ACTION OF THE ENZYMES OF PANCREATIC JUICE 381 



of dilute hydrochloric acid solution in the duodenum, which resulted in a 

 marked increase of pancreatic secretion, explained the phenomenon as a 

 local nerve reflex. 



Doubt has been cast on the whole question of nervous control by the 

 recent discovery of the fact that acid (o . 4 per cent, hydrochloric acid) in the 

 duodenum results in the production of a chemical substance, by the duodenal 

 mucous membrane. This substance secretin, is absorbed into the circulation 

 and acts specifically on the pancreas to produce increased activity of the pan- 

 creatic cells. Acid extracts of the duodenal mucous membrane produce the 

 same effects on the pancreas, in fact this is the current method of experiment- 

 ally stimulating the flow of pancreatic juice at the present time, the secretion 

 being collected from a tube introduced into the duct. 



Under the normal stimulus of food, the flow of pancreatic juice is greatly 

 increased. The increase continues to a maximum in from two to three hours, 

 after which it gradually decreases through the period of digestion. Pawlow 

 has found a certain amount of adaptation, not only of the quantity but of 

 the enzyme composition of the pancreatic secretion, to the kind and character 

 of the food (in dogs). 



Action of the Enzymes of Pancreatic Juice. The secretion of the 

 pancreas accomplishes its digestive action by means of the enzymes given 

 above, viz., trypsin, amylopsin, steapsin, and maltase. 



Trypsin. Trypsin is a proteolytic enzyme. Strange to say, it does not 

 exist in the fresh pancreatic juice as such, but makes its appearance only 

 when there is an admixture with the succus entericus, the secretion of the 

 mucous membrane of the intestine. The succus entericus contains an ac- 

 tivating enzyme, enterokinase, which converts the inactive and stable trypsin- 

 ogen of the pancreatic juice into the active but less stable trypsin. This fact 

 is another of the wonderful series of contributions to the exact knowledge 

 of the subject of digestion made from Pawlow's laboratories. 



Trypsin, like pepsin-hydrochloric acid, converts proteins into proteoses 

 and peptones. The change, however, does not stop here; the hydrolysis 

 with trypsin goes much further. While simple amino-acids, with the excep- 

 tion of traces of tyrosine, are not found in gastric digestions, these are rapidly 

 split off in the tryptic cleavage. Thus in tryptic digestion are formed: tyro- 

 sine, leucine, cystine, amino-valerianic acid, asparaginic acid, glutaminic 

 acid, histidine, lysine, and arginine. A portion of the protein, however, 

 is not completely broken down, the residue consisting of polypeptids 

 containing proline and phenyl-alanine combined with small amounts of 

 alanine, leucine, aspartic acid, and glutaminic acid. Glycocoll, when pres- 

 ent in the protein digested, is also combined in the resistant polypeptids. 

 Crystals of leucine and of tyrosine, especially, can be found in tryptic diges- 

 tion mixtures. The products formed from protein in tryptic digestion may 

 be given in the following graphic scheme: 



