IOO HUMAN BIOLOGY 



hard food is an aid to the teeth by helping the circulation 

 and nerves in the pulp. To take a very hot or very cold 

 drink into the mouth may cause the enamel to crack. If 

 a tooth aches, or a small decayed place is found in it, a 

 dentist should be consulted at once. A tooth is so valu- 

 able to the health that no tooth should be extracted when 

 it can be saved. 



The process of digestion consists in liquefying the food 

 that it may pass through the walls of the food tube into 

 the blood, and through the walls of the blood vessels into 

 the tissues. It is accomplished: (i) by mechanical means, 

 including the chewing muscles, the teeth, and three layers 

 of muscles in the walls of the food tube; (2) by chemical 

 means, or the action of alkalies and acids upon the food ; 

 (3) by organic agency, or the action of ferments. A 

 ferment (or enzyme) is a vegetable substance which has 

 the power of producing a chemical change in large quanti- 

 ties of substance brought in contact with it, without being 

 itself changed. There is one ferment secreted in the mouth, 

 two in the stomach, and three in the small intestine. 



Digestion in the Mouth. — Saliva is formed by six glands : 

 one in the cheek in front of each ear, one at the angle of 

 each lower jaw, and one pair is beneath the tongue. Each 

 gland opens into the mouth by a duct. Saliva is ropy 

 because it is mixed with mucus formed by the mucous 

 membrane lining the mouth ; it usually contains air bub- 

 bles. There is a ferment in the saliva called ptyalin, which 

 has the power of changing starch to malt sugar. If a bit 

 of bread is chewed for a long time, it becomes sweet, 

 because some of the starch is changed to sugar. The flow 

 of saliva is caused by chewing, or by the sight, or even the 

 thought, of agreeable food. Dryness of food is by far 

 more powerful than anything else in causing the saliva to 



