222 CASEINE. 



566. Tlie blood which is sent out from the heart 

 through the arteries all over the body, differs slightly 

 in chemical nature, from the blood which returns 

 through the veins to the heart, undergoing an im- 

 portant change in the course of its circulation : when 

 blood leaves the heart, it is of a bright red color, and 

 contains oxygen, which has been absorbed in the 

 lungs ; when it returns to the heart, it is black, and 

 contains carbonic acid in place of oxygen (606). A 

 pound of blood contains nearly two ounces of solid 

 matter ; of this about ninety grains is inorganic, the 

 rest organic. 



567. The fibrin, which exists in the arteries, is 

 slightly different in chemical properties from that 

 which is obtained from the veins. The latter ap- 

 proaches very nearly in character to albumen, it 

 liquefies under the influence of a warm solution of 

 nitrate of potash, and forms a solution which is co- 

 agulated by heat and metallic solutions. This kind 

 of fibrin is almost identical with the fibrin of flesh. 

 When fibrin is burnt, it leaves a light-colored ash, vary- 

 ing in amount from one to two and a half per cent. ; it 

 consists chiefly of phosphates of lime and magnesia. 



568. Caseine is a substance which very closely re- 

 sembles albumen, in its properties and composition ; 

 it is insoluble in pure water, but dissolves in dilute 

 acid or alkaline liquids, in the latter far more per- 

 fectly than in the former : it exists in milk, in which 

 it is dissolved by a small quantity of alkali; the ad- 

 dition of acid to milk, by neutralizing the alkali, 

 causes the separation of the caseine as a curd. 



