370 



FOOD AND DIGESTION 



parietal cells are peculiarly rich in chlorides, and these chlorides enter into 

 the composition of the secretion and apparently are the final source of the 

 hydrochloric acid formed in the secretion. 



Malay holds that the acid probably results 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 gastric secretion. However, Harvey and Bensley 

 believe that the hydrochloric acid is derived from an organic combination of 

 the chlorides in the secretion, the nature of which is not determined. 



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 o . 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 metaprotein is thrown down. After a while the acid metaprotein 

 disappears, so that no precipitate results on neutralization, and proper 

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

 protein ubstances, 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 re- 

 lative 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 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 



