468 THE HUMAN BODY 



walls and the gastric and pancreatic glands are not themselves 

 digested by the powerful proteolytic enzyms which they produce, 

 in the case of the glands, or which are poured out unto them, in the 

 case of the walls of the digestive organs. It has been shown that 

 the prevention of self-digestion of stomach and intestine depends 

 upon the continuance of life, for animals killed in the midst of di- 

 gesting a meal often do digest great parts of their stomach and in- 

 testinal walls. Just how self-digestion of these structures is 

 normally prevented is not clear, except in so far as the mechanism 

 to be described presently (Chap. XXIX), which limits the out- 

 pouring of the secretions to periods when food is present, may 

 be efficacious. The self-digestion of the pancreatic and gastric 

 glands is, however, prevented by an interesting arrangement 

 which has been recently analyzed. It appears that neither pepsin 

 nor trypsin is formed in the gland as an active enzym but in an 

 inactive pro-enzym or zymogen form, pepsinogen or trypsinogen, 

 which becomes active only when converted into pepsin or trypsin 

 by some activating agent. It has been shown that the conversion 

 of trypsinogen to trypsin occurs only when the pancreatic juice is 

 poured into the small intestine, and that it is brought about 

 through a constituent of the succus entericus, enter okinase. This 

 substance is believed to be an enzym having the sole function of 

 activating trypsinogen to trypsin. The conversion of pepsinogen 

 to pepsin is a similar activation, carried on by the hydrochloric 

 acid of gastric juice. 



