10 cm.) long. The crown is conical and is curved somewhat backward and outward; the embedded part is curved and has a large pulp cavity.[111] The lower canine may reach a length of eight inches or more (ca. 20 cm.); the crown is prismatic and is curved backward and outward in front of the upper one, so that the friction between the two keeps a sharp edge on the lower tooth. In the sow they are much smaller.

The average periods of =eruption= of the teeth are given in the subjoined table.

TOOTH│ ERUPTION │ CHANGE │ │ I_{1}│2 to 4 weeks │12 months I_{2}│upper 2 to 3 months│ │lower 1½ to 2 month│16 to 20 months I_{3}│Before birth │8 to 10 months C │Before birth │9 to 10 months P_{1}│5 months │ ─────┼───────────────────┼─────────────────── P_{2}│5 to 7 weeks │12 to 15 months P_{3}│upper 4 to 8 days │ „ │lower 2 to 4 weeks.│ P_{4}│upper 4 to 8 days │ „ │lower 2 to 4 weeks │ ─────┼───────────────────┼─────────────────── M_{1}│4 to 6 months │ M_{2}│8 to 12 months │ M_{3}│18 to 20 months │

The =cheek teeth= increase in size from before backward. They have (with certain exceptions noted below) complex tuberculate crowns; the term bunodont is applied to this condition, as distinguished from the selenodont structure in horses and cattle, in which there are prominent ridges of enamel. They are short-crowned teeth with a distinct neck and root. The first tooth in each jaw is small, simple, and appears only once; in the lower jaw it is near the canine, in the upper near the second cheek tooth. The next two teeth are larger, laterally compressed, and sectorial. The fourth tooth below is larger, but otherwise like the preceding ones, while the upper one is much wider and is tuberculate.

The =parotid gland= is large and triradiate. It extends very little on to the masseter muscle and its upper angle does not quite reach the base of the ear. It is pale in color, and is embedded in fat in animals in good condition. On its deep face are several large subparotid lymph glands, some of which are only partially covered by the parotid. The =duct= arises on the deep face, has a course similar to that of the ox, and perforates the cheek opposite the fourth or fifth upper cheek tooth. Small accessory glands (Glandulæ parotideæ accessoriæ) may be found along the course of the duct.

The =submaxillary gland= is small, reddish in color, and oval in outline; it is covered by the parotid. Its superficial face is convex, and is marked by rounded prominences. From its deep face a narrow process extends forward about two or three inches (ca. 5 to 7.5 cm.) beneath the mylo-hyoideus muscle along with the duct. The latter opens near the frenum linguæ, but there is no papilla.

The =sublingual gland= has an arrangement similar to that of the ox. The posterior part (Glandula sublingualis grandicanalaris) is reddish-yellow in color, and is about two inches (ca. 5 cm.) long, and half an inch wide; its posterior end is in relation to the submaxillary gland and its duct. The anterior part is much larger, being two or three inches (ca. 5 to 7 cm.) long and about twice the width and thickness of the posterior part. All or most of the ducts from the posterior part unite to form the =ductus sublingualis major=, which opens near the ductus submaxillaris. Eight or ten =ductus sublinguales minores= convey the secretion from the anterior part through the floor of the mouth.

THE PHARYNX

The =pharynx= presents in its posterior part a median cul-de-sac about an inch and a half (ca. 3 to 4 cm.) long, which is situated between the ventral straight muscles of the head and the origin of the œsophagus. Its lower margin is formed by the junction of the posterior pillars of the soft palate, which contain muscular tissue derived from the palatinus and palato-pharyngeus. The vault of the pharynx (Fornix pharyngis) is divided by a median fold of mucous membrane which is a direct continuation of the septum nasi. On either side of this is an infundibulum in which the Eustachian tube opens.

THE ŒSOPHAGUS

The =œsophagus= is short and nearly straight. It has (according to Rubeli) a potential caliber of nearly 3 inches (ca. 7 cm.) at either end, and about 1⁷⁄₁₀ inches (ca. 4.2 cm.) in its middle part.[112] The muscular coat, except near the cardia, is striated. There are mucous glands in the submucosa to about the middle of the tube.

THE STOMACH

The =stomach= is large and pyriform. Its average capacity is about 1½ to 2 gallons (ca. 5.7 to 8 liters). When full it extends backward to the last left intercostal space laterally and the umbilicus ventrally. The left portion is large and rounded, while the right portion (Pars pylorica) is small, and bends sharply upward to join the small intestine. The parietal surface is related to the liver, gall-bladder, and diaphragm. The visceral surface is related to the intestine, the mesentery, and the pancreas. The great curvature is related to the diaphragm, spleen, and abdominal floor. The pyloric end lies on the right lateral lobe of the liver. The left extremity presents a flattened conical blind pouch, the =diverticulum ventriculi=, the apex of which projects backward. The œsophagus joins the stomach very obliquely, almost in the median plane, and about three or four inches (ca. 8 to 10 cm.) below the twelfth thoracic vertebra. The cardiac opening is slit-like and is bounded above and to the left by a fold which contains a thickening of the internal oblique layer of the muscular coat. The opening into the diverticulum is situated above and a little to the left of the cardia; it is transversely oval, and is bounded (except externally) by a thick fold which contains spirally arranged muscular fibers. The mucous membrane may be divided into four regions. Over a quadrilateral area around the cardia (about one inch on the right and two or three inches on the left side of the orifice) it is œsophageal in character, and presents a number of folds. A sharp line of demarcation separates this from the rest of the mucous membrane, which is soft and glandular. The second or cardiac gland region is pale gray in color and thin (ca. 0.5 to 1 mm.); it extends about to the middle of the stomach. The third or fundus gland region is readily distinguished by its thickness (ca. 3 mm.) and its brownish-red mottled appearance.[113] The fourth or pyloric region is pale, thinner than the preceding, and presents a number of irregular folds. At the pylorus there is a remarkable ridge which projects from the wall of the lesser curvature and diminishes considerably the size of the orifice. It is about an inch and a half (ca. 3 to 4 cm.) long and nearly half an inch (ca. 1 cm.) high. Sometimes it is grooved; in other cases it has the form of a rounded eminence attached by a pedicle to the wall. Fibers from the circular muscular coat extend into it.

THE INTESTINE

The =small intestine= is 50 to 65 feet (ca. 15 to 20 m.) long. The mesentery of about the first two feet (ca. 60 cm.) is two to two and a half inches (ca. 5 to 6 cm.) long; this part may be termed duodenum. The remainder (Jejuno-ileum) has a mesentery about six to eight inches (ca. 15 to 20 cm.) long, which is thick and contains a quantity of fat, and numerous large lymph glands at its root; the root is attached in the sublumbar region behind the stomach and blends here with the mesentery of the large intestine. The small intestine is arranged in close coils and lies mainly on the left side and floor of the abdomen, from the stomach to the pelvis; some coils, however, lie against the right flank. The opening of the =bile-duct= is about one or two inches (ca. 2.5 to 5 cm.) from the pylorus, and that of the =pancreatic duct= about four or five inches (ca. 10 to 15 cm.) beyond it. =Peyer’s patches= and =solitary glands= are numerous and very distinct; the patches are usually band-like and prominent; the solitary glands form projecting nodules marked by crypts.

The =large intestine= is about 15 feet (ca. 4 to 5 meters) in length, and is for the most part much wider than the small intestine; it is connected by a mesentery with the dorsal abdominal wall between the kidneys. The =cæcum= is cylindrical, about 8 to 12 inches (ca. 20 to 30 cm.) long, and 3 to 4 inches (ca. 8 to 10 cm.) wide. It lies against the upper part of the right flank, its blind end reaching to within about two inches (ca. 5 cm.) of the pelvic inlet. The ileum joins the cæcum obliquely and projects somewhat into its lumen. The =colon= has at first about the same caliber as the cæcum, but becomes gradually smaller. Most of it is arranged in three close, double spiral coils in the mesentery, in relation chiefly to the right flank externally, and to the small intestine behind and to the left. On emerging from this spiral labyrinth, it passes forward to the stomach and pancreas and then turns backward; this terminal part is narrow, median in position, and closely attached by a short mesentery to the sublumbar region. The =rectum= is usually surrounded by a quantity of fat. The cæcum has three longitudinal muscular bands and three rows of sacculations, which are continued a short distance on the colon. The spiral colon has two bands and two series of sacculations, which, however, gradually disappear in the centrifugal part.

THE LIVER

The liver is relatively large, its average weight being about 4 to 4½ pounds (ca. 2 kg.). It is thick and very strongly curved.[114] It is divided by three deep interlobar incisures into four principal =lobes=—=right lateral=, =right central=, =left central=, =left lateral=; the last of these is usually considerably the largest. On the upper part of the right lateral lobe is the =caudate lobe= and its process. That part of the right central lobe which lies below the portal fissure and to the left of the gall-bladder and cystic duct is homologous with the quadrate lobe of man. The parietal surface is extremely convex, its most anterior part reaching to a plane through the lower part of the sixth or seventh intercostal space. The visceral surface is deeply concave; most of it is related to the stomach, for which there is a correspondingly large and deep gastric impression. There is a duodenal impression on the upper part of the right lateral lobe, but no renal impression, as the right kidney does not touch the liver. The fossa for the gall-bladder is mainly on the right central lobe, but also in part on the adjacent surface of the left central lobe. The posterior vena cava enters the dorsal border of the caudate lobe and soon becomes entirely embedded in the gland substance, emerging only at its passage through the diaphragm. The œsophageal notch is large and is occupied mainly by the large right crus of the diaphragm. The right lateral border extends backward to the upper part of the twelfth intercostal space or thirteenth rib. The left lateral border is opposite the eighth rib and intercostal space. The ventral border extends backward about as far as the umbilicus.

There are no lateral ligaments and the falciform ligament disappears early.

The =gall-bladder= is attached in the fossa vesicæ felleæ on the right central lobe, at a considerable distance from the ventral border. The cystic duct joins the hepatic duct at an acute angle immediately after the emergence of the latter from the portal fissure. The =bile-duct= (Ductus choledochus) opens at the papilla duodeni about one or two inches (ca. 2.5 to 5 cm.) from the pylorus.

Owing to the large amount of interlobular tissue, the =lobules= are mapped out sharply; they are polyhedral in form and are 1 to 2.5 mm. in diameter. For the same reason the gland tissue is much less friable than that of the other animals, from which it is easily distinguished.

THE PANCREAS

The =pancreas= is elongated, and is situated transversely on the dorsal abdominal wall, with its right extremity on the ventral surface of the right kidney, and the left end touching the dorsal end of the spleen and the left kidney. The right portion has two lobes; the posterior lobe bends around the great mesenteric artery and the portal vein; the anterior one is pointed, extends downward and forward about in the median plane, and is attached to the portal fissure and the duodenum. The left portion is prismatic and rests mainly on the left sac of the stomach, its left extremity being included between the left kidney and the dorsal end of the spleen. The pancreatic =duct= passes from the anterior lobe directly through the duodenal wall, opening about five or six inches (ca. 12.5 to 15 cm.) from the pylorus. The interlobular tissue usually contains a good deal of fat.

THE SPLEEN

The =spleen= is long and narrow. Its long axis is nearly dorso-ventral in direction, and is slightly curved to conform to the left part of the greater curvature of the stomach. The dorsal end lies under the vertebral ends of the last three ribs. The ventral end is smaller, reaches to the ventral abdominal wall, and lies against the left lobe of the liver. The visceral surface has a longitudinal ridge on which the hilus is situated; this divides the surface into nearly equal gastric and intestinal areas. The spleen is attached so loosely to the stomach that it may be regarded as being intercalated in the great omentum. In large subjects it may reach a length of 15 to 18 inches (ca. 38 to 45 cm.) and a width of 3 to 4 inches (ca. 8 to 10 cm.).

DIGESTIVE SYSTEM OF THE DOG

THE MOUTH

The size and form of the mouth vary greatly in different breeds, the cavity being in some long and narrow, in others short and wide. The =rima oris= is very extensive, so that the labial commissure is opposite the third or fourth cheek tooth. The =lips= are thin and mobile, and present numerous tactile hairs. The upper lip has a small, central, bare area which forms part of the muzzle, and is marked by a central groove, the philtrum, or (as in the bulldog) a fissure, giving the appearance of harelip. The lateral borders of the lower lip are flaccid and denticulated. The mucous membrane is usually pigmented and forms distinct frena labiorum. The labial glands are small and scanty.

The =cheeks= are loose and capacious, and their mucous lining is smooth and more or less pigmented. The =parotid duct= opens usually opposite the third upper cheek tooth. Near the last tooth are the openings of the four or five ducts from the orbital gland.

The =hard palate= is widest between the fourth pair of cheek teeth. It has eight to ten curved ridges on either side of the raphé. Behind the first pair of incisor teeth is the triangular incisive papilla, at which the naso-palatine ducts open. The mucous membrane is usually pigmented.

The =soft palate= is thick except at its margins. In the resting state of the parts it comes in contact with the oral surface of the epiglottis. Between its anterior and posterior pillars on either side is a marked tonsillar sinus, in which an elongated =tonsil= is situated; this is reddish in color, about an inch long, and projects somewhat into the fauces between two folds of mucous membrane.

The =tongue= is wide, thin, and very mobile. It is not pigmented, but has a bright red color. The upper surface is marked by a median groove (Sulcus medianus linguæ), and is thickly beset with short, pointed, filiform papillæ, the free ends of which are directed backward. On the root the papillæ are long and soft; similar papillæ occur on the lateral walls of the isthmus faucium. The fungiform papillæ are small, and are scattered over the dorsum and sides of the tongue, but are absent on the root. There are usually two or three vallate papillæ on either side. Small foliate papillæ are also present. In the inferior part of the tip of the tongue is the =lyssa=, a fusiform cord, composed of fibrous tissue, muscular tissue, and fat. In large dogs it is about two inches (ca. 4 to 5 cm.) long.

THE TEETH

The formula for the permanent teeth is:

( 3 1 4 2 ) 2 ( I - C - P - M - ) = 42 ( 3 1 4 3 )

All of the teeth have short crowns and distinct necks; they erupt rapidly. The crowns are white, being destitute of cement.

The =incisors= are placed almost vertically and close together in the jaw-bones. They do not correspond to an opposing tooth, but rather to parts of two teeth of the other jaw. The crowns are trituberculate, the central projection being the largest. They increase in size from the first to the third. The labial surface is convex, the lingual slightly concave, and marked off from the neck by a =V=-shaped ridge, the cingulum. The roots are flattened transversely. The lower incisors are smaller than the upper ones. One or two supernumerary teeth may be present.

The =canine teeth= are large, conical, and curved. The upper canine is separated from the corner incisor by an interval into which the lower canine is received when the jaws are closed.[115] The lower canine is close to the corner incisor. The root is about an inch (ca. 2 to 3 cm.) long and is flattened laterally.

