90 PHYSIOLOGY OF ALIMENTATION. 



reaction has come to a standstill a certain amount of the 

 substance will be found unchanged in the reaction mixture.* 

 If, for example, a certain amount of fat and lipase (steapsin) 

 are mixed together and the whole is allowed to stand until 

 the reaction has come to a standstill, it is found that some 

 of the fat has been left in an undigested state. What has 

 been said of fat holds true also for a large number of other 

 ferments in fact, 'all that have thus far been investigated. 



It is only recently that an explanation of this interesting 

 and fundamental fact has been given, and, as will become 

 apparent later, a large number of physiological processes 

 rendered more intelligible in consequence. It used to be 

 said that the products of the fermentation interfered with 

 its further progress. We know now, however, that the reason 

 for the incompleteness of the chemical reaction lies in the 

 fact that the ferment is continually re-forming from the 

 products of the reaction the substance or substances with which 

 the ferment was mixed originally. Thus, in the case of fat, 

 which is split into fatty acid and alcohol by lipase, we 

 have two reactions going on side by side: first, the long- 

 recognized analytical one, by which fatty acid and alcohol 

 are formed from the fat; and, second, a synthetical one, by 

 which fat is formed from the fatty acid and alcohol. Both 

 reactions are catalyzed by the same ferment, the action of 

 which we say is reversible. This reversibility of the action of 

 a ferment is another of its fundamental characteristics. Whether 

 in any given case a ferment acts synthetically or analytically, 

 it will be shown later, is determined solely by the ordinary 

 laws of chemical equilibrium. 



If the products of a chemical reaction which is being catalyzed 

 by a ferment are removed as soon as formed, the reaction 

 will be completed. If, in the illustration cited above, the 

 fat and lipase are put, not into a test-tube, but into a parch- 

 ment bag suspended in a current of water, all the fat will 

 be split into fatty acid and alcohol. Under these circum- 

 stances the fatty acid and alcohol diffuse through the parch- 



