in the saliva changes the starch into sugar. Over 99 per cent of saUva is 

 water, and this water dissolves salts and sugars. 



The amount of enzyme is very small. The digesting of the starch de- 

 pends upon (1) the ferment's reaching every particle of starch and (2) suf- 

 ficient time for the ferment to act. Mixing saliva thoroughly with the food 

 coats the mass with the slippery mucus of the saliva. That makes it easier 

 for the mass to slide along into the throat and down the gullet. 



After the mouthful of food has been thoroughly chewed, it is pushed 

 back by the tongue and passed into the throat chamber, or pharynx. From 

 the pharynx it passes directly into the gullet, or esophagus (see illustration 

 opposite). Muscular rings in the wall of the gullet contract in series and so 

 push the food toward the stomach. If you watch a giraffe or a horse drink- 

 ing water from a pond or from a pail on the ground, you can see him 

 swallow up-yoM can see one wave of contraction after another pass along 

 the gullet, from the head to the trunk. 



The Stomach' When nerve-endings in the mouth or nose are stimu- 

 lated, glands in the stomach wall are aroused. These secrete stomach juice, 

 or gastric juice. The fermentation started by the saliva continues until the 

 mass of food gets into the stomach. Here the action is stopped by the acid 

 stomach juice. The swallowed food is thoroughly mixed with the gastric 

 juice by the churning action of the stomach muscles. 



The gastric digestion breaks proteins into compounds that dissolve in 

 water and diffuse through membranes. As digestion proceeds, the mixture 

 in the stomach becomes more and more liquid and more and more acid. 

 From time to time a quantity of the liquid passes into the intestine. Most 



DIGESTION BY BACTERIA 



In the presence of "food" and under suitable conditions of moisture and tempera- 

 ture, each cell discharges through the eel! wall, by osmosis, one or more enzymes, 

 or ferments. The enzymes digest the food material, changing proteins, for example, 

 into simpler compounds that are soluble in water. The resulting fluid .s then ab- 

 sorbed through the cell wall into the protoplasm, and is then assimilated 



iSee Nos. 5 and 6, p. 183. 

 166 



