NUTRITION 



367 



.P.M 



the food into proper shape, pushes it back toward the gullet, and 

 partly closes the trachea. 



4. It helps to keep clean the inner surface of the teeth. 



5. In man it is one of the organs concerned in speech. 

 The Teeth Structure. The teeth 



are even more familiar and im- 

 portant organs. Each consists of 

 three parts, (1) the crown or ex- 

 posed portion, (2) the neck, a slight 

 narrowing at the edge of the gum, 

 and (3) the root or roots which are 

 attached to the jaw, 



A section cut lengthwise through 

 a tooth shows that the crown is 

 covered by a very hard substance 

 called enamel, which protects the 

 exposed parts. The bulk of the 

 tooth consists of dentine, a softer 

 and more porous substance, while 

 the center is occupied by the pulp 

 which contains the nerves and 

 blood vessels of the tooth. The 

 root is covered by a bone-like coat- 

 ing, the cement, and through the 

 very tip is 'the opening by which 

 the nerves and blood vessels find 

 entrance. 



Number and Kinds of Teeth. 

 It is easily seen that there are four 

 kinds of teeth in the mouth even 

 though the full number may not be there till the 20th year. 



In the full set there are thirty-two, sixteen on each jaw, arranged 

 as follows: In front are eight incisors with sharp edges, whose 

 function is to cut the food, next on each side is one canine, or four 

 in all, which are pointed and which the lower animals use for tear- 

 ing food. In man they assist the incisors. Behind these on each 



FIG. 116. Vertical section of a 

 tooth in jaw. E, enamel ; D, dentine ; 

 P M , peridontal membrane; PC, 

 pulp cavity; C, cement; B, bone of 

 lower jaw; V, vein; A, artery; N, 

 (After Stirling.) From 



