24 BLOOD 



The process is retarded or prevented — ■ 



1. By addition of sodium oxalate, fluoride or citrate; 



2. By cooling ; 



3. By receiving it direct from the interior of a blood- 



vessel into a vessel hned with paraffin ; 



4. By addition of leech extract. 



If freshly drawn blood is treated ^vith sodium oxalate, 

 fluoride or citrate, it fails to clot. Clotting can be induced 

 by addition of calcium in excess. Calcium, therefore, is 

 necessary for the formation of fibrin, the preventive action 

 of the oxalate and fluoride being due to the metal being 

 precipitated, that of the citrate being due to the metal 

 being converted into a non-ionised form. 



From the oxalated plasma there can be precipitated, 

 by half -saturation with sodium chloride, a protein — fibrino- 

 gen. This, on being separated and redissolved, forms 

 fibrin as soon as calcium is added to it. Fibrinogen, then, 

 is or contains the precursor of fibrin. 



If fibrinogen be purified by repeated precipitation, it no 

 longer clots on addition of calcium. Crude fibrinogen, 

 therefore, contains another substance essential to clotting. 



Purified fibrinogen, on addition of serum or clot in the 

 absence of calcium, readily clots. 



Purified fibrinogen, on the addition of fresh oxalated 

 plasma, does not clot. 



From the above facts, these inferences can be drawn. 



1. There is present in clot and serum, but not in fresh 

 blood, a substance which directly causes clotting, even 

 in the absence of calcium. 



This substance is called thrombin. 



2. Thrombin is evidently the substance removed from 

 crude fibrinogen in the process of purification, 



3. Calcium is necessary, not for the conversion of fibri- 

 nogen into fibrin, but for a process anterior to this, the 

 formation of thrombin, from a parent-substance {thrombogen 

 or prothrombin). 



