A guide to the dissection of the blood vessels and nerves of the pectoral and pelvic limbs of the horse

Preface

Every teacher who has had experience with classes in Anatomy appreciates the diffictilties that confront the student in his attempt to dissect any portion of the body with only his text-book of Descriptive Anatomy, for a guide. The present "Guide" has been prepared in the hope of assisting students in the dissection of the blood-vessels and nerves of the pectoral and pelvic limbs. A knowledge of the muscles is presupposed. In our experience, we have found it most desirable that, upon the completion of a part, the student should demonstrate his dissection to the instructor in charge. By no other means can the .student be brought to realize more clearly the vast difference between studying and knowing the actual object itself and the mere memorizing the description of it as given in

his text-book. .

G. S. H.

Dissection of the Blood-Vessels and Nerves of the Arm or Pectoral Limb

Cut the skin along the median Une from the xiphoid cartilage to a point 15 or 20 cm. cephalad of the presternum; also cut at a right angle to this along the middle of the proximal half of the antebrachivini and reflect the skin from the pectoral muscles. Note the large vein (V. cephalica) and the small artery lying in the groove formed by the anterior superficial pectoral muscle and the adjoining border of the M. mastoido-himieralis. Transect the two superficial pectorals close to their sternal attachments. Now dissect up the cervical panniculus, transect it a short distance from its attachment to the sternum and turn it aside. Beneath this portion of the pannicvilus is a mass of fatty tissue in which are numerous lymph nodules — the Prescapular or superficial cervercal lymph gland. The full extent of this gland may be seen when the M. mastoido-humeralis is turned aside. Transect the mastoido-hiuneralis a little above (cephalad) the point of the shoulder and turn it aside sufficiently to expose the underlying prescapular portion of the anterior deep pectoral muscle and the prescapvdar lymph gland. Do not disturb unnecessarily the gland for beneath it are an artery and vein of some surgical importance. Trace the large vein (V. cephalica) and the small artery (a branch of the A. cervicalis inferior) that cross the anterior deep pectoral muscle in the groove between this muscle and the mastoido-hiuneralis, in so far as the vessels are at present accessible. Ligate and cut the vessels. Abduct the arm somewhat and identify the two deep pectoral muscles. Transect the latter muscles close to their attachment to the sternum. Abduct the arm a little and dissect between the deep pectoral muscles and the thoracic wall until the large A. and V. Axillaris are exposed. Carefully clean up the V. Axillaris noting meantime the lymph glands, Posterior cervical or prepectoral, that lie just cephalad or anterior to the V. Axillaris. Cut the vein in order clearly to expose the A. Axillaris which lies immediately dorsal to it. Clean up the A. Axillaris taking care not to cvit any of its branches or the nerves that cross the ventral surface of the artery a short distance (4 to 6 cm.) from the first rib. Cut the artery close to the rib. Occasionally a large vein lies just anterior to the A. Axillaris; if so, this also should be cut.

A. cervicalis inferior arises from the axillaris nearly opposite the first rib. Near the ventral border of the M. scalenus it divides into two nearly equal branches — a ventral and a dorsal branch. The ventral branch crosses the superficial surface of the anterior deep pectoral and then runs in the groove formed by this muscle and the mastoido-humeralis, as already seen, and terminates in the mastoido-himieralis. Small branches are supplied to the posterior cervical lymph glands, the cervical pannicultis, the stemomandibularis, sterno-thyrohyoideus, anterior superficial pectoral and a small branch to the region of the presterntun. The dorsal branch extends antero-dorsally along the anterior border of the anterior deep pectoral muscle. It passes beneath the prescapular lymph gland and between the muscles subscapulo-hyoideus and mastoido-htomeralis to the latter of which it is principally distributed. It also gives off branches to the anterior deep pectoral muscle and to the posterior cervical and prescapular lymph glands.

