420 



LIFE ON THE EARTH 



Salivary 

 Gl&nds X 



portions of this tube are adapted to different processes. In 

 the mouth, the teeth grind the food into small bits and mix 

 it with the saliva. This is an exceedingly important part 

 of the process, because if the food is not ground fine, the 

 digestive juices cannot readily get at it, and the whole process 

 of digestion is greatly retarded. Thus much more energy is 



expended than otherwise 

 would be. The saliva is 

 necessary to digest some of 

 the starch and to aid in the 

 further digestion. 



The food passes from 

 the mouth down the throat 

 and through an orifice to 

 the stomach. This is a 

 large pouch which will hold 

 usually from three to four 

 pints. It has muscular 

 walls which enable it to 

 contract and expand, thus 

 keeping the food mov- 

 ing about so that it is 

 thoroughly mixed with the 

 gastric juice. The gastric 

 juice is secreted by little 

 glands thickly embedded 

 in the lining of the stomach. Artificial gastric juice was 

 made in Experiment 138. Some of the proteins (foods con- 

 taining nitrogen) are digested in the stomach, although the 

 larger part of digestion takes place in the small intestine. 



From the stomach the food passes through a valve into 

 the small intestine. This is a complexly coiled tube which 



FIGURE 135 



