Dorsal Rami (Rami posteriores).—The dorsal ramus of the first nerve (N. suboccipitalis) supplies the short dorsal muscles which move the head and connect the atlas and skull. In the second nerve the dorsal ramus is much larger, forming N. occipitalis major. It sends small branches to the muscles about its origin, then turns craniad on the surface of the obliquus superior muscle, passes through the biventer cervicis and splenius, joins a small branch from the third nerve, and reaches the dorsal surface of the back of the head. It passes craniad, lying beneath the levator auris longus, emerges from between the two divisions of this muscle, and supplies the skin and cutaneous muscles between the two external ears. The dorsal rami of the other cervical nerves supply muscles and integument on the back of the neck.

Ventral Rami (rami anteriores).—These pass ventrad between the transverse processes of the vertebræ, except in the case of the first nerve (Fig. 156, j). This passes ventrad from the atlantal foramen along the groove for the vertebral artery, through the notch in the wing of the atlas, and across the lateral surface of the longus capitis muscle (5). Here it sends a branch caudad to join the second cervical (k), then crosses the vagus (d′) and sympathetic and the carotid artery, giving off communicating branches to the vagus and sympathetic, and uniting with a branch of the descending ramus of the hypoglossal (f). At the lateral surface of the larynx it turns caudad, following the lateral border of the sternothyroid muscle (7), and is distributed to the sternohyoid (8) and sternothyroid (7).

The ventral ramus of the second nerve (Fig. 158, 2) receives a branch from that of the first nerve, passes laterad between the levator scapulæ ventralis (c) and the cleidomastoid (d), receives a communicating branch from the third cervical (3), sends a branch to N. accessorius (1) and numerous small nerves into the sternomastoid (e) and cleidomastoid (d), then turns craniad and divides into N. auricularis magnus (5) and N. cutaneus colli (6).

N. auricularis magnus (5), the great auricular nerve, passes dorsocraniad across the lateral surface of the sternomastoid (e) to the lateral and caudal surface of the external ear and parotid gland (f), where it ramifies. The cutaneus colli (6) is the smaller, ventral, division of the second nerve; it may receive also an accession from the third. It passes to the integument over the ventral part of M. masseter and ventrad of that muscle.

The third nerve (Fig. 158, 3) communicates with the second and supplies the levator scapulæ ventralis (c), cleidomastoid (d), sternomastoid (e), longus capitis, and other muscles of this region and aids in forming the cutaneus colli (6). The fourth (4) and fifth are distributed to the muscles and integument of the sides of the neck. A branch of the fourth supplies the integument in the hollow of the shoulder, and one from the fifth follows the vena cephalica and supplies the integument over the shoulder. The fifth by sending a branch to aid in forming the phrenic nerve (Fig. 157, f) may be considered to enter partly into the brachial plexus.

Owing to the intercommunicating branches between the ventral roots of the first five cervical nerves, these are sometimes considered as forming a loose plexus which receives the name cervical plexus.

The sixth, seventh, and eighth cervical nerves (with a part of the fifth) become interconnected with each other and with the first thoracic to form the brachial plexus.

The Brachial Plexus

(Fig. 159).—The brachial plexus is formed by the ventral rami of the fifth, sixth, seventh, and eighth cervical nerves and the first thoracic. Of the fifth cervical only a small part enters into the plexus, forming part of the phrenic nerve. The formation of the plexus is due to the union of the different nerves by means of strong connecting branches or ansæ. The plexus lies in the axilla, along with the axillary artery and vein; all its component nerves pass laterad in front of the first rib. Its branches supply the arm and shoulder.

The precise arrangement of the different strands is somewhat variable. The plexus is commonly made up in approximately the following manner (Fig. 159). From the fifth cervical nerve (V) a small branch joins a similar one from the sixth to form the phrenic nerve (a); the remainder of the fifth does not enter into the plexus. From the sixth cervical (VI) arise parts of the phrenic nerve (a), the suprascapular (b), the cranial one of the three subscapular nerves (c), the axillary (d), and the musculocutaneous (f). The sixth also gives off close to its origin a nerve (b′) which passes to the inner surface of the levator scapulæ and ramifies over the surface, supplying this muscle and extending to the rhomboideus, which it also innervates. The seventh cervical (VII) is the largest nerve entering into the plexus; it furnishes parts of one or both of the anterior thoracic nerves (k and n), of the posterior thoracic (m), the three subscapular nerves (c, e, and i), the axillary (d), musculocutaneous (f), radial (h), and median (g) nerves. The eighth cervical (VIII) supplies parts of one of the anterior thoracic nerves (k), the caudal one of the subscapular nerves (i), the radial (h), median (g), and ulnar (j) nerves. The first thoracic (1) curves craniad, so as to leave the thorax on the cranial side of the first rib; it supplies the median cutaneous nerve (l) and parts of the radial (h), ulnar (j), and median (g) nerves; sometimes also a part of one (k) of the anterior thoracic nerves.

