THE STORY OF DIGESTION 



39 



The last molars in the jaw receive the name of 

 "wisdom teeth," owing presumably to their late 

 development. The six molars in each jaw are not 

 preceded by any teeth in the milk set, the presence 

 of the twelve molars thus accounting for the differ- 

 ence in number between the teeth of the second and 

 those of the first set. The bulk of each tooth is 

 composed of a substance called dentine or ivory, 

 which is very different from bone and of much 

 denser texture. The top of each tooth is coated 

 with a still harder sub- 

 stance termed enamel, 

 whilst at the root we 

 find a third substance 

 somewhat resembling 

 bone in structure and 

 known as cement. The 

 teeth, it may be added, 

 are typically skin-struc- 

 tures, that is to say, they 

 correspond closely with 

 nails and hairs in their 

 mode of origin and de- 

 velopment, seeing that 

 they originate from 

 the delicate skin layer, 



which, folded inwards at the mouth, constitutes what 

 is popularly known as the gum or mucous membrane. 

 SWALLOWING. Divided by the teeth and mixed 

 with the saliva, the food is ready to be swallowed. 

 This action is somewhat complicated in its nature. 

 It may be briefly described as consisting first in the 

 work of the tongue (Fig. 9) in gathering particles of 

 food from all parts of the mouth and gullet into 

 small masses or boluses suitable for swallowing or 



Fig. 9. THE HUMAN TONGUE 



Showing the papillae or organs 



of taste. 



