244 THE ARM AND FORE-ARM. [chap. 



by the coalescence of two distinct endosteal nuclei ; the 

 lower one includes the whole inferior articular surface with 

 the condyles, and is formed of three or four originally dis- 

 tinct centres of ossification. The lower epiphysis unites to 

 the shaft before the upper one. 



The skeleton of the second segment of the upper limb, the 

 fore-arm or antibrachium, consists of two bones called radius 

 and ulna, placed side by side, articulating with the humerus at 

 their proximal, and with the carpus at their distal, extremity. 



In their primitive or unmodified condition, these bones 

 may be considered as placed one on each border of the 

 limb, the radius being preaxial, and the ulna postaxial. 1 

 The radius articulates above with the preaxial (external) 

 side of the humerus, the ulna with the postaxial (internal) 

 side of the humerus. 



This position is best illustrated in the fore-limb of the 

 Cetacea (see Fig. 99), where the two bones are fixed side by 

 side and parallel to each other, the preaxial border being 

 external, and the postaxial border internal, in their whole 

 extent. 



In the greater number of Mammals, the bones assume a 

 very modified and adaptive position (as will be explained 

 more fully in the chapter on the comparison of the fore and 

 hind limbs), usually crossing each other in the fore-arm, the 

 radius in front of the ulna, so that the preaxial bone 

 (radius), though external (in the ordinary position of the 

 limb) at the upper end, is internal at the lower end : and 

 the hand being mainly fixed to the radius, also has its pre- 



1 So termed (by Prof. Huxley) in relation to the central axis of the 

 limb, when it is extended out from the body in its primitive embryonic 

 position, the extensor surface of the arm and back of the hand being 

 upwards or dorsal, and the flexor surface of the arm and palm of the 

 hand being downwards or ventral. 



