352 



THE HUMAN BODY AND ITS FOOD 



reaction (cf. 218) ; it contains certain salts, also ptyaiin 

 (pronounced ti'-a-lin), a ferment, or enzyme. 



Ferments are substances that by their presence bring 

 about chemical changes (cf. 98). 



Ptyaiin begins the change of starch into sugar (cf. 357) ; hence a 

 cracker becomes sweet when chewed. There are three pairs of salivary 

 glands in the mouth: one pair just under the tongue; another behind 

 the corners of the lower jaws; a third pair in front of the ears. 



The tongue is a flat, movable organ made up of muscular tissue. 

 The back of it is attached to the bottom of the mouth. It has nerves 

 of touch, like those of the finger tips, and also nerves of taste. 



The cheeks and lips are 

 made up of thin muscles 

 covered with skin on the out- 

 side and with mucous mem- 

 brane on the inside. 



360. The Teeth. The 

 adult man or woman 

 has 32 teeth, set into 

 sockets in the jaw bones 

 (Fig. 279). The teeth 

 are of 4 kinds, called 



incisors, canines, bicuspids, and molars. There are 2 

 incisors (" cutters") in each half of each jaw. They 

 are flat, sharp teeth used for biting off pieces of food. 

 Next to the incisors is the canine, or "dog," tooth. 

 This is sharper than the other teeth. In the dog and 

 other carnivorous animals it forms the tusk, or tearing 

 tooth. The fourth and fifth teeth are the bicuspids, so 

 called because each has 2 cusps, or points. The sixth, 

 seventh, and eighth teeth are the molars, or grinding teeth. 



Molars Bicuspids 



FIG. 279. 

 *Teeth in the Lower Jaw. 



