304 THE BLOOD 



Mammalian plasma, if left standing in a tube exposed to the air, 

 clots to a jelly in 2 to 10 minutes. The gel has the same volume 

 as the sol, and no heat is evolved in the process. This is a process 

 common to most emulsoid colloids. In about half an hour the 

 clot contracts and exjjresses a clear straw-yellow liquid, the 

 serum, 



i.e. Plasma = Clot + Serum. 



This process whereby the fluid content of the gel is decreased 

 is common to gels and is called syneresis. 



Historical. 



Observers at first thought that living tissue had a restraining 

 influence. They noticed that in the living tissue the blood did 

 not clot. Lister formed a living test-tube by ligature of the 

 aorta and showed that the blood did not clot until a foreign body 

 such as glass was introduced. This demonstrated that clotting 

 was not due to (1) removal from living vessels, (2) stoppage of the 

 circulation, (3) cooling or (4) to contact with clean air. This latter 

 fact has been confirmed by the observation that blood clots as 

 readily in a vacuum as in air. Nor is clot formation due to cooling. 

 As a matter of fact reduction of temperature lengthens the time 

 taken to produce a clot and, if the plasma is cooled sufficiently, 

 may prevent it altogether. 



The classical work on blood clotting was performed by Andrew 

 Buchanan, Professor of Physiology at Glasgow, who gave explana- 

 tions of the process to the Glasgow Philosophical Society, in March, 

 1844, and February, 1845. Using hydrocele fluid from the tunica 

 vagiiialis of a horse, he showed that it would clot if to it were 

 added a drop or two of blood, of clot washings, of serum or of 

 tissue juice. He compared the process to the curdling of milk 

 by rennin and considered that the white corpuscles or leucocytes 

 were the active agent. 



In 1861 Schmidt, who had devoted some thirty years to the 

 work, proved that : 



(i.) Fibrinogen, the precursor of the clot, was a globulin in the 

 plasma. 



(ii.) There was a fibrin-former in plasma, in serum and in clot 

 washings. 



(iii.) He later showed that this fibrin-former (now called throm- 

 bin) did not exist as such in the blood, but only appeared after 

 treatment with a fibrin-ferment or -kinase. That is, the clotting 

 scheme as he left it appears as follows (modern names) : 



1. Thrombokinase -f Prothrombin — >- Thrombin. 



2. Thrombin + Fibrinogen — > Fibrin (Clot). 



