244 GASTRIC JUICE. [BOOK n. 



trials the same proteid should be used, and the form of proteid 

 most convenient for the purpose is fibrin. If it be desired simply 

 to ascertain whether any given specimen has any digestive powers 

 at all, it is best to use boiled fibrin, since raw fibrin is eventually 

 dissolved by dilute hydrochloric acid alone, probably on account of 

 some pepsin present in the blood becoming entangled with the fibrin 

 during coagulation. But in estimating quantitatively 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 again 

 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, and if the same stock of coloured fibrin 

 be used in a series of experiments, the amount of fibrin dissolved may 

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

 coloured with carmine may be preserved in ether. 



Since, if sufficient 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 are led to take, as a measure of the activity 

 of a spe'cimen 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 part at least, the 

 reason why a fragment of spongy filamentous fibrin is more readily 

 dissolved than a solid. clump of boiled white of egg of the same size. 

 Neutralisation of the juice wholly arrests digestion ; fibrin may be 

 submitted for an almost indefinite time to the action of neutralised 

 gastric juice without being digested. If the neutralised juice be 

 properly acidified, it may again become active ; in gastric juice 

 however which has been made alkaline, and kept at a temperature 

 of 35, the solvent powers are not only suspended but actually 

 destroyed. Digestion is most rapid with dilute hydrochloric acid 

 of '2 p.c. (the acidity of natural gastric juice). If the juice con- 

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

 visibly impaired. Other acids, lactic, phosphoric, &c. may be sub- 

 stituted for hydrochloric; but they are not so effectual, and the 

 degree of acidity most useful varies 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 

 35 40 C. ; at the ordinary temperature it is much slower, and at 

 about C. 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 



