PHYSIOLOGY OF ORGANS 145 



upon proteins, carbohydrates, and fats respectively. The protein- 

 sphtting enzyme is inactive when it emerges from the pancreatic duct, 

 and is then known as trypsinogen; but when it comes in contact with the 

 duodenal surface it is quickly rendered active. This is explained by the 

 assumption that the mucosa of the duodenum produces an enzyme, 

 enterokinase, which reacts with the inactive trypsinogen changing it to an 

 active form, called trypsin. The trypsin by hydrolysis sphts proteins, 

 proteoses, and peptones present in the chyme (partially digested food in 

 a fluid condition) from the stomach into simpler and simpler compounds. 

 As is to be expected from the composition of proteins (described in Chapter 

 III) the end products of protein digestion are amino-acids, of which about 

 twenty are known, and several other compounds. Trypsin works in 

 alkaUne, neutral, or even acid media. It completes the work begun by 

 the pepsin, and works more rapidly and breaks the protein up more com- 

 pletely than does the pepsin. 



The carbohydrate-sphtting enzyme of the pancreatic Imce'mamylopsin. 

 UnHke trypsinogen it requires no activation. It produces a hydrolytic 

 effect on starch very similar to that of ptyalin, producing maltose and 

 a dextrin. These substances are further acted upon and converted into 

 dextrose by an enzyme, maltase, secreted by the intestine. Dextrose is in 

 condition to be absorbed. 



The fat-splitting enzyme of the pancreatic juice is steapsin, sometimes 

 called lipase. Steapsin by inducing hydrolysis splits fats into glycerol 

 (glycerin) and one or more fatty acids of which, as pointed out in Chapter 

 III, fats are composed. The fatty acids combine with alkahne salts 

 that are present in the food or are introduced by bile, the secretion from 

 the liver, and soaps are produced. These soaps and uncombined fats 

 become emulsified, that is, they become broken up into minute globules, 

 and are finally completely hydrolyzed into glycerol and fatty acids. 

 These substances are soluble and are absorbed in this condition. Steapsin 

 is destroyed by acids, and works best when mixed with bile, which being 

 alkaline, corrects the acidity of the food mass and assists in the emulsifi- 

 cation and saponification of the fat. 



The Secretion of the Liver. — Bile, the secretion of the liver, contains 

 no enzymes. It contains water, bile salts and certain excretory materials. 

 The secretion of bile is stimulated by the hormone secretin in the same 

 manner as is the secretion of pancreatic juice. Bile is ordinarily stored 

 in the gall bladder until the acid chyme is ejected by spurts from the stom- 

 ach. Each ejection of chyme is followed by an ejection of bile from the 

 gall bladder. The bile assists the steapsin of the pancreatic juice in the 

 digestion of fats. This action is considered to be due to the bile acids 

 which serve as a solvent for fats and fatty acids. That bile is of impor- 

 tance in the digestion of fats is shown by the fact that if the bile is pre- 

 vented from entering the intestine through the experimental formation 



10 



