Part 211

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=, 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.

FIG. 523.—SUPERFICIAL NERVES OF NECK AND TRUNK OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

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.

FIG. 524.—LUMBAR NERVES OF HORSE, VENTRAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

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.

FIG. 525.—SUPERFICIAL NERVES OF PELVIC LIMB AND POSTERIOR PART OF TRUNK OF HORSE.

a, Cutaneous branches of sixteenth and seventeenth thoracic nerves; b, cutaneous branches of lumbar nerves; c, cutaneous branches of sacral nerves; d, cutaneous branches of coccygeal nerves; e, f, g, cutaneous branches of last intercostal, ilio-hypogastric, and ilio-inguinal nerves; g′, end of external cutaneous nerve of thigh; h, posterior cutaneous nerve of thigh; i, i, cutaneous branches of great sciatic nerve; k, posterior cutaneous nerve of the leg; l, superficial peroneal nerve; m, terminal part of deep peroneal nerve; n, external plantar nerve; 1, obliquus abdominis externus; 2, tensor fasciæ latæ; 3, gluteus superficialis; 4, biceps femoris; 5, semitendinosus; 6, anterior extensor; 7, lateral extensor; 8, flexor tendons; 9, great metatarsal artery. (After Ellenberger, in Leisering’s Atlas.) ]

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.