Part 82

The hypoglossal nerve is involved in the plexus just outside of the jugular foramen, with which the vagus, sympathetic, and accessory nerves are also connected.

2. =Spinal Nerves.=--The origin and general features of the spinal nerves have been described in connection with the account of the spinal cord (page 337). A description of their peripheral distribution will now be given.

A. CERVICAL NERVES.--There are eight pairs of cervical nerves. The first leaves the vertebral canal through the atlantal foramen. The second passes out between the arches of the atlas and axis, not through a special intervertebral foramen, so that its ganglion lies among the muscles of the back of the neck. The others emerge through the intervertebral foramina, the eighth one from between the last cervical and first thoracic vertebræ.

M. clavotrapezius has been partly removed. 1, N. accessorius; 2-4, ventral rami of second to fourth cervical nerves; 5, N. auricularis magnus; 6, N. cutaneus colli. a, M. clavotrapezius (cut); b, M. splenius; c, M. levator scapulæ ventralis; d, M. cleidomastoideus; e, M. sternomastoideus; f, parotid gland.]

=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.

V, VI, VII, VIII, the fifth to eighth cervical nerves. 1, the first thoracic nerve. a, phrenic nerve; b, suprascapular; b′, nerve to serratus anterior and levator scapulæ muscles; c, first or cranial subscapular nerve; d, axillary nerve; e, second subscapular; f, musculocutaneous; g, median; h, radial; i, third subscapular; j, ulnar; k, second anterior thoracic; l, medial cutaneous; m, posterior thoracic; n, first anterior thoracic.]

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.

The pectoral and clavobrachial muscles have been cut and laid aside, their ends being shown; only a part of the nerves are exhibited. 1, M. clavobrachialis; 2, cut end of M. pectoralis major; 3, cut end of M. pectoralis minor; 4, M. biceps; 5, M. teres major; 6, M. epitrochlearis, partly cut and turned back; 7, M. latissimus dorsi, partly cut; 8, M. pectoralis minor; 9, M. pectoralis major; 10, short portion of caput mediale of M. triceps brachii. a, branches of the thyrocervical axis to clavobrachial and clavotrapezius muscles; b, suprascapular nerve; c, first subscapular nerve; d, musculocutaneous nerve; e, median nerve; f, V. axillaris; g, A. axillaris; h, first anterior thoracic nerve, accompanied by the anterior thoracic artery and vein; i, V. subscapularis; j, large muscular branch (to subscapular muscle) of the subscapular vein and brachial artery; l, A. subscapularis; l′, A. circumflexa humeri anterior; m, radial nerve; n, ulnar nerve; o, medial cutaneous nerve; p, A. thoracica longa; q, r, second anterior thoracic nerve; s, V. thoracica longa; t, A. and V. thoracicodorsalis; u, A. profunda brachii; v, branch of brachial artery accompanying medial cutaneous nerve; w, branch of A. collateralis radialis superior; x, A. collateralis radialis superior; y, V. mediana cubiti; z, A. collateralis ulnaris.]

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.