550 MAMMALIAN ANATOMY 



is continuous above with the external surface of the greater tuber- 

 osity. It is gently convex from side to side and slightly depressed 

 at the middle. The surface is twisted upon its own axis in such 

 manner that the upper part faces much more outward than forward 

 and the lower part faces more forward than outward. Along its 

 middle is attached the aponeurosis of the anterior part of the middle 

 pectoral ; lower down, at the apex, is inserted the acromial head of the 

 deltoid muscle, behind and above which, on the deltoid ridge, is the 

 insertion of the spinous head of the deltoid. The upper anterior part 

 of the surface is rough and free from muscular attachment. Above the 

 deltoid ridge, the tendon of origin of the outer humeral head of the 

 triceps is attached. The portion of the external surface remaining is a 

 curved strip everywhere of almost equal width, bounded in front by 

 the deltoid ridge and the anterior border and behind by the external 

 border. It is continuous below with the external surface of the lower 

 extremity ; above it faces outward and downward, and below outward 

 and forward. It is smooth and convex from side to side, more strongly 

 at the middle, and slightly concave from above downward, at the upper 

 and lower ends. The outer part of the area of origin of the posterior 

 humeral head of the triceps occupies a small triangle of the surface 

 at the upper end, below which is the area of origin for the brachi- 

 alis anticus, which is continued downward as a strip along the deltoid 

 ridge and a strip along the external border. The rest of the surface, 

 although covered with muscles, is free from muscular attachment. 



The internal surface (Fig. 429) is the smallest of the three; it 

 lies between the anterior and internal borders, and by reason of their 

 direction is wider above than below. Its upper third is directed back- 

 ward and upward away from the long axis of the lower two-thirds, 

 and passes above into the bicipital groove, the anterior surface of the 

 lesser tuberosity, and the internal surface of the greater tuberosity. 

 Below, it is continuous with the surface of the bony bridge over 

 the supracondyloid foramen. The internal surface is convex from 

 side to side, except above, where it is the continuation downward 

 of the bicipital groove ; from above downward it is gently concave. 

 It faces forward and inward, and is smooth, unmarked by roughened 

 areas for muscular attachment, except on a small line in the upper 

 fourth, behind and parallel with the anterior border, to which line the 

 conjoined tendon of the teres major and the latissimus dorsi muscle 



