692 PHYSIOLOGY OF DIGESTION AND SECRETION. 



body. Pekelharing* has prepared pepsin from gastric juice by 

 dialysis, the substance precipitating as the acid is dialyzed off. The 

 precipitate may be purified by repeated resolutions in acid followed 

 by dialysis. As prepared by this method pepsin is a substance of a 

 proteid nature which contains sulphur and also some chlorin, but no 

 phosphorus. It does not belong, therefore, to the group of nucleo- 

 proteids. 



Pepsin is supposed to be formed in the chief cells of the gastric 

 tubules, but as in other cases it is present in the cells as a zymogen 

 or propepsin which is not changed to the active pepsin until after 

 secretion. The propepsin may be extracted readily from the mucous 

 membrane, and, since it is known that the zymogen is converted 

 quickly to active pepsin by the action of acids, it is evident that in 

 the normal gastric juice the existence of the hydrochloric acid 

 insures that all of the pepsin shall be present in active form. There 

 has been much discussion as to the nature of the secretion of the 

 pyloric glands. Heidenhain isolated this portion of the stomach and 

 collected its secretion. He found that it was alkaline and contained 

 pepsin. Later observers, however, still continue to doubt the secre- 

 tion of a true pepsin in this portion of the stomach. Glaessnerf 

 states that propepsin can not be obtained from extracts of the pyloric 

 glands, and that the proteolytic enzyme that can be shown in this 

 portion of the stomach by self-digestion in acid or alkaline media is 

 not a true gastric pepsin. Whether the pyloric glands take any 

 chemical part in gastric digestion must remain undecided. From 

 the description of the events in the stomach (p. 641) it would seem 

 that the food material which is churned and stirred by the contrac- 

 tions of the pyloric musculature has already been charged with pepsin 

 and hydrochloric acid by the glands of the middle and fundic regions. 



Artificial Gastric Juice. In studying peptic digestion it is not 

 necessary for all purposes to establish a gastric fistula. The active 

 agents of the normal juice are pepsin and acid of a proper strength ; 

 and, as the pepsin can be extracted and preserved in various ways, 

 and the hydrochloric acid can easily be made of the proper strength, 

 an artificial juice can be obtained at any time which may be used in 

 place of the normal secretion for many purposes. In laboratory 

 experiments it is customary to employ a glycerin extract of the 

 gastric mucous membrane, and to add a small portion of this extract 

 to a large bulk of 0.2 per cent, hydrochloric acid. The artificial 

 juice thus made, when kept at a temperature of from 37 to 

 40 C., will digest proteids rapidly if the preparation of pepsin is a 

 good one. While the strength of the acid employed is generally from 

 0.2 to 0.3 per cent., digestion will take place in solutions of greater 



* Pekelharing, "Zeitschrift f. physiol. Chemie," 35, 8, 1902. 

 fGlaessner, " Beitrage zur chem. Physiol. u. Pathol.," 1, 24, 1901. 



