42 THE HORSE AND ITS RELATIVES 



to the whole of the terminal segment of the limb, 

 it properly denotes — at all events, in an anatomical 

 sense — only the dense horny shell or wall investing 

 the front and lateral surfaces, and corresponding 

 to the nail of the human middle finofer or toe and 

 the claw of the same toe in a dog. To the trian- 

 gular, less hard, horny structure projecting from the 

 back into the centre of the lower surface of the 

 foot the name "frog" is applied. 



In all members of the horse tribe the terminal 

 segments of the fore and hind feet are remarkably 

 alike ; more alike, in fact, than in any other animals. 

 In the horse itself this similarity is, however, some- 

 what less marked than in most of the other members 

 of the group, the front hoofs being broader and 

 rounder than the hind pair. In the kiang ^ of 

 Tibet this difference is less marked, and in asses 

 and zebras all the hoofs are relatively small and 

 narrow, so that it is practically impossible to dis- 

 tinguish the front from the hind ones when sepa- 

 rated from the rest of the limb. 



In addition to the difference in the shape of 

 the hoofs of the horse and kiang as compared with 

 those of other members of the family, there are also 

 specific differences in regard to the form of the 

 frog. In the horse, for instance, the frog forms 

 a long narrow ridge, deeply grooved posteriorly, 



' The characteristics of this and other species mentioned in this 

 chapter are given in the sequel. 



