38 THE ARTISTIC ANATOMY OF ANIMALS 



placed in a special group, which is based on that peculi- 

 arity- — that is, the group of ungulate mammals. 



The plantigrades and digitigrades, of which the paws 

 have their surfaces of support strengthened by an epidermic 

 sole and fatty pads, have the free extremities of the third 

 phalanges covered on their dorsal surface by nails or claws ; 

 hence they are named ungniculate mammals. 



The bat and birds have the bones of the forearm so 

 arranged that the radius cannot rotate around the ulna. This 

 is necessary in order that during flight, when the wing is 

 being lowered, the radius and hand shall not be able to turn ; 

 for, if such rotation took place, each stroke of the wing would 

 place it in a vertical position, which would occasion a loss of 

 resistance incompatible with the effect to be obtained. 



The Forearm 



The skeleton of the forearm in quadrupeds is vertical in 

 direction ; consequently, it forms with the arm an angle 

 open anteriorly ; this is well seen on examining the lateral 

 surface. If we examine it on its anterior surface, we find 

 a slight obliquity directed downwards and inwards. In 

 animals in which the bones of the forearm are separate — 

 that is to say, susceptible of supination and pronation — 

 we find a more close resemblance to those of the human 

 skeleton. The ulna, the superior extremity of which always 

 projects beyond that of the radius, has a shaft which gradu- 

 ally narrows from above downwards. Its inferior extremity 

 is terminated by a round head in those animals in which the 

 ulna is fully developed ; in others, as it is atrophied, it ends 

 in a thin, long process. 



The ulna presents at its superior extremity a posterior 

 process, the olecranon, which forms the point of the elbow. 

 We find on the anterior surface of the same, another process, 

 the coronoid. 



It is necessary to dwell on the relations of these parts. 

 In man the head of the radius is situated at the anterior 

 part of the external surface of the superior extremity of the 

 ulna (Fig. 25) ; indeed, the small sigmoid cavity with which 



