448 PEACTICAL PHYSIOLOGY 



Method of Preparation of Pure Gastric Juice. The most satisfactory 

 method for obtaining pure gastric juice, unmixed with semi-digested 

 food, is Pawlow's. This consists in removing the secretion from an 

 isolated pouch of stomach so prepared that the nervous and blood 

 connections are the same as those of the stomach itself (see Part L, 

 Chapter xliv., p. 156). 



The methods employed for obtaining the ferment from the gastric 

 mucosa after death, always yield an impure product on account of 

 the ferment adhering to the proteoses, which are always present. 



To Prepare the Extract 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 infusion 

 contains, the digestion should be allowed to proceed for several days, 

 so that the proteoses may become changed into peptones. The 

 product is saturated with ammonium sulphate crystals the resulting 

 precipitate of undigested proteoses, which carries down the pepsin 

 with it, is pressed free of fluid and again incubated for several days 

 with 0-5 per cent, hydrochloric acid, after which the digest is again 

 saturated with ammonium sulphate, the resulting precipitate being 

 approximately pure pepsin. The ammonium sulphate can be removed 

 from the preparation by dialysis through parchment. 



The scrapings, 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. 



Method of Separation of Products of Gastric Digestion. Fibrin is 

 boiled first with tap water, and then with 0*1 per cent, hydrochloric 

 acid to purify it. It is placed for 1-2 hours in an incubator, along with 

 five times its volume of artificial gastric juice prepared as above, or 

 with five times its bulk of 0-2 per cent, hydrochloric acid and a sixth its 

 bulk of commercial peptic extract. 



The products of digestion can be separated from this digest by the 

 following process : 



(1) Boil the solution in a beaker or basin, cool and separate the 

 coagulated native proteid by filtration. 



(2) Carefully neutralise the filtrate with 1 per cent, sodium 

 carbonate solution ; the acid albumin, which is precipitated, is separated 

 by filtration. 



(3) The resulting filtrate is now saturated in the cold, with ammo- 



