16*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 Rodeiitia, for the purpose of 

 preserving 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 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 

 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, 

 from which a continued secretion and deposition 

 of fresh layers, both of ivory and enamel, take 

 place. The tusks of the Elephant and of the 

 Hippopotamus exhibit the same phenomenon of 

 constant 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 only 

 one of these rows projects and is in use at the 

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



