Part 83

The =ulnar nerve= (Figs. 441, 442, 443, 520) arises with the median from the thoracic components of the brachial plexus. It descends between the brachial artery and vein, accompanied for a short distance by the radial nerve. It then crosses the vein and continues behind the latter along the anterior border of the tensor fasciæ antibrachii and dips under that muscle near the elbow. Here it is joined by the ulnar vessels and passes downward and backward over the internal epicondyle of the humerus. In the forearm it crosses obliquely the deep face of the ulnar head of the flexor carpi medius and descends under the deep fascia with the vessels, at first on the ulnar head of the perforans, and then between the middle and external flexors of the carpus. Near the level of the accessory carpal bone it divides into two terminal branches, superficial and deep. It gives off two principal collateral branches. The =cutaneous branch= (Ramus cutaneus palmaris) is detached just before the nerve passes under the tensor fasciæ antibrachii; it runs downward and backward on that muscle under cover of the posterior superficial pectoral, becomes superficial below the elbow and ramifies on the posterior surface and both sides of the forearm. The =muscular branch= is given off at the elbow joint and divides to supply the flexors of the digit and the middle flexor of the carpus. Of the two terminals, the =superficial branch= (Ramus superficialis) emerges between the tendons of insertion of the external and middle flexors of the carpus and ramifies on the dorso-lateral aspect of the carpus and metacarpus. The =deep branch= (Ramus profundus), after a very short course, unites under cover of the tendon of the flexor carpi medius with the outer branch of the median nerve to form the external metacarpal or volar nerve.

MEDIAN NERVE

The =median nerve= (Figs. 441, 442, 447, 520, 521) derives its fibers chiefly from the first thoracic root of the brachial plexus. It is usually the largest branch and it accompanies the chief arterial trunks to the distal part of the limb. It descends over the inner face of the brachial artery, which it crosses obliquely, and continues down the arm in front of the artery. Near its origin it is joined by a large branch with the musculo-cutaneous nerve, thus forming a loop in which the artery appears to be suspended. Near the elbow it crosses obliquely over the artery (posterior radial) and lies behind it on the internal lateral ligament. Below the joint it again crosses the artery and lies behind the radius and the lower part of the long internal lateral ligament.

At the elbow the nerve is covered by the posterior superficial pectoral muscle and is crossed by the large oblique connection between the cephalic and brachial veins. The variable and often plexiform venous arrangement renders approach to the nerve here undesirable. It can be reached best by an incision just behind the lower part of the long internal lateral ligament; here the nerve lies under the deep fascia in the furrow between the radius and the anterior border of the flexor carpi internus, overlapped by the latter. In some cases the nerve retains its position in front of the artery at the elbow, and in others it crosses the deep face of the artery.

Passing beneath the internal flexor of the carpus, the nerve continues downward in the forearm with the posterior radial vessels, and divides at a variable distance above the carpus into two branches; these are the internal volar or metacarpal nerve and the branch which unites with the ulnar to form the corresponding external nerve.

At the proximal part of the forearm the nerve runs almost straight downward along the posterior border of the long internal lateral ligament, while the artery here inclines somewhat backward. Thus the nerve is superficial to the artery for a short distance, then lies in front of the latter to about the middle of the region, where it inclines a little backward and arrives at the interval between the internal and middle flexors of the carpus. The division may occur about the middle of the region, but commonly takes place in the distal third or fourth.

The collateral branches are as follows:

1. The =musculo-cutaneous branch= is in reality the continuation of the nerve of that name. It is given off about the middle of the arm, passes beneath the lower part of the biceps, and divides into muscular and cutaneous branches. The =muscular branch= enters the brachialis. The =cutaneous branch= emerges between the mastoido-humeralis and the biceps and divides into two branches; these descend on the fascia of the forearm with the cephalic vein and its accessory, and ramify on the front and inner face of the forearm, carpus, and metacarpus.

2. =Muscular branches= to the internal flexor of the carpus and the flexors of the digit are given off just as the nerve passes under the first named muscle.

3. The very small =interosseous nerve= passes through the interosseous space and is distributed chiefly to the periosteum, but in some cases twigs may go to the extensor muscles.

The =metacarpal= or =volar nerves=,[209] internal and external, are the continuations of the median and ulnar nerves in the distal part of the limb.

