588 MAMMALIAN ANATOMY 



ridge and the outer border is a deep pit. The lower margin of the 

 ridge is the anterior border of the distal surface of the radius. 



The posterior surface (Fig. 448) is wider and longer than the 

 anterior. It is convex from side to side, and divided by more or less 

 prominent longitudinal ridges into four grooves. These grooves are 

 converted by ligaments into canals, through which pass the tendons of 

 the extensor muscles. The external groove lies between the base of 

 the styloid process and the external border and lodges the tendon of 

 the extensor ossis metacarpi pollicis. It is the smallest of the four, and 

 runs obliquely forward and downward. The next groove lies between 

 the beginning of the styloid process and a prominent ridge, which 

 appears to be a continuation downward of the posterior border of the 

 shaft. This ridge is the narrowest and highest of all, and lies to the 

 outer side of the longitudinal axis of the bone. The groove is well 

 marked, concave from side to side, and convex from above downward, 

 and holds the tendons of the extensores carpi radialis longior and 

 brevior. The third groove is as wide as the preceding and a little 

 longer. It, also, is concave from side to side, but is almost flat 

 from above downward. It lodges the tendons of the extensor com- 

 munis digitorurn. The ridge which forms the inner boundary of this 

 groove is low and nearly as broad as the groove itself. To the inner 

 side of this ridge is the fourth groove, which, although smaller than 

 the two immediately preceding, is well marked. It runs more obliquely 

 downward and inward, and lodges the tendons of the extensor indicis. 

 The tendons of the extensor minimi digiti lie over the line of articu- 

 lation of the radius with the ulna. 



The distal surface of the lower extremity (Fig. 450), and therefore 

 of the whole bone, is transversely oval, and about twice as wide as deep. 

 It is shorter from before backward at the outer side than at the inner 

 side, and its outer border is prolonged downward and slightly outward 

 into the styloid process. The posterior border is rounded and promi- 

 nent except just within the styloid process, where it is continuous with 

 the groove for the radial extensors of the wrist. Beyond this point, 

 the prominent border is twice emarginated for the outer two grooves 

 on the posterior surface. The anterior border is sharp and prominent 

 except near the styloid process, where it becomes emarginate, smooth, 

 and continuous with the anterior wedge-like surface. The inner border 

 is a narrow curved strip which slopes upward and inward from the 



