35 8 DIGESTION 



digestion. So that when the quantity of protein in the food is 

 increased above that necessary for nitrogen equilibrium (p. 602), none 

 of the excess is assimilated and stored up, as is the case in a normal 

 animal (Levin, etc.). 



Pancreatic Juice. Pancreatic juice, bile, and intestinal juice are 

 all mingled together in the small intestine, and act upon the food, 

 not in succession, but simultaneously. But by artificial fistulae in 

 animals they can be obtained separately ; and occasionally some of 

 them can be procured through accidental fistulae in man. It is said 

 that under certain conditions, especially when fat or oil is introduced 

 into the stomach, the pylorus may remain open long enough to 

 permit the passage of pancreatic juice or bile from the duodenum into 

 the stomach, and this has been recommended as a practical method 

 of obtaining these secretions in man. 



Human pancreatic juice, as obtained from a fistula, is a clear, only 

 slightly viscid liquid of distinctly alkaline reaction to litmus. Its 

 specific gravity is about 1007 to 1010. The total solids constitute 

 about i'5 or 2 per cent., of which a little less than i per cent, is made 

 up of inorganic salts, chiefly sodium carbonate, with small quantities 

 of chlorides. The balance of the solids consists mainly of proteins. 

 The alkaline reaction is due to the sodium carbonate, and it is 

 worthy of remark, as showing the important part taken by this 

 secretion in the neutralization of the chyme, that when titrated 

 against standard acid the alkalinity of the pancreatic juice is not 

 much less than the acidity of the gastric juice when titrated against 

 standard alkali. The quantity of pancreatic juice secreted during 

 the twenty-four hours in an average man has been estimated at 

 500 to 800 c.c. from observations on cases of fistula. Probably 

 under perfectly normal conditions it is greater. A so-called arti- 

 ficial pancreatic juice can be made by extracting the pancreas with 

 water or glycerin. Since better methods of obtaining the natural 

 juice have been developed, these extracts have lost some of their 

 importance. 



Fresh pancreatic juice contains four ferments : (i) Thezymogen or 

 mother-substance, trypsinogen, of a proteolytic or protein-digesting 

 ferment, trypsin ; (2) an amylolytic ferment, or amylase ; (3) a fat- 

 splitting or lipolytic ferment, steapsin ; (4) a milk-curdling ferment. 

 The question whether the last is a different body from the 

 proteolytic ferment has been discussed just as in the case of the 

 gastric rennin (see p. 353). In any case, it cannot be considered as 

 taking any practical share in digestion, since it can hardly ever 

 happen that milk passes through the stomach without being curdled. 



Trypsinogen has no action upon proteins, but in normal digestion 

 it is changed into active trypsin by the enterokinase of the intestinal 

 juice (p. 372). Pancreatic juice collected without contact with 

 intestinal contents or with the mucous membrane of the intestine 



