DIGESTIOX. 



5. Mastication. As soon as solid food is taken into 

 the mouth, it undergoes mastication, or chewing. It is 

 caught between the opposite surfaces of the teeth, and by 

 them is cut and crushed into very small fragments. In 



the movements of chewing, 

 the lower jaw plays the chief 

 part; the upper jaw, having 

 almost no motion, acts sim- 

 ply as a point of resistance, 

 to meet the action of the 

 former. These movements 

 of the lower jaw are of three 

 sorts: a vertical or cutting, 

 a lateral or grinding, and a 

 to-and-fro or gnawing mo- 

 tion. 



6. The teeth are composed 

 of a bone-like material, and 

 are held in place by roots 

 running deeply into the jaw. 

 exposed portion, or 

 is protected by a 

 thin layer of enamel (Fig. 

 17, ), the hardest substance 

 in the body, and, like flint, is capable of striking fire with 

 steel. In the interior of each tooth is a cavity, containing 

 blood-vessels and a nerve, which enter it through a minute 

 opening at the point of the root (Fig. 19). 



7. There are two sets of teeth ; first, those belonging to 

 the earlier years of childhood, called the milk teeth, which 

 are twenty in number and small. At six or eight years of 

 age, when the jaw expands, and when the growing body 

 requires a more powerful and numerous set, the roots of 



PIG. 17. SECTION OF A TOOTH. 



a, Enamel; b. Cavity- cc, Roots: 



d, Body of the Tooth. 



The 



" crown," 



5. Describe the process of mastication ? How many and what movements ? 



6. Composition of the teeth ? Enamel of the teeth ? Interior of teeth * 



7. The milk teeth ? The permanent teeth ? 