The following are the nerves which arise from the brachial plexus:

A. Nerves of the Shoulder and Breast (Fig. 160).

1. Nn. thoracici anteriores.—There are two anterior thoracic nerves. One (Fig. 160, h; Fig. 159, n) arises from the seventh cervical and passes to the pectoral muscles (9) along with the anterior thoracic artery. The other (Fig. 159, k; Fig. 160, q and r) arises usually by two roots, one from the eighth cervical, one from the first thoracic; it passes along with the long thoracic artery to be distributed to the pectoralis muscles (Fig. 160, 8) and sometimes also to the latissimus dorsi (7).

2. N. thoracicus posterior.—The posterior thoracic (or long thoracic) nerve (external respiratory) (Fig. 159, m) arises from the seventh cervical nerve near its beginning. It passes within the scalenus muscle and extends caudad on the outer surface of the serratus anterior muscle, which it supplies.

3. N. suprascapularis (Figs. 159 and 160, b).—The suprascapular nerve arises from the sixth (and sometimes the seventh) cervical. It passes laterad and gives off a branch which passes over the shoulder-joint and penetrates the clavodeltoid muscle to be distributed to the integument on the ventral surface of the upper arm. It then follows the transversa scapulæ artery into the supraspinatus fossa and accompanies the artery to be distributed to the supraspinatus and infraspinatus muscles.

4. Nn. subscapulares.—The subscapular nerves are three. The cranial one (Figs. 159 and 160, c) arises from the sixth and seventh cervical and supplies the subscapular muscle. The middle one (Fig. 159, e) arises from the seventh; it supplies principally the teres major. The caudal one (Fig. 159, i) arises from the seventh and eighth cervical and supplies the latissimus dorsi. (The points of origin from the plexus vary.)

5. N. axillaris (or circumflexus) (Fig. 159, d).—The axillary nerve arises from the sixth and seventh cervical nerves. It passes toward the shoulder-joint, then follows the posterior circumflex artery ventrad of the long head of the triceps to be distributed to the spinodeltoid and acromiodeltoid. A branch of it continues to the clavobrachial, which it supplies.

B. The Phrenic Nerve (Fig. 157, f, page 381).

6. N. phrenicus.—The phrenic nerve (internal respiratory) is formed by the junction of two slender branches, one from the fifth and one from the sixth cervical nerves (Fig. 159, a). (It is said to receive sometimes a branch from the fourth.) It passes caudad into the thorax on the ventral surface of the subclavian artery and then extends to the diaphragm lying at the side of the inferior and superior venæ cavæ. It is the motor nerve of the diaphragm.

C. Nerves of the Arm.

7. N. musculocutaneus (Fig. 160, d).—The musculocutaneous nerve or external cutaneous arises from the ventral surface of the sixth and seventh nerves (Fig. 159, f). It passes toward the shoulder-joint and supplies the biceps (Fig. 160, 4) and coracobrachialis muscles. It then passes distad, resting on the dorsal border of the biceps (4), and supplies the brachialis muscle. Thence it passes dorsad of the biceps to the lateral side, and reaches the integument by passing between the clavobrachialis and pectoantibrachialis muscles near the elbow; it is then distributed to the skin on the radial side of the ventral surface of the forearm, as far as the wrist (Fig. 130, h, page 319).

8. N. cutaneus medialis (Fig. 160, o).—The medial (or internal) cutaneous nerve rises from the first thoracic. It passes distad along the medial side of the biceps, and at the junction of the second and third thirds of the upper arm reaches the integument by passing between the epitrochlearis and pectoantibrachialis muscles. It then curves spirally about the dorsal border of the forearm and is distributed to the integument of the forearm on the ulnar side, extending nearly to the wrist.

