586 MAMMALIAN ANATOMY 



posterior borders, and is wide above and long and narrow below. Its 

 upper part is nearly flat, and faces inward and backward ; it is a sur- 

 face of origin of the supinator brevis. The lower part is convex from 

 above downward and from side to side, and gives origin to a part of 

 the extensor ossis metacarpi pollicis. 



The posterior surface (Fig. 448) lies between the internal and 

 posterior borders. It is narrow above, where these borders lie close 

 together on the inner side of the posterior aspect, but much wider 

 below, where the borders are further apart and parallel. It is convex 

 from above downward and also from side to side, especially in the 

 middle region. It faces backward and inward, giving attachment at 

 its upper part to the extensor ossis metacarpi pollicis. 



The Lower Extremity is moderately well defined from the shaft 

 by its increased width and thickness. It presents a distal smooth artic- 

 ular surface, and anterior, posterior, and internal surfaces, separated by 

 external and anterior and posterior inner borders. Its outer angle is 

 prolonged into the styloid process. 



The external border (Fig. 449) is the continuation of the external 

 border of the shaft. It varies in different bones, but in most specimens 

 is thin and sharp ; it springs suddenly from the lower border of the 

 shaft and runs forward as well as downward. The edge faces forward 

 and outward, and is not continued on the styloid process, but stops 

 abruptly at its base. It forms the outer part of the groove for the 

 tendon of the extensor ossis metacarpi pollicis. 



As has already been explained, the internal or interosseous border 

 of the shaft divides at the lower end into two nearly parallel branches, 

 which become the anterior and posterior inner borders of the extremity 

 and enclose between them the internal surface. 



The internal surface is slightly longer than wide ; it is concave 

 from side to side, and appears strongly concave from above downward, 

 owing to the sudden sloping of the lower part inward and downward. 

 At the lower edge, near the anterior border, is a small facet for articu- 

 lation with the head of the ulna. This facet is transversely oval, 

 slightly concave, and faces forward, upward, and inward. 



The anterior surface (Fig. 446) is a prominent but narrow, trans- 

 verse ridge stretching from the base of the styloid process to the ulnar 

 articular facet on the internal surface. The ridge slopes upward and 

 backward into the lower end of the shaft. In the angle between the 



