CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 327 



Insoluble in dilute saline solutions. 



Keadily soluble in dilute acid (HC1 -1 p.c.) 



Acid-albumin. 

 Alkali-albumin. 



in the cold ~ 



Casein. 



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. 



Milk when treated with gastric juice is first of all "curdled." 

 This is the result partly of the action of the free acid but chiefly 

 of the special action of a particular constituent of gastric juice, of 

 which we shall speak hereafter. The curd consists of a particular 

 proteid matter mixed with fat ; and this proteid matter is sub- 

 sequently dissolved with the same appearance of peptone, albu- 

 mose and other bodies as in the case of other proteids. In fact, 

 the digestion by gastric juice of all the varieties of proteids con- 

 sists in the conversion of the proteid into peptone, with the con- 

 comitant appearance of a certain variable amount of albumose and 

 other bodies. 



182. 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 as well by physical as by chemical 

 characters. Hence in making a series of comparative trials the 

 same proteid should be used, and the form of proteid most con- 

 venient 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 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. 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 