The =internal metacarpal= or =volar nerve= (N. volaris medialis) arises as the inner terminal branch of the median nerve at a variable distance above the carpus (Fig. 442). It descends through the carpal canal along the inner border of the superficial flexor tendon, and lies at first in front of the large metacarpal or common digital artery. It then passes behind the artery to the distal third of the metacarpus, where it lies behind the vein, the artery here becoming deeper in position. In addition to cutaneous twigs, the nerve gives off about the middle of the metacarpus a considerable =anastomotic branch= which winds obliquely downward and outward over the flexor tendons and joins the external nerve at the lower third of the metacarpus (Fig. 447). Near the fetlock the nerve divides into two digital branches.

1. The =dorsal= or =anterior digital branch= (Ramus dorsalis) descends at first between the digital artery and vein, then crosses over the vein and ramifies in the skin and the matrix of the hoof on the dorsal (anterior) face of the digit.

2. The =volar= or =posterior digital branch= (Ramus volaris) is the direct continuation of the trunk. It descends behind the digital artery, which it accompanies in its ramification. A =middle digital branch= is sometimes described as descending behind the vein. In some cases this branch is distinct, but usually there are instead several small twigs derived from the posterior branch, which cross very obliquely over the artery and anastomose in a variable manner with each other and with the anterior branch.

The =external metacarpal= or =velar nerve= (N. volaris lateralis) is formed by the union of the external terminal branch of the median with the deep branch of the ulnar nerve (Fig. 442). It descends with the internal volar or small metacarpal artery in the texture of the posterior annular ligament of the carpus. In the metacarpus it descends along the outer border of the deep flexor tendon behind the external metacarpal vein, and is accompanied by a small artery from the volar subcarpal arch. Toward the distal end of the metacarpus it is joined by the oblique branch from the internal nerve, and beyond this is arranged like the latter. Below the carpus it detaches a deep branch to the suspensory ligament and the interossei, and also supplies twigs to the skin.

Anastomoses are established between the digital branches, and the areas innervated by them are not well defined, but really overlap each other. In certain diseased conditions, however, in which the lesions are confined to the volar structures, relief from pain may be afforded by section of the volar branches only.

THE THORACIC NERVES

The =thoracic nerves= (Nn. thoracales) number eighteen on either side in the horse. They are designated numerically according to the vertebræ behind which they emerge. Most of them are arranged in a very similar manner and therefore do not require separate description. Each divides into a dorsal and a ventral branch, the latter being the larger.

The =dorsal branches= (Rami dorsales) emerge behind the levatores costarum and divide into internal and external branches. The internal branches ascend on the multifidus and supply the dorsal spinal muscles. The external branches run outward under the longissimus and emerge between that muscle and the transversalis costarum; after giving twigs to these muscles they pass through the latissimus dorsi and the lumbo-dorsal fascia and ramify as dorsal cutaneous nerves under the skin of the back (Fig. 523). In the region of the withers they give branches to the serratus anticus and rhomboideus, and their cutaneous terminals pass through these muscles and the dorso-scapular ligament to supply the skin over the ligamentum nuchæ and the scapular cartilage.

The =ventral branches= or =intercostal nerves= (Nn. intercostales) are much larger than the preceding, and are connected with the sympathetic by rami communicantes. The first goes almost entirely to the brachial plexus, but sends a fine branch downward in the first intercostal space which is expended in the muscle there without reaching the lower end of the space. The second ventral branch furnishes a considerable root to the brachial plexus, but its intercostal continuation is typical. The intercostal nerves (Fig. 184) descend in the intercostal spaces with the vessels of like name, at first between the intercostal muscles, and lower down chiefly between the pleura and the internal intercostal muscle. In the anterior spaces the artery lies along the posterior border of the rib, with the nerve in front of it; further back the nerve lies behind the border of the rib, with the artery in front of it. They supply the intercostal muscles, give off lateral perforating branches, and terminate in the following manner: The second to the sixth inclusive emerge through the spaces between the costal cartilages and concur in supplying the pectoral muscles. The second to the eighth give branches to the transversus thoracis. The succeeding ones give branches to the diaphragm, pass between the transverse and internal oblique muscles, give twigs to these, and end in the rectus abdominis. There are three series of cutaneous nerves given off by the intercostal nerves. The upper nerves emerge through the serratus magnus and the external intercostals about parallel with the digitations of the external oblique. The middle set perforate the origin of the latter muscle. The lower ones appear through the abdominal tunic. They supply branches to the abdominal muscles, the panniculus, and the skin. Some of the anterior ones anastomose with the posterior thoracic branches of the brachial plexus. The posterior three supply in part the skin of the flank. The ventral branch of the last thoracic nerve runs outward behind the last rib across the dorsal surface of the psoas major and divides into superficial and deep branches. The superficial branch passes between the obliquus internus and transversus abdominis, perforates the obliquus externus, and ramifies under the skin of the flank (Fig. 525). The deep branch descends on the inner face of the internal oblique to the rectus abdominis, in which it ends.

