208 HUMAN PHYSIOLOGY 



Trypsin differs from pepsin in two obvious ways: 

 First, it does not require the cooperation of an acid. 

 In fact, acid of such a strength as that in the stomach 

 would destroy the pancreatic enzymes. Second, it is 

 able to advance the cleavage rapidly beyond the stage 

 described as that of the peptones. Thus the usual 

 products of peptic digestion are subject to further de- 

 composition in the intestine before they are absorbed. 

 The simple compounds into which the peptones are at 

 length resolved are best called amino-acids. They have 

 been well spoken of as " building stones," for they must 

 evidently be used in new combinations for growth and 

 repair. 



The Intestinal Juice. There is much disagreement as 

 to the properties and importance of this secretion. It 

 is certainly abundant and the general tendency is to 

 attribute to it a larger share in the work of digestion than 

 was formerly granted. It is supposed to change the 

 three common sugars which are not fit for direct intro- 

 duction into the blood (cane sugar, malt sugar, and 

 milk sugar), into sugars of the simpler order which are 

 acceptable to the body cells. It probably contains the 

 enzyme erepsin which cooperates with trypsin in the later 

 stages of protein digestion. 



The Bile. The liver is always secreting bile. It does 

 not have periods of repose like those which seem to be 

 enjoyed by the stomach and the pancreas. But the 

 bile is not always entering the intestine, for there is 

 provision for its temporary storage in the gall-bladder. 

 This is a contractile sac lodged in a hollow of the under- 

 surface of the liver. It is so connected with the system 

 of ducts that the bile flowing from the liver may either 

 pass to the intestine or turn aside to gather in the gall- 

 bladder. At another time a large quantity of bile may 

 be expelled from the bladder and introduced into the 

 alimentary canal. 



Although the. secretion of the bile is always in progress 

 it is accelerated after meals and the hormone, secretin, 



