116 THE STRUCTURE OF THE HORSE 



As it has no surrounding of enamel, it cannot be mis- 

 taken for the pit, or true mark. 



The three incisors of each side of the jaw, beginning 

 at the middle line, are spoken of as 4 central ' or c pincers,' 

 £ lateral ' or 4 intermediate,' and 6 corner ' teeth. For 

 brevity of description they are symbolised as i \ i 2 , and i s . 



The characters of the incisor teeth in the three 

 existing families of Perissodactyles offer an interesting 

 subject for consideration. All originating in a similar, 

 and comparatively simple form, they have all varied from 

 it in totally different directions. Those of the tapirs 

 show the least change from the primitive condition ; 

 those of the rhinoceroses have dwindled down in number 

 and size, to complete disappearance in some species ; 

 those of the horses have undergone changes leading, 

 finally, to a complication of structure unknown in any 

 other existing animal. There can be little doubt but 

 that these changes have all been in adaptation to some 

 peculiarities of the environment of the animals, and 

 that each has been best adapted for the purpose which 

 it has had to fulfil ; but the relations between use and 

 structure are often of such a delicate and intricate cha- 

 racter that they quite escape the recognition 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 