THE LUMBAR NERVES

There are six pairs of =lumbar nerves= (Nn. lumbales) in the horse, the last of which emerge between the last lumbar vertebra and the sacrum. The anterior two or three are about the same size as the thoracic nerves, but the others are much larger.

Their =dorsal branches= are small in comparison with the ventral ones. They are distributed to the muscles and skin of the loins and croup in a fashion similar to those of the thoracic nerves.

The =ventral branches= are connected with the sympathetic by small rami communicantes, and give branches to the sublumbar muscles. Those of the first two nerves are arranged in a manner analogous to the corresponding branch of the last thoracic nerve.

The ventral branch of the =first lumbar nerve= is termed the =ilio-hypogastric nerve= (N. iliohypogastricus). It passes outward between the quadratus lumborum and the psoas major, and divides at the outer border of the latter into an anterior or superficial and a posterior or deep branch. The =anterior= or =superficial branch= passes over the upper edge of the internal oblique, descends between that muscle and the external oblique, perforates the latter, and runs downward and backward and ramifies under the skin of the posterior part of the flank and the outer surface of the thigh. It gives branches to the transversus and obliquus externus abdominis. The =posterior= or =deep branch= is smaller; it runs downward and backward beneath the peritoneum to the outer border of the rectus abdominis, gives branches to the internal oblique, and terminates in the rectus abdominis.

The ventral branch of the =second lumbar nerve= is connected by an anastomotic branch with that of the third nerve. It gives off a large branch to the psoas muscle and an inguinal branch, and is continued as the ilio-inguinal nerve. The =inguinal branch= (N. spermaticus externus) runs backward in the substance of the psoas minor, emerges a short distance in front of the circumflex iliac vessels, and runs backward and downward under the peritoneum to the internal inguinal ring. It gives branches to the internal oblique muscle and descends in the inguinal canal along the outer border of the cremaster (to which it detaches filaments) and ends subcutaneously in the scrotum and prepuce in the male, the mammary gland in the female. The =ilio-inguinal nerve= (N. ilioinguinalis) may be regarded as the continuation of the ventral branch. It runs parallel with the ilio-hypogastric nerve and has a similar arrangement. Its =anterior= or =superficial branch= perforates the external oblique muscle a little in front of the point of the hip, runs downward on the front of the thigh and the outer surface of the stifle, and gives off cutaneous branches. The =posterior= or =deep branch= runs behind and parallel with that of the iliohypogastricus, detaches branches to the internal oblique muscle, and descends the inguinal canal with the inguinal branch, to be distributed to the external genital organs and the surrounding skin in the inguinal region.

The ventral branch of the =third lumbar nerve= is connected by a small anastomotic branch with the second nerve and furnishes a root of the lumbo-sacral plexus. It gives off a branch to the psoas muscles, an inguinal branch, and is continued as the external cutaneous nerve. The =inguinal branch= (N. spermaticus externus) passes backward in the substance of the psoas minor, from which it emerges under cover of or near the circumflex iliac vessels. It then runs external to and parallel with the external iliac artery and descends in the inner part of the inguinal canal. It emerges at the external ring with the external pudic artery and ramifies in the external genital organs and the skin of the inguinal region. The =external cutaneous nerve= of the thigh (N. cutaneus femoris lateralis) runs backward in the substance of the psoas muscles and emerges at the outer border of the psoas minor. It then passes outward and backward on the iliac fascia and accompanies the posterior branch of the circumflex iliac artery. With this vessel it perforates the abdominal wall by passing between the external oblique and the iliacus a short distance below the point of the hip, and descends on the inner face of the tensor fasciæ latæ (near its anterior border) and ramifies subcutaneously in the region of the stifle.

The origin and disposition of some of the foregoing nerves are variable. In some cases the ilio-inguinal nerve ends in the psoas major, and appears then to be absent. The mode of formation of the inguinal nerves is very inconstant. The inner nerve may arise with one of the outer ones from a common trunk, or they may anastomose. The inguinal branch of the ilio-inguinal nerve often receives a twig from the ilio-hypogastric nerve.

The ventral branches of the =fourth=, =fifth=, and =sixth lumbar= nerves concur in the formation of the lumbo-sacral plexus.

LUMBO-SACRAL PLEXUS

This plexus (Fig. 451) results substantially from the union of the ventral branches of the last three lumbar and the first two sacral nerves, but it derives a small root from the third lumbar nerve also. The anterior part of the plexus lies in front of the internal iliac artery between the lumbar transverse processes and the psoas minor. The posterior part lies partly upon and partly in the texture of the sacro-sciatic ligament. From the plexus are derived the nerves of the pelvic limb, which are now to be described.

