The humerus is a long bone with a large medullary cavity. Its shaft is composed of compact tissue and its ends of cancellous tissue. In shape it is like the letter f, that is, convex anteriorly above and concave anteriorly below. At the middle of the bone on its external surface is the rough deltoid eminence for the insertion of the deltoid muscle.

Anterior Surface

Separating the tuberosities above and running down the anterior surface is the bicipital groove. Its external lip receives the insertion of the pectoralis major muscle, its inner lip and floor those of the latissimus dorsi above and the teres major below. On its inner side at and a little below its middle, is the insertion of the coracobrachialis muscle. On the anterior surface from the deltoid eminence to the elbow-joint is the origin of the brachiahs anticus; it has two heads, which embrace the insertion of the deltoid, one being in front and the other behind it (Fig. 277). Posterior Surface. - On the posterior surface, running obliquely across the bone downward and outward, below the insertion of the deltoid, is a shallow groove, called the musculospiral groove {sulcus radialis). It holds the musculospiral radial) nerve and the superior profunda artery. Above the groove and to its outer side is the origin of the outer head of the triceps extensor muscle and the insertion, of the deltoid. To its inner side, below, is the origin of the inner head of the triceps. Therefore the groove separates the inner head of the triceps muscle from the outer (Fig. 278).

Fig. 279.   Muscles of the anterior and internal aspects of the region of the shoulder.

Fig. 279. - Muscles of the anterior and internal aspects of the region of the shoulder.