FIBRIN AND CASEIN. 
21 
sulphur. The salts contained in albumen amount to from 4 to 8 per centum, consisting 
of phosphate and sulphate of lime, and chloride of sodium. 
The importance of albumen is directly indicated by the phenomena which occur in the 
incubation of the egg. It is a familiar fact, that in this process, all parts of the animal 
are developed from the albumen, inasmuch as no other nitrogenous body is present. Bones, 
muscles, feathers, nails, claws, the brain, the membranes and tissues of the body, and the 
blood, are all generated from this substance in a few days. The yolk is albumen inter¬ 
mixed largely with large yellow oil globules. 
Fibrin. Fibrin is so closely allied to albumen, that it is regarded as chemically the 
same, although it presents some physical properties not found in albumen ; thus, fibrin 
coagulates spontaneously from the blood when it ceases to circulate. By fibrin, then, it 
will be understood that I mean that clot or coagulum which forms when blood is drawn 
from a blood-vessel. As it exists in this state, it is mixed largely with the coloring matter 
of the blood. When it is wished to obtain it pure, blood may be shaken in a bottle with 
bits of lead or tin, when the fibrin will adhere in a fibrous mass, which may be washed 
with cold water till all foreign matter is removed. The fat which still adheres to it, may 
be dissolved out by ether. It is then a substance of a pale yellow, and devoid of taste or 
smell. Its chemical composition is the same as that of albumen. The fat associated with 
fibrin varies from 2 to 4 per centum. 
Fibrin, like albumen, is always associated with a certain amount of salts. It is readily 
distinguished from other bodies, by its spontaneous coagulation. 
Casein. Casein is an important constituent of milk. Curd of milk is casein combined 
with some foreign matters. To obtain it pure, milk is evaporated to dryness, and its butter 
dissolved out with hot ether. Dissolve the residue in water, and filter, and then throw 
down the casein with alcohol. When a solution contains casein, a pellicle forms upon the 
surface when heated or boiled : this is regarded as the effect of oxygen upon it. 
The composition of casein, according to Mulder, is 
C400 Nso-j-OiaoH-S. 
Casein is converted into albumen by digestion ; and so, by the same action, albumen is 
converted into casein. It is precipitated from its natural solution by all acids, but is re¬ 
dissolved by the same. The most familiar case of precipitation, or rather coagulation of 
casein is that produced in milk by the mucous membrane of a calf’s stomach : this is the 
method employed in the manufacture of cheese. The addition of an alkali to the milk 
would prevent coagulation. When milk is allowed to stand for some time, a naturally 
coagulated casein surrounds the butter vesicles, which are broken by agitation in stirring 
or churning, and collect in the form of butter. 
Casein contains from 3 to 8 per centum of ash after incineration, consisting of phosphoric, 
carbonic and hydrochloric acids in combination with lime, magnesia and iron. To dis¬ 
tinguish casein from albumen, it may be heated to a little over 167°, when it does not 
coagulate, but a pellicle will be formed upon the surface. 