9. N. medianus (Fig. 160, e).—The median nerve is formed by the junction of three branches, one each from the seventh and eighth cervical with the brachial artery (g) passing between them, and one from the first thoracic nerve. It accompanies the brachial artery (g) lying on its medial side, and passes with it through the supracondyloid foramen; here it is connected by a branch to the musculocutaneous nerve. It passes into the forearm (Fig. 161, b), lying at first beneath the pronator teres (5). Here it gives branches to the pronator teres and flexor muscles (flexor carpi radialis, palmaris longus, five heads of the flexor profundus digitorum, but not to the flexor carpi ulnaris). Continuing along the forearm, lying on the flexor carpi radialis, it sends a posterior interosseous nerve to the pronator quadratus muscle and branches to the deep and superficial divisions of the flexor sublimis digitorum. It thus supplies the flexor and pronator muscles (except the flexor carpi ulnaris). It then passes into the palm beneath the transverse ligament and gives rise to three principal branches. The first of these supplies the integument of the thumb, sending a branch on either side of it; and onto the radial side of the second digit. The second passes along the contiguous sides of the second and third digits and supplies their integument. The third passes along the contiguous sides of the third and fourth digits to their integument. Each sends twigs into the trilobed pad in the palm, so that there are thus seven terminal branches in the palm. From these, branches (probably) pass to the three radial Mm. lumbricales.

10. N. radialis.—The radial (or musculospiral) nerve is formed by the junction of branches from the seventh and eighth cervical and first thoracic nerves. It immediately sends branches to the epitrochlearis, long head of the triceps, and long and intermediate portions of the medial head of the triceps. It then passes between the long and intermediate portions of the medial head of the triceps, following the profunda brachii artery, and curves about the humerus to its cranial side. It supplies there the lateral head of the triceps and the anconeus, and, lying on the brachialis muscle, divides into a superficial and a deep branch. The superficial branch is the superficial radial nerve (Fig. 130, g); the deep one forms the dorsal (or posterior) interosseous nerve. The former is sensory and the latter motor.

The superficial radial nerve (Fig. 130, g, page 319) becomes cutaneous at the junction of the second and third thirds of the upper arm, where it emerges from between the lateral head of the triceps and the brachialis muscle. It then follows the course of the vena cephalica (c) to the wrist and follows the dorsal tributary of the same vein onto the dorsum of the hand. It supplies the integument of the distal part of the ventral surface of the upper arm and that along the forearm. On the hand it is distributed to the dorsal surface in the same manner as the median nerve on the ventral surface, i.e. to the integument of the thumb on both sides and to that of the medial side of the second digit; to that of the contiguous sides of the second and third digits and of the contiguous sides of the third and fourth digits. There are thus seven terminal branches, one for each of these regions.

The dorsal interosseous nerve follows the brachialis muscle and passes onto the forearm between it and the extensor carpi radialis longus. It supplies the supinator and extensor muscles of the forearm (brachioradialis, supinator, extensor carpi radialis longus, extensor carpi radialis brevis, extensor communis digitorum, extensor digitorum lateralis, extensor indicis, extensor brevis pollicis). The dorsal interosseous and radial nerves thus supply all the extensor muscles of the forearm and upper arm and the supinators (modified extensors).

11. N. ulnaris (Fig. 160, n).—The ulnar nerve arises from the eighth cervical and first thoracic and accompanies the brachial artery (g) and median nerve (e) through the upper arm. It does not pass through the supracondyloid foramen, but curves over the dorsal end of the medial epicondyle of the humerus within the short division of the medial head of the triceps, and reaches the dorsal border of the forearm. In the forearm (Fig. 161, a) it lies beneath the flexor carpi ulnaris. It supplies muscular branches to the flexor carpi ulnaris and the first or ulnar head of the flexor profundus digitorum. Near the middle of the forearm it divides into two branches, dorsal cutaneous (c) and palmar (d).

The dorsal cutaneous branch (c) curves about the ulnar side of the wrist to the dorsum of the hand and divides into twigs which supply the dorsal part of the ulnar side of the fifth digit and of the contiguous sides of the fourth and fifth. The palmar branch (d) sends a twig to the integument on the flexor surface of the arm near the wrist, and then divides into deep palmar (e) and superficial palmar (f) branches.

The superficial palmar branch (f) passes into the palm and supplies the ventral portion of the integument on the ulnar side of the fifth digit, and on the contiguous sides of the fifth and fourth.

