CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 371 



Soluble in distilled water. 



Aqueous solutions not coagulated on boiling. 



Diffusible ...... Peptone. 



Not diffusible ..... Album ose. 

 Aqueous solutions coagulated on boiling . Albumin. 



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 acid (HC1 -1 p.c.) 

 - J .-, , -, ' 



in the cold . . . . . - r\ - 



[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 arid parapeptone as in the case of other proteids. In fact, 

 the digestion by gastric juice of all the varieties of proteids 

 consists in the conversion of the proteid into peptone, with the 

 concomitant appearance of a certain variable amount of albumose 

 and parapeptone. 



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- 



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