THE HEAD AND NECK 



119 



such close contact, by broad surfaces fitting tightly 

 against each other, that they form together one solid 

 mass, presenting a grinding-surface composed of sub- 

 stances of various densities, and therefore projecting at 

 slightly different levels, interwoven in such an intricate 

 pattern as to form one of the most efficient natural 

 millstones imaginable. 



A distinction must be pointed out among these 

 teeth. In a great many animals their form differs so 

 much that they are readily separated into an anterior 

 set of simple character, and a posterior set, larger, 

 broader, and with more complex crowns and roots ; and 

 when it was discovered that these also presented a 

 constant difference in their mode of development — 

 the first set being preceded in their places by other 

 teeth of the milk, or deciduous series, and the last set 

 coming up behind the last of the milk-teeth, without 

 any predecessors — the distinction was thought to be 

 of sufficient importance to give them different names, 

 the first being called ' premolars,' or ' false molars,' and 

 the last, ' true molars.' In the horse there is no differ- 

 ence in form or size between the premolars and true 

 molars, and it is only by the analogy of other animals, 

 and by a knowledge of their early history in the horse 

 itself, that we can divide them, and know that the great 

 mill-like mass of cheek-teeth consists of three premolars 

 and three molars. 



