Part 34

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.

[Illustration: FIG. 162.--THE LUMBAR AND SACRAL NERVES, AS SEEN IN A VIEW OF THE DORSAL WALL OF THE ABDOMINAL CAVITY, AFTER REMOVAL OF THE VISCERA.

On the right side the iliopsoas, psoas minor, and quadratus lumborum have been removed. The symphysis pubis has been split and the two parts divaricated, to show the pelvic cavity; on the right side parts of the innominate bones and the levator ani and coccygeus muscles have been removed. _II-VII_, second to seventh lumbar nerves; 1-3, first to third sacral nerves. _a_, second lumbar nerve; _b_, lateral and medial branches of third lumbar nerve; _c_, N. genitofemoralis (_c′_, its lateral branch; _c″_, its medial branch); _d_, N. cutaneus femoralis lateralis; _e_, muscular branch of N. femoralis; _f_, N. femoralis; _g_, N. obturatorius; _h_, N. ischiadicus or great sciatic nerve; _i_, N. gluteus superior; _j_, N. gluteus inferior; _k_, N. cutaneus femoris posterior; _l_, N. pudendus; _m_, N. hemorrhoidalis inferior; _n_, part of the sympathetic (incompletely shown, merely to indicate connections with the spinal nerves); 4, M. transversus abdominis; 5, 6, cut edges of M. obliquus internus and externus; 7, dorsal border of the diaphragm; 8, M. iliopsoas (mostly removed on the right side); 9, M. psoas minor; 10, M. pyriformis; 11, M. levator ani; 11′, M. iliocaudalis; 12, M. flexor caudæ longus; 13, M. flexor caudæ brevis; 14, edge of pubic symphysis; 15, cut end of M. obturator internus; 16, cut ramus of ischium; 17, cut ramus of pubis.]

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_).

[Illustration: FIG. 163.--DISSECTION OF LEG, LATERAL VIEW, TO SHOW DISTRIBUTION OF THE GREAT SCIATIC NERVE.

The biceps, caudofemoralis, gluteus maximus, gluteus medius, tensor vaginæ femoris, and part of the peroneus longus have been removed. _a_, N. ischiadicus or great sciatic nerve; _b_, muscular branch; _c_, N. suralis; _d_, N. peroneus communis; _e_, N. tibialis; _f_, N. peroneus profundus; _g_, N. peroneus superficialis and its terminal branches; _h_, branches of N. cutaneus femoris posterior; _i_, N. gluteus inferior; _j_, N. gluteus superior; _k_, vena saphena parva; _k′_, communicating branch to V. glutea inferior. 1, M. sartorius; 2, cut origin of M. gluteus medius; 3, cut origin of M. gluteus maximus; 4, cut origin of M. caudofemoralis; 5, M. gluteus minimus; 6, M. gemellus superior; 7, M. pyriformis; 8, M. flexor caudæ longus; 9, M. obturator internus; 10, M. quadratus femoris; 11, tuberosity of the ischium; 12, great trochanter of femur; 13, M. rectus femoris; 14, M. vastus lateralis; 15, M. adductor femoris; 16, M. semimembranosus; 17, M. semitendinosus (the slender ribbon-like muscle crossing this is M. tenuissimus); 18, M. vastus intermedius; 19, M. plantaris; 20, M. gastrocnemius; 21, M. peroneus longus (cut); 22, M. tibialis anterior; 23, M. extensor longus digitorum; 24, transverse ligament; 25, M. peroneus brevis; 26, M. soleus; 27, tendon of Achilles; 28, M. extensor brevis digitorum; 29, tendon of M. peroneus tertius.]

_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).