382 PKACTICAL PHYSIOLOGY 



indicator. In doing this, the gastric contents are mixed with the standard alkali 

 in a small evaporating dish, samples of the mixture being removed with a glass 

 rod for testing with the reagent. 



It is from the glands of the cardiac end of the stomach that the 

 hydrochloric acid is secreted. These glands differ from those of the 

 pyloric end in having parietal as well as central cells, and there is much 

 evidence to show that it is the special function of the former to 

 separate the hydrochloric acid from the blood. After the injection of 

 solutions of neutral salts of iron into the blood, the parietal cells, when 

 treated with potassium ferro-cyanide solution, turn blue, because of the 

 formation of Prussian blue in them. No other cells in the body show 

 this reaction, because they do not contain the necessary acid. 



The Organic Matter. If pure gastric juice be cooled to 0C., a 

 precipitate falls down. On analysis, this precipitate is found to have 

 nearly the same percentage composition as protein ; and on testing its 

 action on a solution of protein, it is found to be pepsin. Pepsin of 

 similar composition can also be prepared by saturating gastric juice 

 with ammonium sulphate, which precipitates it. Whether the actual 

 ferment pepsin is what we obtain by these methods is uncertain. The 

 methods employed for obtaining ferment from the gastric mucosa after 

 death yield a still more impure product, on account of the ferment 

 adhering to the proteoses, etc., which are always present in the final 

 precipitate. 



To Prepare an Extract of Gastric Mucosa containing large quantities 



of Pepsin, the thoroughly washed stomach of the pig is taken, and the mucosa 

 is scraped off with a knife. The scrapings are mixed with a large excess (100 

 times their bulk) of 0*4 per cent, hydrochloric acid, and the mixture is digested for 

 several hours in the incubator. The extract is then filtered through muslin, and 

 may be employed for general work without further purification. In order to 

 separate the pepsin from the excess of proteoses which this extract contains, the 

 digestion should be allowed to proceed for several days longer, so that the 

 proteoses may become changed into peptones. The product is then saturated 

 with ammonium sulphate crystals ; the resulting precipitate of proteoses, which 

 carries down the pepsin with it, is pressed free of fluid, and again incubated for 

 a few days with several volumes of 0*5 per cent, hydrochloric acid, after which the 

 digest is again saturated with ammonium sulphate. This final precipitate is 

 approximately pure pepsin. The ammonium sulphate can be removed from the 

 preparation by dialysis through parchment. 



The scrapings of mucosa, after being treated with weak acid to convert the 

 pepsinogen into pepsin, can also be extracted with glycerin. This is the method 

 which is most used commercially. The various commercial preparations of 

 pepsin are very suitable for the experiments about to be described. 



Prior to its secretion, pepsin exists in an inactive form as granules in 



