30 CASEIN AND FIBEIN. 



another. Albumen forms basic, neutral, and acid compounds. It is a 

 basic albuminate of soda which is found in the egg and in serum of 

 blood. In certain diseased conditions the blood contains the neutral al- 

 buminate. 



Casein presents nearly the same constitution as albumen, but differs 

 from it in its physical properties ; for, while a solution of albumen 

 is coagulable by heat, one of casein is not, but lactic and acetic 

 acids coagulate it, -though they have no such effect on albumen. While, 

 so far as their protein nucleus is concerned, the two substances agree in 

 composition, they differ in this respect, that casein appears to contain a 

 less proportion of sulphur, and no phosphorus. It is interesting to re-* 

 mark that, during incubation, casein arises from albumen in the eggs of 

 birds. 



Closely allied to albumen and casein, and having the same protein nu- 

 cleus, is fibrin, which likewise exists in two states, soluble and 

 insoluble. Its solidification or coagulation can be produced by 

 the action of sulphuric ether, which does not affect albumen. Moreover, 

 in the coagulated state fibrin decomposes the deutoxide of hydrogen, 

 but albumen does not. The most important difference between them is, 

 that in the act of coagulation albumen shows no disposition to assume a 

 definite structure, but fibrin does fibrillating, as it is termed. The 

 analogy of constitution and closeness of relation of the two substances is 

 demonstrated by the fact that y nitrate of potash coagulated fibrin may 

 be changed into albumen, and the same conversion is accomplished in the 

 stomach by the digestive juices. 



It is generally supposed, however, that fibrin contains a larger pro- 

 portion of oxygen than albumen, a conclusion which seems to be confirm- 

 ed by physiological considerations respecting its origin. For this reason, 

 Mulder describes it as a higher oxide of his hypothetical protein. It al- 

 ways is associated with fat, or, perhaps more correctly, with soaps of 

 ammonia and lime. 



Fibrin is found in the chyle, lymph, and blood. In the latter fluid 

 its quantity varies in different parts of the circulation. The blood of the 

 portal vein yields it in smaller proportion than that of the jugular. It is 

 also affected very much by diet : thus Lehmann found that under an ani- 

 mal diet there was much more fibrin in his blood than under a vegeta- 

 ble one, a result which has been confirmed by experiments on dogs. It 

 has also been observed that its quantity is increased during starvation. 

 But the blood of herbivorous animals contains more than that of carnivo- 

 rous ones, and that of birds contains the most of all. 



These remarks on the composition and physical properties of casein, 

 albumen, and fibrin, have been introduced for the purpose of illustrating 

 the facility with which these bodies are mutually convertible, and more 



