DIGESTION AND ABSORPTION 177 



pancreas where it acts as a stimulus causing the flow of pan- 

 creatic juice. 



The pancreatic juice then passes from the pancreas through 

 the duct of Wirzung to the duodenum. It is a colorless alkaline 

 solution amounting in twenty-four hours to about 650 c.c. in 

 man. It has a specific gravity of about 1.008 and contains 

 about 1.3 per cent solid matter. 



Enzymes. — The most important constituents of the pan- 

 creatic juice are the enzymes. These enzymes are of three 

 classes, (a) amylolytic or starch-hydrolyzing, viz. amylopsin, 

 which we have already considered, (b) protein enzymes, viz. a 

 protein-coagulating enzyme, pancreatic rennin, similar to gastric 

 rennin, and a protein-hydrolyzing enzyme known as trypsin^ 

 resembling pepsin, {c) a fat-hydrolyzing enzyme which we shall 

 consider later. 



Rennin. — The enzyme pancreatic rennin, present in the 

 pancreatic juice, acts exactly like gastric rennin of the gastric 

 juice, coagulating the caseinogen of milk. 



Trypsin. — The most important enzyme of the pancreatic 

 juice is the proteolytic enzyme trypsin. This enzyme, like 

 pepsin, does not exist in the digestive juice as the enzyme it- 

 self, but as the mother substance or zymogen known in this case 

 as trypsinogen. When the pancreatic juice reaches the intestine, 

 it is unable to hydrolyze protein as has been shown by drawing 

 off the pancreatic juice from the pancreas by means of a fistular 

 opening. In order that the trypsinogen present in the pan- 

 creatic juice may become active by conversion into trypsin it 

 must be activated by another enzyme which is present in the 

 intestinal juice and known as enterokinase. This substance is 

 probably an enzyme though differing from them in a certain 

 particular in that it seems to act quantitatively upon a definite 

 amount of trypsinogen. It is also found to be present in the 

 intestinal juice only when pancreatic juice is present. When, 

 therefore, pancreatic juice containing trypsinogen becomes 

 mixed with intestinal juice, the enterokinase of the latter acti- 

 vates the trypsinogen and the enzyme trypsin is produced. 



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