88 THE BLOOD AND THE LYMPH 



by volume or about 55 per cent by weight consists of plasma; and from 

 40 to 30 per cent by volume or 45 per cent by weight consists of cor- 

 puscles. 



THE PROTEINS OF THE BLOOD 



The plasma obtained by centrifuging the blood rendered noncoagula- 

 ble by oxalates, hirudin or other means (see page 100), contains 5 to 8 

 per cent of c.oagulable proteins. These proteins are serum albumin, 

 serum globulin, and fibrinogen. They can be separated from each other 

 by the use of acids and neutral salts. Their proportion varies under dif- 

 ferent conditions, but is approximately as follows: 



Fibrinogen 0.15-0.6% 



Serum globulin 3.8% 



Serum albumin 2.5% 



The amount of fibrinogen is subject to the greatest variation (Mathews). 



Fibrinogen 



The least soluble of the blood proteins is fibrinogen. The plasma is 

 almost freed of it by half-saturation with sodium chloride, or with a 

 small amount of acetic acid. It is precipitated as fibrin in the process 

 of blood coagulation (see page 102), and is estimated by weighing the 

 amount of fibrin which it produces. 



Serum Globulin and Serum Albumin 



Globulins are ordinarily defined as being insoluble in distilled water, 

 and albumins as being soluble. It is, however, impossible to separate 

 serum globulin and albumin satisfactorily in this manner. The globu- 

 lin obtained by dialysis can be returned to solution by the addition of 

 a suitable amount of water, which makes the salt adherent to the pre- 

 cipitate a weak saline solution. In neutral or acid solutions it is coag- 

 ulated by heat at about 75 C. But it does not act as an individual pro- 

 tein, since a portion of it is precipitated by dialysis or by carbon diox- 

 ide. Probably serum globulin really consists of two or more proteins. 



The serum albumin remaining in solution after saturation with am- 

 monium sulphate likewise does not represent a chemical entity. It is 

 possible by carefully heating the solution of serum albumin to distin- 

 guish three separate coagulation temperatures. This fact has been in- 

 terpreted as meaning that the serum albumin consists of at least three 

 closely related proteins. 



Since the refractive index of the blood depends primarily upon the 



