52-4 DIGESTION IN THE INTESTINES [CH. XXXIV. 



time after the operation. He found that this juice, like that of the 

 dog, contains a substance which renders pancreatic juice active. He 

 could not find that it exercised any activating influence on the 

 fat-splitting and amylolytic enzymes of the pancreas, but its action 

 on the tryptic enzyme was most marked. His quantitative experi- 

 ments do not bear out Pawlow's view that the active substance 

 in the intestinal juice is an enzyme, for it is unable, like an enzyme, 

 to act on an unlimited amount of pancreatic juice. Starling, however, 

 supports Pawlow's view ; provided sufficient time is allowed to 

 elapse, it will activate any amount of pancreatic juice. 



Delezenne has advanced a hypothesis on the lines of Ehrlich's 

 explanation of the action of hsemolysins (see p. 475). He regards 

 trypsinogen as the amboceptor which enables the enterokinase to 

 become effective. 



Starling's subsequent work did not support this view. We 

 may therefore best explain the action of enterokinase as an 

 activating agent, by the fact that it is capable of transforming the 

 zymogen trypsinogen into the effective enzyme trypsin. How it does 

 this is explained by J. Mellanby and Woolley in the following 

 way : Trypsinogen is a complex consisting of trypsin united by a 

 protein moiety, and so long as the enzyme is combined in this way 

 it is inactive ; enterokinase is a proteoly tic enzyme which adsorbs 

 and then digests this protein moiety, and thus liberates the trypsin. 



The mixture of pancreatic and intestinal juice is extraordinarily 

 powerful. If secretin is administered to a fasting animal, the juice 

 secreted, having no food to act upon, will produce erosion and 

 inflammation of the intestinal wall. (Starling.) 



Dixon and Hamill's recent work has made clearer the mechanism 

 of pancreatic secretion. There are in the pancreas three precursors 

 of enzymes, namely, protrypsinogen, proamylopsin, and prolipase. 

 Secretin combines chemically, or at any rate acts chemically, on all 

 three ; it liberates amylopsin and lipase from their precursors, and 

 these two active enzymes pass into the pancreatic juice. It liberates 

 trypsinogen from protrypsinogen, and trypsinogen passes into the 

 juice; finally trypsinogen is converted into the active enzyme trypsin 

 by the enterokinase of the succus entericus. 



Another discovery in connection with succus entericus has been 

 made by Otto Cohnheim. The juice has no action on native proteins 

 such as fibrin and egg-white, but it acts on proteoses and peptone. 

 It rapidly breaks them up into simpler substances, of which ammonia, 

 leucine, tyrosine, and the hexone bases have been identified. Cohn- 

 heim has named the enzyme to which this is due erepsin. Ham- 

 burger found that erepsin is also present in the human juice ; it is 

 not identical with enterokinase, because erepsin is destroyed by heat- 

 ing the juice to 59 C. for three hours ; enterokinase is not destroyed 



