THE BLOOD 85 



convert prothrombin into thrombin. We may therefore represent 

 the process of clotting in the following tabular way. 



In the plasma a proteid substance From the colourless corpuscles a 



exists, called nucleo-proteid is shed out, called 



FrBRiNOGEN. Prothrombin! 



By the action of calcium salts 

 prothrombin is converted into fibrin- 

 ferment, or 



Thrombin. 



Thrombin acts on fibrinogen in such a way that two new substances are 



formed. 



One of these is unimportant, viz. The other is important, viz. 



a globulin (fibrino-globulin) which Fibrin, which entangles the cor- 



remains in solution. Its amount is puscles and so forms the Clot. 

 very small. 



THE PLASMA AND SERUM 



The liquid in which the corpuscles float may be obtained by 

 employing one or other of the methods already described for pre- 

 venting the blood from coagulating. The corpuscles, being heavy, 

 sink, and the supernatant plasma can then be removed by a pipette 

 or siphon ; the separation can be effected more thoroughly by the 

 use of a centrifugal machine (see fig. 60, Lesson XXI.). 



On counteracting the influence which has prevented the blood from 

 coagulating, the plasma then itself coagulates. Thus plasma obtained 

 by the use of cold clots on warming gently ; plasma which has been 

 decalcified by the action of a soluble oxalate clots on the addition of 

 a calcium salt ; plasma obtained by the use of a strong solution of 

 salt coagulates when this is diluted by the addition of water, the 

 addition of fibrin ferment being necessary in most cases ; where co- 

 agulation occurs without the addition of fibrin ferment, no doubt some 

 is present from the partial disintegration of the corpuscles 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 