The =cheek teeth= are typically ⁶⁄₇, but in brachycephalic breeds they are commonly reduced to ⁵⁄₇, and in extreme cases even to ⅘. The reduction occurs at either end or at both ends of the series.[116] The first tooth appears only once. The fourth tooth of the upper row and the fifth of the lower row are much larger than the rest, and termed =sectorial= or =carnassial= teeth. From these the teeth diminish in size both forward and backward. The upper and lower teeth do not correspond, but rather dovetail. The teeth behind the sectorial ones are tuberculate, i. e., have rounded eminences on the masticatory surface. The others are all sectorial in character, i. e., have sharp-edged, pointed projections, the middle one being the most prominent. The premolars are laterally compressed, and are separated by intervals from the canines and from each other, except in the brachycephalic breeds. The upper molars have wide, somewhat quadrangular crowns, and three roots. The crown of the upper fourth premolar (Dens sectorius) is divided into two pointed lobes and has an antero-internal tubercle; it has three roots. The crown of the first lower molar (Dens sectorius) is compressed laterally and has two pointed, sharp-edged lobes, behind which are one or two tubercles; it has two roots.

The average periods of eruption are given below.

Tooth Eruption │ Change │ I_{1} 4 to 5 weeks │4 to 5 months I_{2} 4 to 5 weeks │ „ I_{3} 4 weeks │ „ C 3 to 4 weeks │4 to 5 months P_{1} 4 to 5 months│5 to 6 months P_{2} 4 to 5 weeks │ „ P_{3} 3 to 4 weeks │ „ P_{4} 3 to 4 weeks │ „ M_{1} 4 months │ upper 5 to 6 │ M_{2} months │ lower 4 ½ to │ 5 months │ M_{3} 6 to 7 months│

THE SALIVARY GLANDS

The =parotid gland= is small. Its upper part is wide and is divided into two lobes by a deep notch into which the base of the ear is received. The lower end is small and overlaps the submaxillary gland. The =duct= leaves the gland at its anterior border, crosses the masseter muscle, and opens into the mouth opposite the third upper cheek tooth. Small accessory glands (Glandulæ parotidæ accessoriæ) are sometimes found along the course of the duct.

The =submaxillary gland= is usually larger than the parotid. In large dogs it is about two inches (ca. 5 cm.) long and an inch or more (ca. 3 cm.) wide. It is rounded in outline and of a pale yellow color. Its upper part is covered by the parotid, but it is otherwise superficial, and is palpable in the angle of junction of the jugular and external maxillary veins. The =duct= leaves the deep face of the gland, passes along the surface of the digastricus and stylo-glossus, and opens into the mouth near the frenum linguæ on a very indistinct papilla.

The =sublingual gland= is divided into two parts. The posterior part (Glandula sublingualis grandicanalaris) lies on the digastricus muscle in intimate relation with the submaxillary gland, but clearly separable from it after removal of the common fibrous capsule. It has a pointed anterior process. The =duct= (Ductus sublingualis major) accompanies the submaxillary duct, and usually opens beside it, but in some cases joins it. The anterior part (Glandula sublingualis parvicanalaris) is long and narrow; it lies on the stylo-glossus muscle. It has a number (8 to 12) of small ducts (Ductus sublinguales minores), some of which open directly into the mouth, while others join the large duct.

The =orbital= or =zygomatic gland= (Glandula zygomatica) represents the superior molar glands of herbivora. It is prismatic and is situated in the orbital region, between the zygomatic arch and the internal pterygoid muscle, in contact with the periorbita. It has four or five =ducts= (Ductus zygomatici) which open near the last upper cheek tooth; one of them (canal of Nuck) is almost as large as the parotid duct; the others are small.

THE PHARYNX

The fornix is narrow. The orifices of the Eustachian tubes are small and slit-like; the end of the tube causes a rounded projection of the mucous membrane (Torus tubarius). The œsophageal opening is relatively small and is encircled by a fold of the mucous membrane. The hyo-pharyngeus muscle is clearly divided into a kerato-pharyngeus and a chondro-pharyngeus.

THE ŒSOPHAGUS

The =œsophagus= is relatively wide and dilatable except at its origin. The constriction at its origin is termed the =isthmus œsophagi=. The cardiac end is slightly dilated; it joins the stomach under the tenth or eleventh thoracic vertebra, slightly to the left of the median plane. The muscular tissue is striated and consists mainly of two layers of spiral fibers which cross each other; near the cardia, however, the fibers are longitudinal and circular. There are mucous glands and lymph glands in the submucosa throughout.

THE STOMACH

The =stomach= is relatively large. Its capacity in a dog weighing about 40 pounds is about six to seven pints.

Colin estimates the average capacity at about 3 liters (ca. 6½ pints), with a range between 0.6 and 8 liters (ca. 1⅓ to 17½ pints). Neumayer gives the capacity as 100 to 250 c.c. per kilogram of body-weight (ca. 2.7 ounces per pound). The average capacity of the human stomach is estimated at 35 to 40 ounces—only about one-half of that of a dog of medium size.

When full it is irregularly pyriform. The left or cardiac part (Corpus ventriculi) is large and rounded, while the right or pyloric part (Pars pylorica) is small and cylindrical. When empty, or nearly so, the left sac is strongly contracted; the pyloric part is not materially affected by variations in the amount of ingesta.

The parietal surface of the full stomach is very extensive, strongly convex, and faces partly forward, but largely to the left. It is related to the liver, the left part of the diaphragm, and the left and ventral abdominal wall as far back as a transverse section through the second or third lumbar vertebra.

The visceral surface is much less extensive and is considerably flattened; it faces chiefly to the right, and is related to the intestine, pancreas, and left kidney.

The upper part of the lesser curvature is nearly straight and vertical, but the lower part forms a deep, narrow angle, due to the fact that the pyloric part is directed sharply forward and upward.

The greater curvature is nearly four times as long as the lesser curvature. In the full stomach it extends considerably behind the left costal arch (having carried the spleen backward); ventrally, it lies on the abdominal wall, almost midway from the xiphoid cartilage to the pubis.

The left extremity or fundus is large and rounded; it is the most dorsal part of the organ, and lies under the dorsal ends of the eleventh and twelfth ribs.

The pyloric extremity is small and is directed forward and somewhat upward; it lies usually about an inch or two (ca. 3.5 to 5 cm.) below the cardia, a little to the right of the median plane. It is related to the portal fissure of the liver and to the pancreas.

The cardia is situated about two to three inches (ca. 5 to 7 cm.) from the left extremity, and is oval; it lies a little to the left of the median plane, below the tenth or eleventh thoracic vertebra.

When empty, or nearly so, the stomach is separated from the ventral abdominal wall by the liver and small intestine, and the greater curvature extends back to the eleventh or twelfth rib. In this state there is not rarely a marked constriction between the right and left portions.

The longitudinal muscular fibers are found on the curvatures and on the pyloric part. The oblique fibers are arranged in two layers: the external layer is a continuation of the longitudinal fibers of the œsophagus onto the body and fundus. The circular layer covers the whole stomach except the fundus, and forms a pyloric sphincter and an antral sphincter. The internal oblique layer is arranged as in the horse, and forms a similar loop-like, cardiac sphincter.

Three regions of the mucous membrane exist. Cardiac glands are found in a very narrow pale zone around the cardiac opening, and also scattered along the lesser curvature. The fundus gland region has a thick, reddish-brown mucous membrane, which lines about two-thirds of the organ. The pyloric mucous membrane is thinner and pale; in the dead subject it is often stained by regurgitated bile.

THE INTESTINE

The intestine is short—about five or six times the length of the body.

The =small intestine= has an average length of about thirteen feet (ca. 4 meters). It occupies most of the abdominal cavity behind the liver and stomach. The duodenum begins at the portal fissure and passes backward and upward, at first on the visceral surface of the liver, then in contact with the right flank. Near the pelvis it turns inward and sharply forward, passes along the inner border of the left kidney, and joins the jejunum to the left of the root of the mesentery. The mesoduodenum is given off from the common mesentery; it is a relatively wide fold, except at the terminal part. Here there are two short folds, one of which joins the mesocolon; the other passes on to the terminal part of the colon. The first part of the mesoduodenum contains the right branch of the pancreas. Its root is blended with the mesocolon to form a common mesentery. The remainder of the small intestine forms numerous coils, and is attached by a wide mesentery to the sublumbar region. The =pancreatic= and =bile-ducts= open into the duodenum about two or three inches (ca. 5 to 8 cm.) from the pylorus; the =accessory pancreatic duct= opens an inch or two (ca. 2.5 to 5 cm.) further back. The mucous membrane has very long villi. The duodenal (Brunner’s) glands occur only near the pylorus. Peyer’s patches are numerous (about twenty in young subjects), and begin in the duodenum. They are usually elliptical in outline, but the last one is band-like, reaches to the end of the ileum and is four to sixteen inches (ca. 10 to 40 cm.) long in young dogs (Ellenberger). There is an ileo-cæcal valve.[117] The muscular coat is relatively thick.

The =large intestine= is two to two and a half feet (ca. 60 to 75 cm.) long. Its caliber is about the same as that of the small intestine, and it has neither longitudinal bands nor sacculations.

The =cæcum= is about three or four inches (ca. 8 to 10 cm.) long, and is twisted in a spiral fashion. The twist is maintained by the peritoneum, which attaches it to the ileum. It is situated usually against the middle of the upper part of the right flank, below the duodenum and the right branch of the pancreas.[118]

The =colon= is attached to the sublumbar region by a mesentery, the mesocolon. It presents three parts, which correspond to the ascending, transverse, and descending colon of man. The first or right part (Colon ascendens) is very short. It begins at the junction of the ileum and cæcum, and passes forward along the inner surface of the duodenum and the right branch of the pancreas till it reaches the pyloric part of the stomach; here it turns inward and crosses the median plane, forming the transverse part (Colon transversum). The third or left part (Colon descendens) passes backward in the sublumbar region along the inner border or ventral surface of the left kidney; then inclines toward the median plane and joins the rectum.[119] The caliber of the colon is about the same throughout. It has no bands nor sacculations.

The mucous membrane of the cæcum contains numerous solitary glands; they are found also in the first part of the colon.

The =rectum= is almost completely covered with peritoneum, the line of peritoneal reflection being under the second or third coccygeal vertebra. At the junction of the rectum and anus the mucous membrane has a stratified, squamous epithelium, and contains the =anal glands=. A small opening on either side leads into two lateral =anal sacs= (Sinus paranales); these are usually about the size of a hazelnut, and contain a dirty gray, fatty substance, which has a peculiar and very unpleasant odor. The skin which lines these pouches contains coil glands. Further back the skin contains large sebaceous glands and peculiar =circumanal glands=.

The retractor ani muscle is extensive. It arises from the shaft of the ilium, the pubis, and the symphysis pelvis, and passes upward and backward to end on the first coccygeal vertebræ and the sphincter ani externus. The retractors, together with the coccygei, form a sort of pelvic diaphragm, analogous to that of man.

THE LIVER

The liver is relatively large, weighing usually about 5 per cent. of the body-weight. It is divided into six or seven =lobes= by fissures which converge at the portal fissure. When the gland is examined in the soft condition, all the lobes are visible on the visceral surface, but usually only four on the parietal surface.

The =left lateral lobe= is the largest, and is oval in outline. The =left central lobe= is much smaller and is prismatic. The =right central lobe= is second in size, and presents a somewhat tongue-shaped quadrate lobe, marked off by the deep fissure in which the gall-bladder lies. The =right lateral lobe= is third in size, and is oval in outline. On its visceral surface is the large =caudate lobe=; this consists of two parts—on the right, the caudate process; on the left, the papillary process or lobe, both often being subdivided by fissures.

When hardened in situ (with the stomach full), the gland presents the following characters:

The visceral surface is marked by a somewhat oblique ridge (corresponding to the lesser curvature of the stomach), which separates two deep cavities. The depression to the left of the ridge is occupied by the body and fundus of the stomach. The depression on the right is smaller, and is occupied by the pyloric part of the stomach, the origin of the duodenum, and the right branch of the pancreas. The gall-bladder is not visible.

When the stomach is empty and contracted, the visceral surface of the liver is strikingly different. There is then a shallow impression for the left part of the stomach on the left lobe, and a large convex area, related to the small intestine and a mass of omentum. The pyloric and duodenal impressions are not much changed. The gall-bladder is visible.

The portal fissure is very deep and might well be termed a fossa. It is bounded on the right (externally) by the caudate process, on the left (internally) by the papillary process or lobe, which is directed ventrally. The hepatic artery enters at the dorsal end and the portal vein centrally, while the bile-duct leaves at the ventral part. The left central lobe is not visible, and the right central one is largely concealed.

The parietal surface is extremely convex, being adapted to the diaphragm and the adjacent part of the ventral wall of the abdomen. On it all the lobes except the papillary are completely or partially visible. The gall-bladder is visible in the fissure between the quadrate and chief portions of the right central lobe.

The dorsal border presents a renal impression on its right portion. The posterior vena cava passes downward and forward at first in a deep groove on the caudate lobe, then largely embedded in the parietal surface of the right lateral lobe; it receives two large hepatic veins just before piercing the diaphragm.

The œsophageal notch is large, and is occupied on the right by the thick margin of the hiatus œsophageus. The left border is opposite to the ninth and tenth ribs; it is thin and is marked by numerous indentations. The caudate process extends usually a little beyond the last right rib, between the right kidney and the right branch of the pancreas. The ventral border lies on the ventral abdominal wall a variable distance behind the xiphoid cartilage.

The =gall-bladder= lies in the fossa vesicæ felleæ, between the two parts of the right central lobe; it does not reach to the ventral border of the liver. The cystic duct joins the hepatic duct at the lower part of the portal fissure, forming with it the bile duct (Ductus choledochus); the latter passes to the right and opens into the duodenum, about two or three inches (ca. 5 to 8 cm.) from the pylorus.

Of the ligaments, the coronary and right lateral are well developed, but the left lateral and falciform are small.

THE PANCREAS

The =pancreas= is =V=-shaped, consisting of two long narrow branches, which meet at an acute angle above the pylorus. The =right branch= extends backward above the first part of the duodenum, below the caudate lobe of the liver and the right kidney, and ends usually a short distance behind the latter. The =left branch= passes inward and backward on the visceral surface of the stomach, and ends under the left kidney. There are two =ducts=: the larger one unites with the bile duct, while the smaller one enters the duodenum an inch or two (ca. 3 to 5 cm.) further back.

THE SPLEEN

The =spleen= is bright red in color in the fresh state. It is somewhat falciform, long, and narrow; the ventral part is the widest. Its position varies in accordance with the fullness of the stomach. When that organ is moderately full the long axis of the spleen corresponds to the direction of the last rib. Its parietal surface is convex and lies largely against the left flank. The visceral surface is concave in its length, and is marked by a longitudinal ridge, on which the vessels and nerves are situated. The dorsal end lies against the left kidney and left crus of the diaphragm. The ventral end is a little further back, and reaches to the ventral wall of the abdomen. It is so loosely attached by the great omentum as to be regarded as an appendage of the latter.