The ventral portion of the M. scalenus intervenes between the axillary blood-vessels and the nerves that form the brachial plexus. The brachial plexus is formed by the ventral divisions of the 6th, 7th and 8th, cervical and the ist and 2d thoracic nerves, as will be demonstrated later. Cut the dorsal branch of the A. cervicalis inferior and carefully remove the connective tissue that surroimds the large nerve trtmks of the brachial plexus.

N. phrenicus or diaphragmatic nerve originates from the ventral divisions of the 6th and 7th cervical nerves ; occasionally it receives a small branch from the 5th cervical also. The two or three radicals of the phrenic nerve extend obliquely across the superficial face of the M. scalenus and at its ventral border unite to form a single trunk, just as the nerve enters the thorax, or a short distance within the thoracic cavity. Dissect up from the surface of the scalenus the two or three radicals of the N. phrenicus. Immediately after crossing the A. cervicalis inferior the N. phrenicus receives a small communicating branch from the inferior cervical ganglion of the sympathetic nerve. The course of the phrenic nerve through the thoracic cavity to the diaphragm will be studied when the thoracic portion, of the vagus and sympathetic nerves are traced.

Exposure of the brachial plexus and removal of the thoracic limb.

Expose as follows, the ventral divisions of the nerves that form the brachial plexus, viz., the 6th (not constant), 7th and 8th cervical and the I St and 2d thoracic. Cut the N. phrenicus at the first rib. Remove a section of the ventral portion of the M. scalenus opposite the 6th, 7th, and 8th intervertebral foramina. Cut the skin and the Mm. panniculus and latissimus, some distance (30 to 50 cm.) posterior to the scapula; cut the skin and both portions of the trapezius close to the ligamentum nuchae;

also cut the serratus magnus, and rhomboideus close to their attachments to the scapula thus leaving the arm attached to the body only by the nerves of the brachial plexus. Before attempting to expose farther the ventral divisions of the 6th, if present, 7th and 8th cervical and the 1st and 2d thoracic nerves, first note on a skeleton the relation of the first two ribs to the 6th, 7th and 8th cervical and the ist and 2d thoracic intervertebral foramina, through which the above mentioned nerves emerge from the spinal cord. Now place the arm in such a position that the whole of the thoracic portion of the M. serratus magnus is exposed. Carefully remove the small portion of the M. scalenus that lies dorsal to the brachial plexus taking care not to cut any of the nerves. The large nerve to the thoracic portion of the serratus magnus arises from the 7th and 8th cervicals (or from the 8th only) and extends posteriorly on the surface of the serratus magnus to which it is distributed. This nerve, more or less covered by connective tissue, may emerge from between the dorsal and ventral portions of the scalenus, or it may pass through the small dorsal portion of the muscle. After tracing the nerve, cut it 15 to 20 cm. from its origin. The ventral divisions of the 7th and 8th cervical nerves are readily exposed ; the 1st and 2d thoracic nerves however, are covered by the ribs of which the first one must be cut in order to expose the nerves. Remove enough of the serratus magnus to expose the first rib. Cut the rib a little ventral to the A. Axillaris, also cut it close to the tuberosity of the rib and remove the detached portion. The A. cervicalis inferior should now be traced to its place of origin from the A. Axillaris. With a little care the ventral divisions of the first two thoracic nerves may be found and cut close to the intervertebral foramina ; the 7th and- 8th cervical nerves also should be cut close to the vertebrae and the arm removed.

