CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 323 



acid-albumin or alkali-albumin suspended in water, or paraglo- 

 bulin suspended in water or dissolved in a dilute saline solution, 

 are heated to a temperature, which for the whole group may be 

 put down at about 75 80 C., each of them becomes coagulated, 

 and after the change is insoluble in water, saline solutions, dilute 

 acids &c., in fact in everything but very strong acids. Myosin 

 and fibrinogen undergo a similar change at a lower temperature, 

 viz. about 56 C. We may, for present purposes, speak of all 

 these proteids thus changed under the one term of coagulated 

 proteids. 



To the above list we may now add two other proteids, viz. : 

 7. A kind of albumin which forms the great bulk of the proteid 

 matter present in raw ' white of egg,' and which, since it differs 

 in minor characters from the albumin of blood and of the tissues, 

 is called egg-albumin. 8. The peculiar proteid casein, an impor- 

 tant constituent of milk. This may perhaps be regarded as a 

 naturally occurring alkali-albumin since it has many resemblances 

 to the artificial alkali-albumin ; but for several reasons it is desir- 

 able to consider it as an independent body. 



Egg-albumin like serum-albumin becomes coagulated at a 

 temperature of about 75 80 C., and though casein as it natu- 

 rally exists in milk is not coagulated on boiling, when separated 

 out in a special way, and suspended in water in which it is in- 

 soluble, it becomes coagulated at about 75 80 C. 



It will be observed that all these proteids form, as regards 

 their solubilities, a descending series, in the following order. 

 Coagulated Proteids. Fibrin. Acid-albumin with Alkali-albu- 

 min, and Casein. Myosin, Globulins. Serum-albumin with Egg- 

 albumin. 



We must now return to the action of gastric juice. 



If a few shreds of fibrin, obtained by whipping blood, after 

 being thoroughly washed and boiled and thus by the boiling 

 coagulated, be thrown into a quantity of gastric juice, and the 

 mixture be exposed to a temperature of from 35 to 40 C., the 

 fibrin will speedily, in some cases in a few minutes, be dissolved. 

 The shreds first swell up and become transparent, then gradually 

 dissolve, and finally disappear with the exception of some granular 

 debris, the amount of which, though generally small, varies accord- 

 ing to circumstances. If raw, that is unboiled, uncoagulated fibrin 

 be employed the same changes may be observed, but they take 

 place much more rapidly. 



If small morsels of coagulated albumin, such as white of egg, 

 be treated in the same way, the same solution is observed. The 

 pieces become transparent at their surfaces ; this is especially seen 

 at the edges, which gradually become rounded down ; and solution 

 steadily progresses from the outside of the piece inwards. 



If any other form of coagulated albumin (e.g. precipitated 

 acid- or alkali-albumin, suspended in water and boiled) be treated 



