PANCREATIC DIGESTION. 169 



testinal cavity and the blood current. It changes proteids into 

 peptones, it profoundly modifies fatty substances, and converts 

 starch into soluble sugar. The ferments to which its activity is 

 due may be separately described. 



I. Action of Pancreatic Juice on Proteids. The ferment which 

 produces peptones is trypsin. Some of the conditions required for 

 its perfect operation are the same as those necessary for the action 

 of the gastric ferment, pepsin; namely, a certain degree of dilu- 

 tion, and a temperature of about 40 C. But it differs from 

 pepsin in the most important characteristic of its action. While 

 the presence of an acid is absolutely necessary for peptic proteo- 

 lysis, we find that an alkaline reaction is required for this action 

 of the pancreatic ferment, and as the peptic peptones has to pass 

 through preliminary stages in which it closely resembles acid albu- 

 min, so the tryptic peptone is first produced from alkali albumin, 

 which has been formed as a preliminary step by the alkali of the 

 pancreatic juice. The addition of the sodium carbonate aids the 

 action, and, indeed, seems to play a part which closely corresponds 

 to that taken by the hydrochloric acid in gastric digestion. 



The change to alkali albumin and peptone as accomplished by 

 the trypsin, is not accompanied by any swelling of the albumin, 

 such as occurs in the formation of the acid albumin in the 

 stomach, but the proteid is gradually eroded from the surface, 

 and thus diminished in size. 



Moreover, the alkali albumin is not made directly into pep- 

 tone, but passes through a stage in which it resembles globulin, 

 and is soluble in solutions of sodium chloride. 



Besides these differences between the mode of action of pepsin 

 and trypsin in producing peptones, trypsin has a peculiar power 

 upon proteids, which has no analogue in the peptic action. While 

 the pancreatic peptone is being produced, a further change occurs, 

 which gives rise to the formation of two crystallizable nitrogenous 

 bodies known as leucin and tyrosin, the former belonging to the 

 fatty acid, and the latter to the aromatic acid group. These sub- 

 stances, which are commonly found together as a result of the de- 

 composition of peptones, seem inseparable from pancreatic diges- 

 tion, and increase in amount toward the later stages of the action. 

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