DENTITION. 145 



a less extent, in the Sus JEthiopicns* This mode 

 of dentition appears to be peculiar to animals of 

 great longevity, and which subsist on vegetable 

 substances containing a large proportion of tough 

 fibres, or other materials of great hardness ; and 

 requiring for their mastication teeth so large as not 

 to admit of both the old and new tooth being con- 

 tained at the same time in the alveolar portion of 

 the jaw. 



An expedient of a different kind has been re- 

 sorted to in the Rodentia, for the purpose of pre- 

 serving the long chisel-shaped incisors in a state fit 

 for use. By the constant and severe attrition to 

 which they are exposed, they wear away very 

 rapidly, and would soon be entirely lost, and the 

 animal would perish in consequence, were it not 

 that nature has provided for their continued growth, 

 by elongation from their roots, during the whole of 

 life. This growth proceeds in the same manner, 

 and is conducted on the same principles, as the 

 original formation of the simple teeth already des- 

 cribed; but in order to effect this object, the roots 

 of these teeth are of great size and length, and are 

 deeply imbedded in the jaw, in a large bony socket 

 provided for that purpose ; and their cavity is 

 always filled with the vascular pulp, within which 

 a continued secretion and deposition of the materials 

 both of ivory and enamel, take place. The tusks 

 of the Elephant and of the Hippopotamus exhibit 

 the same phenomenon of constant and uninter- 

 rupted growth. 



In the Shark, and some other fishes, the same 

 object is attained in a different manner. Several 



* Home, Phil. Trans, for 1799, p. 237 ; and 1801, p. 319. 

 VOL. II. L 



