46 THE ARTISTIC ANATOMY OF ANIMALS 



one bone ; this, the canon bone, articulates with two 

 digits. 



The number of metacarpals and digits diminishes in pro- 

 portion as the limbs cease to be organs of prehension, and 

 become more exclusively organs of support and locomotion. 



The number of phalanges is two for the thumb and three 

 for each of the other digits ; except in the cetaceans, in 

 which they are more numerous. 



In the bat, the metacarpals and phalanges are very long, 

 and form the skeleton of the wing ; these phalanges are 

 not furnished with nails ; the thumb, which is very short, 

 is alone provided with one (Fig. 8). 



With regard to the relative dimensions of the bones of 

 the metacarpus, it is necessary to remember that, in the 

 human being, the second metacarpal is the longest ; then, 

 in the order of decrease, come the third, fourth, fifth, and 

 first. In quadrupeds we shall also find differences in 

 length (see the chapter relating to the anterior limbs in 

 certain animals), but the order ol decrease is not always 

 that which we have just mentioned. 



In man the articular surface, situated at the inferior ex- 

 tremity of each of the metacarpals, is rounded, and is 

 called the head. This allows the first phalanx, which is 

 in relation with that surface, to be displaced in every 

 direction ; indeed, this phalanx can not only be flexed and 

 extended, but it can also be moved laterally ; this latter 

 movement allows of the fingers being separated and drawn 

 together. 



In quadrupeds which can only perform the movements of 

 flexion and extension of the digits — for example, the horse 

 — the inferior extremity of the metacarpal has not a rounded 

 head of a regular outline ; it is marked by a prominent 

 median crest, directed from before backwards, so that the 

 articular surfaces, which fit more exactly, form a sort of hinge 

 which allows of backward and forward movements only, and 

 permits no lateral displacement. In man, at the level of the 

 inferior extremity of the first metacarpal, in the vicinity of the 

 articulation of this bone with the first phalanx of the thumb, 

 we find two sesamoid bones — small bones developed in 



