330 FOOD AND DIGESTION 



water with a little organic matter and mineral salts besides 

 the hydrochloric acid just mentioned. In addition there 

 are present two enzymes: Pepsin and rennin. "When the 

 food first enters the stomach it is alkaline in character due 

 to the admixture of saliva. The action of ptyalin continues 

 as long as the reaction is alkaline. As soon as the food 

 becomes thoroughly mixed with the acid gastric juice the 

 action of ptyalin ceases. 



The enzyme pepsin, for which the acid solution is necessary, 

 hydrolyzes proteins to proteoses and peptones which are 

 soluble and diffusible decomposition products of proteins. 

 Not all of the proteins are acted upon in this way, for the 

 food does not remain in the stomach long enough for the 

 complete solution of all proteins to take place. This solvent 

 action of pepsin is also of secondary importance in that it 

 dissolves the protein cell walls of fats, thus disintegrating 

 fatty material and setting free the drops of fat. 



Rennin, the other enzyme in the stomach, acts on the 

 caseinogen of milk (Section 244, c), which is a soluble com- 

 pound, changing it or "curdling" it to a solid compound, 

 casein. Just why this is necessary is not apparent. After 

 the change takes place the coagulated casein is dissolved by 

 the pepsin. No other food constituents are acted upon in 

 the stomach. There is no evidence that the hydrochloric 

 acid inverts sucrose, as might be expected (Section 54). 

 The combined action of water, hydrochloric acid, and pepsin, 

 together with the mixing and churning motions of the 

 stomach has now changed the solid elements of food material 

 to a semi-liquid form called chyme. 



(c) Intestinal Digestion. — The chyme is discharged into 

 the small intestine where the next processes of digestion 

 take place. The food material here is mixed with three 

 different fluids, intestinal juice, pancreatic juice, and bile. 



Intestinal juice is secreted by certain glands in the walls 

 of the small intestine and is a watery, light yellow, slightly 

 opalescent, alkaline liquid, containing at least carbonates. 

 Because it is very difficult to obtain it in the pure state, its 

 composition is not accurately known except that it does 

 contain certain enzymes which are active in hydrolyzing 



