286 DIGESTION. 



and adding to the infusion a few drops of hydrochloric 

 acid about 3-3 grains to half an ounce of the mixture, 

 according to Schwann. Portions of food placed in such 

 fluid, and maintained with it at a temperature of about 

 1 00, are, in an hour or more, according to the toughness 

 of the substance, softened and changed in just the same 

 manner as they would be in the stomach. 



The nature of the action by which the mucous mem- 

 brane of the stomach and its secretion work these changes 

 in organic matter is exceedingly obscure. The action of 

 the pepsin may be compared with that of a ferment, which 

 at the same time that it undergoes change itself, induces 

 certain changes also in the organic matters with which it 

 is in contact. Or its mode of action may belong to that 

 class of chemical processes termed " catalytic," in which 

 a substance excites, by its mere presence, and without 

 itself undergoing change as ordinary ferments do, some 

 chemical action in the substances with which it is in 

 contact. So, for example, spongy platinum, or charcoal, 

 placed in a mixture, however voluminous, of oxygen and 

 hydrogen, makes them combine to form water ; and 

 diastase makes the starch in grains undergo transforma- 

 tion, and sugar is produced. And that pepsin acts in 

 some such manner appears probable from the very minute 

 quantity capable of exerting the peculiar digestive action 

 on a large quantity of food, and apparently with little 

 diminution in its active power. The process differs from 

 ordinary fermentation, in being unattended with the for- 

 mation of carbonic acid, in not requiring the presence of 

 oxygen, and in being unaccompanied by the production of 

 new quantities of the active principle, or ferment. It 

 agrees with the processes of both fermentation and organic 

 catalysis, in that whatever alters the composition of the 

 pepsin (such as heat above 100, strong alcohol, or strong 

 acids), destroys the digestive power of the fluid. 