The deep palmar branch (e) passes into the palm, beneath the ligament of the pisiform bone. Just distad of the pisiform bone it curves toward the radial side and breaks up into a number of small branches which are distributed to the short muscles in the palm.

B. Thoracic Nerves.

Dorsal Rami.—The dorsal rami of the thoracic nerves are small and supply the muscles and integument of the back. Each gives off a branch directly dorsad to the spinal muscles, and a lateral branch which reaches the integument some distance from the middle line.

Ventral Rami.—The ventral ramus of the first thoracic nerve enters into the brachial plexus, as already described. The ventral rami of the other thoracic nerves form the intercostal nerves. Each of these passes ventrolaterad, lying close to the caudal border of a rib, in company with the intercostal artery. Branches are given to the intercostal muscles, and at about the middle of the length of the rib a large lateral branch is given off, which passes to the more superficial muscles of the thoracic wall (serrati posteriores, obliquus externus, etc.), its main branches running dorsad and ventrad. The main intercostal nerve extends ventrad to the transversus costarum and rectus abdominis, supplying these muscles.

C. Lumbar Nerves.

—There are seven lumbar nerves, one passing from the vertebral canal caudad of each lumbar vertebra.

Dorsal Rami.—The dorsal rami are similar to those of the thoracic region, sending one branch dorsad to the muscles of the vertebral column, another dorsolaterad to reach the integument at about the lateral border of the longissimus dorsi. The dorsal rami are somewhat smaller caudad.

Ventral Rami.—The last four lumbar nerves are interconnected to form the lumbar or lumbosacral plexus. The first three are distinct, and will therefore be described separately.

The first three lumbar nerves are directed strongly caudad (as well as ventrad), so that on leaving the intervertebral foramen they pass ventrad of the transverse process of the vertebra immediately succeeding. Each communicates with the sympathetic system and gives off near its origin branches to the muscles on the ventral side of the vertebræ,—the first to the crus of the diaphragm, the second and third (Fig. 162, II and III) to the quadratus lumborum and psoas muscles. Each divides three to five centimeters from its origin into a lateral and a medial branch, the first having a more cranial, the latter a more caudal course. The first three nerves of the cat are represented in man by the iliohypogastric and the ilioinguinal nerves. There seems no good ground for applying these names to two of the three in the cat, in preference to the third, so that we shall speak of these nerves in the cat as simply the first, second, and third lumbar nerves.

1. The first lumbar nerve arises from the intervertebral foramen caudad of the first lumbar vertebra. Its lateral division passes between the transversus and obliquus internus muscles, then between the obliquus internus and externus. At about the middle of the abdomen it pierces the obliquus externus and is distributed to the integument of the middle of the ventral surface of the abdomen. The medial division passes between the obliquus internus and transversus, crosses the lateral division of the second (lying mediad of it), and extends to the rectus abdominis, which it supplies.

2. The second lumbar nerve (Fig. 162, a) divides, like the others, into two branches. The lateral branch pierces the muscles of the abdominal wall at the lateral border of the longissimus dorsi and passes, lying just beneath the integument, ventrocaudad nearly to the pubis. It supplies the integument of the caudal half of the abdomen and of the fold between the thigh and abdomen. The medial branch passes at first almost directly caudad, lying on the medial surface of the transversus abdominis; it curves gradually ventrad, lying in the substance of the transversus, and reaches the rectus abdominis three or four centimeters craniad of the pubis.

3. The third lumbar nerve (b) is not united with the fourth, as in many animals, but remains distinct. It divides one or two centimeters from its origin. The lateral branch passes caudad to the outer surface of the abdominal muscles, and extends, lying just beneath the integument, to that portion of the abdominal wall which is partly covered by the thigh. The medial branch passes caudad on the lateral surface of the iliopsoas muscle, follows the iliolumbar artery for a short distance, and gives branches to the caudal part of the transversus and rectus abdominis muscles.

The Lumbar Plexus

(Fig. 162).—The fourth, fifth, sixth, and seventh lumbar nerves are interconnected by short branches, forming thus the lumbar plexus. They are also connected with the sacral plexus, so that the two are often considered together as the lumbosacral plexus (Fig. 162). In some other animals the first three lumbar nerves form also a part of the plexus; but they are not connected with it in the cat.

