DIGESTION AND ABSORPTION 



30s 



into the nasal cavity which lies above the palate (see fig. 157). 

 The teeth and tongue act mechanically upon the food, 



masticating it or breaking it up. 



The teeth. — The teeth are important structures since 



they initiate the work of digestion. Those that grow in 



during the first two years 

 of life are called milk 

 teeth. They have small- 

 er roots than the per- 

 manent teeth and ap- 

 pear while the jaws are 

 small. As the jaws 

 enlarge (during the sixth 

 and seventh years) the 

 second set or perma- 

 nent teeth grow in and 

 one by one replace the 

 milk teeth. The perma- 

 nent teeth need much 

 care and attention as 

 they must last through- 

 out life. 



Fig. 1S7. Diagram of the buccal cavity Kinds of teeth. 



showing the manner of closing the There are eight teeth in 



posterior nares during deglutition, the front of the mOUth 



(After Landois & Stiriing.) ^^^^ ^^ ^^^^ ^^^^ ^^jj^^ 



incisors, to bite and cut the food. The sharp pointed teeth 

 on either side of the incisors are the canines. On either 

 side of the canines there are two large teeth with two-point- 

 ed surfaces. These are the bicuspids or premolars. On 

 either side of the premolars are three large molars. The 

 premolars and molars are grinders. The third molars fre- 

 quently do not appear until after the twentieth year, and 

 are commonly called the wisdom teeth. 



Structure of the teeth (fig. 158). — The teeth, though as 