Nn. pectorales. Before attempting to demonstrate the nerves to the pectoral muscles first carefidly remove the connective tissue and fat from the nerves of the brachial plexus using for this purpose the forceps, the back of the scalpel, or some other non cutting instrument. The origin of the pectoral nerves vary somewhat in different individuals. The following disposition however, is common. The most cephalic or anterior of the pectoral nerves originates from the 7th and 8th cervicals and is distributed wholly to the anterior deep pectoral muscle (PL I, 27). The nerve enters the muscle a little dorsal to the cephalic vein where it crosses the anterior deep pectoral. A communicating branch between this nerve and the phrenic is quite commonly present. The nerves to the superficial pectoral muscles arise from the loop or arch formed by the N. musculo-cutaneous (PL I, 29) and the N. medianus (PL I, 32) immediately beneath the A. axil-

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laris and from the brachial plexus, in common with the N. medianus (PI. I, 30, 31). Carefully dissect up the overlying posterior portion or sternal extremity of the anterior deep pectoral and trace the nerves to their respective places of distribution in the superficial pectoral muscles. The two or more nerves to the posterior deep pectoral and the panniculus are readily found. They arise from the brachial plexus apparently in common with the Nn. medianus and ulnaris. The smaller and more anterior of the two nerves (in the present specimen) soon divides into three or four branches which enter the muscle near its humeral end along with the artery and vein supplied to the muscle. The larger and more posterior nerve also divides into several branches some of which are distributed to the posterior deep pectoral and some to the panniculus ; note particularly the large branch which accompanies the large V. thoracica externa which runs along the dorsal margin of the posterior deep pectoral and is continued posteriorly in the panniculus.

A. suprascapularis arises from the A. axillaris near the anterior border of the tendon of the subscapularis, and close to the N. musculo-ctitaneous where it crosses the A. axillaris. About one centimeter from its origin the A. suprascapularis divides into two branches, of which one is distributed to the subscapularis, anterior deep pectoral and the supraspinatus ; the other branch is distributed to the anterior deep pectoral, supraspinatus, the shoulder joint, and the proximal tendon of the biceps. The artery is accompanied by the V. suprascapularis.

N. suprascapularis (PI. I, 28; PL II, 21) originates from the 7th and 8th cervical nerves but principally from the 7th; occasionally some fibres originate from the 6th cervical nerve also. This large nerve passes between the adjacent borders of the supraspinatus and subscapularis. Carefully dissect the supraspinatus from the adjoining subscapularis, on one side of it, and the infraspinatus, on the other, and transect the muscle opposite the N. suprascapularis. Reflect the muscle sufficiently to expose the two or three large branches of the nerve to the supraspinatus, which branches are given off just as the nerve crosses the anterior border of the scapula. Trace the nerve across the supraspinus fossa and note its width, i to 2 cm. opposite the spine of the scapula. Just as the nerve enters the M. infraspinatus it divides into 5 or 6 branches, all of which are distributed to this muscle.

Nn. subscapulares (PI. I, 47). There are usually two or more of these nerves. They pass directly to the M. subscapularis but before reaching it they divide into numerous branches which diverge somewhat like the rays of a fan.

The nerve to the M. latissimus arises from near the middle of the brachial plexus, most of its fibres coming from the 8th cervical (PI. I, 48). It passes obliquely across the deep face of the Mm. subscapularis and teres major and is distributed wholly to the latissimus.

Nerves to the M. teres major {PI. I, 7). There are from i to 3 small nerves supplied to this muscle. They arise from the N. axillaris or rather are branches of it, and pass directly to the muscle, close to its anterior border.