The fourth lumbar nerve forms the genitofemoral (c) and part of the lateral cutaneous (d) nerves, and is connected by a short strand with the fifth. The fifth aids in forming the lateral cutaneous (d) and femoral (f) nerves, and is connected by a short branch with the sixth. The sixth lumbar is large; it forms the major part of the femoral (f) and a large part of the obturator nerve (g), and sends a large connecting branch caudad to join the seventh and thus pass into the sacral plexus. The seventh passes caudad to join the sacral plexus and aid in forming the great sciatic nerve (h); by its connection with the sixth it aids in forming also the obturator nerve (g).

4. N. genitofemoralis (or N. lumboinguinalis) (c, c′, c″).—This is a direct continuation of the fourth lumbar nerve. Its medial branch (c″) passes along the medial surface of the iliopsoas (8) and the psoas minor (9) to the external iliac artery, accompanies this, lying on the ventral surface, from its origin to the point where it gives off the profunda femoris, then accompanies the latter artery and passes onto that branch of it that spreads out under the integument of the ventral pelvic region. It crosses the spermatic cord and ramifies in the skin of the proximal part of the medial side of the thigh. The lateral branch of the genitofemoralis (c′) pierces the psoas minor (9), appearing on its ventral surface opposite the fifth lumbar vertebra. It passes caudad on the ventral surface of this muscle, crosses the iliolumbar artery, then turns caudolaterad, passes through the abdominal wall, and is distributed to the craniomedial surface of the thigh and to the adjacent abdominal wall.

This nerve is variable in origin and in distribution. Its lateral branch is sometimes lacking.

5. N. cutaneus femoris lateralis (d).—The lateral cutaneous nerve arises from the connecting strand between the fourth and fifth nerves, most of its fibres coming from the fifth. It passes caudoventrad between the iliopsoas and psoas minor and accompanies the iliolumbar artery across the ventral surface of the iliopsoas. It pierces the abdominal wall in company with the artery and passes onto the cranial border of the thigh. Its terminal branches are distributed to the integument along with the branches of the iliolumbar artery,—supplying the lateral surface of the femur and extending as far as the knee.

6. N. femoralis (f).—The femoral is a large nerve which rises by strong roots from the fifth and sixth lumbar nerves. It gives branches to the iliopsoas (8) and passes through that muscle to its ventral surface. It leaves the abdominal cavity lying on the ventral surface of the iliopsoas, and at the point where it pierces the abdominal wall it divides into three (or four) branches. One (or two) of these pass to the sartorius muscle. Another passes between the rectus femoris and vastus medialis muscles, and divides into numerous branches which supply these muscles and the vastus intermedius.

The third branch of the femoral is N. saphenus or the long saphenous nerve (Fig. 127, g, page 310). This passes distad along with the femoral artery and vein, giving a few twigs to the integument. Where the long saphenous artery (f) separates from the femoral artery the nerve follows the former and passes with it along the medial side of the lower leg. At the knee and in the lower leg it gives off a number of cutaneous branches, and finally divides just distad of the middle of the lower leg into two main branches. Both of these pass to the concavity of the ankle-joint and divide into fine branches, which form a sort of plexus on the ankle and dorsal surface of the foot; branches from this can be traced almost to the toes.

7. N. obturatorius (Fig. 162, g).—The obturator nerve arises from the connecting band between the sixth and seventh nerves. It lies at first laterad, then dorsad, of the common iliac vein, and passes almost directly caudad to the cranial border of M. obturator internus (15). Here it turns sharply laterad about the pubis, and passes through the obturator foramen. It gives twigs to the obturator externus muscle, and divides into several branches, which innervate the adductor femoris, adductor longus, pectineus, and gracilis.

D. Sacral Nerves and Sacral Plexus

(Fig. 162).—A large band passes caudad from the sixth and seventh lumbar nerves to connect with the sacral nerves; this constitutes the so-called lumbosacral cord. The sacral nerves are three in number. Their dorsal rami pass dorsad out of the two posterior sacral foramina and the foramen caudad of the sacrum, to the muscles and skin dorsad of the sacrum. The ventral rami pass from the two anterior sacral foramina and from the foramen between the last sacral and first caudal vertebræ. The first is the largest; it joins the lumbosacral cord to form the great sciatic nerve (N. ischiadicus) (h) and the superior (i) and inferior (j) gluteal, and sends a branch caudad to connect with the second sacral. The second and third sacral nerves are small; they unite with the connecting branch from the first to form a network from which arises the pudendus (l), cutaneus femoris posterior (k), hemorrhoidalis inferior (m), and a small branch to the great sciatic (h).

