372 GASTRIC DIGESTION. [BOOK n. 



tivly the peptic power of two specimens of gastric juice under 

 different conditions, raw fibrin prepared by Griitzner's method is 

 the most convenient. 



Portions of well washed fibrin are stained with carmine and a^ain 

 washed to remove the superfluous colouring matter. A fragment of 

 this coloured fibrin thrown into an active juice on becoming dissolved, 

 gives up its colour to the fluid. Hence if the same stock of coloured 

 fibrin be used in a series of experiments, and the same bulks of fibrin 

 and of fluid be used in each case, the amount of fibrin dissolved may 

 be fairly estimated by the depth of tint -jiven to the fluid. Fibrin thus 

 coloured with carmine may be preserved in ether. 



Since, if siitHeient time be allowed, even a small quantity of 

 gastric juice will dissolve at least a very large if not an indefinite 

 quantity of fibrin, we arc 1 led to take, as a measure of the activity 

 of a specimen of gastric juice, not the quantity of fibrin which it 

 will ultimately dissolve, but the rapidity with which it dissolves a 

 given quantity. 



The greater the surface presented to the action of the juice, the 

 more rapid the solution : hence minute division and constant move- 

 ment favour digestion. And this is probably, in par! at lea-t. the 

 reason why a fragment of spongy filamentous fibrin is more readily 

 dissolved t han a solid clump of boiled white of egg of the same si/e. 

 Neutralisation of the juice wholly arrests digestion ; fibrin may be 

 Submit ted for an almost in definite time to t he action ofneul ralised 

 gastric juice without being digested. If the neutralised juice be 

 properly acidified, it may again become active: when gastric juice 

 however has been made alkaline, and kept for some time at a 

 temperature of :!.">", its solvent powers are not only suspended but 

 actually de.-troyed. Digestion is most rapid with dilute hydrochloric 

 acid of "2 p.c. (the acidity of natural gastric juice). If the juice 

 contains much more or much less free acid than this, its activity is 

 distinctly impaired. Other acids, lactic, phosphoric, &c. may be 

 substituted for hydrochloric ; but they are not so effectual, and tin- 

 degree of acidity most useful vari>-- with the different acids. The 

 presence of neutral salts, such as sodium chloride, in excess is 

 injurious. The action of mammalian gastric juice is most rapid at 

 3o 40 C.; at the ordinary temperature it is much slower, and at 

 about 0C. ceases altogether. The juice may be kept however at 

 0C. for an indefinite period without injury to its powers. The 

 gastric juice of cold-blooded vertebrates is relatively more active 

 at low temperatures than that of warm-blooded mammals or birds. 



At temperatures much above 40 or 45 the action of the juice 

 is impaired. By boiling for a few minutes the activity of the most 

 powerful juice is irrevocably destroyed. The presence in a concen- 

 trated form of the products of digestion hinders the process of solu- 

 tion. If a large quantity of fibrin be placed in a small quantity of 

 juice, digestion is soon arrested ; on dilution with the normal hy- 



