DIGESTION AND ABSORPTION IN THE STOMACH. 775 



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 

 meml)rane, and, since it is known that the zymogen is converted 

 quickly to active pepsin l^y 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 nmch discussion as to the nature of the secretion of the 

 pyloric glands. Heidenliain 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. Glaessner* 

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

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

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

 not a true gastric pepsin. The possibility that a special secretin 

 (hormone) is formed in the pyloric mucous membrane has been 

 referred to above (p. 772). From the description of the events 

 in the stomach (p. 717) it would seem that the food material which 

 is churned and stirred by the contractions of the pyloric musculature 

 has already been charged with pepsin and hydrochloric acid by the 

 glands of the middle and fundic regions before reaching the 

 antrum pylori. 



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 an acid of a proper strength ; 

 and, as the pepsin can be extracted and preserved in various w%ays 

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

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

 Dlace of the normal secretion for many purposes. In laboratory ex- 

 periments it is customary to employ a glycerin or commercial prep- 

 aration of the gastric mucous membrane, and to add a small portion 

 of this preparation 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 proteins rapidly if the preparation of pepsin 

 is a good one. 



The Pepsin-hydrochloric Acid Digestion of Proteins. — It has 

 long been known that solid proteins, when exposed to the action of a 

 normal or an artificial gastric juice, swell up and eventually pass 

 into solution. The soluble protein thus formed was known not to be 

 coagulated by heat, and was remarkable also for being more diffus- 

 ible than other forms of soluble proteins. This end-product of diges- 

 tion was formerly conceived as a soluble protein with properties 

 fitting it for rapid absorption, and the name of peptone was given to 

 it. It was quicldy found, however, that the process is complicated — 

 * Glaessner, "Beitriige zur chem. Physiol, u. Pathol.," 1, 24, 1901. 



