416 THE BLOOD [CH. XXVI. 



which has already occurred. Pericardial and hydrocele fluids 

 resemble pure plasma very closely in composition. As a rule, 

 however, they contain few or no white corpuscles, and do not clot 

 spontaneously, but after the addition of fibrin-ferment, or liquids like 

 serum that contain fibrin-ferment, they always yield fibrin. 



Pure plasma may be obtained from horse's veins by what is known 

 as the " living test-tube " experiment. If the jugular vein is ligatured 

 in two places so as to include a quantity of blood within it, then 

 removed from the animal and hung in a cool place, the blood will not 

 clot for many hours. The corpuscles settle, and the supernatant 

 plasma can be removed with a pipette. 



The plasma is alkaline, yellowish in tint, and its specific gravity 

 is about 1026 to 1029. 1000 parts of plasma contain : 



Water 902*90 



Solids 97-10 



Proteids : 1. yield of fibrin . . . s . . . 4-05 



2. other proteids 78 '84 



Extractives (including fat) . . . . . . . 5 -66 



Inorganic salts .......... 8*55 



In round numbers, plasma contains 10 per cent, of solids, of which 

 8 are proteid in nature. Note, however, the comparatively small 

 yield of fibrin. 



Serum contains the same three classes of constituents proteids, 

 extractives, and salts. The extractives and salts are the same in 

 both liquids. The proteids are different, as is shown in the following 

 iable : 



Proteids of Plasma. Proteids of Serum. 



Fibrin ogen. Serum globulin. 



Serum globulin. Serum albumin. 



Serum albumin. Fibrin-ferment (nucleo-proteid). 



Fibrino-globulin. 



The gases of plasma and serum are small quantities of oxygen, 

 nitrogen, and carbonic acid. The greater part of the oxygen of the 

 blood is combined in the red corpuscles with haemoglobin; the 

 carbonic acid is chiefly combined as carbonates. The gases of the 

 blood have already been considered under Kespiration (see p. 378). 



We may now study one by one the various constituents of the 

 plasma and serum. 



A. Proteids. Fibrinogen. This is the substance acted on by 

 fibrin-ferment. It yields, under this action, an insoluble product 

 called fibrin, and a soluble proteid of the globulin class (fibrino- 

 globulin). 



Fibrinogen is a globulin. It differs from serum globulin, and 

 may be separated from it, by making use of the fact that half- 

 saturation with sodium chloride precipitates it. It coagulates by 

 heat at the low temperature of 56 C. 



