CH. XXVI.] 



THE PLASMA AND SERUM. 



411 



This change is brought about by the activity of a sjjecial 

 wtori/anised ferment called the fibrin-ferment or thrombin. 



This ferment does not exist in healthy blood contained in healthy 

 blood-vessels, but is one of the products of the disintegration of the 

 white corj>uscles and blood tablets that occurs when the blood leaves 

 the vessels or comes into contact with foreign matter. 



To this it may be added, as the result of recent research, that 

 a soluble calcium salt is essential for the formation of the 

 ferment ; that the fibrin-ferment belongs to the class of nucleo- 

 proteids ; that other nucleo-protcids (Wooldridge's tissue-fibrino- 

 gens) obtained from most of the cellular organs of the body 

 produce iutravasuular clotting when injected into the circulation 

 of a living animal. 



The substance which is converted into fibrin-ferment or thrombin 

 by the action of a calcium salt may be conveniently termed 

 prothrombin. 



The process of fibrin formation may therefore be represented in 

 the following tabular way : 



In the plasma a proteid substance 

 exists, called 



FlBKlNOGEN. 



From the colourless corpuscles a 

 nucleo-proteitl is shed out, called 

 PROTHROMBIN. 



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



formed. 



One of these is unimportant, viz. 

 a globulin which remains in solution. 

 Its amount is very small. 



The other is important, viz. 

 FIBRIN, which entangles the cor- 

 puscles and so forms the CLOT. 



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 

 preventing the blood from coagulating. The corpuscles, being 

 heavy, >ink, and the supernatant plasma can then be removed by 

 :i pipette or siphon, or more thoroughly by the use of a centrifugal 

 machine (see fig. 341). 



On counteracting the influence which has prevented the blood 

 from coagulating. the plasma then itself coagulates. Tims plasma 



