160 MANUAL OF PHYSIOLOGY. 



things, to zymogen, which in the presence of an acid begets 

 trypsiu. 



Pancreatic Digestion. The pancreatic juice is, of all the 

 digestive fluids, the most general solvent. It acts upon the three 

 great classes of food-stuffs which require modification to enable 

 them to pass through the barrier that intervenes between the in- 

 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 pep- 

 sin in the most important characteristic of its action. Whilst the 

 presence of an acid is absolutely necessary for peptic proteolysis, 

 we find that an alkaline reaction is required for this action of the 

 pancreatic ferment, and as the peptic peptone has to pass through 

 preliminary stages in which it closely resembles acid albumin, 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 peptone, 

 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 



