DIGESTION IN THE STOMACH 137 



lyzed by dilute hydrochloric acid, and it is probable that this 

 process takes place at least to some extent in the stomach. The 

 hydrochloric acid also kills bacteria and parasites in the food, 

 and thus protects the body from their attack. 



Enzymes of the Gastric Juice. Three important enzymes 

 are secreted in the gastric juice, pepsin, rennin and a lipase. 



Pepsin is formed in glands of the stomach wall, particularly 

 in the fundic region but it exists there probably in an inactive 

 form called pepsinogen, which becomes active only after secre- 

 tion. Pepsinogen is stored up during the period of inactivity 

 between times of secretion. The transformation of pepsinogen 

 into pepsin is brought about by the hydrochloric acid of the 

 gastric juice. Little is known of the nature of pepsin. Prob- 

 ably it is similar in nature to the proteins, or perhaps only 

 attached to a protein, but this is still uncertain. 



Pepsin, which digests proteins, acts best in weak acid solu- 

 tion. For pepsin from man, the most favorable acidity is about 

 0.3% hydrochloric acid or a hydrogen-ion concentration of 

 about L7X10' 2 N. to 3X10' 2 N. At 1Q- 4 N. digestion almost stops. 

 The most. favorable acidity for pepsin corresponds closely with 

 the normal acidity of the gastric juice of the animal from which 

 the pepsin is obtained. Other acids may be used in place of 

 hydrochloric, but they are not so favorable. Pepsin is very 

 sensitive even to slight amounts of alkali, and is rendered in- 

 active if its solution is made alkaline. For the action of pepsin 

 it is not necessary that the hydrochloric acid be free. If the acid 

 is combined with the protein acted upon, a neutral pepsin solu- 

 tion will digest the protein quite readily. Hydrochloric acid 

 alone will digest proteins, but much more slowly than pepsin 

 and acid together. 



There are various methods for estimating the activity of a 

 pepsin solution. The use of Mett's tubes is one of the most 

 satisfactory. Egg white is drawn up into narrow glass tubes 

 and coagulated by heating. The tubes then are cut into short 

 lengths (ca. 2 cm.) and suspended in the enzyme solutions. These 

 are placed in the incubator for 24 hours. The amount of diges- 