N. axillaris (PL I, 46) arises from the 7th and 8th cervical nerves. It passes through the triangular space formed by the Mm. subscapularis, teres major, and the caput longum tricipitis. To demonstrate the distribution of the nerve first remove the skin and the panniculus from the lateral or external surface of the scapidar region. Dissect up the borders of the M. deltoideus and transect it about 15 to 18 cm. from its attachment to the deltoid tuberosity. Carefully reflect the humeral portion of the muscle in order to expose the N. axillaris which enters the deep face of the muscle directly opposite the above mentioned triangular space (PI. II, 22). Beneath the muscle, the nerve divides into several branches which are distributed as follows: several branches enter the deltoideus; one large branch passes obliquely across the superficial surface of the teres minor (near its humeral end) to the anterior face of the brachium and enters the mastoido-humeralis several centimeters below (distal to) the point of the shoulder (PL I, 24); another large branch crosses the superficial surface of the caput laterale tricipitis or sometimes, is embedded in the substance of this muscle, and emerges from beneath the posterior border of the M. deltoideus at the level of the deltoid tuberosity where it becomes subcutaneous (PL II, 23) ; from here, this subcutaneous branch extends to the antero-intemal aspect of the antebrachiiim where it breaks up into several cutaneous branches some of which extend nearly to the carpus. One branch of the N. axillaris is distributed to the teres minor. This branch may be readily demonstrated by transecting the muscle, a little proximal to the N. axillaris, and drawing aside the muscle. One or more branches are supplied to the teres major, one branch to the subscapularis and a very small filament to the M. capsularis. The branches to the subscapularis and the capsularis may arise from the main nerve trunk either separately or by a common branch.

Clean up the V. axillaris and the several veins from the pectoral, panniculus, and latissimus muscles also clean up the portion of the V. subscapularis that lies in the space between the Mm. subscapularis and teres major. Demonstrate the axillary lymph gland (PL I, 49), which lies on the tendon of the teres major in the angle formed by the Vv. subscapularis and brachialis. Now slit the posterior superficial pectoral longitudinally near its middle. Turn aside the anterior portion of the muscle, together with the posterior deep pectoral, in order to expose the Mm. coraco-brachialis and the biceps brachii.

N. musculo-cutaneous (PL I, 29, 29') is a large nerve and originates from the 7th and 8th cervical nerves. Immediately after crossing the lateral face of the A. axillaris (PL I, 21) a large portion of the nerve joins the N. medianus by a large short branch that passes beneath the A. axillaris and with the N. medianus forms a loop around it. Occasionally this nerve loop is entirely wanting. From the loop thus formed, one or more branches are given ofi to the superficial pectoral muscles as already demonstrated. The larger portion of the N. musculo-cutaneous however, passes through the M. coraco-brachialis (PL I, 13, 29'), the overlying portion of which should be cut just proximal to the nerve, and drawn aside. As the nerve enters the coraco-brachialis it supplies a branch to it. Trace the main nerve through the coraco-brachialis to its termination in the biceps brachii near to the proximal end of the fleshy portion of the muscle (PL I, 29'). The connective tissue and aponeurosis covering the vessels and nerves on the medial face of the brachial region should now be removed. Note that here the vessels and nerves have the following relations, viz., the N. medianus lies most anteriorly and is followed in order by the A. brachialis, V. brachialis, N. ulnaris, and finally, in the proximal part of the brachium, the large N. radialis. Note also the small mass of lymph glands lying on the artery and vein near the distal third of the brachium (PL I, 49').

N. radialis (PL I, 40) lies posterior to the Nn. medianus and ulnaris and is the largest nerve of the brachial plexus. It originates from the 7th and 8th cervical and the ist thoracic nerves, but principally from the 8th cervical and I St thoracic. The nerve passes into the triangular space formed by the tendon of the teres major and latissimus, the caput longum tricipitis and the proximal end of the brachiahs. An artery and vein also enter this space and should be left intact. As the N. radialis enters the above mentioned triangular space it gives off several branches which are distributed as follows: (a) one branch to the tensor fasciae antebrachii (PL I, 44). To demonstrate this branch, transect the tensor fasciae antebrachii near its distal third and turn the muscle aside but do not injure the nerve (a branch of the N. ulnaris) which runs along the anterior border (PL I, a), or sometimes crosses obliquely, the superficial surface of the distal half of the muscle. Sometimes a small communicating branch passes from

