OSTEOLOGY AND ARTHROLOGY 



39 



the head articulates is situated on the outer side of the 

 coronoid process, and this apophysis is placed in front. 

 In the plantigrades and digitigrades the head of the radius 

 is placed still more forward, so much so that it is situated 

 almost in front of the superior extremity of the ulna (Fig. 26). 

 In the unguligrades it is placed directly in front of this 

 latter (Fig. 27). 



Further, the displacement of the radius is made at the 

 expense of tlie superior extremity of the neighbouring bone ; 

 the radius appears to appropriate more and more the parts 

 which in man belong exclusively to the ulna — for example, 

 the coronoid process. '- In the plantigrades and the digiti- 



FiG. 25. — Superior Extremity of 

 THE Bones of the Human Fore- 

 arm : Left Side, Superior Sur- 



FACE. 



Fig. 26. — Superior Extremity of 

 THE Bones of the Forearm of 

 THE Dog: Left Limb, Superior 

 Surface. 



I, Radius; 2, ulna; 3, olecranon i, Radius; 2, ulna; 3, olecranon 

 process ; 4, coronoid process. process ; 4, coronoid process. 



grades half of the process still belongs to the ulna and the 

 remainder to the radius. In the ungulates — the horse, for 

 example — the coronoid process belongs to the radius ; the 

 ulna, situated behind the latter, is correspondingly dimin- 

 ished in size. 



In brief, when we study this region of the skeleton in 

 plantigrades, then in digitigrades, and finally in unguligrades, 

 we find a kind of progressive absorption of one of the two 

 bones (ulna) by the other (radius), which thus becomes the 

 more developed. 



It is easy to explain this partial disappearance of the 

 ulna. When the forearm is capable of performing the 



