92 THE DIGESTION OF FOOD 



stances which pass through the walls of the stomach and the 

 intestines. 



Digestion is a mechanical as well as a chemical process. Food 

 must be crushed and ground into bits by the teeth in order to 

 be swallowed and sent to the stomach, and it must be kneaded 

 by the muscular walls of the stomach and pushed forward into 

 the intestine. Food must also be crushed and ground up by 

 the teeth in order that the saliva may be thoroughly mixed with 

 it, it must be moved up and down and back and forth by the 

 stomach muscles in order that the gastric juice may come in 

 contact with its whole mass, and finally it must be squeezed and 

 turned by the intestinal muscles and brought into contact with 

 the intestinal fluids. Whether our digestion is good or bad 

 depends largely upon the manner in which we eat. When we eat 

 slowly and chew our food thoroughly, it passes into the stomach 

 well mixed with saliva and so softened and diluted that the 

 gastric juice reaches every particle of it. When we eat rapidly, 

 our food is not thoroughly ground, and it passes into the 

 stomach poorly mixed with saliva and so coarse and unsoftened 

 that it is unfitted to mix readily with the gastric juice. 



Digestion begins in the mouth. Saliva is the watery secretion 

 of the salivary glands. It is important in digestion because it 

 softens and moistens foods and makes them easier to swallow 

 and because it acts chemically upon starchy foods and changes 

 them into sugar which can pass easily through the cell walls 

 and mingle with the blood. The digestive or chemical action 

 of the saliva on starch is due to a ferment, called ptyalin. But 

 although ptyalin changes starch to sugar, it is powerless to bring 

 about chemical changes in protein and fat. The protein and 

 fats which we eat pass into the stomach unaltered chemically. 



The action of saliva on starch is the first chemical step in 

 digestion, and it is within our power to increase or to decrease 

 the amount of saliva secreted by the salivary glands. We can 



