DIFFERENT KINDS OF TEETH. 



171 



cutting edges, which work against each other like the blades 

 of a pair of scissors (fig. 94) ; whilst in animals that live on 

 insects, these teeth are raised into conical points, which lock 



Fig. 05. 

 Fig. 94. TEETH OF CARNIVOROUS ANIMAL TEETH OF INSECTIVOROUS ANIMAL. 



into corresponding depressions in the teeth of the opposite 

 jaw (fig. 95). When the nourishment of the animal con- 

 sists principally of soft fruits, these teeth are simply raised 

 into rounded elevations (figs. 97, 98) and when they are 



Fig. 96. 

 TEETH OF HERBIVOROUS AMMAL. Fig. 97. TEETH OF FRUGIVOROUS ANIMAL. 



destined to grind harder vegetable substances, they are termi- 

 nated by a large flat and roughened surface (figs. 96, 99). 

 The roughness of this surface is maintained by the peculiar 

 arrangement of the three substances of which the tooth is 

 composed. The enamel, instead of covering its crown, is 

 arranged in tipright plates, which are dispersed through the 

 tooth ; and the space between them is filled up by plates of 

 ivory and of cementum ( 54). These last, being softer than 

 the enamel, are worn down the soonest ; and thus the plates 

 of enamel are left constantly projecting, so as to form a rough 

 surface admirably adapted to the grinding action which the 

 tooth is destined to perform. The mode in which these 

 plates are disposed, affords a most characteristic distinction 

 between the two species of Elephant at present existing, 



