CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 395 



Insoluble in distilled water. 



Readily soluble in dilute saline solutions 



(NaCl 1 per cent.) .... Globulins. 



Soluble only in stronger saline solutions 



(NaCl 5 to 10 p.c.) . . . . Myosin. 



Insoluble in dilute saline solutions. 



Readily soluble in dilute acids and alkalis . Acid and Alkali- 

 albumin. 



Soluble with difficulty in dilute acid, that is 

 at high temperature (60 C.) and after 

 prolonged treatment only . . . Fibrin. 



Insoluble in dilute acids, soluble only in 



strong acids Coagulated Proteid. 



204. Circumstances affecting gastric digestion. The solvent 

 action of gastric juice on proteids is modified by a variety of cir- 

 cumstances. The nature of the proteid itself makes a difference, 

 though this is determined probably by physical rather than by 

 chemical characters. Hence in making a series of comparative 

 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 previously present in the blood becoming entangled 

 with the fibrin during clotting. But in estimating quantita- 

 tively the peptic power of two specimens of gastric juice under 

 different conditions, raw fibrin prepared by Grutzner'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. 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 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 specimen of gastric juice, not the quantity of fibrin which it 

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

 given quantity. 



262 