The nerves arising from the sacral plexus are the following:

1. N. gluteus superior (Fig. 162, i; Fig. 163, j).—The superior gluteal nerve arises from the lumbosacral cord and the first sacral nerve; it passes dorsad in the notch between sacrum and ilium, caudolaterad of the first anterior sacral foramen, curves over the dorsal border of the ilium between the gluteus medius and pyriformis muscles (Fig. 163, 7), passes between the gemellus superior (6) and gluteus minimus (5) and ventrad of the latter, and reaches the medial surface of the tensor fasciæ latæ, which it innervates. In its course it sends branches to the gluteus medius, gluteus minimus (5), and gemellus superior.

2. N. gluteus inferior (Fig. 162, j; Fig. 163, i).—A small nerve from the lumbosacral cord and the first sacral nerve. It passes dorsocaudad, lying on the dorsal surface of the great sciatic nerve (Fig. 162, h). After leaving the pelvis by the great sciatic notch it divides beneath the pyriformis (Fig. 163, 7) into two branches; one goes to M. caudofemoralis (4), the other to M. gluteus maximus (3).

3. N. ischiadicus (Fig. 162, h; Fig. 163, a).—The great sciatic nerve, the largest nerve in the body, arises from the lumbosacral cord (formed chiefly by the sixth and seventh lumbar nerves), the first sacral nerve and a small branch from the second sacral. It passes caudodorsad and leaves the pelvis by passing across the great sciatic notch, between M. pyriformis and M. gemellus superior. It passes across the tendon of M. obturator internus (Fig. 163, 9) near the insertion, and here sends caudad a large muscular branch (b) beneath the biceps. This branch innervates the biceps, semitendinosus (17) and semimembranosus (16). Other twigs in this region pass to the quadratus femoris (10) and tenuissimus. The great sciatic nerve (a) now passes across the quadratus femoris (10), adductor femoris (15), and semimembranosus (16), lying beneath the biceps femoris. It thus enters the popliteal space and approaches the popliteal artery and vein. Here it gives off one or two small muscular branches to the distal part of the biceps, and sends a slender branch, N. suralis (c), along the medial surface of the biceps to the lateral surface of the lateral head of M. gastrocnemius (20). The sural nerve (c) becomes cutaneous at the distal (ventral) border of the biceps, and divides three or four centimeters proximad of the ankle into two branches. One passes over the tendon of Achilles (27) to the proximal end of the calcaneus and ramifies in this region. The other passes onto the lateral surface of the foot, and supplies the integument in this region over the tarsus and part of the metatarsus.

The great sciatic nerve now divides in the popliteal space into the peroneus communis (d) and the tibialis (e).

a. N. peroneus communis (d).—This is the more lateral of the two divisions of the great sciatic nerve. It passes along the medial surface of the biceps to the lateral surface of the lateral head of M. gastrocnemius (20), where it extends to a point just distad of the head of the fibula. Here it passes beneath that part of the gastrocnemius which has origin on the fascia of the shank, passes between the soleus and peroneus longus (21), then between the peroneus longus and peroneus tertius. It gives off a number of small muscular branches, then divides two or three centimeters distad of the head of the fibula into two main branches, N. peroneus superficialis (g) and N. peroneus profundus (f).

(1) N. peroneus superficialis (g).—The superficial peroneal nerve passes distad between the peroneus longus and peroneus tertius. Near the ankle it becomes superficial, passing along the concavity of the ankle across the transverse ligament which binds down the tendons of the extensor longus (23) and tibialis anterior (22), and thus reaches the dorsal side of the foot. Two or three minute twigs are given off to the ankle, then the nerve divides into four divisions. These pass distad to the toes, subdividing so as to send a nerve to each side of each toe.

