DIGESTION IN THE INTESTINE 143 



300 c.c. of juice, using litmus as indicator. The concentration 

 of hydroxyl-ions is about .0001 N. It contains protein in quan- 

 tities sufficient to cause a turbidity on boiling, and several 

 enzymes, chiefly trypsin, lipase, amylase, nuclease, maltase and, 

 at least in young animals, a lactase. 



Trypsin. Trypsin is a proteolytic enzyme. When secreted 

 by the gland it has very little digestive action, but if pancreatic 

 juice is mixed with the secretion of the walls of the small intes- 

 tine, it becomes very much more active, and digests proteins 

 vigorously. This fact is generally believed to indicate that tryp- 

 sin is secreted in an inactive form, called trypsinogen, which is 

 converted into active trypsin by a substance, " enterokinase, " 

 in the intestinal secretion. The nature of the activation is not 

 understood. Perhaps trypsin is liberated from some other sub- 

 stance with which it is combined. Some authors report also 

 that the bile has a favorable action on tryptic digestion. It has 

 been reported that some other substances have the power of 

 activating trypsinogen. 



There is some difference of opinion as to the most favorable 

 reaction for tryptic digestion, which is usually stated to be a 

 slightly alkaline reaction. It is certain, however, that trypsin 

 will act in alkaline, neutral or even faintly acid solution. The 

 reaction of the duodenal contents varies in fact, being at first 

 acid, until it is neutralized or made alkaline by the various 

 digestive juices poured into the intestine. 



Activated trypsin attacks most proteins very vigorously, 

 breaking them down into proteoses, peptones, peptids and even 

 amino acids. The digestion is by no means complete, however, 

 and there still are many of the partially digested products. The 

 digestion is more vigorous and far reaching than that of pepsin, 

 and differs from it also in the fact that it will take place in 

 alkaline solution. The previous action of pepsin on certain pro- 

 teins appears to make them more vulnerable to the attack of 

 trypsin. 



The activity of a trypsin solution may be studied by methods 



