DIGESTION. 



247 



cold solution. The addition of alcohol throws down the pepsin in greyish- 

 white flocculi. Glycerine also has the property of dissolving out the fer- 

 ment; and if the mucous membrane be finely minced and the moisture 

 removed by k absolute alcohol, a powerful extract may be obtained by 

 tin-owing into glycerine. 



Functions. The digestive power of the gastric juice depends on the 

 pepsin and acid contained in it, both of which are, under ordinary cir- 

 cumstances, necessary for the process. 



The general effect of digestion in the stomach is the conversion of 

 the food into chyme, a substance of various composition according to the 

 nature of the food, yet always presenting a characteristic thick, pultace- 

 ous, grumous consistence, with the undigested portions of the food mixed 

 in a more fluid substance, and a strong, disagreeable acid odor and taste. 



The chief function of the gastric juice is to convert proteids into pep- 

 tones. This action may be shown by adding a little gastric juice (natural 

 or artificial) to some diluted egg-albumin, and keeping the mixture at a 

 temperature of about 100 F. (37*8 C.); it is soon found that the albu- 

 min cannot be precipitated on boiling, but that if the solution be neutral- 

 ized with an alkali, a precipitate of acid-albumin is thrown down. After 

 a while the proportion of acid-albumin gradually diminishes, so that at 

 last scarcely any precipitate results on neutralization, and finally it is 

 found that all the albumin has been changed into another proteid sub- 

 stance which is not precipitated on boiling or on neutralization. This is 

 called peptone. 



Characteristics of Peptones. Peptones have certain characteristics 

 which distinguish them from other proteids. 1. They are diffusible, i.e., 

 they possess the property of passing through animal membranes. 2. 

 They cannot be precipitated by heat, nitric, or acetic acid, or potassium 

 ferrocyanide and acetic acid. They are, however, thrown down by tannic 

 acid, by mercuric chloride and by picric acid. 3. They are very soluble 

 in water and in neutral saline solutions. 



In their diff visibility peptones differ remarkably from egg-albumin, 

 and on this diffusibility depends one of their chief uses. Egg-albumin as 

 such, even in a state of solution, would be of little service as food, inas- 

 much as its indiffusibility would effectually prevent its passing by absorp- 

 tion into the blood-vessels of the stomach and intestinal canal. Changed, 

 however, by tha action of the gastric juice into peptones, albuminous 

 matters diffuse readily, and are thus quickly absorbed. 



After entering the blood the peptones are very soon again modified, 

 so as to re-assume the chemical characters of albumin, a change as neces- 

 sary for preventing 'their diffusing out of the blood-vessels, as the previous 

 change was for enabling them to pass in. This is effected, probably, in 

 great part by the agency of the liver. 



Products of Gastric Digestion. The chief product of gastric 



