254 THE TISSUES AND MECHANISMS OF DIGESTION. 



the particular acid-albumin into which the myosin of muscle is changed, 

 being sometimes called syntonin. If the reagent used be not dilute acid, 

 but dilute alkali, the product is called alkali-albumin. The two bodies, 

 acid-albumin and alkali-albumin, are very parallel in their characters, and 

 may readily be converted, the one into the other, by the use of dilute alkali 

 or dilute acid respectively. Their most important common characters are 

 insolubility in water and in saline solutions and ready solubility in dilute 

 acids and alkalies. 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 paraglobulin suspended in water or dis- 

 solved in a dilute saline solution, are heated to a temperature which for the 

 whole group may be put down at about 75 to 80 C., each of them be- 

 comes coagulated, and after the change is insoluble in water, saline solu- 

 tions, dilute acids, etc., 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 important constituent of milk. This may perhaps be 

 regarded as a naturally occurring alkali-albumin, since it has many resem- 

 blances 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 to 80 C., and though casein as it naturally exists in milk is 

 not coagulated on boiling, when separated out in a special way, and sus- 

 pended in water in which it is insoluble, it becomes coagulated at about 75 

 to 80 C. 



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

 ties, a descending series in the following order : coagulated proteids ; fibrin ; 

 acid-albumin with alkali-albumin, and casein ; myosin, globulin ; 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 thor- 

 oughly washed and boiled and thus by the boiling coagulated, be thrown 

 into a quantity of gastric juice, and the mixture be exposed to a tempera- 

 ture 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 gran- 

 ular debris, the amount of which, though generally small, varies according 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 trans- 

 parent 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 inward. 



If any other form of coagulated albumin (e. g., precipitated acid- or 

 alkali-albumin, suspended in water and boiled) be treated in the same way, 

 a similar solution takes place. The readiness with which the solution is 

 effected, will depend, cceteris paribus, on the smallness of the pieces, or rather 

 on the amount of surface as compared with bulk, which is presented to the 

 action of the juice. 



