438 DIGESTION. 



quantity of fibrin to some 0.2 per cent, hydrochloric acid in a 

 test-tube, :md place it in the water-bath at 40 C. for several 

 hours. If all the fibrin is digested, add more, and repeat the 

 addition until at last it remains undissolved, however long it 

 may be digested. 



'The arrest of digestion in this experiment is not due to de- 

 struction of the pepsin, but to the accumulation of the products 

 of digestion in the liquid, and to the want of acid. Dilute the 

 mixture with water, and put it in the water-bath again, and di- 

 gestion will go on for a while and then stop. If again diluted, 

 it will go on again, but the action will be slow from the dilu- 

 tion of the acid. If more acid be added, digestion will proceed 

 more quickly, and by adding fresh quantities of acid, a very 

 large quantity of fibrin may be digested. 



The same tiling may be shown by putting the fibrin and di- 

 gestive lluid in a dialyzer and letting the peptones diffuse out. 

 The digestive fluid is then to be evaporated to its original bulk, 

 and acidulated, when it will digest the same amount of fibrin 

 as it did at first. It is well to keep an excess of fibrin always 

 in the dialyzer. This experiment is interesting, because diges- 

 tion in the stomach takes place under somewhat similar con- 

 ditions, the peptones being absorbed by the gastric vessels. 

 A stronger acid is required for digestion if the products of di~ 

 -/it in quantity in the solution. When digestion 

 stops, as in the previous experiment, it may be renewed by 

 acidulating the solution more strongly with hydrochloric acid 

 instead of diluting with water, and when it stops a second time 

 a second addition of acid will set it on again. As too strong 

 hydrochloric acid arrests digestion, a limit is soon put to the 

 addition of acid, but if phosphoric acid is used instead, diges- 

 tion may be- kept up for a considerable time by fresh additions 

 of acid. 



* 125. Pepsin Test. The power of pepsin to dissolve albu- 

 minous substances and convert them into peptones, has been em- 

 ployed as a test for its presence. For this purpose either fibrin 

 or coagulated white of egg may be used. The process is given 

 by r.riickc in " Moleschotts Untersuchungen" for 1860, p. 490, 

 and from this the following description has been taken: 



f J'c) min Test with Fibrin. To test for the presence of pep- 

 sin in any substance or organ (as for example, any part of the 

 digestive system of an invertebrate animal), it must be finely 

 divided, treated with distilled water, and then allowed to stand 

 for some time, with frequent stirring, and filtered. If the filtrate 

 is alkaline it must be neutralized, after which as much hydro- 

 chloric acid must be added to it as will bring the percentage 

 of acid to one-tenth. A bit of fibrin is then thrown into it ; if 

 it swells it is allowed to stand, but if it does not swell, dilute 

 acid is added by drops at intervals till the edges and free fibres 



