en. xxix.] 



COAGULATION OF THE BLOOD 



451 



We are thus justified in two conclusions: 



(1) That the antithrombin (normally present in healthy blood in 

 sufficient quantities to prevent intravascular clotting) is formed in 

 the liver. 



(2) That commercial peptone, in virtue of the proteoses it 

 contains, stimulates this action to such an extraordinary degree 

 that the accumulation of antithrombin in the blood becomes so 

 great that the blood does not clot even after it is shed. 



We will conclude by considering only one more of the hindrances 

 to coagulation, and that by no means the least interesting. The 

 leech lives by sucking the blood of other animals ; from the leech's 

 point of view it is therefore necessary that the blood should flow 

 freely and not clot. The glands at the head end of the leech, often 

 spoken of roughly as its salivary glands, secrete something which 

 hinders the blood from coagulating, and everyone knows by experi- 

 ence, who has been treated by leeches, how difficult it is to prevent 

 a leech-bite from bleeding after the leech has been removed ; com- 

 plete cleansing is necessary to wash away the leech's secretion from 

 the wound. Now if an extract of leeches' heads is made with salt 

 solution and filtered, that fluid will prevent coagulation whether it is 

 injected into the blood-stream or added to shed blood. The sub- 

 stance in question is believed to be antithrombin itself. The 

 purified material obtained from leech extract is called hirudin. 

 Blood so obtained can be made to clot by the addition of thrombin, or 

 of such a fluid as serum, that contains thrombin in sufficient amount. 



We may summarise this view of the causes of coagulation in 

 the following tabular way : 



From the platelets, and 

 to a lesser degree from the 

 leucocytes, a material is 

 shed out, called 



TH ROM BOG EN. 



I 



From the formed ele- 

 ments of the blood, but 

 also from the tissues over 

 which the escaping blood 

 flows, is shed out an acti- 

 vating agent, called 



THROMBOKINASE. 



I 



In the blood plas- In the presence of CALCIUM salts, thrombokinase 



ma a protein sub- activates thrombogen in such a way that an active 

 stance exists, called enzyme is produced, which is called 



FlBRlNOGEN. 



I 



THROMBIN. 



I 



Thrombin or fibrin-ferment acts on fibrinogen in such a way that it is trans- 

 formed into the insoluble stringy material which is called 



FIBRIN. 



Differing hypotheses are held as to the exact role played by each of the factors 

 in fibrin-formation, and the views enunciated in the preceding paragraphs are in the 



