DIGESTION 



171 



STUDENT REPORT 



Animals eat a large variety of things, parts of which serve to furnish 

 energy or to nourish the body. In the following report, work out the 

 sources from which the animals derive their food. To what extent ;ue 

 they alike ? 



146. Digestion. — Digestion begins in the mouth. The 

 teeth break up the food and mix it with the fluid of the 

 mouth, the saliva. During this process, sugars and 

 starches are changed into soluble sugars. The fluids of 

 the mouth are usually slightly alkaline (al'ka-lm or lin, 

 a chemical term, the opposite to sour or acid), but as soon 

 as the food passes into the stomach it enters an acid (sour) 

 medium, and the digestive action of the saliva is destroyed 

 in a short time by the stomach fluid. For this reason, the 

 sugar and starch undergo no further digestive changes 

 until they reach the intestines. 



Into this acid medium of the stomach, the gastric glands 

 (Figure 179) pour out the gastric juice (a digestive fluid), 

 and the pepsin in this juice acts on the proteins so that 

 they can later pass through the walls of the intestines. In 

 the stomach the heat of the body dissolves some of the fats 

 into oils, but many of the fats used as food remain solid 

 at body temperature and are unchanged in 1 lie stomach. 



After one or two hours the food passes into the intes- 

 tine and undergoes further changes in another alkaline 

 medium. Here the pancreatic juice, which is made in the 



