THE ARM. 



267 



THE HUMERUS. 



The humerus is a long bone with a large medullary cavity. Its shaft is com- 

 posed 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 mid- 

 dle 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 



Coracoid process 



Pectoralis major 



Pectoralis minor 



Pectoralis major 



Long head of biceps 



Short head of biceps 



Coracobrachialis, 



Subscapularis 

 Teres major 



Latissimus dorsi 



Inner head of triceps 



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



eminence to the elbow-joint is the origin of the brachialis 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). 



MUSCLES OF THE ARM. 



In order to operate intelligently it is necessary to know the muscles and inter- 

 spaces, for the latter carry important structures. The arm possesses four sets of 

 muscles. One, an external set, abducts it, the deltoid ; another, or internal set, ad- 

 ducts it (and rotates it inward), the pectoralis major, teres major, latissimus dorsi, 

 and coracobrachialis ; another, anterior set, flexes the forearm, the biceps, and the 

 brachialis anticus; and the last, or posterior set, extends the forearm, the triceps, with, 

 sometimes, the subanconeiis beneath it. 



