CHAP. II.] LABORATORY WORK ON GASTRIC DIGESTION. 185 



diluted solutions fades with time ; this is somewhat delayed, however, 

 by corking the test tubes and keeping them in the dark. 



" When extracts which dissolve fibrin rapidly appear to dissolve it 

 at nearly the same rate, they should be diluted and again tested, 

 when a distinct difference in the amount of pepsin may sometimes be 

 ascertained. In these experiments, test tubes of the same diameter 

 must be used, and the conditions, as to temperature and percentage 

 of acid employed, must be the same. 



" In order to find out how much more pepsin one extract contains 

 than another, the extract which has been found to contain more 

 pepsin should be diluted until it digests at the same rate as the other 

 extract; the dilution may conveniently be carried out thus: 



" Supposing there are two extracts, of which the one a contains 

 more pepsin than the other b. 



"Take 2 c.c. of a + 18 c.c. of dil. HC1 (0'2 p. c.), and mix in a 

 beaker: of this put 10 c.c. in test tube (1). 



"Add 10 c.c. of the same HC1 to fluid in beaker, and of this put 

 10 c.c. in test tube (2). 



"Add 10 c.c. of the same HC1 to the fluid in the beaker, and of 

 this put 10 c.c. in test tube (3), and proceed in the same way as far 

 as may be considered necessary. 



" Test tubes (1), (2), (3) contain 1 c.c., 0*5 c.c., 0'25 c.c., respectively, 

 of extract a. 



" Take 2 c.c. of extract 6 + 8 c.c. of dilute HC1 (0'2 p. c.). Add to 

 this and to each of the various pepsin solutions made with extracts a, 

 1 c.c. of carmine-stained and already swollen fibrin (which has 

 been previously measured and placed aside in watch glasses), note 

 which of the (a) mixtures digests at the same rate as (6) ; if, for 

 example, the (a) mixture in test tube (3) digests at the same rate 

 as (6), then the original (a) extract contains eight times as much 

 pepsin as (b). 



" 2. In comparing the amount of pepsin which can be extracted 

 from equal weights of ditferent stomachs, or of different parts of any 

 one stomach, the mucous membrane should be rapidly washed with 

 salt solution, and the salt solution then sopped up with blotting 

 paper. The stomach should, then, be spread out on glass and the 

 muscular coat removed; the mucous membrane should then be dried 

 first at about 25 C. and then over sulphuric acid. Weighed portions 

 of mucous membrane should then be cut into small pieces and treated 

 with a 2 p. c. HC1, in the proportion of 500 c.c. of the diluted acid for 

 each gramme of dried mucous membrane. Digestion should be 

 allowed to go on at 38 C. for one day (though much the greater part 

 of the pepsin is extracted in two or three hours), the mixture then 

 filtered, and the amount of pepsin in the filtrate determined. If 

 there is any appreciable residue left on the filter, this may again be 

 treated as before with HC1, &c. 



" 3. Compare the amounts of pepsin that can be extracted from 

 the gastric mucous membrane of guinea-pig or rabbit (1) taken from 



