DIGESTION AND USES OF FOOD. 163 



THE COURSE OF DIGESTION. The food is first bitten off and 

 taken into the mouth, where it is cut up and ground fine by 

 the teeth. At the same time a liquid called the saliva is set 

 free from glands in the cheeks and under the tongue. This 

 saliva not only moistens the food so that it can slip down the 

 throat or guilet, but it also acts upon the starch, converting it 

 into sugar, thus changing it from an insoluble to a soluble 

 form. Thus digestion begins in the mouth. Thorough 

 chewing of the food not only breaks up the food fine so that 

 it can be acted upon by the juices of the body, but also helps 

 to set free saliva and mix it with the food to digest the starch. 

 When we remember that starch forms a very large portion of 

 most of our vegetable foods, we see that thorough mastication 

 the food is very necessary to good digestion, and " bolting '' 

 the food by man and many other animals a common cause of 

 indigestion. 



The food passes from the mouth into the gullet, which is a 

 tube formed of tough elastic rings that can contract and 

 expand as required. Through the gullet it passes into the 

 stomach. Here it comes in contact with the gastric juice, 

 which is a secretion of the stomach. The gastric juice acts 

 principally upon the albuminoids, changing them into soluble 

 forms. Some of the soluble and digested food here passes 

 into the blood, but most of it goes on through into the intes- 

 tines. Just below the stomach, and on the right side, is the 

 liver, which builds up or secretes a liquid called bile. This bile 

 flows into the intestines and acts upon the fat of the food, 

 forming with it soluble compounds. Other secretions come in 

 contact with the food, acting upon the albuminoids and starch 

 to complete the digestion ; and through the walls of the in- 

 testines the soluble foods now pass in large quantities into the 

 blood. The rest of the food that cannot pass into the blood 

 moves on and is ex{)elled from the body, forming the solid 

 excrement. The solid excrement therefore consists of the 



