THE HEAD AND NECK. 125 



that each has been best adapted for the purpose 

 which it has had to fulfill ; but the relations between 

 use and structure are often of such a delicate and 

 intricate character that they quite escape the recog- 

 nition of our limited powers of observation. 



The Canines. — Separated from the incisors by a 

 short interval are the teeth called in the general 

 language of zoology " canines/' but usually spoken of 

 in the horse as the "tusks/' or, more often, "tushes" 

 (Fig. 16, c). They correspond exactly with the tusks 

 of the boar and the great corner teeth of the lion 

 and dog; but in all the Equidw they play a very 

 subordinate part, not being required either as a 

 means of defense or for the purpose of seizing prey. 

 Following a very general rule among the Mammalia, 

 especially marked in the Ungulates and other groups 

 (as monkeys) in which these teeth are not a necessity 

 for procuring food, they are much more developed 

 in the male than in the female. Indeed, they are 

 practically absent in the latter sex, as, when they do 

 occur as an exception, they are in a more or less 

 rudimentary condition. As the canines were present 

 in both sexes in the Eocene and Miocene Ungulates, 

 their loss in the females of the existing Equidce must 

 be reckoned, like the loss of the anterior premolar, 

 among the numerous instances of specialization 

 which this group has acquired. 



