CHEMICAL CHANGES IN PLANTS AND ANIMALS. 425 



into albuminoids, carbohydrates, and fats. As a rule, the inorganic 

 foods are taken into the body without chemical change. Before the 

 organic foods can be absorbed, they have to undergo digestion. This 

 is the process by which organic compounds, capable of acting as foods, 

 are so altered that they may be absorbed. 



The first part of the process of digestion is accomplished in the 

 mouth and consists in the breaking up of the food by the teeth and 

 mixing it with saliva, the process being known as mastication. In 

 addition, the saliva, to a limited extent, converts starch into maltose. 

 This action of the saliva is due to its ferment ptyalin. Other func- 

 tions of the saliva are to keep the mucous membrane of the mouth 

 moist and to lubricate the food bolus. 



After being masticated, the food is passed into the stomach, where 

 it comes in contact with the gastric juice. The active principles of 

 the gastric juice are free hydrochloric acid, which is present in from 

 0.1 to 0.2 per cent., and the ferments pepsin and rennet. The pro- 

 teids are the only compounds aifected by the gastric juice. The free 

 acid first converts them into syntonin, which is soluble in dilute acids, 

 but is insoluble in water or solutions of neutral salts. The pepsin 

 causes syntonin to take up water, converting it into albumoses and 

 peptones, which latter, as stated in chapter 51, are dialyzable, and 

 soluble in water, dilute acids, dilute alkalies,, and neutral solutions. 

 Rennet ferment has the power of coagulating milk in neutral solu- 

 tion that is, of precipitating the casein. Starch cells and fat 

 globules are set free by the gastric juice acting upon their albu- 

 minous envelopes. 



After the food has been acted upon by the gastric juice it forms a 

 very turbid mixture, chyme, which, by the contraction of the stomach, 

 is forced through the pyloric orifice into the small intestine. Here 

 it soon comes in contact with the bile and pancreatic juice. 



The functions of the bile as a digestive fluid are : to assist in the 

 em unification of neutral fats ; to promote the absorption of fats ; by 

 its diastatic ferment to convert starch and glycogen into sugar ; to 

 stimulate intestinal peristalsis ; to assist in the evacuation of the 

 feces, to which it furnishes the coloring matter ; and, finally, to act 

 as an intestinal antiseptic. 



Pancreatic juice is alkaline in reaction and contains most likely 

 four ferments, trypsin, steapsin, amylopsiu, and one unnamed. Much 

 the larger portion of fats are simply emulsified by the pancreatic 

 juice. Under the influence of steapsin a small portion is broken up 

 into fatty acids and glycerin. For example : 



