CHYME. 225 



at a temperature of about 100, 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 membrane 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 con- 

 tact. 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 transformation, 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 quan- 

 tities 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 diges- 

 tive power of the fluid. 



Changes of the Food in the Stomach. 



The general effect of digestion in the stomach is the conver- 

 sion of the food into chyme, a substance of various composition 

 according to the nature of the food, yet always presenting a 

 characteristic thick, pultaceous, grumous consistence, with the 

 undigested portions of the food mixed in a more fluid substance, 

 and a strong, disagreeable acid odor and taste. Its color de- 

 pends on the nature of the food, or on the admixture of yellow 

 or green bile which may, apparently, even in health, pass into 

 the stomach. 



Reduced into such a substance, all the various materials of 

 a meal may be mingled together, and near the end of the diges- 

 tive process hardly admit of recognition ; but the experiments 

 of artificial digestion, and the examination of stomachs with 