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the N. ulnaris to this branch of the N. radiahs supphcd to the tensor fasciae antebrachii. (b) A large fasciculus composed of several branches, some of which recurve slightly around the tendon of the teres major and latissimus; all the branches of this fasciculus are distributed to the caput longum tricipitis (PI. I, 41). (c) Branches to the caput laterale tricipitis, caput mediale tricipitis, and anconeus. Of these branches the small one to the caput mediale is readily found and traced to its muscle; the branches to the caput laterale and anconeus, however, are more or less completely hidden by the caput longum and caput laterale and often can be satisfactorily demonstrated only after the caput laterale has been dissected as follows: Turn aside the skin and the panniculus from the outer surface of the triceps. The posterior surface of the caput laterale may now be freely separated from the caput longum, but in dissecting up the anterior border of the muscle care should be taken not to cut the cutaneous nerve (a branch of the N. radialis) which emerges from beneath the muscle at, or close to, its anterior border (6 to 8 cm. distal to the cutaneous branch of the N. axillaris, or 8 to 10 cm. from the posterior edge of the M. deltoideus, PL II, c). Transect the caput laterale a little distal to its middle and freely reflect the distal portion of the muscle; the proximal portion of the muscle can be turned aside only a little distance as the nerve and blood-vessels to the muscle enter its deep or anterior face.

Trace this nerve and also the cutaneous branch to their origin from the N.. radialis. Now trace the main trunk of the N. radialis in its course along the musculo-spiral groove to the point where it passes beneath the M. extensor carpi radialis at its origin, near the proximal end of the external epicondyle of the humerus (PL II, 1 1 , 25) . Remove the skin of the antebrachivim as far as to the carpus. Trace the cutaneous branches of the N. axillaris and the N. radalis, the former to the antero-internal side of the antebrachium and the latter to the antero-extemal side of the antebrachium. Both of these cutaneous branches may readily be traced to near the carpus. The M. extensor carpi radialis should now be dissected as follows: Transect it near the middle and freely turn aside the distal portion, but in turning aside the proximal part of the muscle care must be exercised to avoid cutting or breaking the nerves and blood-vessels that enter the deep face of the muscle (near the posterior border of the distal end of the M. brachialis). Now trace the N. radialis beneath the proximal portion of the extensor carpi radialis taking care not to cut the branches of this nerve, usually three in number, that enter the deep face of the muscle (PL II, d). The main nerve now passes beneath the extensor digitalis communis, near the origin of the muscle. Transect the extensor digitalis communis near the middle of the antebrachium and as the proximal portion is reflected avoid breaking the blood-vessels and the two or three branches of the N. radialis supplied to the muscle (PI. II, e). Beneath this muscle the N. radialis gives off a small branch to the extensor carpi obliquus (PI. II, f) ; this branch lies directly upon the anterior-lateral face of the radius, at the side of the A. radialis anterior, and beneath the deep face of the extensor digitalis communis which must be turned aside sufficiently to expose the extensor carpi obliquus. Shortly before the N. radialis passes beneath the extensor digitalis lateralis, close to its origin, it gives off a small branch to this muscle (PL II, g). Slit open the aponeurotic sheath of the muscle and trace the branch some little distance. Now transect the muscle at the level of the N. radialis and trace the latter into the flexor carpi extemus where the nerve terminates (PL II, h). It will be seen from the above that the N. radialis supplies the following muscles; the triceps (caput longum, laterale and mediale), the tensor fasciae antebrachii, the anconeus, the extensor carpi radialis, the extensor digitalis communis, the extensor digitalis lateralis, the extensor carpi obliquus, and the flexor carpi externus, and one or more cutaneous branches to the skin of the antebrachium.

Blood-vessels of the Brachial Region

The large V. axillaris and several of the vessels opening into it, viz., V. thoracico-dorsalis from the deep pectoral muscles; the V. suprascapularis, the V. thoracico-externa together with the branches from the latissimus and panniculus that communicate with the V. thoracico-externa a short distance from its termination, have already been exposed. Additional veins should be noted as they are uncovered in the further dissection.

