CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 391 



the one into the other by the use of dilute alkali or dilute acid 

 respectively. Their most important common characters are in- 

 solubility in water and in saline solutions and ready solubility in 

 dilute acids and alkalis. 6. Coagulated proteids. As we have 

 seen, when fibrin suspended in water, serum-albumin in solution, 

 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 pur- 

 poses, 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 important 

 constituent of milk. This, though it has a superficial resemblance 

 to alkali-albumin in being precipitated by acids, is in reality a 

 wholly different body. We shall speak of it later on. 



Egg-albumin like serum-albumin becomes coagulated at a 

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

 exists in milk is not coagulated on boiling, it does become co- 

 agulated under certain conditions. 



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-albumin. 

 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. 



