_Insertion_ (Fig. 82, _f_) by a tendon common to it and the latissimus dorsi (Fig. 79, _d′_), as already described.
_Relations._--Outer surface with the caput longum (Fig. 75, _g_) and the long portion (Fig. 80, _f_) of the caput mediale of the triceps, the latissimus dorsi (Fig. 77, _e_), and the cutaneus maximus. Inner surface with the serratus anterior (Fig. 73, _i_), the scalenus (Fig. 73, _f_), the transversus costarum (Fig. 73, _j_), and the biceps (Fig. 77, _g_). Dorsal border with the subscapularis (Fig. 77, _a_) and infraspinatus (Fig. 75, _c_).
_Action._--Rotates the humerus inward and flexes it in opposition to the infraspinatus, teres minor, and the deltoidei.
[Illustration: FIG. 78.--MEDIAL SURFACE OF THE SCAPULA WITH THE AREAS OF ATTACHMENT OF MUSCLES.
_a_, M. subscapularis; _b_, M. teres major; _c_, M. serratus anterior; _d_, M. levator scapulæ; _e_, M. rhomboideus; _f_, M. occipitoscapularis; _g_, M. biceps; _h_, M. coracobrachialis; _i_, caput longum of M. triceps.]
2. =Muscles of the Brachium or Upper Arm.=
M. clavobrachialis and the pectoralis group (Fig. 65, page 109) lie partly in this region; they have been described.
=M. coracobrachialis= (Fig. 77, _f_; Fig. 79, _c_).--A very short muscle covering the inner (medial) surface of the capsule of the shoulder-joint. It extends from the coracoid process to the proximal end of the humerus.
_Origin_ (Fig. 78, _h_).--By a round tendon from the tip of the coracoid process.
_Insertion_ (Fig. 82, _e_) usually by fleshy fibres on an area five to eight millimeters long and about half as wide, which lies on the medial side of the humerus parallel with the ridge which runs from the dorsal end of the lesser tuberosity to the shaft and close to it. The proximal end of the area is about one centimeter from the proximal end of the head of the humerus.
_Relations._--Medial surface with the biceps (Fig. 77, _g_), pectoralis minor (Fig. 65, _o_), and teres major (Fig. 77, _c_). Lateral surface with the capsule of the shoulder-joint. Dorsal border with the subscapularis (Fig. 77, _a_) and the long portion of the caput mediale of the triceps (Fig. 77, _j_).
The part of the coracobrachialis just described is known as the _short_ head. A _long_ head is sometimes found. It is a conical bundle of fibres of varying size, which arises from the tendon of origin of the short head. It passes distad into a long and extremely slender tendon, the insertion of which varies greatly in different individuals. It is commonly on the humerus in the region of the supracondyloid foramen.