A. axillaris (PL I, 21). Upon emerging from the thorax the A. axillaris

crosses the tendon of origin of the coraco-brachialis and the insertion of the subscapularis, at the postero-internal side of the scapulo-humeral joint. From here the artey is continued by the A. brachialis (PL I, 23) which extends obliquely across the middle of the humerus to the antero-intemal side of the distal third of the shaft of the bone where it divides into two terminal branches — the A. radialis anterior and the A. radialis posterior.

Branches of the A. axillaris,

A. cervicalis inferior.

A. suprascapularis.

A. subscapularis.

The A. cervicalis inferior and the suprascapularis have already been traced, pp. 5, 8. The large A. subscapularis (PL I, 22) arises from the axillaris at the posterior border of the M. subscapularis. Before exposing the A. subscapularis first trace the branch, A. thoracico-dorsalis, that arises from the subscapularis i to 2 cm. from its origin (PI. I, immediately below 22). This artery extends obliquely across the inner face of the teres major supplies small branches to it and to the axillary lymph gland and terminates in the latissimus and panniculus. Transect the M. teres major near the middle and turn it aside. Now trace the A. subscapularis between the Mm. subscapularis and teres major across the inner surface of the caput longtim tricepitis to the posterior angle of the scapula. The A. subscapularis gives ofif the following branches: »

(i) A. thoracico-dorsalis already dissected. See above.

(2) A. circumflexa humeri posterior arises i to 2 cm. peripheral to the A. thoracico-dorsalis. It accompanies the N. axillaris to the deep face of the M. deltoideus and is distributed to this muscle and also to the caput longum, the caput laterale, the brachialis, the teres minor and a small twig to the capsule of the scapulo-humeral joint.

(3) A. circumflexa scapulae arises from the subscapularis 5 to 8 cm. peripheral to the A. circumflexa humeri posterior and extends directly to the posterior border of the scapula; here it divides into two branches of which one passes to the outer and the other to the inner surface of the scapula. The outer branch crosses, at nearly a right angle, the outer face of the scapula, a little below the spine, and is distributed to the infra and supraspinati and the teres minor. The inner branch is distributed to the subscapularis. The general course of these vessels is indicated by the vasciilar grooves of the scapula. (See cleaned scapula.)

(4) Rami musculares. Numerous small branches of the A. subscapularis are supplied to the IVIm. subscapularis and teres major. Two or more large branches of the A. subscapularis are given off to the caput longum, the tensor fasciae antebrachii, the infraspinatus and the deltoideus. The V. subscapularis accompanies the artery.

A. circumflexa humeri anterior arises from the brachialis i to 4 cm. peripheral to the subscapularis. It passes between the hixmerus and the M. coraco-brachialis, supplies a branch to this muscle and terminates in the biceps brachii and mastoido-hiuneralis. (The biceps may now be transected.) It gives off a branch which extends towards the scapiilo-humeral joint and is distributed to the biceps brachii and its s^^novial tendon sheath. Occasionally the A. circumflexa humeri anterior is a small vessel supplying only the coraco-brachialis.

A. profunda brachii (PI. I, 24) is a large but very short trunk, i to 3 cm. in length, which arises from the brachialis near the middle of the brachium, or at the level of the lower border of the tendon of the teres major and latissimus. The artery divides into several branches which are distributed to the caput longtmi, caput laterale and the brachialis. Not infrequently this artery is double at its origin.

A muscular branch to the biceps brachii originates from the anterior side of the brachialis, 6 to 8 cm. distal to the A. profunda brachii and nearly opposite the A. ulnaris, and enters the biceps a little below the middle of the muscle (PI. I). Occasionally it gives off a branch of considerable size to the M. brachialis. Turn aside the distal portion of the tensor fasciae antebrachii taking care not to ctit the cutaneous nerve (a branch of the ulnaris) which lies at the anterior border of this portion of the muscle.

