Relations.—Outer surface with the spinodeltoideus ([Fig. 75], e), acromiodeltoideus ([Fig. 75], f), and the infraspinatus ([Fig. 80], b). Inner surface with the lateral ([Fig. 75], h) and long ([Fig. 75], g) heads of the triceps and the capsule of the joint.

Action.—Assists the infraspinatus to rotate the humerus outward.

B. Muscles on the Medial Surface of the Shoulder.

([Fig. 77].)

M. subscapularis ([Fig. 77], a).—A triangular mass from the subscapular fossa to the lesser tuberosity of the humerus.

Origin ([Fig. 78], a).—From the whole subscapular fossa except along the fusiform area for the attachment of the levator scapulæ and the serratus anterior near the vertebral border, and except over a quadrangular area about one centimeter long near the glenoid angle. The origin is by fleshy fibres directly from the periosteum except along two or three lines marked by oblique ridges. To these lines are attached tendinous fibres. At the glenoid border the area of origin sometimes occupies part or the whole of the surface of the fossa for the origin of the teres major, so that the teres arises from the fascia on the surface of the subscapularis. At the coracoid border some of the fibres may take origin from the adjacent fascia of the supraspinatus ([Fig. 77], d).

Insertion ([Fig. 82], b).—The fibres converge to the glenoid border, and the insertion is by a strong, flat tendon into the dorsal border of the lesser tuberosity of the humerus.

[Fig. 77].—Muscles on the Medial Side of the Arm.

M. epitrochlearis, M. clavobrachialis, and the breast-muscles have been removed. a, M. subscapularis; b, insertion of M. levator scapulæ and of M. serratus anterior; c, M. teres major; d, M. supraspinatus; e, M. latissimus dorsi (e′, cut edge of that part which becomes united with the bicipital arch); f, M. coracobrachialis; g, M. biceps brachii (the capsule of the joint has been laid open to show its tendon); h, cut insertions of pectoralis muscles; i, caput longum of M. triceps brachii; j, long portion of caput mediale of M. triceps brachii; k, intermediate portion of caput mediale of M. triceps brachii; l, short portion of caput mediale of M. triceps brachii; m, cut end of M. clavobrachialis; n, M. brachioradialis; o, M. extensor carpi radialis longus; p, M. extensor carpi radialis brevis; q, M. pronator teres; r, M. flexor carpi radialis; s, M. palmaris longus (s′, its tendons); t, M. flexor carpi ulnaris (t, ulnar head; t′, humeral head); u, third head of M. flexor profundus digitorum; u′, fifth head of M. flexor profundus digitorum; u″, one of the tendons of M. flexor profundus digitorum; v, M. pronator quadratus; w, M. abductor brevis pollicis; x, ulnar part of M. flexor sublimis digitorum. 1, transverse ligament of the wrist.