m] 



THE DIGESTIVE ORGANS 



27 



the same time the teeth are gromid down through wear and 

 tear. B3' taking advantage of this fact the age of an animal can 

 be determined after it has acquired its full mouth of permanent 

 teeth. At an earlier period of life, the age can be ascertained 

 by noting the number and arrangement of the temporary or 

 milk teeth. 



Enamel 

 Dentine 

 Cement 



Pulp cavity - 



Fig. 19. Longitudinal and cross-sections of incisor tooth of horse. 

 Cross sections A, B, C illustrate the surface appearance of the 

 tooth at various stages of wear.' Note that the peripheral cement 

 quickly disappears from the newly erupted tooth in situations 

 subject to wear. 



The movements which the jaws undergo to admit of mastica- 

 tion vary somewhat in different animals. In the dog there is a 

 simple up-and-down movement of the lower jaw, and for this 

 purpose the articulation of the jaw is of the simplest kind. Hardly 

 any effort is required for opening the mouth, for this movement 

 merely involves a depression of the lower jaw, but powerful 

 muscles are provided for closing the mouth, that is, for elevating 

 the jaw. This statement applies to the horse and ox and other 

 animals besides the dog, but the horse, ox and sheep differ from 



