_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.
=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.
[Illustration: FIG. 76.--LATERAL SURFACE OF THE SCAPULA, WITH THE AREAS OF ATTACHMENT OF MUSCLES.
_a_, M. infraspinatus; _b_, M. supraspinatus; _c_, origin of M. biceps; _d_, M. acromiodeltoideus; _e_, M. teres minor; _f_, M. levator scapulæ ventralis; _g_, M. spinodeltoideus; _h_, M. acromiotrapezius; _i_, M. rhomboideus; _j_, M. teres major; _k_-_l_, line of insertion of M. spinotrapezius.]
_Origin_ (Fig. 76, _a_).--By fleshy fibres from the whole infraspinatus fossa, and by a raphe between it and the teres major (Fig. 75, _d_), and sometimes by fibres from the teres minor (Fig. 80, _c_), triceps, spinodeltoideus (Fig. 75, _e_), spinotrapezius (Fig. 68, _j_), and subscapularis (Fig. 77, _a_).