DIGESTION IN THE INTESTINE 169 



thus has an important part to play in neutralizing the acid 

 chyme. This is by no means the only role of the intestinal juice, 

 however, for it contains substances of the greatest importance in 

 intestinal digestion. It contains enterokinase which greatly 

 increases the action of pancreatic juice on protein. This is 

 usually believed to be due to activation of the inactive trypsino- 

 gen, by conversion into trypsin. Enterokinase may be extracted 

 from the intestinal mucosa from almost the entire length of the 

 intestine to the rectum, but it is present in largest amount in 

 the upper regions of the small intestine. Enterokinase is itself 

 probably an enzyme. 



Erepsin. A second enzyme found in the intestinal secretion 

 is erepsin. This enzyme is important, for it is responsible for 

 the last stage in the digestion of proteins. Erepsin does not act 

 on proteins themselves, with one or two exceptions, e.g., casein, 

 but on the intermediate digestion products, proteoses, peptones 

 and polypeptids, which it reduces to amino acids, the final diges- 

 tion products of the proteins. Erepsin is found in the mucous 

 membrane of the intestinal wall, and in fact, in most of the body 

 tissues. Erepsin acts best in weak alkaline solution. 



Other Enzymes. Three enzymes which have the power of 

 splitting disaccharides also are present in the intestinal juice, 

 maltase, which splits maltose; lactase, which splits lactose, and 

 is found in the intestinal juice of young mammals and in adults 

 of animals which take milk in their diet, and invertase, which 

 splits cane sugar. 



The intestinal juice also contains a nuclease, which splits the 

 nucleic acids of the nucleoproteins. 



Excretory Function of Intestinal Secretion. Aside from the 

 digestive enzymes, the intestinal juice carries various substances 

 into the intestine which are thrown out of the blood as undesir- 

 able, or waste materials. It thus serves in the capacity of an 

 excretion as well as a secretion. 



Prom the walls of the large intestine a secretion is poured 

 into the intestinal cavity. This contains mucus as its chief con- 



