120 



DIGESTION 



simply as a point of resistance, to meet the action of the 

 former. These movements of the lower jaw are of three 

 sorts : an up-and-down or cutting, a lateral or grinding, and 

 a to-and-fro or gnawing motion. 



8. The teeth are composed of a bone-like material called 

 dentine, and are held in place by roots running deep into the 

 v jaw. The exposed portion, or 



" crown,' 7 is protected by a thin 

 layer of enamel (Fig. 23, a), 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. 25). 



9. 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 the milk teeth 

 are absorbed, and the latter are " shed/ 7 or fall out, one after 

 another (Fig. 24), to make room for the permanent set. 



10. There are thirty-two teeth in the permanent set, an 

 equal number in each jaw. Each half-jaw has eight teeth, 

 similarly shaped and arranged in the same order; thus, two 

 incisors, one canine, two bicuspids, and three molars. The 

 front teeth are small, sharp, and chisel-edged, and are well 

 adapted for cutting purposes ; hence their name incisors. The 



8. Composition of the teeth ? Enamel of the teeth ? Interior of the teeth ? 



9. The milk teeth ? The permanent teeth ? 



10, 11. The number of the teeth ? How arranged ? 



FIG. 23. SECTION or A TOOTH 



a, Enamel ; ft, Cavity ; c, c, Roots : 



d, Body of the Tooth 



