366 THE HUMAN BODY. 



to undergo this change, before the pepsin and acid of the 

 gastric juice digest it. 



The most important change effected by the gastric juice 

 is that of the proteids. This may be studied either on natu- 

 ral juice obtained from the stomach of an animal through an 

 opening (gastric fistula) or on an artificial juice prepared by 

 extracting the mucous membrane of a fresh stomach with 

 glycerine, and 'adding a large quantity of dilute (0.2$) hydro- 

 chloric acid. If blood-fibrin or boiled white of egg be placed 

 in such a mixtttre and kept at a temperature of about 38 C. 

 | (100 F.) these bodies swell, become transparent, and soon 

 dissolve ; and all other solid proteids undergo similar changes. 

 If the solution be now neutralized a small white precipitate 

 of parapeptone (which is probably only ordinary acid albu- 

 min) is obtained. The filtrate from this gives no precipitate 

 on boiling, but an abundant one of albumose on the addition 

 of ammonium sulphate. The filtrate from this precipitate 

 yields an abundant precipitate of peptone when alcohol is 

 added. Peptone is dialyzable, though not so easily as saline 

 bodies, and in this differs from albumose and parapeptone 

 and all other proteids. The parapeptone is probably a bye- 

 product due to the action of the acid of the juice alone: the 

 albumose and peptone are true products of peptic digestion of 

 proteids, due to their breaking up with concomitant hy- 

 dration, the peptone being the more finished or complete 

 digestive product. If instead of solid proteids we use solu- 

 tion of white of egg or of serum albumin, the earlier stages of 

 the process cannot be followed by the eye, but the final prod- 

 ucts are the same: the original proteid disappears, giving 

 origin to some parapeptone, to albumose, and to peptone; and 

 prolonged artificial peptic digestion causes no further breaking 

 up of the albumose or peptone. Peptone is very soluble in 

 water, and its solutions are not coagulated by boiling. A 

 very small amount of pepsin can, if some acid be added from 

 time to time, convert a very large amount of proteid: it is de- 

 stroyed by boiling. 



Gastric Digestion. The process of swallowing is contin- 

 uous, but in the stomacli the onward progress of the food is 

 stayed for some time. The pyloric sphincter, remaining con- 

 tracted, closes the aperture leading into the intestine, and the 

 irregularly disposed muscular layers of the stomach keep its 

 semi-liquid contents in constant movement, maintaining a 



