THE BLOOD 3 



great variation. The composition of the blood is also affected 

 by the source from which it is derived ; the blood from an artery 

 does not exactly represent that from a vein. 



Plasma forms about 66 per cent, of the blood, and consists of 

 three proteins, which are held in solution — viz., fibrinogen, para- 

 globulin (serum-globulin), and serum-albumin. The two former 

 belong to the globulin group of proteins ; the latter to the albu- 

 minous group, of which egg-albumin is typical. 



The three proteins may be separated by the employment of 

 certain neutral salts. Serum-albumin is precipitated by satura- 

 tion with ammonium sulphate. Its solution in a neutral or acid 

 medium is thrown down by heat. The temperatures of coagula- 

 tion, yo° to 75 C. (158 to 167 F.), would appear to indicate that 

 there are two or three different proteins classed as serum- album in. 



This substance is found not only in blood plasma, but in lymph. 

 It also forms a part of such secretions as milk. Its source is 

 believed to be the protein substances of the food which are taken 

 up from the intestinal canal, though there is no experimental 

 proof of the correctness of this view. Serum-albumin obtained 

 from the blood of the horse may readily be made to crystallise. 



Paraglobulin or serum-globulin is distinguished by being; 

 precipitated by saturation with magnesium sulphate, or half 

 saturation with ammonium sulphate. In neutral or faintly acid 

 solutions it is coagulated by heating to 75 C. (167 F.), and there 

 is reason to think from the behaviour under analysis of the para- 

 globulin obtained from serum that this substance is probably a 

 mixture of two or three allied proteins. It is supposed that the 

 source of paraglobulin is twofold — first from the protein sub- 

 stances of the food ; and secondly from the disintegration of 

 the white cells of the blood ; but in both cases proof is wanting. 



Fibrinogen constitutes but a small proportion of the total 

 protein of plasma. It is precipitated from solution by half 

 saturation with sodium chloride. It belongs to the group of 

 proteins known as globulins, and is a substance of remarkable 

 interest, for on its conversion from a fluid to a solid condition 

 depends the phenomenon of the coagulation of the blood. Like 

 the other proteins in plasma, it coagulates on heating, but 

 at a much lower temperature, for fibrinogen is coagulated at 

 56 to 6o° C. (133 to 140 F.). Blood so treated is no longer 

 capable of clothing, owing to its fibrinogen being coagulated. 

 The source of fibrinogen in the body is unknown ; it is supposed 

 to be connected with the destruction of leucocytes, and there is 

 also some evidence of the liver being concerned in its formation. 



Perhaps the nearest approach to pure plasma is the fluid found 

 in the pericardial and abdominal cavities. That which is effused 

 into the chest during an attack of pleurisy is plasma to start with, 