THE RESPIRATORY SYSTEM

The organs of respiration (Apparatus respiratorius) comprise the =nasal cavity=,[120] the =pharynx=, the =larynx=, the =trachea=, the =bronchi=, and the =lungs=. The lungs are the central organs in which the exchange of gases between the blood and the air takes place; the other parts of the system are passages by which the air passes to and from the lungs. The nasal cavity opens externally at the nostrils, and communicates behind with the pharynx through the posterior nares (Choanæ); it contains the peripheral part of the olfactory apparatus, which mediates the sense of smell. The pharynx is a common passage for the air and food—a remnant of the primitive embryonic arrangement; it has been described as a part of the digestive tube. The larynx is a complex valvular apparatus which regulates the volume of air passing through the tract; it is also the chief organ of voice. The trachea, and the bronchi formed by its bifurcation, are permanently open conducting tubes. The thorax, the pleural sacs which it contains, and the muscles which increase or diminish the size of the cavity are also parts of the system. The bones, joints, and muscles of the thorax have already been described.

For topographic reasons two ductless glands, the =thyroid= and the =thymus=, are usually described in this section, although they are in no sense a part of the respiratory system.

RESPIRATORY SYSTEM OF THE HORSE

THE NASAL CAVITY

The =nasal cavity= (Cavum nasi), the first segment of the respiratory tract, is a long, somewhat cylindrical passage, inclosed by all the facial bones except the mandible and hyoid. It is separated from the mouth ventrally by the palate. It opens externally at the nostrils, and communicates posteriorly with the pharynx through the posterior nares.

The =nostrils= or =anterior nares= (Nares) are somewhat oval in outline, and are placed obliquely, so that they are closer together below than above. They are bounded by two =alæ= or =wings= (Alæ nasi), which meet above and below, forming the =commissures=. The outer ala is concave; the inner one is convex above, concave below. The upper commissure is narrow, the lower one wide and rounded. If the finger is passed into the nostril at the upper commissure, it enters the so-called =false nostril=, which leads backward to a blind cutaneous pouch. This cul-de-sac, the =diverticulum nasi=, extends backward to the angle of junction of the nasal bone and the nasal process of the premaxilla. The posterior part of the diverticulum is blind, but the false nostril communicates below and internally with the vestibule of the nasal cavity. The external opening of the =naso-lacrimal duct= is seen when the nostril is dilated; it is situated on the floor of the vestibule, perforating the skin close to its junction with the mucous membrane, about two inches (ca. 5 cm.) from the lower commissure. (It is not rare to find one or two accessory orifices further back.)

=Structure.=—The =skin= around the nostrils presents long tactile hairs as well as the ordinary ones. It is continued around the alæ and lines the vestibule. In the posterior part of the diverticulum the skin is thin and usually black, and is covered with exceedingly fine hairs; it is provided with numerous sebaceous glands. The inner wing is supported by the =alar cartilage= (Cartilago alaris), which is shaped somewhat like a comma, the convex margin being direct medially. The cartilages are attached by fibrous tissue to the extremity of the septal cartilage. Each consists of an upper, quadrilateral curved plate, the =lamina=, and a narrow =cornu= which curves downward and outward, supporting the inner wing and the lower commissure, but not entering into the formation of the outer wing. The lamina causes the projection of the upper part of the internal wing, from which the thick =alar fold= (Plica alaris) passes backward along the external wall of the nasal cavity to cover the cartilaginous prolongation of the inferior turbinal bone. This fold separates the true from the false nostril. The extremity of the cornu usually causes a slight projection of the skin a short distance behind and below the lower commissure.

The muscles of the nostrils have been described.

=Blood-supply.=—Palato-labial, superior labial, and lateral nasal arteries.

=Nerve-supply.=—Trigeminal and facial nerves.

The nasal cavity is divided into two symmetrical halves by the median =septum nasi=. The osseous part of the septum (Septum osseum) is formed behind by the perpendicular plate of the ethmoid and below by the vomer. A few ridges on the former correspond to the ethmoidal meatuses. The major part of it, however, is formed by the =septal cartilage= (Cartilago septi nasi). The surfaces of the cartilage are marked by faint grooves for the vessels and nerves which course over it. The =dorsal border= is attached along the frontal and nasal sutures, and extends beyond the apices of the nasal bones about two inches (ca. 5 cm.). From this border a thin, narrow plate, the =parietal cartilage= (Cartilago parietalis) curves outward for a short distance on either side. Near the nostrils these plates are somewhat wider, partially making good the defect in the bony roof of the cavity in this situation. The =ventral border= is thick and rounded; it lies chiefly in the groove of the vomer, but anteriorly it occupies the space between the premaxillary bones. The =posterior border= is continuous with the perpendicular plate of the ethmoid bone.[121] The alar cartilages are attached to the anterior extremity by fibrous tissue in such a manner as to allow very free movement—in fact, an actual joint may be found. The ventral part of the cartilage is about half an inch (ca. 1 cm.), the middle part about one-tenth of an inch (ca. 2.5 mm.), and the dorsal part about a quarter to a third of an inch (ca. 6 to 8 mm.) in thickness.

The two =turbinal bones= (Conchæ nasales) project from the lateral wall, and divide the outer part of the cavity into three =meatuses=—superior, middle, and inferior.

The =superior meatus= (Meatus nasi dorsalis) is a narrow passage, bounded dorsally by the roof of the cavity, and ventrally by the superior turbinal bone; its posterior end is closed by the junction of the inner plate of the frontal bone with the cribriform plate and lateral mass of the ethmoid. It transmits air to the upper part of the olfactory region.

The =middle meatus= (Meatus nasi medius) is between the two turbinal bones. It is somewhat larger than the superior meatus, and does not extend back so far; it ends near the great ethmoturbinal and the ethmoidal meatuses. In its posterior part is found the extremely narrow, slit-like interval by which the maxillary sinus communicates with the nasal cavity—the =naso-maxillary fissure= (Aditus naso-maxillaris). The fissure is not visible from the nasal side, being concealed by the overhanging superior turbinal. A fine flexible probe, passed outward and somewhat backward between the turbinal bones, enters the maxillary sinus; if introduced a little further in the same direction, it usually passes through the orifice of communication between the maxillary and the frontal sinus and enters the latter. A small part of the fissure usually brings the anterior division of the maxillary sinus into communication with the nasal cavity. The spaces inclosed by the coiled portions of the turbinals also open into the middle meatus. This passage may be characterized as the sinus-meatus, but it also conducts air to the olfactory region.

The =inferior meatus= (Meatus nasi ventralis) is situated between the inferior turbinal and the floor of the cavity. It is much larger than the other two, and is the direct passage between the nostrils and the pharynx.

The =common meatus= (Meatus nasi communis) is situated between the septum and the turbinals, and is continuous externally with the other meatuses. It is very narrow dorsally, but widens ventrally.

The lateral masses of the ethmoid bone project forward into the posterior part (fundus) of the nasal cavity (Fig. 347). Between the ethmoturbinals of which each mass is composed there are three principal and numerous small passages, the =ethmoidal meatuses= (Meatus ethmoidales).

The =posterior nares= (Choanæ) are two elliptical orifices by which the nasal cavity and pharynx communicate. They are in the same plane as the floor of the nasal cavity, and are separated from each other by the vomer. They are, taken together, about two inches (ca. 5 cm.) wide and three to four inches (ca. 8 to 10 cm.) long.

The =nasal mucous membrane= (Membrana mucosa nasi) is thick, highly vascular, and is, in general, firmly attached to the underlying periosteum and perichondrium. It is continuous in front with the skin which lines the nostrils, and behind with the mucous membrane of the pharynx. It is also continuous at the naso-maxillary fissure with the very thin and only slightly vascular mucous membrane which lines the sinuses. In the anterior part of the cavity it forms prominent thick folds on the lateral wall, which extend from the turbinal bones to the nostril. There are usually two =superior turbinal folds= which unite anteriorly. The upper one incloses a thin plate of cartilage which is continuous with the superior turbinal bone. The =inferior turbinal fold= is curved, and incloses an ᔕ-shaped cartilaginous plate which prolongs the inferior turbinal bone; this fold is continuous with the alar fold of the nostril, and forms with it the upper margin of the entrance from the true nostril to the nasal cavity. Below this there is a rounded ridge produced by the nasal process of the premaxilla. The mucous membrane of the greater part of the cavity (Regio respiratoria) is red in color, and is covered with a stratified ciliated epithelium. It contains numerous acinous =nasal glands= (Glandulæ nasales). The submucosa contains rich venous plexuses which form in certain situations a sort of cavernous tissue (Plexus cavernosus), composed of several strata of freely anastomosing veins, between which there are unstriped muscle-fibers. This arrangement is most marked in the turbinal folds, on the lower part of the inferior turbinal bone, and the lower part of the septum. On the posterior part of the lateral masses of the ethmoid and the adjacent part of the superior turbinal and the septum (Regio olfactoria) the mucous membrane is brownish-yellow in color and thicker; it contains the olfactory nerve-endings in a special non-ciliated epithelium. In it are numerous =olfactory glands= (Glandulæ olfactoriæ); these are long, tubular, and often branched.

The =vomero-nasal organ= of Jacobson (Organon vomeronasale) is situated on the floor of the nasal cavity, on either side of the ventral border of the septum (Fig. 345.) It consists of a tubular cartilage lined with mucous membrane, to which fibers of the olfactory nerve may be traced. Its anterior part communicates with the nasal cavity by a slit-like orifice.

The =paranasal sinuses= are described in the Osteology.

THE LARYNX

The =larynx= is a short tube which connects the pharynx and trachea. It is a complex valvular apparatus, which regulates the volume of air in respiration, prevents aspiration of foreign bodies, and is the chief organ of voice.

It is situated partly between the internal pterygoid muscles, partly in the neck between the parotid glands.[122] Its long axis is practically horizontal when the head and neck are extended (Fig. 254).

It is related dorsally to the pharynx and the origin of the œsophagus. Ventrally it is covered by the skin, fascia, and sterno-hyoid and omo-hyoid muscles. Laterally it is related to the parotid and submaxillary glands and to the internal pterygoid, stylo-maxillaris, digastricus, stylo-hyoid, and pharyngeal constrictor muscles.

It is attached to the body and thyroid cornua of the hyoid bone, and thus indirectly to the base of the cranium.

Its cavity communicates anteriorly with the pharynx and (potentially) with the mouth; posteriorly, with the trachea.

The wall of the larynx consists of a framework of cartilages, connected by joints and ligaments or membranes, and moved by extrinsic and intrinsic muscles. The cavity is lined with mucous membrane.

CARTILAGES OF THE LARYNX

There are three single cartilages and one pair; the single cartilages are the cricoid, thyroid, and epiglottis; the arytenoid cartilages are paired.

The =cricoid cartilage= (Cartilago cricoidea) is shaped like a signet ring. The dorsal part is a broad, thick, quadrilateral plate termed the =lamina=; the outer (dorsal) surface of this is marked by a median ridge separating two shallow cavities, which give attachment to the posterior crico-arytenoid muscles. On either side of these depressions are two articular facets. The =anterior facets= (Facies articulares arytænoideæ) are placed at the anterior border, are oval and convex, and articulate with the arytenoid cartilages. The =posterior facets= (Facies articulares thyreoideæ) are situated on the front of a ridge, a short distance from the posterior border; they are concave, and articulate with the posterior cornua of the thyroid cartilage. The ventral and lateral parts of the ring are formed by a curved band, called the arch (Arcus), which is narrowest below. The lateral surfaces of the arch are grooved for the crico-thyroid muscle. The anterior border of the lamina is thick and slightly concave; the posterior border is thin and irregularly notched. The anterior border of the arch is concave ventrally and gives attachment to the crico-thyroid membrane; laterally it is thicker and gives attachment to the crico-arytenoideus lateralis muscle. The posterior border is attached to the first ring of the trachea by an elastic membrane. The inner surface is smooth and is covered with mucous membrane.

The =thyroid cartilage= (Cartilago thyreoidea) consists of a median thickened portion, termed the body, and two lateral laminæ or wings. The =body= forms ventrally a slight prominence (Prominentia laryngea), which can be felt, but is not visible in the living subject; it supports the base of the epiglottis, which is attached to it by an elastic ligament. The =wings= or =laminæ= spring from the body on either side and form a large part of the lateral wall of the larynx. Each is a rhomboid plate, presenting a slightly convex outer surface, which is divided into two areas by an oblique line (Linea obliqua), on which the thyro-hyoid and thyro-pharyngeus muscles meet. The superior (dorsal) border is nearly straight; it gives attachment to the pharyngeal fascia and the palato-pharyngeus muscle, and bears a cornu at each end. The =anterior cornu= (Cornu orale) is attached to the thyroid cornu of the hyoid bone; below it is a notch, which is converted into a foramen (Foramen thyreoideum) by a fibrous band, and transmits the superior laryngeal nerve to the interior of the larynx. The =posterior cornu= (Cornu caudale) articulates with the cricoid cartilage. The inferior (ventral) border is fused with the body in front; behind, it diverges from its fellow to inclose a triangular space (Incisura thyreoidea caudalis), which is occupied by the crico-thyroid membrane. The anterior border is slightly convex, and is attached to the hyoid bone by the thyro-hyoid membrane. The posterior border overlaps the arch of the cricoid cartilage, and gives attachment to the crico-thyroid muscle. The inner surface is concave, and is covered by mucous membrane to a small extent.

The =epiglottis= (Cartilago epiglottica) is situated above the body of the thyroid cartilage and curves toward the root of the tongue. It is shaped somewhat like a pointed ovate leaf, and presents two surfaces, two borders, a base, and an apex. The =oral= (or anterior) =surface= is concave in its length, convex transversely; the =pharyngeal= (or posterior) =surface= has the reverse configuration. The =borders= are thin, denticulated, and somewhat everted. The =base= is thick, and is attached to the dorsal surface of the body of the thyroid cartilage by elastic tissue. From each side of it a cartilaginous bar projects backward; these processes correspond to the cuneiform cartilages of man. The =apex= is pointed and curved ventrally. Nearly all of the epiglottis is covered with mucous membrane. It lies sometimes in front of, sometimes behind, the soft palate.

The =arytenoid cartilages= (Cartilagines arytenoideæ) are situated on either side, in front of the cricoid, and partly between the alæ of the thyroid cartilage. They are somewhat pyramidal in form, and may be described as having two surfaces, two borders, a base, and an apex. The =internal surface= is concavo-convex, smooth, and covered by mucous membrane. The =external surface= is divided by a ridge into two areas: the lower flat area faces outward, and is covered by the thyro-arytenoid and lateral crico-arytenoid muscles; the upper concave area faces chiefly forward, and is covered by the transverse arytenoid muscle. The =anterior border= is convex, and at the base of the cartilage forms a projection termed the =vocal process= (Processus vocalis); it is so named because it gives attachment to the vocal or thyro-arytenoid ligament, which forms the basis of the true vocal cord. The =posterior border= is straight, and extends from the vocal process to the facet on the base. The =dorsal border= forms a deep notch with the apex. The =base= is concave and faces chiefly backward; it presents an oval, concave facet for articulation with the anterior border of the lamina of the cricoid cartilage. The external angle of the base forms a strong eminence termed the =muscular process= (Processus muscularis), which gives attachment to the crico-arytenoid muscles. The internal angle is attached to its fellow by the transverse arytenoid ligament. The =apex= (Cartilago corniculata) curves upward and backward, forming with its fellow the pitcher-shaped lip, from which the cartilages derive their name.

