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.
[Fig. 160].—Blood-vessels and the more Ventral Nerves of the Axilla, Ventral View.
![[Fig. 160]](#3418956513767812439_illo389lg.jpg.id-4662866794067388513.wrap-0.html.html){kind=link}
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.