424 



INTRODUCTION TO ZOOLOGY. 



of three substances of different degrees of hardness; the 

 "dentine,'' which constitutes the principal component; the 

 "enamel,'' which is a much harder substance; and the 

 "cement," which is a softer one, and serves to unite the 

 plates of which the tooth is composed. The unequal density 

 causes the surface to wear away in an unequal manner, and 

 hence the property which makes a mill-stone most valuable is 

 secured. The arrangement to make good what a mechanic 

 would call "the wear and tear" of the apparatus is not less 

 effectual. The teeth are ever growing, not as in the Rodentia 



Fig. 322. INDIAN ELEPHANT. 



(ante, p. 404), by a deposit of new matter at the base, but 

 by the development of new teeth. We are accustomed to see 

 a new tooth come forth in the mouth of a child from the place 

 where the former tooth had been shed; but in the young 

 Elephant the plan of development and succession is altogether 

 different. Each tooth is formed in a membraneous bag, 

 enclosed in a chamber of bone, forming part of the massive 



