414 THE BLOOD [CII. XXVI. 



7. Addition of leech extract. This acts in virtue of a proteose it 

 contains. 



The theory generally received which accounts best for the coagula- 

 tion of the blood is that of Hammarsten, and it may be briefly stated 

 as follows : 



When blood is in the vessels one of the constituents of the plasma, a 

 proteid of the globulin class called fibrinogen, exists in a soluble form. 



When the blood is shed the fibrinogen molecule is split into two 

 parts : one part is a globulin, which remains in solution ; the other is 

 the insoluble material fibrin. 



This change is brought about by the activity of a special unorganised 

 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 

 corpuscles and blood platelets 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-proteids (Wooldridge's tissue-fibrinogens) obtained from 

 most of the cellular organs of the body produce intravascular 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 From the colourless corpuscles a . 



exists, called nucleo-proteid is shed out, called 



FIBRIVOGEN. 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-globuliri) 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- 



