172 



THE STEUCTUEE OF THE HOESE 



generally convex, the tissue of which, it is composed 

 being thicker at the middle than at the edges. The 

 hinder one is rather smaller and less elevated. Its pos- 

 terior margin is nearly straight or regularly convex ; 

 its anterior margin is excavated in its upper third. It 

 is therefore more pointed above than below. The upper 

 end is about four inches below the point of the hock 

 (tuber calcis). The natural colour of both is dark slate, 

 but when much dry epidermis collects on the surface 

 they have a lighter or yellowish appearance. 



In all the species of asses and zebras the hinder one 

 is absent ; but the one on the fore limb always exists, 

 although in a modified form. It is broader or more oval in 

 shape, and with a smoother and scarcely elevated surface. 

 In the zebra it assumes the form of a large circular flat 

 black patch of bare skin, nearly two inches in diameter. 



The signification and utility of these structures are 

 complete puzzles. Various suggestions have been 

 made, none of which will bear examination. One 

 of the most plausible, especially in the light of modern 

 comparative anatomy, is that they are rudiments or 

 vestiges of the inner toe — the thumb or pollex of the 

 fore limb, the great toe or hallux of the hind limb — 

 which, as already shown, is not otherwise represented in 

 the horse. There are, however, many objections to this 

 theory. The inner toe is always the first to disappear 

 in all mammals, and no traces of it are found in any 



