DIGESTION 
411 
man is living on a mixed and varied diet. This is to be 
regarded as an acquired habit and one that is questionable 
when carried to an extreme. The question of how much 
to eat is a modern problem, and on its solution depend our 
health, length of life, and energy for work. 
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 are 
they alike? 
Para¬ 
mecium 
Hydra 
Earth¬ 
worm 
Frog 
Man 
Flies 
Minute plants . . 
Minute animals . 
Plants .... 
Flies. 
Add food of man . 
328. Digestion. — Digestion begins in the mouth. The 
teeth break up the food and mix it with the fluid of the 
mouth, the saliva which contains the enzyme, ptyalin. Dur¬ 
ing this process, sugars and starches are changed into soluble 
sugars. The fluids of the mouth are usually slightly alkaline 
(al'ka-lm or lln, 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 373) pour out the gastric juice (a digestive fluid), 
and the enzyme 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 
