Origin.—Its superficial fibres are continuations of the clavotrapezius ([Fig. 68], d); other fibres have origin from the clavicle and from a raphe laterad of the clavicle which is common to this muscle and the clavotrapezius. It passes distad along the cranial surface of the arm, growing narrower as it approaches the convexity of the elbow.
Insertion.—Eight to ten millimeters from the ulna it joins the brachialis muscle ([Fig. 79], i) to be inserted with it by a flat tendon ([Fig. 79], i; [Fig. 87], c) upon a rough area on the medial surface of the ulna just distad of the semilunar notch, and about midway between the dorsal and ventral borders.
Relations.—Outer surface with the integument. Inner surface with the pectoralis major ([Fig. 65], l), biceps ([Fig. 77], g), lateral head of the triceps ([Fig. 75], h), acromiodeltoid ([Fig. 75], f), and brachialis ([Fig. 80], h). Medial border with the pectoantibrachialis ([Fig. 65], m). Lateral border free except near the insertion, where it is in relation with the brachialis.
Action.—Flexor of the antibrachium.
M. supraspinatus ([Fig. 75], a) occupies the whole of the supraspinatus fossa of the scapula. It is covered by strong fascia which stretches from the free edge of the spine to the coracoid border of the scapula and to the coracoid half of its vertebral border.
Origin ([Fig. 76], b) by fleshy fibres from the whole surface of the supraspinatus fossa, from the above-mentioned fascia, and from the subscapularis ([Fig. 77], a) craniad of the coracoid border of the scapula.
Insertion ([Fig. 83], a).—It passes over the capsule of the shoulder-joint, to which it is closely attached, and is inserted into the free border of the great tuberosity ventrad (or proximad) of the fossa for the infraspinatus ([Fig. 83], c).
Relations.—Outer surface with the spinotrapezius ([Fig. 68], j), acromiotrapezius ([Fig. 68], h), levator scapulæ ventralis ([Fig. 68], f), clavotrapezius ([Fig. 68], d), and cleidomastoid ([Fig. 65], h). Inner surface with the scapula. The distal end of the coracoid border is closely related to the pectoralis minor ([Fig. 79], f). The glenoid border is related to the origin of the deltoidei.
Action.—Extends the humerus after it has been flexed on the scapula.
M. infraspinatus ([Fig. 75], c).—This fills the infraspinatus fossa, its fibres converging to the insertion on the great tuberosity of the humerus.