100 PROXIMATE PRINCIPLES OF THE THIRD CLASS. 



at present most thoroughly acquainted. The first seven are fluid, 

 or nearly so, and either colorless or of a faint yellowish tinge. 



1. FIBRIN. Fibrin is found in the blood ; where it exists, in the 

 human subject, in the proportion of two to three parts per thousand. 

 It is fluid, and mingled intimately with the other ingredients of the 

 blood. It occurs also, but in much smaller quantity, in the ly_rnph. 

 It is distinguished by what is called its " spontaneous" coagulation ; 

 that is, it coagulates on being withdrawn from the vessels, or on the 

 occurrence of any stoppage to tho circulation. It is rather more 

 abundant in the blood of some of the lower animals than in that of 

 th human subject. In general, it is found in larger quantity in 

 the blood of the herbivora than in that of the carnivora. 



2. ALBUMEN. Albumen occurs in the blood, the lymph, the 

 fluid of the pericardium, and in that of the serous cavities gene- 

 rally. It is also present in the fluid which may be extracted by 

 pressure from the muscular tissue. In the blood it occurs in the 

 proportion of about seventy -five parts per thousand. The white of 

 egg, which usually goes by the same name, is not identical with the 

 albumen of the blood, though it resembles it in some respects ; it is 

 properly a secretion from the mucous membrane of the fowl's ovi- 

 duct, and should be considered as a distinct organic substance. 

 Albumen coagulates on being raised to the temperature of 160 F.; 

 and the coagulum, like that of all the other proximate principles, is 

 soluble in caustic potassa. It coagulates also by contact with alco- 

 hol, the mineral acids, ferrocyanide of potassium in an acidulated 

 solution, tannin, and the metallic salts. The alcoholic coagulum, if 

 separated from the alcohol by washing, does not redissolve in water. 

 A very small quantity of albumen has been sometimes found in the 

 saliva. 



3. CASEIN. This substance exists in milk, in the proportion of 

 about forty parts per thousand. Ic coagulates by contact with all 

 the acids, mineral and organic ; but is not affected by a boiling 

 temperature. It is coagulated also by the juices of the stomach. 

 It is important as an article of food, being the principal organic 

 ingredient in all the preparations of milk. In a coagulated form, 

 it constitutes the different varieties of cheese, which are more or 

 less highly flavored with various oily matters remaining entangled 

 in the coagulated casein. 