The cricoid and thyroid cartilages, and the greater part of the arytenoid cartilages, consist of hyaline cartilage. The apices and vocal processes of the arytenoid cartilages and the epiglottis (including the cuneiform processes) consist of elastic cartilage; they show no tendency toward ossification at any age. The thyroid and cricoid cartilages regularly undergo considerable ossification; the process begins in the body of the thyroid, and often involves the greater part of the cartilage.

JOINTS, LIGAMENTS, AND MEMBRANES OF THE LARYNX

The =crico-thyroid joints= (Articulationes crico-thyreoideæ) are diarthroses formed by the apposition of the convex facets on the ends of the posterior cornua of the thyroid cartilage with corresponding facets on the sides of the cricoid cartilage. The =capsule= is thin, but is strengthened by accessory bands dorsally, externally, and internally. The movements are chiefly rotation around a transverse axis passing through the centers of the two joints.

The =crico-arytenoid joints= (Articulationes crico-arytænoidea) are also diarthrodial. They are formed by the apposition of the convex facets on the anterior border of the cricoid cartilages with the concave facets on the bases of the arytenoid cartilages. Each has a very thin, loose =capsule=, strengthened by accessory bundles dorsally and internally. The movements are gliding and rotation. In the former the arytenoid cartilage glides inward or outward; in the latter it rotates around a longitudinal axis, so that the vocal process swings outward or inward.

The =thyro-hyoid joints= (Articulationes hyo-thyreoideæ) are formed by the anterior cornua of the thyroid cartilage and the extremities of the thyroid cornua of the hyoid bone. They permit rotation around a transverse axis passing through the two joints.

The =crico-thyroid membrane= (Ligamentum crico-thyreoideum) occupies the thyroid notch and extends backward to the arch of the cricoid cartilage. It is triangular in form, and is attached by its base to the anterior border of the arch of the cricoid cartilage, while its borders are attached to the margins of the thyroid notch. It is strong, tightly stretched, and composed largely of elastic tissue. It is reinforced ventrally by longitudinal fibers, dorsally by fibers which stretch across the thyroid notch.

The =thyro-hyoid membrane= (Membrana hyo-thyreoidea) connects the anterior border of the thyroid cartilage with the body and thyroid cornua of the hyoid bone.

The =hyo-epiglottic ligament= (Ligamentum hyo-epiglotticum) attaches the lower part of the oral surface of the epiglottis to the body of the hyoid bone. It forms an elastic sheath for the hyo-epiglottic muscle. Ventrally it blends with the thyro-hyoid membrane, and dorsally it is not well defined.

The =thyro-epiglottic ligament= (Ligamentum thyreo-epiglotticum) is strong and thick; it is composed chiefly of elastic tissue. It connects the base of the epiglottis with the body and the adjacent inner surface of the laminæ of the thyroid cartilage. Other fibers attach the cuneiform processes somewhat loosely to the thyroid laminæ.

The =transverse arytenoid ligament= (Ligamentum arytænoideum transversum) is a slender band which connects the inner angles of the bases of the arytenoid cartilages.

The =vocal= (or inferior thyro-arytenoid) =ligament= (Ligamentum vocale) forms the basis of the true vocal cord (Fig. 254). It is elastic and is attached ventrally to the body of the thyroid cartilage and the crico-thyroid membrane close to its fellow of the opposite side, and ends dorsally on the processus vocalis of the arytenoid cartilage.

The =ventricular= (or superior thyro-arytenoid) =ligament= (Ligamentum ventriculare) is included in the false vocal cord. It consists of loosely arranged bundles which arise chiefly from the cuneiform process and end on the outer surface of the processus vocalis and adjacent part of the arytenoid cartilage.

The =crico-tracheal membrane= (Ligamentum crico-tracheale) connects the cricoid cartilage with the first ring of the trachea.

MUSCLES OF THE LARYNX

A. EXTRINSIC MUSCLES

The =sterno-thyro-hyoideus= is described on p. 226.

The =thyro-hyoideus= (M. hyo-thyreoideus) is a flat, quadrilateral muscle, which lies on the outer surface of the thyroid lamina and the lateral part of the thyro-hyoid membrane. It arises from the thyroid cornu of the hyoid bone, and ends on the oblique line on the lamina of the thyroid cartilage. Its action is to draw the larynx toward the root of the tongue.

The =hyo-epiglotticus= (M. hyo-epiglotticus) is a feeble muscle which occupies a median position above the central part of the thyro-hyoid membrane, inclosed by an elastic sheath, the hyo-epiglottic ligament. It arises from the body of the hyoid bone by two branches which unite and are inserted into the front of the base of the epiglottis. Its action is to draw the epiglottis toward the root of the tongue.

B. INTRINSIC MUSCLES

The =crico-thyroideus= (M. crico-thyreoideus) is a short muscle which fills the groove on the lateral surface of the cricoid cartilage; its fibers are directed dorsally and somewhat forward. It arises on the lower half of the lateral surface and posterior edge of the cricoid cartilage, and is inserted into the posterior border and adjacent part of the surfaces of the lamina of the thyroid cartilage. Its action is to draw the thyroid cartilage and the ventral part of the cricoid cartilage together. In this action the cricoid cartilage is probably rotated, carrying the bases of the arytenoid cartilages with it and thus tensing the vocal cords.

The =crico-arytenoideus dorsalis s. posterior= (M. crico-arytænoideus dorsalis) is a strong, somewhat fan-shaped muscle, which, with its fellow, covers the dorsal surface of the lamina of the cricoid cartilage. It is partially divisible into two layers. It has a broad origin on half of the lamina of the cricoid cartilage, and its fibers converge to be inserted into the processus muscularis of the arytenoid cartilage. Its action is to dilate the rima glottidis by rotating the arytenoid cartilage so as to carry the vocal process and cord outward.

The =crico-arytenoideus lateralis= (M. crico-arytænoideus lateralis) lies on the inner face of the thyroid lamina. It arises from the anterior border of the lateral part of the arch of the cricoid cartilage. The fibers pass in a dorsal direction and converge on the processus muscularis of the arytenoid cartilage. It closes the rima glottidis by rotating the arytenoid cartilage inward.

The =arytenoideus transversus= (M. arytænoideus transversus) is an unpaired muscle which stretches across the concave dorsal surface of the arytenoid cartilages. Its fibers are attached on either side to the processus muscularis and the ridge which extends forward from it. The right and left parts of the muscle meet at a fibrous raphé which is connected with the transverse arytenoid ligament. Filters of the thyro-arytenoideus muscle overlap its anterior part. It narrows the rima by drawing the arytenoid cartilages together.

The =thyro-arytenoideus= (Mm. ventricularis et vocalis) is situated in the lateral wall of the larynx, covered by the lamina of the thyroid cartilage. It consists of two parts, between which the mucous membrane of the lateral ventricle pouches to form the saccule. The =anterior part= (M. ventricularis) arises from the anterior part of the crico-thyroid membrane and the ventral border of the thyroid lamina. Its fibers pass upward and backward to end partly on the processus muscularis, partly on the arytenoideus transversus, meeting its fellow. The =posterior part= (M. vocalis) has a more extensive origin on the crico-thyroid membrane. Its direction corresponds with that of the true vocal cord. It is inserted into the outer surface of the arytenoid cartilage below the processus muscularis. The muscle closes the rima and slackens the vocal cords. With the arytenoideus transversus and crico-arytenoideus lateralis it forms a sphincter which closes the entrance to the larynx.

CAVITY OF THE LARYNX

The =cavity= of the larynx (Cavum laryngis) is smaller than one would expect from its external appearance. On looking into it through the pharyngeal end two folds are seen projecting from its lateral walls. These are termed the =vocal cords=, and they divide the cavity into two portions.

The =aditus laryngis=, or pharyngeal aperture, is a large, oblique, oval opening, which faces forward and upward into the ventral part of the pharynx. It is bounded in front by the epiglottis, behind by the arytenoid cartilages, and laterally by the aryteno-epiglottic folds of mucous membrane (Plicæ aryepiglotticæ), which stretch between the edges of the epiglottis and the arytenoid cartilages.

The =vestibule= of the larynx (Vestibulum laryngis) is that part of the cavity which extends from the aditus to the vocal cords. On its lateral walls are the =false vocal cords= (Plicæ ventriculares), each of which consists of a fold of mucous membrane covering the anterior part of the thyro-arytenoid ligament and the cuneiform process. Between the false and true vocal cord there is a pocket-like depression termed the =lateral ventricle= of the larynx (Ventriculus lateralis laryngis). This is the entrance to the =laryngeal saccule= (Appendix ventriculi laryngis), a cul-de-sac of the mucous membrane which is about one inch (ca. 2 to 3 cm.) long and extends upward and backward on the inner surface of the thyroid lamina. There is a small =middle ventricle= (Ventriculus laryngis medianus) at the base of the epiglottis.

The middle, narrow part of the cavity is termed the =glottis= or =rima glottidis=.[123] It is bounded on either side by the true vocal cord and the inner surface of the arytenoid cartilage. The =true vocal cords= (Labia vocales) are situated behind the false cords and the lateral ventricles. They extend from the angle between the body and laminæ of the thyroid cartilage to the vocal processes of the arytenoid cartilages. They are prismatic in cross-section, and their free edges look forward and somewhat upward. The mucous membrane of the cord (Plica vocalis) is very thin and smooth, and is intimately attached to the underlying ligament. In ordinary breathing the rima is somewhat lanceolate in form; when dilated, it is diamond-shaped, the widest part being between the vocal processes. The narrow part of the glottis between the vocal cords is termed the glottis vocalis (Pars intermembranacea), while the wider part between the arytenoid cartilages is the glottis respiratoria (Pars intercartilaginea).

The =posterior compartment= of the laryngeal cavity is directly continuous with the trachea. It is inclosed by the cricoid cartilage and the crico-thyroid membrane. It is oval in form, the transverse diameter being an inch and a half to two inches (ca. 4 to 5 cm.), and the dorso-ventral diameter two to two and a half inches (ca. 5 to 6 cm.).

The =mucous membrane= which lines the larynx (Tunica mucosa laryngis) is reflected around the margin of the aditus to become continuous with that of the pharynx, and behind it is continuous with that which lines the trachea. It is closely attached to the pharyngeal surface of the epiglottis, but elsewhere in the aditus and vestibule it is loosely attached by submucous tissue which contains many elastic fibers.[124] It is thin and very closely adherent over the vocal cords and the inner surfaces of the arytenoid cartilages. The epithelium is of the stratified squamous type from the aditus to the glottis, beyond which it is columnar ciliated in character. There are numerous =mucous glands= (Glandulæ laryngeæ), except over the glottis and the pharyngeal surface of the epiglottis, in which situations they are scanty.

=Blood-supply.=—Thyro-laryngeal artery.

=Nerve-supply.=—Superior laryngeal and recurrent laryngeal nerves (from the vagus).

THE TRACHEA

The =trachea= extends from the larynx to the roots of the lungs, where it divides into the right and left bronchi. It is kept permanently open by a series of about fifty to fifty-five incomplete cartilaginous rings embedded in its wall. It occupies a median position, except near its termination, where it is pushed a little to the right by the arch of the aorta. It is approximately cylindrical, but its cervical portion is for the most part depressed dorso-ventrally by contact with the longus colli muscle, so that the dorsal surface is flattened. The average caliber is about two to two and one-half inches (ca. 5 to 6 cm.), but in the greater part of the neck the transverse diameter is greater and the dorso-ventral smaller. It is inclosed by a fascia propria.

In its =cervical part= the trachea is related dorsally to the œsophagus for a short distance, but chiefly to the longus colli muscle. Laterally it is related to the thyroid gland, the carotid artery, the jugular vein, the vagus, sympathetic, and recurrent laryngeal nerves, and the tracheal lymph ducts and cervical lymph glands. The œsophagus lies on its left face from the third cervical vertebra backward. The sterno-cephalicus muscles cross it very obliquely, passing from the ventral surface forward over its sides, and diverging to reach the angles of the jaw. The omo-hyoidei also cross it very obliquely, passing over the lateral surfaces of the tube, and converging ventrally to the body of the hyoid bone. The sterno-thyrohyoideus lies on the ventral surface.[125] The scaleni lie on either side near the entrance to the thorax.

The =thoracic part= of the trachea passes backward between the pleural sacs and divides into the two bronchi over the left atrium of the heart. It is related dorsally to the longus colli for a short distance, and beyond this to the œsophagus. Its left face is crossed by the aortic arch, the left brachial artery, and the thoracic duct. Its right face is crossed by the vena azygos, the dorso-cervical and vertebral vessels, and the right vagus nerve. Ventrally it is related to the anterior vena cava, the brachiocephalic and common carotid trunks, and the left recurrent nerve.

The =bifurcation= of the trachea (Bifurcatio tracheæ) is situated opposite to the fifth intercostal space, and about five or six inches (ca. 12 to 15 cm.) below the seventh and eighth thoracic vertebræ.

The trachea is composed of—(1) A fibro-elastic membrane in which are embedded (2) the cartilaginous rings; (3) a muscular layer; (4) the mucous membrane. The =elastic membrane= is intimately attached to the perichondrium of the rings. In the intervals between the latter it constitutes the annular ligaments (Ligamenta annularia). The =rings= of the trachea (Cartilagines tracheales) are composed of hyaline cartilage. They are incomplete dorsally, and when their free ends are drawn apart, resemble somewhat the letter C. In the cervical part the thin wide free ends overlap, while in the thoracic part they do not meet; here the deficiency is made up by thin plates of variable size and form, embedded in a membrana transversa. Ventrally the rings are about one-half inch (ca. 1 to 1.2 cm.) wide and much thicker than dorsally. The first ring is attached to the cricoid cartilage by the crico-tracheal membrane, and is usually fused dorsally with the second ring. Various irregularities, such as partial bifurcation or partial or complete fusion with an adjacent ring, are common. The arrangement in the terminal part is very irregular. The =muscular layer= (Musculus trachealis) consists of unstriped fibers which stretch across the dorsal part of the tube. It is separated from the ends of the rings and the membrana transversa by a quantity of areolar tissue. When it contracts the caliber of the tube is diminished. The =mucous membrane= is pale, normally, and presents numerous fine longitudinal folds, in which are bundles of elastic fibers. The epithelium is stratified columnar ciliated. Numerous tubular =mucous glands= (Glandulæ tracheales) are present.

=Blood-supply.=—Common carotid arteries.

=Nerve-supply.=—Vagus and sympathetic nerves.

THE BRONCHI

The two =bronchi=, right and left (Bronchus dexter, sinister), are formed by the bifurcation of the trachea. Each passes backward and outward to the hilus of the corresponding lung. The right bronchus is a little wider and less oblique in direction than the left. They are related ventrally to the divisions of the pulmonary artery, and dorsally to the branches of the bronchial artery and the bronchial lymph glands. Their structure is similar to that of the trachea.

