([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.
Fig. 159.—Diagram of the Right Brachial Plexus.
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.