ACTION OF PEPSIN AND HYDROCHLORIC ACID 351 



reduced to the point at which they are tenaciously held the hydrochloric 

 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 di- 

 sodic 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 ac- 

 tivity 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 o . 2 per cent, soda solution kept at 

 40 C. retains for hours its power to digest protein 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 protein 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 proteins 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 precipi- 

 tate of acid albumin is thrown down. After a while the acid albumin dis- 

 appears, so that no precipitate results on neutralization, and proper analysis 

 will show that all the fibrin or albumin has been converted into other protein 

 substances, viz., proteases and peptones. The process, as in the case of sali- 

 vary 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 prod- 

 uct is found (as an insoluble residue) in artificial gastric digestion which 

 gives practically all the protein 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 physiological 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. 



