22 CHEMICAL COMPOSITION OF HUMAN BODY. 



solidifies on cooling much less firmly, and, unlike gelatin, it is 

 precipitable with acetic and the mineral and other acids, and 

 with alum, persulphate of iron, and acetate of lead. 



Albuminous substances, or proteids, as they are sometimes 

 called, exist abundantly in the human body. The chief among 

 them are albumen, fibrin, casein, syntonin, myosin,and globulin. 



Albumen exists in most of the tissues of the body, but es- 

 pecially in the nervous, in the lymph, chyle, and blood, and 

 in many morbid fluids, as the serous secretions of dropsy, pus, 

 and others. In the human body it is most abundant, and 

 most nearly pure, in the serum of the blood. In all the forms 

 in which it naturally occurs, it is combined with about six per 

 cent, of fatty matter, phosphate of lime, chloride of sodium, 

 and other saline substances. Its most characteristic property 

 is, that both in solution and in the half-solid state in which it 

 exists in white of egg, it is coagulated by heat, and in thus 

 becoming solid, becomes insoluble in water. The temperature 

 required for the coagulation of albumen is the higher the less 

 the proportion of albumen in the solution submitted to heat. 

 Serum and such strong solutions will begin to coagulate at 

 from 150 to 170, and these, when the heat is maintained, 

 become almost solid and opaque. But weak solutions require 

 a much higher temperature, even that of boiling, for their 

 coagulation, and either only become milky or opaline, or pro- 

 duce flocculi which are precipitated. 



Albumen, in the state in which it naturally occurs, appears 

 to be but little soluble in pure water, but is soluble in water 

 containing a small proportion of alkali. In such solutions it is 

 probably combined chemically with the alkali ; it is precip- 

 itated from them by alcohol, nitric, and other mineral acids, 

 by ferrocyanide of potassium (if before or after adding it the 

 alkali combined with the albumen be neutralized), by bi- 

 chloride of mercury, acetate of lead, and most metallic salts. 



Coagulated albumen, i. e., albumen made solid with heat, is 

 soluble in solutions of caustic alkali, and in acetic acid if it be 

 long digested or boiled with it. With the aid of heat, also, 

 strong hydrochloric acid dissolves albumen previously coag- 

 ulated, and the solution has a beautiful purple or blue color. 



Fibrin is found most abundantly in the blood and the more 

 perfect portions of the lymph and chyle. It is very doubtful, 

 however, whether fibrin, as such, exists in these fluids whether, 

 that is to say, it is not itself formed at the moment of coagula- 

 tion. (See chapter on the Blood.) 



If a common clot of blood be pressed in fine linen while a 

 stream of water flows upon it, the whole of the blood-color is 

 gradually removed, and strings and various pieces remain of 