(2) N. peroneus profundus (f).—The deep peroneal nerve passes between the tibialis anterior and extensor longus digitorum muscles, gives branches to these muscles, and passes distad with the tibialis anterior artery, lying on the inner surface of the tibialis anterior muscle. It passes onto the dorsum of the foot, lying on the inner surface of the tendon of the muscle. On the tarsus it divides into two branches. The lateral branch passes into the extensor brevis digitorum, while the medial one extends in the dorsal groove between metatarsals four and five to the toes; here it divides into two nerves which supply the contiguous sides of digits four and five.

b. N. tibialis (e).—The tibial nerve passes distad parallel with the peroneal nerve, but mediad of it. It passes between the lateral and medial heads of the gastrocnemius, giving off muscular branches to these, and to the plantaris and soleus. It then passes between the plantaris and the medial head of the gastrocnemius, and reaches the space between the plantaris and flexor longus hallucis, where it passes distad. It gives off, in the region just described, muscular branches to the flexor longus digitorum, flexor longus hallucis, and tibialis posterior. Below the middle of the lower leg the nerve becomes superficial (Fig. 127, i, page 310), lying on the ventral surface of the flexor longus hallucis. It passes in the depression between the heel and the medial malleolus onto the plantar surface of the tarsus, sends a small branch to the plantar surface of the heel, and divides into two branches, the medial and the lateral plantar nerves.

(1) N. plantaris medialis.—The medial plantar nerve passes along the medial border of the flexor brevis digitorum and divides into two branches. One passes to the medial (second) digit, supplying both sides; the other supplies the third digit in the same way. Both send branches to the fibrous pad on the sole of the foot.

(2) N. plantaris lateralis.—The lateral plantar nerve passes laterad across the tendon of the flexor longus digitorum to its lateral edge. At about the proximal end of the metatarsi it divides into two branches. The lateral branch passes distad to the ventral surface of the fifth digit. The medial branch passes beneath the tendon of the flexor longus digitorum and transversely across the interossei; it divides into numerous tendons which innervate the interossei and other short muscles of the sole of the foot.

4. N. pudendus (Fig. 162, l; Fig. 112, 8, page 265).—This arises (Fig. 162, l) from the sacral plexus caudad of the great sciatic nerve. It has two roots, one from the second sacral, one from the third, and it may also receive a small accession from the great sciatic nerve (h) three or four centimeters caudad of the origin of the latter. It passes at first dorsolaterad, laterad of the coccygeus muscle, then curves mediad toward the anus. It divides into two branches: one passes to the base of the penis, sends twigs into the compressor urethræ muscle, and passes onto the dorsal surface of the penis. This branch is N. dorsalis penis; it extends with the artery of the same name to the glans. The second branch of N. pudendus (N. hemorrhoidalis medius) passes to the anus and innervates the muscles and other structures about the caudal end of the rectum. In the female, branches of N. pudendus supply the urogenital sinus and adjacent structures (Fig. 112, 8).

5. N. cutaneus femoris posterior (Fig. 162, k; Fig. 163, h).—This arises from the sacral plexus, its roots coming chiefly from the second and third sacral nerves, and passes caudodorsad, at first in close connection with N. pudendus. It then accompanies the posterior gluteal artery and vein, sends branches (perineal nerves) into the fat at the sides of the anus, and branches onto the lateral surface of the biceps muscle. One of the latter follows the communicating vein (Fig. 163, k′) from V. saphena parva, and may be traced as far distad as the popliteal space.

6. N. hemorrhoidalis inferior (or posterior) (Fig. 162, m).—This arises by two roots, from the second and third sacral nerves, and passes with the inferior hemorrhoidal artery ventrad across the lateral surface of the rectum to the urethra. Here it divides into two parts: one passes craniad to the bladder, the other caudad onto the ventrolateral surface of the rectum.

A small nerve passes from the sacral plexus, especially from the third sacral nerve, into M. levator ani (Fig. 162, 11). Another small nerve from the same region passes to M. coccygeus, and a third to the proximal end of M. tenuissimus.

E. Nerves of the Tail. Coccygeal Nerves.

—From the intervertebral foramina of the first seven or eight caudal vertebræ spinal nerves of the usual type are given off. The dorsal rami innervate the dorsal muscles of the tail. The ventral rami are interconnected with each other and with the last sacral nerve by a longitudinal cord; they innervate the muscles and integument of the ventral side of the tail.

3. The Sympathetic Nervous System. Systema nervorum sympathicum.

The sympathetic system consists essentially of a chain of ganglia on each side of the ventral surface of the vertebral column, interconnected by longitudinal nerve-cords, and stretching from the base of the skull to the tail. The ganglia are connected to the spinal nerves by communicating branches, and numerous branches pass from them to the abdominal and thoracic viscera, and to the walls of the lymph- and blood-vessels, forming complicated plexuses.

