The Anterior Ljmb Skeleton. 



a similar point of origin for the flexor muscles 

 of the ventral or volar surface. Between the 

 epicondyles the extremity of the bone is 

 greatly excavated, so that the projecting 

 portions of the radius in front and of the 

 ulna behind are received into depressions of 

 the surface when the forearm is greatly flexed 

 or extended. On the anterior side is the 

 radial fossa (fossa radialis) ; on the posterior 

 side the olecranon fossa (fossa olecrani), so- 

 called because it accommodates the olecranon 

 process of the ulna. 



THE RADIUS AND ULNA. 



The radius (Fig. 39) is the shorter of the 

 two bones of the forearm, since its proximal 

 extremity does not extend backward beyond 

 the front of the elbow joint. It is antero- 

 dorsal in its general position, but is crossed 

 on the ulna in such a way that its proximal 

 extremity tends to be lateral, while its distal 

 extremity is medial. The proximal extremity, 

 termed the head of the radius (capitulum 

 radii) is immovably articulated with the ulna. 

 It bears an extensive articular surface, meet- 

 ing both the trochlea and capitulum of the 

 humerus, and thus forming a considerable 

 portion of the elbow-joint. The body of the 

 bone is solidly united with the ulna by the 

 interosseous ligament of the forearm. The, 

 distal extremity is largely formed by an 

 epiphysis, which is well-marked even in older 

 animals. It bears a grooved, carpal articular 

 surface (facies articularis carpea), for articula- 

 tion with the navicular and lunate bones. 



The ulna (Fig. 39) is a somewhat S-shaped 

 bone, the shaft of which is vertically flattened, 

 so that it possesses two main surfaces, respec- 

 tively anterodorsal and posteroventral. The 

 former, in conjunction with the related sur- 

 face of the radius, continues the area of origin 

 of the extensor muscles of the hand from the 

 lateral epicondyle of the humerus distad on 

 to the forearm, while the latter has a similar 

 function with respect to the flexor, muscles. 

 The proximal portion of the bone is laterally 

 compressed. It bears a crescentic depression, 

 the semilunar notch (incisura semilunaris), 



101 



Fig. 39. Skeleton of the fore- 

 arm and hand from the dorsal 

 surface: R, radius; U, ulna; C, 

 carpus; M, metacarpus; P, 

 phalanges ; I-V, metacarpal 



bones; c, central bone; cp. f 

 capitate; c.r., head of radius; 

 i. a. a, carpal articular surface of 

 radius ; h., hamate bone ; i.s., 

 semilunar notch of the ulna; 

 1., lunate bone; mi., lesser mul- 

 tangular; mj., greater mul- 

 tangular; n., navicular; ol., 

 olecranon; p.s., styloid process 'of 

 the ulna; tr., triquetral bone; 

 u., ungual phalanges. 



