THE SWALLOWING OF FOOD 



353 



The child's first set of teeth ("milk teeth") is temporary. There 

 are 20 in the set. When the child is 6 or 7 years old, the first permanent 

 teeth appear. These are the fii^t of the permanent molars. The 

 molars farthest back in each jaw 

 are "cut" last, ancl are called the 

 "wisdom teeth." 



A tooth consists of a crown of 

 very hard enamel, and a neck and 

 root of material called dentine, The 

 roots are covered with "cement." 

 The interior of the tooth is called 

 the pulp cavity; it contains the 

 nerves and blood vessels of the 

 tooth. The teeth are precious, 

 and ought to be given every care. 

 They should be cleaned, every day, 

 with a soft brush, and if cavities 

 are formed they should be filled 

 by a good dentist. Hard sub- 

 stances, such as nuts, thread, and 

 brittle candy, should not be bitten; 

 for they may break the enamel. 

 If bacteria get into the interior of 

 the teeth, they cause a rapid decay. 



361. The Swallowing of 

 Food. When food is ready 

 to be swallowed, the tongue 

 pushes it back to the pharynx 

 (Fig. 280). This is a mus- 

 cular tube, about , 4 inches 

 long, having a number of 



openings. At its top there are 2 openings into the 

 nose, and 2 into the ears. The openings leading to 



FIG. 280. 



Paths Taken by Food and by Air as They 

 Enter the Body. 