Cervical portion (Fig. 156, page 379).—The sympathetic system begins just caudad of the tympanic bulla as the superior cervical ganglion (G. cervicale superius) (Fig. 156, e). This is a large ganglion closely applied to the ganglion nodosum (d) of the vagus, and lying on its ventrocranial side. From the superior cervical ganglion the sympathetic trunk passes caudad usually closely bound up with the vagus (d′), so that the two can scarcely be distinguished. They lie (i) at the side of the trachea (6), close against the lateral surface of the carotid artery. A few centimeters before reaching the first rib the two separate (Fig. 157, page 381); the sympathetic (c) is here the smaller and more dorsal of the two. Just craniad of the first rib the sympathetic usually forms a small swelling, the middle cervical ganglion (Fig. 157, d), beyond which it divides into two portions. The larger dorsal division passes dorsocaudad and joins a large ganglion, the inferior cervical (e), which lies at the head of the first rib, on the lateral surface of the longus colli muscle. The ventral branch (g′) passes caudad, turns laterad about the medial and caudal surface of the subclavian artery, and likewise reaches the inferior cervical ganglion.

In some cases the middle cervical ganglion (d) is lacking.

Branches of the Sympathetic in the Cervical Region.—From the cranial end of the superior cervical ganglion several nerves pass craniad. Some of these become connected immediately with the cranial nerves emerging from the jugular foramen, forming a plexus in this region. Others pass with the internal carotid artery craniad, forming a plexus about that artery; from the plexus twigs arise which pass to the fifth and sixth cranial nerves; doubtless also to the seventh and eighth. A specially large branch, the deep petrosal nerve (see page 373), passes from the superior cervical ganglion along with the internal carotid artery to the pterygoid canal, thence through this to the sphenopalatine ganglion. Before separating from the vagus the sympathetic gives off one or two cardiac nerves (Fig. 157, i). These are closely bound up with the vagus (b), so that they appear usually to have origin from the vagus at about the level of the first rib. These pass with the vagus to the heart and take part in the formation of the cardiac plexus (k).

At the middle cervical ganglion (d) usually a communicating branch is given to the vagus. The branches of the inferior cervical ganglion (e) fall in the thoracic region, and are described in that connection.

Thoracic Portion of the Sympathetic (Fig. 157, page 381).—The inferior cervical ganglion (e) lies entirely within the thoracic cavity. It has the following branches:

1. One or two small cardiac branches (h), which pass to the heart (r).

2. A large communicating branch (e′), which passes craniad from the cranial end of the ganglion (e) along the lateral surface of the longus colli muscle, and gives communicating branches to the sixth, seventh, and eighth cervical nerves (VI, VII, and VIII).

3. One or two small communicating branches to the first thoracic nerve (I′).

4. A communicating branch to the second thoracic nerve.

The arrangement of these communicating branches is somewhat variable; they may arise in various combinations, as at first united, or as more or less completely separated nerves.

The sympathetic trunk (c) continues caudad from the caudal end of the inferior cervical ganglion, lying at first on the lateral surface of the longus colli, then on the lateral surface of the centra of the vertebræ. For each vertebra there is a slight enlargement, forming one of the thoracic ganglia of the sympathetic, and from each ganglion one or two communicating branches are given off to the corresponding spinal nerve. Just caudad of the last rib (but while still in the thoracic cavity) the sympathetic gives off the large N. splanchnicus major or great splanchnic nerve (n), which passes ventrocaudad and pierces the diaphragm. The sympathetic itself then passes through the diaphragm laterad of the crus.

Abdominal Portion of the Sympathetic (Fig. 164).—The great splanchnic nerve (b) passes through the diaphragm (2) laterad of the crus, extends two or three centimeters caudoventrad, and joins a large ganglion lying just caudad of the cœliac artery (4), almost on the lateral surface of the superior mesenteric (5). This is the cœliac (or semilunar) ganglion (d). From it nerves branch in all directions, forming the cœliac plexus (c), a part of the large solar plexus. Less than a centimeter ventrocaudad of the cœliac ganglion, on the caudal surface of the superior mesenteric artery (5), is the somewhat smaller superior mesenteric ganglion (f); the two are connected by two strong nerve-trunks.