THE THORACIC CAVITY

The =thoracic cavity= (Cavum thoracis) is the second in point of size of the body cavities. In form it is somewhat like a truncated cone, much compressed laterally in its anterior part, and with the base cut off very obliquely.

The =dorsal wall= or =roof= is formed by the thoracic vertebræ and the ligaments and muscles connected with them.

The =lateral walls= are formed by the ribs and the intercostal muscles.

The =ventral wall= or =floor= is formed by the sternum, the cartilages of the sternal ribs, and the muscles in connection therewith. It is about one-half as long as the dorsal wall.

The =posterior wall=, formed by the diaphragm, is very oblique and is strongly convex.

The =anterior aperture= or =inlet= (Apertura thoracis cranialis) is relatively small, and of narrow, oval form. It is bounded dorsally by the first thoracic vertebra and laterally by the first pair of ribs. It is occupied by the longus colli muscles, the trachea, œsophagus, vessels, nerves, and the prepectoral lymph glands.

The cavity is lined by the =endothoracic fascia= and by the =pleuræ=.

A longitudinal =septum=, termed the =mediastinum= (Septum mediastinale), extends from the dorsal wall to the ventral and posterior walls, and subdivides the cavity into two lateral chambers which contain the lungs. Each of these chambers is lined by a serous membrane called the pleura, and is called a =pleural cavity= (Cavum pleuræ). The mediastinum is, for the most part, not median in position; this is largely due to the fact that the largest organ contained in it, the heart, is placed more on the left side; consequently the right pleural cavity and lung are larger than the left. Practically all the organs in the thorax are in the mediastinal space with the exception of the lungs, the posterior vena cava, and the right phrenic nerve. The part in which the heart and the pericardium are situated, together with that above it, is usually called the middle mediastinal space; the parts before and behind this are termed the anterior and posterior mediastinal spaces respectively.

THE PLEURÆ

The =pleuræ= are two serous sacs which line the pleural cavities and are reflected at the roots of the lungs to invest those organs. We, therefore, distinguish =parietal= and =visceral= parts of the pleuræ.

The =parietal pleura= (Pleura parietalis) lines the cavity in which each lung is situated (Cavum pleuræ). On the lateral thoracic wall it is adherent to the ribs and intercostal muscles and is termed the =costal pleura= (Pleura costalis). Behind it is closely attached to the diaphragm, forming the =diaphragmatic pleura= (Pleura diaphragmatica). The part which is in apposition with the opposite sac or covers the mediastinal organs is termed the =mediastinal pleura= (Pleura mediastinalis); where this layer is adherent to the pericardium, it is distinguished as =pericardiac pleura= (Pleura pericardiaca).[126]

The pleura is reflected at the root of the lung, which it covers, constituting the =visceral= or =pulmonary pleura= (Pleura pulmonalis). Behind the root of the lung a considerable triangular area is not covered by the pleura, the two lungs being attached to each other by connective tissue in this situation.

The =ligament of the lung= (Lig. pulmonale) is a fold formed by the reflection of the pleura from the mediastinum and the diaphragm to the lung, behind the triangular area of adhesion just mentioned. It is seen when the base of the lung is drawn outward. It contains elastic tissue, especially in its posterior part.

The right pleura forms a special =sagittal fold= (Plica venæ cavæ) about a handbreadth to the right of the median plane, which incloses the posterior vena cava in its upper edge and gives off a small accessory fold for the right phrenic nerve. The fold arises from the thoracic floor and from the diaphragm below the foramen venæ cavæ, and intervenes between the mediastinal lobe and the body of the right lung. It is delicate and lace-like.

The posterior mediastinum is very delicate below the œsophagus, and usually appears fenestrated; when these apertures are present, the two pleural cavities communicate with each other.[127]

The pleural sacs contain a serous fluid, the =liquor pleuræ=; in health there is only a sufficient amount to moisten the surface, but it accumulates rapidly after death.

THE LUNGS

The =lungs= (Pulmones) occupy much the greater part of the thoracic cavity. They are accurately adapted to the walls of the cavity and the other organs contained therein. The two lungs are not alike in form or size, the right one being considerably larger than the left; the difference is chiefly in width, in conformity with the projection of the heart to the left.

The lung is soft, spongy, and highly elastic. It crepitates when pressed between the finger and thumb, and floats in water. When the thoracic cavity of the unpreserved subject is opened, the lung collapses immediately to about one-third of its original size, and loses its proper form; this is due to its highly elastic character and the fact that the tension of the lung tissue caused by the air pressure in its cavities has been relieved by the external air pressure.[128]

The =color= varies according to the amount of blood contained in the lung. During life the lung has a pink color, but in subjects which have been bled for dissection it is light gray or faintly tinged with red. In unbled subjects it is dark red; the depth of color varies, and is often locally accentuated by gravitation of blood to the most dependent parts (hypostasis).

The =fœtal lung=, since it contains no air and has a relatively small blood-supply, differs from that of an animal which has breathed in the following respects: (1) It is much smaller; (2) it is firmer and does not crepitate; (3) it sinks in water; (4) it is pale gray in color.

In =form= the lungs are like casts of the pleural cavities in which they are situated. When well hardened in situ, their surfaces present impressions and elevations corresponding exactly to the structures with which they are in contact. Each lung presents two surfaces, two borders, a base, and an apex.

The =costal= (or external) =surface= (Facies costalis) is convex, and lies against the lateral thoracic wall, to which it is accurately adapted.

The =mediastinal= (or internal) =surface= (Facies mediastinalis) is less extensive than the costal surface. It is molded on the mediastinum and its contents. It presents a large cavity adapted to the pericardium and heart; this is termed the =cardiac impression= (Impressio cardiaca), and is larger and deeper on the left lung than on the right. Above and behind this is the =hilus=, at which the bronchus, vessels, and nerves enter the lungs. The bronchial lymph glands are also found here. Behind this the two lungs are adherent to each other over a triangular area. Above this is a groove for the œsophagus (Sulcus œsophageus), which is deepest on the left lung. A groove for the aorta (Sulcus aortæ) curves upward and backward over the hilus, and passes backward near the dorsal border of the lung; the curved part of the groove for the aortic arch is absent on the right lung, on which there is a groove for the vena azygos. Anteriorly there are grooves for the trachea, the anterior aorta, the anterior vena cava, and other vessels.

The =dorsal= (or superior) =border= (Margo obtusus) is long, thick, and rounded; it lies in the groove alongside of the bodies of the thoracic vertebræ.

The =ventral= (or inferior) =border= (Margo acutus) is thin and short. It presents, opposite to the heart, the =cardiac notch= (Incisura cardiaca). On the left lung this notch is opposite to the third, fourth, and fifth ribs, so that a considerable area of the pericardium here lies in direct contact with the chest-wall. On the right lung the notch is less extensive, and is an intercostal space further forward.

The =base= of the lung (Basis pulmonis) is oval in outline; its surface (Facies diaphragmatica) is deeply concave in adaptation to the thoracic surface of the diaphragm. Laterally and posteriorly it is limited by a thin =basal border= which fits into the narrow recess (Sinus phrenico-costalis) between the diaphragm and the chest-wall. The position of this border, of course, varies during respiration. In the deepest inspiration it may reach the bottom of this recess; in ordinary inspiration it lies about four or five inches (ca. 10 to 12 cm.) from the costal arch, to which it is nearly parallel; in ordinary expiration it lies about twice as far from the costal arch.

The =apex= of the lung (Apex pulmonis) is prismatic, narrow, and flattened transversely. It is partially marked off from the rest of the lung by the cardiac notch. It curves downward, and is related internally to the anterior mediastinum and behind to the anterior part of the pericardium.

In the horse the lungs are not divided into distinct =lobes= by deep fissures, as in most mammals. In the case of the left lung there is no lobation, but the right lung presents a =mediastinal lobe=, separated from the body of the lung by a fissure which forms in its dorsal part a canal for the posterior vena cava and the right phrenic nerve, inclosed in a special fold of the right pleura.[129]

The =root= of the lung (Radix pulmonis) is composed of the structures which enter or leave the lung at the hilus on the mediastinal surface. These are: (1) The =bronchus=; (2) the =pulmonary artery=; (3) the =pulmonary veins=; (4) the =bronchial artery=; (5) the =pulmonary nerves=; (6) the =lymph vessels=, which open into the bronchial lymph glands. The bronchus is situated dorsally, with the bronchial artery on its upper surface and the pulmonary artery immediately below it. The pulmonary veins lie chiefly below and behind the artery.

The =lobulation= of the lungs is not very evident on account of the small amount of interlobular tissue. The lobules appear on the surface or on sections as irregular polygonal areas of different sizes.[130]

=Bronchial Branches.=—Each bronchus at its entrance into the lung gives off a branch to the apex of the lung. It is then continued backward as the main stem bronchus, parallel with the dorsal border of the lung, giving off branches (Rami bronchiales) in monopodic fashion; these branch similarly and reach all parts of the lung. The right bronchus gives off a special branch to the mediastinal lobe.

The arrangement of the larger bronchi is not the same in the two lungs. The right bronchus, after detaching the apical branch, gives off a large ventral branch which shortly bifurcates. One of the divisions passes ventrally into the part of the lung behind the cardiac notch (homologue of cardiac lobe), while the other passes downward and backward into the lower part of the base. The stem bronchus then gives off the branch to the mediastinal lobe, and divides, about six inches from the hilus of the lung, into two nearly equal branches; the dorsal branch is a direct continuation of the stem, the ventral one passes into the middle part of the base. The left bronchus, after a short course, divides into two nearly equal branches. The dorsal branch may be regarded as the continuation of the stem; it passes backward parallel with the dorsal border of the lung, giving off a ventral branch much further back than in the right lung. The ventral division of the stem bronchus divides, after a very short course, into two branches; the anterior one passes ventrally into the homologue of the cardiac lobe, while the posterior one passes downward and backward into the base of the lung.

The =structure= of the larger bronchial tubes is, in general, similar to that of the trachea. Their walls contain irregular plates of cartilage instead of rings. There is a continuous layer of unstriped muscle, composed of circularly arranged bundles. The mucous membrane presents numerous longitudinal folds; it contains many elastic fibers, mucous glands, and lymph nodules, and is lined by ciliated columnar epithelium. As the tubes diminish in size the coats become thinner and the cartilages smaller; in tubes about 1 mm. in diameter the cartilages and mucous glands are absent.

By repeated branching the =interlobular bronchi= are formed, and from these arise the =lobular bronchioles=. The latter enter a lobule and branch within it, forming the =respiratory bronchioles=; these give off the =alveolar ducts=, the walls of which are pouched out to form hemispherical diverticula, the =alveoli= or =air-cells=.

A =lobule= of the lung, the unit of lung structure, is made up of a lobular bronchiole with its branches, and their air-cells, blood- and lymph vessels, and nerves. Between the lobules is the =interlobular tissue=, which forms the supporting framework of the lung.

=Vessels of the Lungs.=—The branches of the =pulmonary artery= carry venous blood to the lungs. They accompany the bronchi, and form rich capillary plexuses on the walls of the alveoli. Here the blood is arterialized, and is returned to the heart by the =pulmonary veins=. The =bronchial arteries= are relatively small vessels which carry arterial blood for the nutrition of the lungs. The branches of these arteries accompany the bronchial ramifications as far as the alveolar ducts, but do not extend to the alveoli. The œsophageal artery also supplies pulmonary branches (in the horse) which reach the lung by way of the ligament of the latter. These branches vary much in size, and ramify chiefly in the subpleural tissue of the basal part of the lung.[131] The =lymph vessels= are numerous, and are arranged in two sets. The superficial set forms close networks in and under the pleura, while the deep set accompanies the bronchi and pulmonary vessels. All converge to the root of the lung and enter the =bronchial lymph glands=.

=Nerve-supply.=—The pulmonary nerves come from the vagus and sympathetic nerves. They enter at the hilus and supply branches to the bronchial arteries and the air-tubes.

THE THYROID GLAND

The =thyroid gland= (Glandula thyreoidea) is a very vascular ductless gland, situated on the trachea close to the larynx. It is red brown in color, and consists of two =lateral lobes= and a connecting =isthmus= (Fig. 354).

The =lateral lobes= are situated on either side of the first and second or second and third rings of the trachea, to which they are loosely attached. Each is about the size of a walnut, and has a convex superficial face which is related to the parotid gland and the omo-hyoid muscle, and a slightly concave, deep face, applied to the trachea.

The =isthmus= in the adult horse is usually very rudimentary. It may occur as a very narrow glandular band which connects the posterior extremities of the lateral lobes, extending across the ventral surface of the trachea, but it is frequently only a small strand of connective tissue, and sometimes is entirely absent. It may be represented by a small tail-like process of one lobe.

The lateral lobes vary in size and position, and are often dissimilar on the two sides. The anterior extremity is the larger; it is rounded and usually lies about a finger’s breadth behind the larynx, but may be in contact with it. The posterior extremity is often produced to form a narrow tail-like process, which is continuous with the isthmus when one is present. In the ass and mule the isthmus is usually well developed. There is sometimes a narrow band of thyroid tissue which extends forward from the isthmus for a variable distance; this is termed the pyramidal lobe. The position of the lateral lobe is indicated approximately by the junction of the external maxillary and jugular veins.

=Structure.=—The gland is enveloped by a thin, fibrous =capsule=, from which =trabeculæ= pass into the gland, dividing it into =lobules=. The lobules consist of non-communicating =alveoli= or =follicles= of varying form and size. The alveoli are lined by cubical epithelium, and contain a viscid colloid substance.

=Blood-supply.=—The arteries are relatively very large; there are usually two thyroid arteries, which arise from the carotid and enter each extremity of the gland.

=Nerve-supply.=—The nerves are derived from the sympathetic system.

Nodules of thyroid tissue of variable size, sometimes as large as a pea, may be found near the anterior extremity of the lateral lobes, on the course of the anterior thyroid artery or on the trachea, even at a considerable distance from the thyroid region. They are termed =accessory thyroids= (Glandulæ thyreoidæ accessoriæ).

The =parathyroids= are small glandular bodies found in the thyroid region. They are paler and not so dense as the thyroid tissue, but often cannot be differentiated by their naked-eye appearance. In the horse there is usually only one on each side in the connective tissue over the anterior extremity of the lateral lobe of the thyroid. They resemble in structure embryonic thyroid tissue.

THE THYMUS GLAND

The =thymus= is a ductless gland which has a close resemblance to the lymphoid tissues. It is well developed only in late fœtal life and for a few months after birth. After this it undergoes rapid atrophy, fatty infiltration, and amyloid degeneration, so that in the adult it is represented by a thin remnant in the lower part of the anterior mediastinum or has entirely disappeared. In the new-born foal it is of a grayish-pink color, and consists of two =lateral lobes= (Fig. 358). The greater part of the gland is situated in the anterior mediastinum, but the two lobes are continued into the neck by a chain of lobules which lie along the course of the carotid artery, extending sometimes as far as the thyroid gland.[132]

=Structure.=—The thymus is inclosed in a thin, connective-tissue =capsule=, and consists of secondary =lobules= (Lobuli thymi), of varying shape and size, held together by areolar tissue and fat, and all attached to a cord of adenoid tissue termed the =tractus centralis=. These lobules are subdivided into polyhedral primary lobules, which are composed of lymphoid tissue, and present a dark vascular =cortex=, and a lighter =medulla= that contains Hassal’s corpuscles.

