ACTION OF PEPSIN AND HYDROCHLORIC ACID 325 



acid is no longer secreted. One can only guess at the detail by which the 

 parietal cells secrete the acid. 



The acid probably results (Maly) from a combination of common salt 

 with monosodic phosphate, NaH 2 PO 4 + NaCl = Na 2 HPO 4 + HC1; the 

 disodic phosphate is then reconverted by the action of carbonic acid and 

 water, Na 2 HPO 4 + CO 2 + H 2 O = NaH 2 PO 4 + NaHCO 3 . All these 

 salts are found in the blood. 



The Pepsin. The pepsin of the gastric juice is derived from the 

 activity of the chief cells of the fundic glands. The zymogen pepsinogen, 

 which is its immediate precursor, is in all probability represented by the gran- 

 ules of the resting cells. The ferment pepsin does not exist as such in the cells, 

 for an extract of peptic glands in 0.2 per cent soda solution kept at 40 C. retains 

 for hours its power to digest proteid when added to 0.2 per cent hydrochloric 

 acid. If the extract be first treated with acid till it is active, then neutralized 

 and kept, it quickly loses its power to digest. The enzyme is destroyed by 

 the treatment, but the pro-enzyme is not so injured. 



Digestive Action of Pepsin and Hydrochloric Acid. The chief func- 

 tion of gastric juice is to alter the proteid food stuffs so that they may 

 be readily absorbed. Less important functions are the antiseptic action of 

 the hydrochloric acid, and the coagulation of milk. The chief digestive power 

 of the gastric juice depends on the pepsin and acid contained in it, both of 

 which are necessary for the process in the stomach. 



This action on proteids may be shown by adding a little gastric juice 

 (natural or artificial) to some flakes of fibrin or to diluted egg albumin, and 

 keeping the mixture at a temperature of about 37.8 C. (100 F.). It is soon 

 found that the fibrin goes into solution and that the albumin cannot be pre- 

 cipitated on boiling. If the solution be neutralized with an alkali, a precipitate 

 of acid albumin is thrown down. After a while the acid albumin disappears, 

 so that no precipitate results on neutralization, and proper analysis will show 

 that all the fibrin or albumin has been converted into other proteid substances, 

 viz., proteases and peptones. The process, as in the case of salivary digestion, 

 is never complete and the final result is always a mixture of peptones with 

 proteoses which cannot be further peptonized. The relative proportions, of 

 course, depend on the duration of the process. A side product is found (as 

 an insoluble residue) in artificial gastric digestion which gives practically all 

 the proteid reactions and is soluble in dilute alkali, though insoluble in water, 

 sodium chloride, or dilute acid. This is known as anti-albumid and may be 

 changed into peptone by prolonged digestion; it does not occur in physiologi- 

 cal gastric digestion. The commonest proteose is the one formed from albumin 

 and is known as albumose, or by the more general name proteose; this name 

 is used in the subsequent descriptions of the digestive processes. 



All classes of proteids are digested by gastric juice, leading to the produc- 

 tion of proteoses and peptones. The change is indicated best by the characters 



