182 THE STKUCTURE OF THE HOESE. 



They are patches on which no hair grows, but in 

 which the papillae of the derm or true skin are much 

 enlarged and covered with an abundant and thick- 

 ened epidermis, which becomes dry and horny and 

 sometimes accumulates in considerable quantity on 

 the surface, occasionally even making a horn-like 

 projection. Their structure, in fact, is much like 

 that of a wart or corn, but they are not the results 

 of pathological changes, though often treated as such 

 in old works on veterinary surgery. Even so en- 

 lightened a writer as Youatt includes them among 

 diseases, and prescribes remedies both external and 

 internal for the purpose of getting rid of them. They 

 are, however, perfectly normal structures ; they ex- 

 ist at birth, are equally developed in both sexes, and 

 (allowing for certain limited individual variations) 

 constant in form, size, and position. They consti- 

 tute, moreover, one of the characteristic distinctions 

 by which the species Equus caballus is distinguished 

 from the other members of the genus. 



They differ in form in the two limbs, but in both 

 are placed upon the inner surface and nearer the 

 hinder than the front border. That on the fore limb 

 is above the carpal or wrist joint ("knee" of the 

 horse), that on the hind limb beloiv the ankle or 

 "hock" joint. The former is about two inches long 

 and three-quarters of an inch wide, pointed at each 



