Part 35

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.