THE CHEMISTRY OF DIGESTION 465 



undergo this change, before the pepsin and acid of the gastric 

 juice digest it. 



Since muscle-fibers are enclosed within connective tissue (al- 

 buminoid) envelopes, it is necessary that the albuminoid cover- 

 ings be digested off before the protein contents are exposed to the 

 action of the digestive enzyms. There is reason to think that 

 pepsin, which converts proteins, including albuminoids, into pro- 

 teoses and peptones, soluble substances, but does not carry the 

 digestion to completion, has as an important part of its function 

 this removal from animal proteins of their albuminoid coverings. 



The Pancreatic Juice. In the intestine the food is subjected 

 to the action of the pancreatic juice. This is clear, watery, alka- 

 line, and much like saliva in appearance. The Germans call the 

 pancreas the "abdominal salivary gland." In digestive prop- 

 erties, however, the pancreatic secretion is far more important 

 than the saliva, or even the gastric juice. It contains three di- 

 gestive enzyms; amylopsin, a starch-splitting enzym whose action 

 is identical with that of salivary ptyalin, and which is thought to 

 be, perhaps, itself identical with ptyalin; lipase, a fat-splitting 

 enzym, converting fats to fatty acid and glycerin; trypsin, a 

 protein-splitting (proteolytic) enzym whose action is much more 

 powerful than that of pepsin, as it is able to carry the process of 

 protein hydrolysis clear to the amino acid stage. It acts upon 

 such proteins as escape the influence of pepsin in the stomach. 



The Bile. This fluid, which is poured into the intestine from 

 the liver does not contain any digestive enzym, but it does have 

 an important role in connection with fat digestion; it has been 

 shown that pancreatic lipase splits fats several times as rapidly 

 when bile is present as when it is absent. 



The Succus Entericus (Intestinal juice). This fluid, which is 

 secreted by the minute glands of the intestinal wall, is the last of 

 the digestive fluids to come in contact with the food, and by its 

 enzyms whatever foods are not completely digested must be 

 finally prepared for absorption. By the enzyms thus far described 

 none of the carbohydrate digestion is carried to completion, and 

 only part of the proteins are made ready for use, for proteose 

 and peptone are not end products, but only intermediate products 

 of digestion. Fats are the only foods which do not require the 

 aid of the succus entericus for their complete digestion. 



