1()2 THE VITAL FUNCTIONS. 



contained at the same time in the alveolar por- 

 tion of the jaw. 



An expedient of a different kind has been 

 resorted to in the Rodentia, for the purpose of 

 preserving the long chisel-shaped incisors in a 

 state fit for nse. 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 conse- 

 quence, 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 described : but, in 

 order to effect this object, the roots of these 

 t^eth are of great size and length, and are 

 deeply imbedded in the jaw, in a large bony 

 canal provided for that purpose; and their cavity 

 is always filled with the vascular pulp, from whicli 

 the continued secretion and deposition of fresh 

 layers, both of ivory and enamel, take place. 

 The tusks of the Elephant and of the Hippo- 

 potamus exhibit the same phenomenon of con- 

 stant and uninterrupted growth. 



In the Shark, and some other fishes, the same 

 object is attained in a different manner. Several 

 rows of teeth are lodged in each jaw, but one 

 only of these rows projects and is in use at the 

 same time ; the rest lying flat, but ready to rise 