=Blood-supply.=—Internal thoracic and carotid arteries.

=Nerve-supply.=—Sympathetic and vagus nerve.

RESPIRATORY SYSTEM OF THE OX

THE NASAL CAVITY

The =nostrils=, situated on either side of the muzzle, are relatively small, and are much less dilatable than those of the horse. The alæ are thick and firm. The superior commissure is narrow. There is no false nostril or diverticulum nasi. The skin is smooth, bare, and moist, and there is no clear line of demarcation between it and the nasal mucosa. The opening of the naso-lacrimal duct is not visible, as it is on the outer wall of the nostril, and is concealed by the cartilaginous prolongation of the inferior turbinal. There are two =parietal= or =lateral cartilages= on either side, united by fibrous tissue. The dorsal cartilages are thin laminæ which curve outward and downward from the dorsal margin of the septal cartilage; they are prolongations of the nasal bones, and carry the alar cartilages on their anterior extremities. The ventral pair are lateral continuations of the basal lamellæ of the inferior turbinals. They lie along the nasal processes of the premaxillæ; anteriorly, they become thicker, turn a little upward, and each blends with the cartilaginous prolongation of the upper coil of the inferior turbinal. The =alar cartilages= are of peculiar form. The lamina is oblong, and curves outward and downward from the anterior extremity of the dorsal parietal cartilage, with which it is connected. The cornu springs from the outer part of the lamina and curves outward and upward into the outer wing of the nostril; it carries on its extremity a small transverse bar, thus having some resemblance to the fluke of an anchor.

The =nasal cavity= is short, wide anteriorly, narrow behind. It is not completely divided by the septum, which in its posterior third is separated from the floor of the cavity by an interval that increases from before backward. The middle meatus is very narrow, and divides posteriorly into two branches; the upper division leads to the ethmoidal meatuses, and communicates with the frontal sinus and the cavity of the superior turbinal. Anterior to the division are communications with the upper cavity of the ventral turbinal and the maxillary sinus. The main facts in regard to the sinuses were stated in the osteology.

THE LARYNX

The =larynx= is more compact than in the horse, and presents numerous differential features. The =cricoid cartilage= is compressed laterally. The lamina is not distinctly marked off from the arch; it slopes downward and backward and has a large median ridge. The =thyroid cartilage= is complete ventrally, i. e., the laminæ are united throughout to form a long plate which is notched in front and behind; the laryngeal prominence is small and is situated posteriorly. The height and thickness of the cartilage increase from before backward. The posterior cornua are about an inch long, and form syndesmoses with the cricoid cartilage; the short anterior cornua unite similarly with the hyoid bone. The =epiglottis= is oval in outline and its apex is rounded; its base rests on the thyro-hyoid membrane, to which it is rather loosely attached. The cuneiform cartilages are absent. The =arytenoid cartilages= present only slight differences. The muscular process is well developed, and the vocal process is narrow and long. The =vocal cords= project very little from the wall, so that the rima glottidis is wide; the vocal ligament is attached ventrally to the crico-thyroid membrane; it has the form of a half tube, open behind. The =lateral ventricles= are extremely shallow, and the saccules are absent. The thyro-arytenoideus muscle is not divided; it is fan-shaped, thick, and narrow dorsally; ventrally, it is attached to the base of the epiglottis, the angle of the union of the thyroid laminæ, and the crico-thyroid membrane. The hyo-epiglotticus muscle is large and bifid.

THE TRACHEA

The caliber of the trachea is relatively small, its width being about an inch and a half (ca. 4 cm.) and its height one and a half to two inches (ca. 4 to 5 cm.). The rings are smaller, and number about fifty. In the neck their free ends are separated by a varying interval, so that the tube is flattened and membranous dorsally. In the thorax the ends are in apposition, so that they form a ridge dorsally. The trachea is adherent to the right lung from the second rib backward. The bifurcation is opposite the fourth intercostal space.

THE BRONCHI

There are three chief bronchi. The bronchus for the apical and the anterior cardiac lobe of the right lung is detached from the trachea at a considerable distance anterior to the bifurcation.

THE THORACIC CAVITY AND PLEURA

The thoracic cavity is relatively small; it is especially short dorsally, and is diminished laterally by the mode of attachment of the diaphragm to the ribs (vide Myology). The endothoracic fascia is better developed than in the horse, and is distinctly elastic. The pleura is also thick, and there are no perforations of the mediastinum. The pleural sacs are more unequal in extent than in the horse, and the ventral part of the mediastinum is further to the left; in front of the pericardium it lies for the most part on the left wall of the chest.

THE LUNGS

The difference in size between the two lungs is greater than in the horse, the right lung weighing about half as much again as the left one. The average weight of the lungs is about 7½ pounds (ca. 3 to 4 kg.); they form about ¹⁄₁₇₀ of the body-weight.

They are divided into lobes by deep =fissures= (Incisuræ interlobares). The =left lung= is divided into =three lobes=, named from before backward =apical=, =cardiac=, and =diaphragmatic=. The =right lung= has =four= or =five lobes=; the =apical lobe= is much larger than that of the left lung, and occupies the space in front of the pericardium, pushing the mediastinum against the left wall;[133] the =mediastinal lobe= resembles that of the horse, while the =cardiac lobe= is often divided by a fissure into two parts. The apical lobe of the right lung receives a special bronchus from the trachea opposite the third rib. The right lung is adherent to the trachea from the second rib backward.

The interlobar fissures begin at the ventral margin of the lung and pass toward the root. Those of the left lung lie opposite to the fourth and sixth ribs. The diaphragmatic lobe is the largest, and has the form of a three-sided pyramid with its base resting on the diaphragm. The cardiac lobe is prismatic and forms the posterior margin of the cardiac notch; its long axis corresponds to the fifth rib. The apical lobe of the left lung is small and pointed; its ventral margin lies on the pulmonary artery and the anterior aorta; below this the space in front of the heart is occupied by the apical lobe of the right lung. The cardiac lobe of the right lung is much larger than that of the left lung, and covers the right face of the pericardium, so that the latter has no contact with the lateral wall of the thorax.

The =lobulation= is very distinct on account of the extremely large amount of interlobular tissue.

In the =sheep= the costal attachment of the diaphragm resembles that of the horse, and the basal part of the lung corresponds thereto. The lobation of the lungs resembles that of the ox, but the lobulation is very indistinct. The pleural sacs extend under the first lumbar vertebra.

THE THYROID GLAND

The lobes of the thyroid gland are more extensive and paler in color than in the horse; they extend further forward, overlapping the crico-pharyngeus muscle to a small extent, and come in contact dorsally with the œsophagus. They are flattened, and have an irregular, lobulated surface. In the young subject the isthmus is well developed, being about half an inch (ca. 1 to 1.5 cm.) in width.

Accessory thyroids may be found, and parathyroids also occur near the posterior extremity or inner surface of the lateral lobes of the thyroid.

THE THYMUS GLAND

The thymus is pale and distinctly lobulated. It is much larger in the calf than in the foal, weighing at five or six weeks about 15 to 25 ounces (ca. 425 to 600 grams). The cervical part is specially large, thick, and compact; it extends to the thyroid gland. It undergoes atrophy slowly and remnants of the thoracic part often remain even in advanced age.

RESPIRATORY SYSTEM OF THE PIG

THE NASAL CAVITY

The =nostrils= are small, and are situated on the flat anterior surface of the =rostrum= or snout (Rostrum suis). The latter is a short cylindrical projection, with which the upper lip is fused, and is circumscribed by a prominent circular margin. The skin on the snout is thin and highly sensitive; it presents small pores, and scattered over it are fine short hairs. In the snout between the nostrils is the =os rostri=, which is to be regarded as a special development of the extremity of the septum nasi in adaptation to the habit of rooting. A plate of cartilage, representing the lamina of the alar cartilage of the horse, curves outward and downward from the upper part of the os rostri and a pointed bar of cartilage curves upward from the lower part of the bone in the outer wing of the nostril. The notch between the nasal bone and the premaxilla is closed in by parietal cartilages which resemble those of the ox.

The =nasal cavity= is long and narrow. It is divided behind by a horizontal plate into an upper olfactory part, which leads to the ethmoidal meatuses, and a lower respiratory part, which is a direct continuation of the inferior meatus. The posterior part of the septum is membranous.

The =turbinal bones= resemble in general those of the ox. The =superior meatus= is exceedingly small. The =middle meatus= is a deep fissure between the two turbinals: it divides posteriorly into two branches; one of these extends upward and backward between the lateral mass of the ethmoid bone and the superior turbinal; the other widens and joins the inferior meatus. The opening between the maxillary sinus and the middle meatus lies in a plane through the last cheek teeth. The middle meatus is continuous with the space inclosed by the dorsal part of the inferior turbinal and with the cavity of the superior turbinal. The dorsal division of the middle meatus presents several openings into the frontal sinus. The =inferior meatus= is relatively roomy; it communicates with the space inclosed by the ventral coil of the inferior turbinal. The opening of the naso-lacrimal duct is found in the posterior part of the inferior meatus. The naso-palatine duct and the vomero-nasal organ (of Jacobson) resemble those of the ox.

THE LARYNX

The larynx is remarkable for its great length and mobility. The cartilages are more loosely attached to each other than in the other animals. The =cricoid cartilage= is thick and compressed laterally; its lamina is long and narrow; its arch is directed obliquely downward and backward. The =thyroid cartilage= is very long; its laminæ are united ventrally and form a median ridge. The anterior cornua being absent, there are no joints formed with the hyoid bone. The posterior cornua are broad, bent inward, and articulate with the cricoid cartilage. The =epiglottis= is relatively very large, and is more closely attached to the hyoid bone than to the rest of the larynx. The middle part of its base is turned forward, and rests on the thyro-hyoid membrane; it is closely connected with the body of the hyoid bone by the hyo-epiglottic ligament and the strong hyo-epiglottic muscle.[134] The apex of the =arytenoid cartilage= is very large, and is divided into two parts at its extremity; the inner part is fused with that of the opposite cartilage. There is a small =interarytenoid cartilage= in the transverse ligament. The =rima glottidis= is very narrow. The =vocal cords= are directed obliquely downward and backward; and each is pierced by a long, slit-like opening, which leads into the large laryngeal saccule. The thyro-arytenoid or vocal ligament is similarly divided into a larger anterior and a smaller posterior part. There is a middle ventricle near the base of the epiglottis. The thyro-arytenoid muscle is very strong and is undivided. The arytenoideus is very small. The crico-thyroid consists of two strata; the superficial layer corresponds to the muscle of the other animals; the deep part consists of transverse fibers.

TRACHEA AND BRONCHI

The trachea is circular in cross-section, and contains thirty or more rings which almost meet dorsally. A special bronchus is detached for the apical lobe of the right lung, as in the ox.

THE LUNGS (Figs. 324, 325)

The =right lung= has =four= or =five lobes=, according to whether the apical is subdivided or not. The =left lung= resembles that of the ox in form and lobation. The lobulation is distinct, but the interlobular septa are thinner than in the ox.

THE THYROID GLAND

The two lobes of the thyroid are long and flattened; they are situated close together, on the ventral surface of the trachea, extending as far forward as the cricoid cartilage. The gland is dark red in color.

THE THYMUS

The thymus is very large, extending to the larynx or even to the submaxillary space in young subjects.

RESPIRATORY SYSTEM OF THE DOG

THE NASAL CAVITY

The =nostrils= are situated on the muzzle, with which the upper lip blends. They are shaped somewhat like a comma, with the broad part next to the septum and the narrow part directed backward and outward. The skin around the nostrils is bare, usually black, and in health moist and cool. The muzzle is marked by a median furrow (philtrum) or a deep fissure in some breeds. The cartilaginous framework is formed essentially by the septal cartilage and the parietal cartilages which proceed from it. The septal cartilage projects beyond the premaxilla, and is much thickened at its extremity; it gives off from its upper and lower margins the two parietal cartilages, which curve laterally and toward each other. A grooved plate extends outward from the septal cartilage in the floor of the nostril, and another lamina supports the alar fold of the inferior turbinal bone.

The length of the =nasal cavity= varies greatly in different breeds, corresponding, of course, to the length of the face. The cavity is roomy, but is very largely occupied by the turbinals and the lateral masses of the ethmoid bone. The =middle meatus= is short and narrow, and divides posteriorly into two branches; the upper branch leads to the ethmoidal meatuses; the lower branch joins the inferior meatus. The =inferior meatus= is very small in its middle part, owing to the great development here of the inferior turbinal. The posterior part of the nasal cavity is divided by a horizontal plate, the lamina transversalis, into an upper olfactory part and a lower respiratory part. The sinuses have been described (vide Osteology).

THE LARYNX

The larynx is relatively short. The lamina of the =cricoid cartilage= is wide; the arch is grooved laterally. The laminæ of the =thyroid cartilage= are high, but short; they unite ventrally to form the body, on which there is anteriorly a marked prominence, and posteriorly a deep notch. The oblique line on the outer surface of the lamina is prominent. There is a rounded notch (Fissura thyreoidea) below the short anterior cornua for the passage of the superior laryngeal nerve. The posterior cornu is strong, and has a rounded surface for articulation with the cricoid cartilage. The =arytenoid cartilages= are relatively small, and have between them a small =interarytenoid cartilage=. The =epiglottis= is quadrilateral; its lower part or stalk is narrow, fitting into the angle of the thyroid cartilage. The =cuneiform cartilages= are large and somewhat crescent-shaped; they are not blended with the epiglottis.

The =false vocal cords= extend from the cuneiform cartilages to the thyroid. The =true vocal cords= are large and prominent. The entrance to the large lateral ventricle and saccule is a long slit parallel with the anterior margin of the true vocal cord.

The crico-thyroid muscle is thick. The hyo-epiglotticus is well developed, and is double at its hyoid attachment. The anterior part of the thyro-arytenoideus arises on the cuneiform cartilage. Hence Lesbre has suggested the name cuneo-arytenoideus for it.

THE TRACHEA AND BRONCHI

The trachea is very slightly flattened dorsally. It contains forty to forty-five =U=-shaped rings; the ends of the rings do not meet dorsally, so that here the wall of the trachea is membranous and is composed of a layer of transverse smooth muscle-fibers outside of the rings, the fibrous membrane, and the mucous membrane.

The stem bronchi diverge at an obtuse angle, and each divides into two branches before entering the lung. The anterior branch goes to the apical and cardiac lobes, the posterior one to the diaphragmatic lobe, supplying also the mediastinal lobe of the right lung. The anterior branch of the left bronchus crosses under the pulmonary artery.

