DIGESTION OF FOOD IN THE MOUTH 63 



tube contracts just above the morsel. This contraction 

 runs down to the stomach, forcing the food before it as 

 though a tight iron ring were slipped down over the esopha- 

 gus. A contraction of a tube within the body in a regular 

 manner, producing an onward movement of its contents, 

 is called peristalsis. While a horse is drinking, the peri- 

 stalsis of the esophagus may be plainly seen along its 

 neck. 



SUMMARY 



1. Anything which taken inside the body supplies it with 



weight or heat or energy is food. 



2. All foods are composed of one or more of the five sub- 



stances : water, albumin, fat, starch or sugar, and 

 mineral matter. 



3. Food must become liquid in form and enter the blood 



tubes before it can reach the cells of the body. 



4. Cooking softens the food and develops its flavors. It 



also destroys many poisons in food. 



5. In the mouth food is ground fine between the teeth 



and mixed with the saliva so as to form a thin paste. 



6. Saliva contains a ferment which changes some of the 



starch of the food to sugar. 



7. The tongue pushes the chewed food backward into 



the pharynx. The pharynx then closes all its 

 openings except the one into the esophagus. The 

 pharynx then squeezes the food into the esophagus, 

 and the esophagus forces it into the stomach. 



8. All cavities of the body which have an opening leading 



to the air are lined with a kind of soft skin called 

 tmicous membrane. 



9. Mucous membrane is a network of cells and fibers 



covered with flat cells called epithelium. 



