STRUCTURE OF TEETH. 



151 



trace of gelatin : it is the hardest of all animal 

 substances, and is capable of striking fire with 



steel. It exhibits a fibrous structure, approach- 

 ing to a crystalline arrangement ; and the direc- 

 tion of its fibres, as shown by the form of its 

 fragments when broken, is every where perpen- 

 dicular to the surface of the ivory to which it is 

 applied. The ends of the fibres are thus alone 

 exposed to the friction of the substances on 

 which the teeth are made to act ; and the effect 

 of that friction in wearing the enamel is thus 

 rendered the least possible. 



In the teeth of some quadrupeds, as of the 

 Rhinoceros^ the Hippopotamus^ and most of the 

 Rodentia, the enamel is intermixed with the 

 ivory ; and the two so disposed as to form jointly 

 the surface for mastication. In the progress of 

 life, the layers of enamel, being the hardest, are 

 less worn down by friction than those of the 

 ivory, and therefore form prominent ridges on 