THE LUNGS

The lungs (Figs. 342, 343) differ in shape from those of the horse and ox in conformity with the shape of the thorax, which is relatively very wide in the dog; the lateral thoracic walls are strongly curved, and the costal surface of the lungs is correspondingly convex. There is no cardiac notch on the left lung, although a small part of the pericardium is not covered by the lung at the lowest part of the fifth and sixth intercostal spaces. On the right side the cardiac notch is opposite the fourth and fifth intercostal spaces. Each lung is divided by deep fissures into =three lobes=—=apical=, =cardiac=, and =diaphragmatic=; the right lung has in addition a =mediastinal lobe=. The apical lobe of the right lung is considerably larger than that of the left. The latter is often more or less fused with the cardiac lobe. Variable accessory lobes may be found. On account of the small amount of interlobular tissue the lobulation is not distinct.

THE THYROID GLAND

The =lateral lobes= of the thyroid gland are long and narrow, and have a flattened, ellipsoidal form; they are placed on the lateral surfaces of the trachea near the larynx. The extremities are small, the posterior one often being pointed. The =isthmus= is inconstant and variable; in large dogs it usually has the form of a glandular band which may be nearly half an inch (ca. 1 cm.) wide; in small dogs it is usually absent.

=Accessory thyroids= are frequently present; three or four may be found on either side, as well as a median one near the hyoid bone.

The =parathyroids=, about the size of millet or hemp seed, are four in number usually. Two are placed on the deep face of the thyroid lobes; the others are placed laterally near the anterior extremity of the thyroid.

THE THYMUS

The thymus is relatively small, and is situated almost entirely in the thorax. The left lobe is much larger than the right, and extends back as far as the sixth rib at the time of its greatest size. According to Baum the gland increases in size during the first two weeks after birth, and atrophies rapidly in the next two or three months. Traces of it are present at two or three years of age, and may be found even in old subjects.

THE UROGENITAL SYSTEM

This system (Apparatus urogenitalis) includes two groups of organs, the urinary and the genital. The =urinary organs= elaborate and remove the chief excretory fluid, the urine. The =genital organs= serve for the formation, development, and expulsion of the products of the reproductive glands. In the higher vertebrates the two apparatus are independent except at the terminal part, which constitutes a urogenital tract, and includes the vulva in the female and the greater part of the urethra in the male.

THE URINARY ORGANS

The =urinary organs= (Organa uropoiëtica) are the kidneys, ureters, bladder, and urethra. The =kidneys= are the glands which secrete the urine; they are red-brown in color, and are situated against the dorsal wall of the abdomen, being in most animals almost symmetrically placed on either side of the spine. The =ureters= are tubes which convey the urine to the =bladder=. The latter is an ovoid or pyriform sac, situated on the pelvic floor when empty or nearly so; it is a reservoir for the urine. The urine accumulates in the bladder until that organ is full, and is then expelled through the =urethra=.

THE URINARY ORGANS OF THE HORSE

THE KIDNEYS

Each kidney (Ren) presents two surfaces, two borders, and two extremities or poles, but they differ so much in form and position as to require a separate description of each in these respects.[135]

The =right kidney= (Ren dexter) in outline resembles the heart on a playing card, or an equilateral triangle with the angles rounded off. It lies ventral to the upper parts of the last two or three ribs and the tip of the first lumbar transverse process. The =dorsal= (or superior) =surface= (Facies dorsalis) is strongly convex; it is related chiefly to the diaphragm, but also to a small extent posteriorly to the psoas muscles. In well hardened specimens, especially those from thin subjects, impressions of the last two ribs and the tip of the first lumbar transverse process are usually visible. The =ventral= (or inferior) =surface= (Facies ventralis) is in general slightly concave, and is related to the liver, pancreas, cæcum, and right adrenal; it either has no peritoneal covering, or only a narrow area externally is so covered. The =internal border= (Margo medialis) is convex and rounded; it is related to the right adrenal and the posterior vena cava. It presents about its middle a deep notch, the =hilus= (Hilus renalis); this is bounded by rounded margins, and leads into a space termed the =renal sinus= (Sinus renalis). The vessels and nerves reach the kidney at the hilus, and the sinus contains the pelvis or dilated origin of the ureter. The =external border= (Margo lateralis) is rounded, and is thinner than the internal one. It consists of two parts, anterior and posterior, which meet at an external angle; the anterior part fits into the renal impression of the liver. The duodenum curves around the external border. The =anterior extremity= (Extremitas cranialis), thick and rounded, lies in the renal impression of the liver. The =posterior extremity= (Extremitas caudalis) is thinner and narrower.

The =left kidney= (Ren sinister) is bean-shaped. It is considerably longer and narrower than the right one, and is situated nearer the median plane and further back, so that the hilus of the left kidney is about opposite to the posterior extremity of the right one. It lies usually under the upper part of the last rib and the first two or three lumbar transverse processes. The =dorsal surface= is convex, and is related to the left crus of the diaphragm, the psoas muscles, and the base of the spleen. The =ventral surface= is convex and irregular; the greater part of it is covered by the peritoneum. It is in relation with the origin of the small colon, the terminal part of the duodenum, the left adrenal, and the left extremity of the pancreas. The =internal border= is longer, straighter, and thicker than that of the rightkidney. It is related to the posterior aorta, the adrenal, and the ureter. The =external border= is related chiefly to the base of the spleen. The =anterior extremity= extends almost to the saccus cæcus of the stomach; it is related to the left end of the pancreas, and the splenic vessels. The =posterior extremity= is usually larger than the anterior one.

The form of the left kidney is variable. In some cases its outline is similar to that of the right kidney, but its ventral surface is convex and is often marked by several furrows which diverge from the hilus. In well hardened specimens the three areas of the dorsal surface are often distinct. The psoas area is flat, parallel with the inner border, and widens behind. The diaphragmatic area is small and convex; it is crescentic and is confined to the anterior pole. The splenic area, convex and external, is often so extensive and distinct as really to constitute a third surface.

=Fixation.=—The kidneys are held in position chiefly by the pressure of adjacent organs and by the renal fascia. The latter is a special development of the subperitoneal tissue, which splits into two layers to inclose the kidney, together with the perirenal fat or capsula adiposa. On account of its relations with the liver, pancreas, and the base of the cæcum, the right kidney is much more strongly attached than the left one. It is, therefore, not surprising that the latter varies somewhat in position; its posterior pole may be found ventral to the third or fourth lumbar transverse process. The position of the right kidney seems to be very constant, excluding its movements during respiration.

=Weight and Size.=—The right kidney is usually one or two ounces heavier than the left. The average weight of the right kidney is about 23 to 24 ounces (ca. 700 gm.); that of the left, about 22 to 23 ounces (ca. 670 gm.). The relation of the weight of both kidneys to the body-weight is about 1 ∶ 300–350.

Chauveau gives as an average 750 grams for the right kidney, and 710 grams for the left. Ellenberger and Baum (24 cases) give the right kidney as varying between 430 and 840 grams; and the left between 425 and 780; this is an average of 635 grams (about 22½ ounces) for the right kidney, and 602.5 grams (about 21½ ounces) for the left. They give the relation of the weight of both kidneys to the body-weight as 1 ∶ 255–344. In a Percheron mare weighing about 2000 pounds the right kidney weighed 4 pounds 3 ounces, and the left one 4 pounds. In a horse of medium size the right kidney is about six inches (ca. 15 cm.) in length, about the same in width, and about two inches (ca. 5 cm.) thick. The left kidney is about seven inches (ca. 18 cm.) long, four to five inches (ca. 10 to 12 cm.) wide, and two to two and a half inches (ca. 5 to 6 cm.) thick.

=Structure.=—The surface of the kidney is covered by a thin but strong =fibrous capsule= (Tunica fibrosa), which is in general easily stripped off the healthy kidney. It is continued along the hilus and lines the renal sinus. Sections through the kidney show it to consist of an external cortical substance and an internal medullary substance.[136] The =cortical substance= (Substantia corticalis) is red brown in color and has a granular appearance. It is dotted over with minute dark points; these are the =renal= (or Malpighian) =corpuscles= (Corpuscula renis), each consisting of the dilated origin of a uriniferous tubule (Capsula glomeruli), with an invaginated tuft of capillaries (Glomerulus) inclosed by it. The =medullary substance= (Substantia medullaris) is more resistant and presents a distinct radial striation. Its central part is pale, but its periphery, the =intermediate zone=, is of a deep red color; in the latter are seen, at fairly regular intervals, sections of the relatively large arciform vessels, which are taken to represent the demarcation between the primitive lobes.[137] Between the vessels the medulla is prolonged somewhat toward the periphery, forming the bases of the =renal= (or Malpighian) =pyramids=. These are not very pronounced in the kidney of the horse, especially as the gland is not papillated. Between the bases of the pyramids processes of the cortex dip in toward the sinus, forming the =renal columns=.[138] The inner central part of the medulla forms a concave ridge or crest which projects into the pelvis of the kidney; the crest is marked by numerous small openings at which the renal tubules terminate.

Examination with a pocket lens shows that the cortex is imperfectly divided into =lobules= (Lobuli corticales). Each lobule consists of an axial =radiate part= (Pars radiata), surrounded by a =convoluted part= (Pars convoluta). The former (formerly termed pyramids of Ferrein) appear as ray-like prolongations from the bases of the pyramids (hence also termed medullary rays), and consist largely of narrow, straight or slightly flexuous, tubules (limbs of the loops of Henle). The convoluted part (formerly termed the labyrinth) is granular in appearance, and consists largely of the renal corpuscles and convoluted tubules.

The =pelvis= of the kidney (Pelvis renalis) is the dilated origin of the excretory duct. It lies in the sinus of the kidney, and it is funnel-shaped, but flattened dorso-ventrally. The =renal crest=[139] (Papilla communis) projects into the outer part of the pelvis in the form of a horizontal ridge with a concave free edge. The tubules of the middle part of the medullary substance open on this crest into the pelvis. The tubules from each end of the kidney do not open into the pelvis proper (Recessus medius), but into two long, narrow diverticula (Recessus terminales), which proceed from it toward the poles of the kidney. The wall of the pelvis consists of three layers. The external =fibrous coat= or adventitia is continuous with the supporting tissue of the kidney. The =muscular coat= consists of longitudinal and circular fibers. The =mucous coat= does not cover the renal crest, nor is it continued into the diverticula of the pelvis. It has a yellowish tinge, and presents numerous folds. It contains glands (Glandulæ pelvis renalis) which secrete the thick mucus always found in the pelvis.

=Renal Tubules=—The parenchyma or proper substance of the kidney is composed of the small =renal= or =uriniferous tubules= (Tubuli renales), which are very close together and have a complicated course. Each tubule begins in a thin-walled, spherical dilatation or =capsule=, which is invaginated to receive a tuft of looped capillaries termed a =glomerulus=; these two structures constitute a =renal= (or Malpighian) =corpuscle=; the corpuscles are visible as minute red or dark spots in the convoluted part of the cortex. Succeeding this is a short narrow neck, beyond which the tubule becomes wide and convoluted, forming the proximal convoluted tubule, and enters the radiate portion of the cortex. It then gradually narrows and enters the intermediate zone; becoming very narrow and nearly straight, it descends for a variable distance into the medullary substance, turns sharply upon itself, and returns to the cortex, forming thus the loop of Henle, with its descending and ascending limbs. In the convoluted part of the cortex, it widens and becomes tortuous, constituting the distal convoluted tubule. The tubule then narrows, enters a medullary ray, and opens with other tubules into a straight collecting tubule; this passes axially through a pyramid, and unites with other collecting tubules to form the relatively large papillary ducts, which open into the renal pelvis.

=Stroma.=—The interstitial tissue forms a reticulum throughout which supports the tubules and blood-vessels. It is very scanty in the cortex, much more abundant in the medulla, in which it increases in amount toward the pelvis.

=Blood-supply.=—The kidneys receive a large amount of blood through the =renal arteries=. Branches of these enter at the hilus and on the ventral surface of the gland, and reach the intermediate zone, where they form anastomotic arches (Arteriæ arciformes). From these arciform arteries branches pass into the cortex and medulla. The cortical branches (Arteriæ interlobulares) have in general a radial course between the cortical lobules, and give off short lateral branches, each of which ends as the =afferent vessel= (Vas afferens) of a renal corpuscle. The blood is carried from the glomerulus by a smaller =efferent vessel=, which breaks up immediately into capillaries which form networks around the tubules. The medullary branches descend in the pyramids, forming in them bundles of straight twigs (Arteriolæ rectæ).

The =veins= correspond in general to the arteries. In the superficial part of the cortex the veins form star-like figures (Venæ stellatæ) by the convergence of several small radicles to a common trunk.

=Lymph Vessels.=—These may be divided into two sets, capsular or superficial, and parenchymatous or deep.

=Nerve-supply.=—The nerves are derived from the renal plexus of the sympathetic, and form a plexus around the vessels.

THE URETERS

The =ureters= are the narrow part of the excretory ducts of the kidneys. Each begins at the renal pelvis and terminates at the bladder. It is about ⅓ to ¼ inch (ca. 6 to 8 mm.) in diameter, and its average length is about 28 inches (ca. 70 cm.). The =abdominal part= (Pars abdominalis) of each ureter emerges ventrally from the hilus of the kidney, and curves backward and inward toward the lateral face of the posterior vena cava (right side) or the posterior aorta (left side). They then pass almost straight backward in the subperitoneal tissue on the surface of the psoas minor, cross the external iliac vessels, and enter the pelvic cavity. The =pelvic part= (Pars pelvina) passes backward and a little downward on the lateral wall of the pelvic cavity, turns inward, and pierces the dorsal wall of the bladder near the neck.

In the male the pelvic part enters the urogenital fold and crosses the vas deferens. In the female the ureter is situated in the greater part of its course in the dorsal part of the broad ligament of the uterus.

The wall of the ureter is composed of three coats. The outer =fibrous coat= (Tunica adventitia) is composed of loose fibrous tissue. The =muscular coat= (Tunica muscularis) consists of inner and outer layers of longitudinal fibers, with a stratum of circular fibers between them. The =mucous membrane= (Tunica mucosa) is covered with transitional epithelium; glands (Glandulæ mucosæ ureteris) resembling those of the renal pelvis occur in the first three or four inches of the ureter.

THE URINARY BLADDER

The =urinary bladder= (Vesica urinaria) (Figs. 270, 271, 272) differs in form, size, and position according to the amount of its contents. When empty and contracted, it is a dense, pyriform mass, about the size of a fist, lies on the ventral wall of the pelvic cavity, and does not reach to the inlet. When moderately filled, it is ovoid in form, and extends a variable distance along the ventral abdominal wall. Its physiological capacity varies greatly, but may be estimated approximately at about three or four quarts.

The anterior rounded blind end is termed the =vertex=;[140] on its middle is a mass of cicatricial tissue (Centrum verticis), a vestige of the urachus, which in the fœtus forms a tubular connection between the bladder and the allantois. The =body= or middle part (Corpus vesicæ) is rounded, and is somewhat flattened dorso-ventrally, except when distended. It presents two surfaces, dorsal and ventral, the former being the more strongly convex, especially in its posterior part in front of the entrance of the ureters.[141] The posterior narrow extremity, the =neck= (Collum vesicæ), joins the urethra.

