DIGESTION 



117 



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- 

 f ro or gnawing motion. 



6. The teeth are composed of a bone-like material, and are 

 held in place by roots running deep into the jaw. The 

 exposed portion, or " crown," 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 tootli is a cavity, contain- 

 ing blood-vessels and a nerve, 

 which enter it through a minute 

 opening at the point of the root 

 (Fig. 25). 



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 the milk teeth 



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

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



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

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

 similarly shapecL and arranged in t^ e same qrxier j 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 



6. Composition of the teeth ? Enamel of the teeth ? Interior of toe^^/t -* 



T. The milk teeth ? The permanent teeth ? 



8, 9. Number of teeth ? How arranged ? /'' 



FIG. 23. SECTION OF A TOOTII 



a, Enamel ; b, Cavity ; c, c, Roots ; 

 d, Body of the Tooth 



