THE PANCREATIC SECRETION 01 



when the concentration is sufficient, precipitates the casein. 

 The value of the curdling is not at all clear unless it is assumed 

 that under the conditions that exist in the body casein is better 

 digested by being converted to a solid form. Rennin is found 

 elsewhere than in the gastric mucosa. Wherever proteolytic 

 enzymes are found there is evidence of a curdling action on 

 milk, so that some observers have urged the view that milk 

 coagulation is not due to a specific ferment, but is due to the 

 pepsin. The curd formed by the action of rennin has been 

 much more profoundly affected than the precipitate formed 

 by acid. The latter may be redissolved and later curdled by 

 rennin, but a procedure in the reverse direction is not pos- 

 sible. 



Lipase. In the stomach the protein substances which form 

 the envelopes of fat cells and the substances which keep the 

 fat globules of milk apart are dissolved by the pepsin-hydro- 

 chloric acid. The freed fats, liquefied by the heat of the body, 

 then run together and, mixed mechanically with other food 

 substances, pass from the stomach as chyme. In addition, 

 a considerable amount of the fat is split up. Gastric juice 

 will produce this result, in vitro, in media of either neutral or 

 acid reaction, but is very sensitive to an alkaline reaction. 

 A glycerin extract of the gastric mucous membrane is much 

 more resistant to an alkaline reaction, but is very sensitive 

 to acid. This indicates that the ferment lipase exists in 

 the mucous membrane in a different form from that in which 

 it is secreted. In other words, it exists in a zymogen form. 



Digestion in the Small Intestine. In this portion of the 

 alimentary tract the pancreatic juice, the bile, and the intesti- 

 nal juice act upon the food, which as a result undergoes here 

 its greatest digestive changes. 



The pancreatic secretion is a clear, slightly viscid liquid and 

 of an alkaline reaction, due mainly to sodium carbonate. Its 

 neutralizing power is not much less than that of an equal 

 amount of gastric juice. The solid constituents form about 

 2 per cent., being chiefly inorganic salts and proteins. The 

 quantity secreted by man, per day, varies from 500 c.c. to 

 800 c.c. Pancreatic juice contains four ferments: (1) Tryp- 

 sinogen, the precursor of a proteolytic ferment, trypsin; (2) 

 a starch-splitting ferment, amylopsin; (3) a fat-splitting 