The =relations= of the bladder vary according to the degree of fullness of the organ, and also differ in important respects in the two sexes. The ventral surface lies on the ventral wall of the pelvis, and extends forward on the abdominal wall as the bladder fills. The dorsal surface in the male is related to the rectum, the urogenital fold, the terminal parts of the vasa deferentia, the vesiculæ seminales, and the prostate; in the female it is in contact, instead, with the body of the uterus and the vagina. The vertex of the full bladder is related to coils of the small intestine and small colon, and to the pelvic flexure of the large colon.

=Fixation.=—Displacement of the bladder is limited chiefly by three peritoneal folds, termed the middle and lateral ligaments (Figs. 257, 272). The =middle ligament= (Plica umbilicalis media) is a median triangular fold, formed by the reflection of the peritoneum from the ventral surface of the bladder on to the ventral wall of the pelvis and abdomen. In the new-born animal it is extensive and reaches to the umbilicus; in the adult it is usually much reduced in length relatively. It contains elastic and muscular fibers in its posterior part. The =lateral ligaments= (Plicæ umbilicales laterales) stretch from the lateral aspects of the bladder to the lateral pelvic walls. Each contains in its free edge a round, firm band, the =round= or =umbilical ligament= (Lig. umbilicale s. teres); this is the remnant of the large fœtal umbilical artery, the lumen of which in the adult is very small. The retroperitoneal part of the bladder is attached to the surrounding parts by loose connective tissue, in which there is a quantity of fat. It is evident that the posterior part of the bladder has a definite fixed position, while its anterior part is movable.

=Structure.=—The wall of the bladder consists of a partial peritoneal investment, the muscular coat, and the mucous lining.

The =serous coat= (Tunica serosa) covers the greater part of the dorsal surface, from which it is reflected in the male to form the urogenital fold; in the female it passes on to the vagina, forming the vesico-genital pouch. Ventrally the peritoneum covers only the anterior half or less of the bladder, and is reflected posteriorly on to the pelvic floor.

The =muscular coat= (Tunica muscularis) is relatively thin when the bladder is full. It is unstriped, pale, and not clearly divided into layers, but has rather a plexiform arrangement. Longitudinal fibers occur on the dorsal and ventral surfaces, but laterally they become oblique and decussate with each other. A distinctly circular arrangement is found at the neck, where the fibers form a =sphincter= (Annulus urethralis).

The =mucous coat= (Tunica mucosa) is pale and thin. It is in general attached by a highly elastic submucosa to the muscular coat, and forms numerous folds when the organ is empty and contracted. It is modified dorsally in the vicinity of the neck over a triangular area, termed the =trigonum vesicæ=; the angles of this space lie at the orifices of the two ureters and the urethra, which are close together. Here the mucous membrane is closely attached and does not form folds. From each =ureteral orifice= (Orificium ureteris) a fold of mucous membrane (Plica ureterica) passes backward and inward, uniting with its fellow to form a median crest (Crista urethralis) in the first part of the urethra. The ureteral orifices are a little more than one inch (ca. 3 cm.) apart. The terminal part of the ureter, after piercing the muscular coat of the bladder, passes for a distance of about an inch (ca. 2 to 3 cm.) between the muscular and mucous coats before piercing the latter; this arrangement constitutes a valve which prevents absolutely the return of the urine from the bladder into the ureter. The =internal urethral orifice= (Orificium urethæ internum) lies at the apex of the trigonum, and is about an inch and a half (ca. 4 cm.) behind the ureteral orifices. The mucous membrane is covered with transitional epithelium like that of the ureter and renal pelvis. It contains lymph nodules.

=Blood-supply.=—This is derived chiefly from the vesico-prostatic branch of the =internal pudic artery=. It is also supplied by small twigs from the =obturator= and =umbilical arteries=. The =veins= terminate chiefly in the =internal pudic veins=. They form plexuses posteriorly.

=Lymph Vessels.=—These form plexuses on both surfaces of the muscular coat. They go to the internal iliac and lumbar glands.

=Nerves.=—The nerves are derived from the =pelvic plexus= (sympathetic and ventral branches of third and fourth sacral nerves). They form a plexus in the submucosa which presents microscopic ganglia.

In the fœtus and new-born animal the bladder is situated chiefly in the abdomen. It is long, narrow, and fusiform. Its abdominal end lies at the umbilicus, through which it is continued by the urachus to the extra-embryonic part of the allantois. As the pelvis increases in size and the large intestine grows, the bladder retracts into the pelvis and changes its form.

The urethra will be described with the genital organs.

THE ADRENAL BODIES

The =adrenal bodies= or =suprarenal glands= (Glandulæ suprarenales)[142] are two small, flattened organs, which lie in contact with the anterior part of the inner border of the kidneys (Figs. 376, 377). They are ductless.

In the horse they are red-brown in color, about three and a half to four inches (ca. 9 to 10 cm.) long, one to one and a half inches (ca. 3 to 4 cm.) wide, and about half an inch or more (ca. 1 to 5 cm.) in thickness. The weight varies from one to two ounces (ca. 28 to 56 gm.).

The =right adrenal= is related internally to the posterior vena cava, to which it is adherent. Its anterior part curves dorsally around the inner border of the right kidney. Its posterior part is flattened and is related ventrally to the pancreas and cæcum, dorsally, to the right renal vessels. The anterior extremity is concealed in the renal impression of the liver; the posterior is related to the ureter.

The =left adrenal= is a little shorter than the right one, and its extremities are rounded; it is usually curved, so that its internal border partly embraces the anterior mesenteric artery. Its dorsal surface is related to the kidney, the renal artery, the aorta, and the left cœliaco-mesenteric ganglion. The ventral surface is in relation with the left extremity of the pancreas and the root of the great mesentery. The posterior extremity often curves inward behind the anterior mesenteric artery; it is related behind to the left renal vein.

When hardened in situ the adrenals present several features not evident in the soft organs. The anterior part of the right adrenal is twisted upward and outward over the inner margin of the kidney, so that this part is prismatic and has three surfaces; of these, the concave outer one is applied to the kidney, the dorsal one is related to the right crus of the diaphragm and the liver, and the inner one is in contact with the posterior vena cava. The anterior mesenteric artery is nearly always more or less enlarged as a result of verminous arteritis, and the form and degree of curvature of the left adrenal seem to vary in conformity with the condition of the artery.

=Structure.=—The =fibrous capsule= adheres intimately to the surface of the organ. It contains elastic fibers, and in its deep part unstriped muscle-fibers. From it trabeculæ pass radially into the substance, blending with the fine supporting reticulum. The =parenchyma= consists of a cortical and a medullary portion. The =cortical substance= (Substantia corticalis) is red-brown in color, and is clearly distinguishable from the yellow =medullary substance= (Substantia medullaris). A large =central vein= (Vena centralis) is visible on cross-sections.

The cells of the cortex are arranged in chains of one or two rows. In the peripheral portion the cells are of high cylindrical shape, and the chains form connecting loops; Günther has proposed the name zona arcuata for this part, instead of the usual term zona glomerularis. More deeply the chains are distinctly palisade-like, and this region is called the zona fasciculata. Next to the medulla is the zona reticularis, in which the chains form a network. The cells in these two zones are polygonal and contain a brown pigment.

The cells of the medulla are arranged in irregular groups or form sheaths around the veins. They react to chromic salts by assuming a yellow or yellow-brown color, and are termed chromaffin cells; they share this peculiarity with certain cells of the sympathetic ganglia and paraganglia, with which they are probably related genetically. The alkaloid adrenalin appears to be formed in the medullary cells.

=Vessels and Nerves.=—The adrenals receive a relatively large blood-supply through the =adrenal arteries= (Aa. suprarenales), which arise from the renal arteries or from the aorta directly. The =veins= terminate in the posterior vena cava and the left renal vein. The =lymph vessels= go to the renal lymph glands. The numerous =nerves= are derived from the sympathetic system through the solar and renal plexuses. The fibers form a rich interlacement, especially in the medullary substance. Ganglion cells are found chiefly in the medulla, but also occur in the deeper part of the cortex.

URINARY ORGANS OF THE OX

The =kidneys= are superficially divided into polygonal lobes by fissures of variable depth. The lobes vary in size, and are commonly about twenty to twenty-five in number. The fissures are filled with fat.

The =right kidney= has an elongated elliptical outline, and is flattened dorso-ventrally. It lies ventral to the last rib and the first two or three lumbar transverse processes. The dorsal surface is rounded, and is in contact chiefly with the sublumbar muscles. The ventral surface is less convex, and is related to the liver, pancreas, duodenum, and terminal part of the colon. The hilus is situated on the anterior part of this surface near the inner border. The internal border is nearly straight, and lies parallel with the posterior vena cava. The external border is convex. The anterior extremity occupies the renal impression of the liver, and is capped by the adrenal body.

The =left kidney= occupies a remarkable position, and when hardened in situ, differs strongly in form from the right one. When the rumen is full, it pushes the kidney backward and across the median plane, so that it is situated on the right side, behind and at a lower level than the right kidney. It then lies usually under the third, fourth, and fifth lumbar vertebræ. When the rumen is not full, the left kidney may lie partly to the left of the median plane. It has three surfaces. The dorsal surface is convex, and presents on its antero-external part the hilus, which opens outward. The ventral surface is related to the intestine. The third face is often more or less flattened by contact with the rumen, and may be termed the ruminal surface. The anterior extremity is small, the posterior large and rounded.

The preceding statements refer to the adult subject, and are based on investigations made on living subjects, and studies of frozen sections and material hardened in situ. In the young calf the kidneys are almost symmetrically placed, but as the rumen grows it pushes the left kidney to the right and backward pari passu. It also usually causes a rotation of the kidney, so that the primary dorsal surface comes to lie almost in a sagittal plane. Further, the gland is bent so that the hilus is largely closed up and faces outward (to the right). In very fat subjects the three-sided appearance of the kidney may be absent, and about one-third or more may remain to the left of the median plane, even where the rumen is pretty well filled.

The kidneys are embedded in a large amount of perirenal fat. The weight of a kidney is about 20 to 25 ounces (ca. 570 to 700 gm.), the left one being usually an ounce or more the heavier. The two form about ¼ per cent. of the body-weight.

The right kidney measures about 8 to 10 inches (ca. 20 to 25 cm.) in length, 4 to 5 inches (ca. 10 to 12 cm.) in width, and 2½ to 3 inches (ca. 7 cm.) in thickness. The left kidney is one or two inches (ca. 2 to 5 cm.) shorter, but its posterior part is much thicker than the right one.

=Structure.=—The hilus is equivalent to the hilus and sinus of the kidney of the horse; in the right kidney it is an extensive elliptical cavity, in the left, a deep fissure. The pelvis is absent. The ureter begins at the junction of two wide, thin-walled tubes, the =calyces majores=; the anterior calyx is usually the larger. Each calyx major gives off a number of branches, and these divide into several funnel-shaped =calyces minores=, each of which embraces a renal papilla. The space not occupied by the calyces and vessels is filled with fat.

On section through the kidney the =renal pyramids= are easily made out. The blunt apex of each pyramid, the =papilla renalis=, projects into a calyx minor. On each papilla are small orifices (Foramina papillaria) by which the terminal renal tubules (Ductus papillares) open into the calyx. The renal columns are much more distinct than in the horse.

At the hilus the renal artery is dorsal, the vein in the middle, and the ureter ventral; a quantity of fat surrounds these structures in the hilus.

The kidneys of the =sheep= are bean-shaped and smooth, without any superficial lobation. The soft organ is regularly elliptical in form, with convex dorsal and ventral surfaces and rounded extremities; its length is about 2½ to 3 inches (ca. 5 to 8 cm.), its width about 1½ to 2 inches (ca. 4 to 5 cm.), and its thickness a little more than 1 inch (ca. 3 cm.). In position they resemble those of the ox, except that the right one is usually a little further back, and lies under the first three lumbar transverse processes. The average weight of each is about four ounces. The hilus is in the middle of the inner border. There is a renal crest or common papilla formed by the fusion of twelve to sixteen pyramids.

When the rumen is full, the left kidney (which is attached by a short mesentery) usually lies entirely to the right of the median plane, under the third, fourth, and fifth lumbar transverse processes. The primitive dorsal surface has become ventro-medial, and is somewhat flattened by contact with the rumen.

The =ureters= are, in general, like those of the horse, except in regard to the first part of the left one, which has a peculiar course, in conformity with the remarkable position of the kidney. It begins at the ventral part of the hilus (which faces toward the right), curves upward and inward over the outer aspect of the kidney to its dorsal surface, crosses the median plane, and runs backward on the left side.

The =bladder= is longer and narrower than that of the horse, and extends further forward on the abdominal floor. The peritoneal coat extends backward further than in the horse.

THE ADRENAL BODIES

The =right adrenal= lies against the inner part of the anterior pole of the right kidney. When hardened in situ it is pyramidal in form. Its inner surface is flattened and is in contact with the right crus of the diaphragm. The outer surface is convex and lies in the renal impression of the liver. The ventral surface is grooved for the posterior vena cava; on this surface a relatively large vein emerges near the apex. The base is concave and rests obliquely against the anterior pole of the kidney. The apex fits into the angle between the posterior vena cava and the dorsal border of the liver.

The =left adrenal= lies on the inner face of the posterior vena cava just behind the anterior mesenteric artery, and is therefore practically median in position. It is flattened, and irregularly triangular or heart-shaped in outline. Its left face is related to the dorsal sac of the rumen. Its right face is related to the vena cava, and presents a large emergent vein. The posterior border or base is deeply notched.

It is interesting to note that the left adrenal does not migrate with the kidney, but retains its primitive position; it lies usually about two inches (ca. 5 cm.) in front of a transverse plane through the anterior pole of the left kidney.

The adrenals of the =sheep= are both bean-shaped. The right one lies along the anterior part of the inner border of the kidney, at the angle of the junction of the right renal vein and the posterior vena cava. It is a little over an inch (ca. 3 cm.) long, and about an inch (ca. 12 to 15 mm.) wide. The left adrenal is longer, flatter, and somewhat bent. It lies across the left renal vein, to which it is attached, but is not in contact with the kidney.

URINARY ORGANS OF THE PIG

The =kidneys= are smooth and bean-shaped; they are more flattened dorso-ventrally, more elongated, and smaller at the extremities than those of the dog. The length is about twice the width. They are usually almost symmetrically placed on either side of the first four lumbar vertebræ. (Variations in position are not rare, and involve the left kidney oftener than the right. The former has been found near the pelvic inlet.) The external border lies against the flank parallel with the edge of the longissimus muscle. The posterior extremity usually lies about midway between the last rib and the external angle of the ilium. The anterior extremity of the right kidney usually extends as far forward as the last rib, but has no contact with the liver.

The weight of the kidney of a medium-sized pig is about seven to nine ounces (200 to 250 grams). The ratio of their combined weight to that of the body is about 1 ∶ 150–200.

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