582 MAMMALIAN ANATOMY 



backward away from the long axis of the shaft. It appears also as if 

 rotated about its long axis in such manner that its longest diameter is 

 not transverse, as is the corresponding diameter of the shaft, but passes 

 obliquely from the inner side in front backward and outward. The 

 upper extremity consists of a head, a neck, and a tubercle. 



The head is a small, terminal, oval disk which furnishes surfaces 

 for articulation with the humerus and the ulna. It presents an oval 

 proximal surface and a narrow marginal surface at right angles to it, 

 which for most of the circumference is sharply divided from the neck 

 below. The transverse diameter of the head is about one-fourth 

 greater than the antero-posterior diameter. It does not lie exactly 

 transversely, but is directed from without backward as well as inward. 

 The degree of this obliquity varies in different bones. 



The oval proximal surface (Fig. 447) forms a shallow cup which 

 faces upward and also outward, is lined with cartilage, and articulates 



FIG. 447. 



Back. 



Inside. "^iN Outside. 



Front. 



HEAD OF RADIUS, PROXIMAL SURFACE. 



with the capitellum of the humerus. It is limited by well-defined 

 margins. Of these the front margin is sharp and forms also the front 

 margin of the head. The margins at the sides and back are at some 

 little distance from the edge of the head, a rounded border intervening 

 and passing into the corresponding sides of the head. This rounded 

 border is interrupted behind, at about the middle, by a notch, the con- 

 tinuation of a shallow vertical groove, on the dorsal surface. 



The front of the marginal surface, the anterior surface of the* head 

 (Fig. 446), is narrow, strap-like, convex from side to side, and directed 

 backward and downward from the sharp margin above. It is a smooth 

 articular surface rotating in the lesser sigmoid cavity of the ulna. At 

 the outer side this articular surface turns downward and ends obliquely, 

 the rounded margin of the proximal surface cutting it off by turning 

 downward. At the inner side it passes on the posterior surface. 



