Chap. XV] DIGESTIVE PROCESSES 295' 



The action of gastric juice upon food. — The action of gastric 

 juice upon food is dependent on (1) hydrochloric acid, (2) pepsin, 

 and (3) rennin. 



(1) Hydrochloric acid. — The hydrochloric acid found in the 

 gastric juice is supposed to be secreted by special cells in the in- 

 termediate portion of the stomach, from chlorides found in the 

 blood. The chief chloride is sodium chloride (NaCl), and by some 

 means this is decomposed ; the chlorine (CI) combines with hydro- 

 gen (H), and is then secreted upon the free surface of the stomach 

 as hydrochloric acid (HCl). Besides giving an acid medium, 

 which is necessary for the pepsin to carry on its work, it serves : 

 (1) to swell the protein fibres and thus give easier access to pep- 

 sin, (2) it helps in the inversion of sugar, i.e. changing complex 

 sugars to simple ones, (3) it acts as a disinfectant and kills many 

 bacteria that enter the stomach, and (4) it helps to regulate the 

 opening and closing of the pyloric valve. 



(2) Pejysin. — The property of converting proteins into peptones 

 is dependent upon the enzyme pepsin. Whatever the protein 

 may be, whether the albumin of eggs, the gluten of flour in bread, 

 the myosin in flesh, the result is the same ; pepsin, in conjunc- 

 tion with an acid at the temperature of the body, transforms them 

 into peptones. This process of converting an insoluble protein 

 to a soluble peptone is complicated, as the protein under digestion 

 passes through a number of intermediate stages. Peptones 

 readily dissolve in water, and pass with ease through animal 

 membranes. 



(3) Rennin. — So far as is known, this ferment acts only upon 

 the soluble protein of milk, which is called caseinogen. It converts 

 this substance into a clotted mass called curd, which is later 

 prepared for absorption by the action of the enzyme pepsin. 



The gastric juice has no action upon starch, and upon fats it has 

 at most a limited action. The fats are set free from their mixture 

 with other food-stuffs by the dissolving action of the gastric juice, 

 they are liquefied by the heat of the body, and are scattered 

 through the chyme in a coarse emulsion by the movements of the 

 stomach. In addition to pepsin and rennin, various authorities 

 describe other enzymes in the gastric juice, but there is a good deal 

 of uncertainty regarding them. It is probable that a third enzyme, 

 called gastric lipase, acts upon fats that are ingested in an emulsi- 



