HOW THE ANIMAL LIVES 215 



simple machine, which can, in all cases, turn out 

 a product exactly corresponding to the chemical 

 food elements which are turned into it. The 

 vital element has always to be reckoned with. 

 One animal demands a little more of this class of 

 aliment, and another a little more of that, in 

 order to secure the best results ; while in all cases 

 palatability and facility of digestion have a 

 controlling influence. 



3. Digestion of Food 

 3a. What digestion is 



377. Digestion is the process by means of 

 which the food becomes dissolved so as to be 

 taken up by the blood. It takes place in the 

 alimentary canal, the mouth, stomach, and 

 intestines. 



378. Digestion takes place under the action 

 of different secretions, each of which operates 

 on special constituents of the food. Considered 

 in the order in which they mingle with the 

 food, these digestive secretions are : (a) saliva; 

 (6) gastric juice; (c) bile, (d) pancreatic juice, 

 (e) intestinal juice. 



36. The saliva 



379. Saliva is furnished by a group of glands 

 located under the tongue, in the cheeks, and 



