I5 6 THE VITAL PROCESSES 



most important of the digestive fluids. It acts with vigor 

 on all of the nutrients insoluble in water, producing the 

 following changes : 



1. It converts the starch into maltose, completing the 

 work begun by the saliva. This action is due to the 

 amylopsin?- which is similar to ptyalin but is more 

 vigorous. 



2. It changes proteids into peptones and proteoses, 

 completing the work begun by the gastric juice. This is 

 accomplished by the trypsin, which is similar to, but more 

 active than, the pepsin. 



3. It digests fat. In this work the active agent is the 

 steapsin. 



The necessity of a milk-curding enzyme, somewhat 

 similar to the rennin of the gastric juice, is not under- 

 stood. 



Digestion of Fat. Several theories have been proposed 

 at different times regarding the digestion and absorption 

 of fat. Among these, what is known as the " solution 

 theory " seems to have the greatest amount of evidence 

 in its favor. According to this theory, the fat, under the 

 influence of the steapsin, absorbs water and splits into two 

 substances, recognized as glycerine and fatty acid. This 

 finishes the process so far as the glycerine is concerned, 

 as this is soluble in water ; but the fatty acid, which (from 

 certain fats) is insoluble in water, 2 requires further treat- 

 ment. The fatty acid is now supposed to "be acted on in 

 one, or both, of the following ways : I. To be dissolved as 

 fatty acid by the action of the bile (since bile is capable 



1 Amylopsin is absent from the pancreatic juice of infants, a condition which 

 shows that milk and not starch is their natural food. 



2 The fact that butter is more easily digested than other fatty substances is 

 probably due to its consisting largely of a kind of fat which, on splitting, forms a 

 fatty acid (butyric) which is soluble in water. 