FEMORAL NERVE

The =femoral= (or anterior crural) nerve (N. femoralis) is derived chiefly from the fourth and fifth lumbar nerves, but commonly receives a fasciculus from the third nerve also (Fig. 524). It is the larger of the two nerves which are given off from the anterior part of the lumbo-sacral plexus. It runs backward at first between the psoas major and minor, then crosses the deep face of the tendon of insertion of the latter, and descends under cover of the sartorius in the furrow between the two heads of the iliacus. It gives off the saphenous nerve, crosses the lower part of the outer head of the iliacus, and divides into several branches which dip into the interstice between the rectus femoris and the vastus internus (Fig. 450). These branches are accompanied by the anterior femoral vessels and innervate the quadriceps femoris. Collateral muscular branches are supplied to the ilio-psoas and sartorius.

The =saphenous nerve= (N. saphenus) (Figs. 450, 451, 457) is given off as the parent trunk crosses the furrow between the psoas major and the outer head of the iliacus. It descends with the femoral vessels in the femoral canal, and gives branches to the sartorius, gracilis, and pectineus. About the middle of the thigh it divides into several branches which emerge from between the sartorius and gracilis, perforate the deep fascia and ramify on the inner surface and the front of the limb as far downward as the hock. The longer posterior branches accompany the saphenous vessels, while the anterior branches deviate forward toward the stifle and the anterior surface of the leg.

OBTURATOR NERVE

This nerve (N. obturatorius) is derived from the ventral branches of the last three lumbar nerves (Fig. 524). It runs downward and backward, at first above and then upon the external iliac vein, inclines inward across the obturator vein, and passes through the anterior part of the obturator foramen in front of that vessel (Fig. 451). It continues downward through the obturator externus, and divides into several branches which innervate the obturator externus, pectineus, adductor, and gracilis muscles (Fig. 456).

ANTERIOR GLUTEAL NERVE

This nerve (N. glutæus cranialis) is derived chiefly from the last lumbar and first sacral nerves. It divides into four or five branches which emerge through the great sacro-sciatic foramen with the divisions of the anterior gluteal artery and supply the gluteal and tensor fasciæ latæ muscles. The nerve to the latter and the anterior part of the superficial gluteus passes between the deep part of the gluteus medius and the deep gluteal muscle, and is accompanied on the iliacus by branches of the external circumflex vessels.

POSTERIOR GLUTEAL NERVE

The =posterior gluteal nerve= (N. glutæus caudalis) is derived mainly from the sacral roots of the lumbo-sacral plexus (Figs. 451, 526). It divides into two trunks which emerge above the great sciatic nerve. The upper nerve passes backward on the sacro-sciatic ligament and divides into branches which enter the two heads of the biceps femoris; it supplies a branch to the posterior part of the middle gluteus, and a nerve which turns around the posterior border of the latter and enters the posterior head of the superficial gluteus. The inferior nerve runs downward and backward on the sacro-sciatic ligament and divides into the =posterior cutaneous nerve= and muscular branches which supply the semitendinosus. The former (N. cutaneus femoris caudalis) passes through the biceps femoris, emerges between that muscle and the semitendinosus a little below the level of the tuber ischii, and ramifies subcutaneously on the outer and posterior surfaces of the hip and thigh (Fig. 525).

GREAT SCIATIC NERVE

The =great sciatic nerve= (N. ischiadicus) (Figs. 451, 455, 526), the largest in the body, is derived chiefly from the last lumbar and the sacral roots of the lumbo-sacral plexus, but may receive a fasciculus from the third sacral nerve also. It emerges through the great sacro-sciatic foramen as a broad flat band—blended at first with the posterior gluteal nerve—which passes downward and backward on the lower part of the sacro-sciatic ligament and on the origin of the deep gluteus muscle. It turns downward in the hollow between the trochanter major and the tuber ischii over the gemellus, the tendon of the obturator internus, and the quadratus femoris. In its descent in the thigh it lies between the biceps femoris externally and the adductor, semimembranosus, and semitendinosus internally, and it is continued between the two heads of the gastrocnemius as the tibial nerve. Its chief branches are as follows:

1. In the pelvic part of its course the sciatic nerve supplies small branches to the obturator internus, gemellus, and quadratus femoris; the branch to the obturator internus reaches the muscle by passing through the anterior end of the lesser sacro-sciatic foramen.