124 THE STRUCTURE OF THE HORSE. 



infundibulum, another spot, having a cloudy-yellow 

 color, always situated in front of the pit when the 

 two coexist, and continuing, after the obliteration of 

 the former, quite to the base of the root. This is 

 caused by the pulp-cavity, which has become filled 

 up by an irregular deposit of dentine. As it has no 

 surrounding of enamel, it cannot be mistaken for 

 the pit, or true mark. 



The three incisors of each side of the jaw, begin- 

 ning at the middle line, are spoken of as " central " 

 or " pincers," " lateral n or " intermediate," and " cor- 

 ner" teeth. For brevity of description they are 

 symbolized as i l , i 8 , and {?. 



The characters of the incisor teeth in the three 

 existing families of Perissodactyles offer an inter- 

 esting subject for consideration. All originating 

 in a similar, and comparatively simple form, they 

 have all varied from it in totally different direc- 

 tions. Those of the. tapirs show the least change 

 from the primitive condition 5 those of the rhinoce- 

 roses 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 



