DIGESTION AND THE DIGESTIVE AGENTS. 339 



starches which are left entirely unacted upon in the stomach, 

 are now subjected anew to ferments. The ptyalin of the 

 saliva continues its action, while the amylopsin added from 

 the pancreatic juice soon completes the digestion of the 

 starches and effects their entire change into soluble mal- 

 tose. There is no good reason for calling the ptyalin of the 

 pancreatic juice by a new name unless it be that the term 

 "amylopsin" indicate the source from which the ptyalin is 

 derived. 



Steapsin. Steapsin is a ferment of the pancreatic juice 

 which has the property of splitting fats into a fatty acid and 

 glycerine. The chemical nature of this change is the same 

 as that of ptyalin. Under the action of the steapsin the fat 

 is made to combine with more water and the resulting 

 molecule is then split up into glycerine and a free fatty acid. 

 It has not been possible to isolate this steapsin, and so we 

 know at present nothing about its chemical nature. The 

 splitting up of fats into a fatty acid and glycerine is, how- 

 ever, a very common occurrence and familiar to every one. 

 When butter becomes old, it acquires an offensive, rank 

 odor. This is due to the fact that the butter, which is a 

 pure fat, has been split, in this case by an organic ferment, 

 into an acid called butyric acid, and glycerine. It is the 

 butyric acid which has the disagreeable odor. The same 

 thing is true of other fats. Exposed for a long time they 

 become, as we say, strong and rank, the explanation of 

 which is found in the fact that these pure fats have been 

 split into a fatty acid, hence their odor, and glycerine. 

 These fatty acids may be made to combine easily with some 

 form of alkali and so soap produced. Soap is, in fact, 

 nothing but the combination of a fatty acid with some suit- 

 able alkali. In the once very common manufacture of 

 household soap, the rank fats were boiled in a kettle with 

 some form of lye, the resulting combination being soap. 



The physiological value of the ferment, steapsin, is 

 found not immediately in the fact that it splits these fats up 

 into glycerine and fatty acid, but in the succeeding fact, that 