A. ulnaris (PI. I, 25) arises from the brachialis near the nutrient foramen, at the distal third of the shaft of the humerus. The artery passes beneath the tensor fasciae antebrachii and along the anterior border of the caput mediale to the posterior part of the humero-radial joint where it passes beneath the antebrachial aponeurosis and the small ulnar head of the M. flexor carpi medius. Its further course along the antebrachium should be deferred until later.

Branches of A. ulnaris. Close to its origin it gives off a long small branch that winds obliquely across the internal and anterior faces of the tendon of the biceps brachii and extends to the posterior superficial pectoral and to the antebrachial aponeurosis. (Sometimes this small branch arises from the A. brachialis) (PI. I, just below 34')- About i to 2 cm. peripheral to the above branch the A. ulnaris gives off a larger, long, branch to the middle and posterior portions of the posterior superficial pectoral. The A. ulnerus also gives off one or more small branches to the tensor fasciae antebrachii and a branch of considerable size enters the olecranon fossa and supplies the joint capsule. The V. ulnaris accompanies the artery.

A. nutritia humerii (nutrient artery of the hiunerus).

The nutrient artery of the humerus sometimes arises directly from the brachialis and sometimes from the ulnaris. It passes directly into the nutrient foramen of the humerus.

Remove the skin from the inner side of the antebrachium, if it has not already been removed, and slit longitudinally the antebrachial aponeurosis. Identify the small, thin, ulnar head of the flexor carpi medius (PI. I, 17');

turn this aside taking care not to cut the very smah filament supphcd to it from the N. ulnaris which Has immediately under the ulnar head of this muscle (PL I, e). Now trace the A. ulnaris (vein and nerve also) along the antebrachium noting that in the proximal half of this region the artery lies between the ulnar and humeral heads of the flexor perforans and in the distal half of the antebrachium it lies between the flexor carpi medius and the flexor carpi extemus. Note that the nerve, artery and vein lie side by side and are immediately beneath the aponeurosis. Transect the flexor carpi medius near its middle and reflect the distal portion in order to expose the supracarpal arch which is formed by the anastomosis of the A. ulnaris with a branch from the A. metacarpae volaris lateralis, a little proximal to the carpus (PL III, i6). In demonstrating this vascular arch be careful not to cut any of the vessels arising from it. Two to four centimeters below the origin of the A. ulnaris the A. brachialis divides into its' two terminal branches — the A. radialis anterior and the A. radialis posterior.

N. ulnaris (PL I, 39) arises from the ist and 2d thoracic nerves. It lies posterior to the brachial artery and vein and between the median and radial nerves, the median being anterior and the radial posterior to it. Near the middle of the brachium the nerve passes beneath the tensor fasciae antebrachii and extends to the humero-radial joint where, with the A. and V. ulnaris, it dips beneath the small ulnar head of the flexor carpi medius (PL I, 17')- Near the middle of the brachium, a little before passing beneath the tensor fasciae antebrachii, the nerve gives off a cutaneous branch which extends along the anterior border of the tensor fasciae antebrachii and between it and the posterior superficial pectoral (PL I, a). The nerve does not supply the posterior superficial pectoral but passes through it and extends subcutaneously along the posterior face of the antebrachium nearly to the carpus. At the humero-radial joint the N. ulnaris gives off several branches the smallest of which supplies the ulnar head of the flexor carpi medius (PL I, e); a larger branch goes to the ulnar head of the flexor perforans (PL I, d) and the two largest branches are distributed one to the humeral head of the flexor carpi medius, and the other to the flexor perforatus. Occasionally the humeral head of the flexor perforans receives a branch from the N .ulnaris. These branches are all easily demonstrated and need no special directions for their dissection other than to turn aside the proximal portion of the flexor carpi medius in order to see the place where the branch that supplies it enters its deep face near the proximal end of the muscle. Now trace the N. ulnaris along its antebrachial course, noting carefully its relation to the aponeurosis, to the flexor perforans to the middle and i6