204 VETERINARY PHYSIOLOGY 



red fluid blood which is left, consisting of blood cells and 

 serum, is said to be defibrinated. 



Fibrin is a protein substance. It is slowly dissolved in 

 solutions of neutral salts. It is coagulated by heat, and is 

 precipitated when an excess of a neutral salt is added. It is 

 therefore a globulin. 



The plasma before clotting and the serum squeezed out from 

 the clot both contain in the same proportion an albumin 

 (sepum albumin) and a globulin, or series of globulins which 

 may be classed together as serum globulin. But the plasma 

 contains a small quantity about 0'4 per cent. of another 

 globulin (flbrinogen) which coagulates at a low tempera- 

 ture, and which is absent from serum. It is this which 

 undergoes the change from the soluble form to the insoluble 

 form in coagulation. If, by taking advantage of the fact that 

 it is more easily precipitated by sodium chloride than the 

 other proteins, it is separated from them, it may still be 

 made to clot. (Chemical Physiology.) 



The substance which usually causes clotting appears to be 

 an enzyme, which is formed by the action of calcium salts or 

 ions on a pro-enzyme set free in the blood. The enzyme may 

 be called thrombin, and its precursor prothrombin. Oxalates, 

 when added to blood, precipitate the soluble calcium salts, and 

 prevent the formation of thrombin, and thus prevent coagula- 

 tion. (Chemical Physiology.) 



It is not yet quite clear what brings about this change of 

 prothrombin into thrombin. Calcium salts or calcium ions 

 and prothrombin may exist together without thrombin being 

 formed, and it has been suggested that another enzyme is 

 required to activate the thrombin a thrombokinase as it has 

 been called. This may be liberated from the cellular elements 

 of the blood and tissues. 



The steps in the process of clotting might therefore be 

 represented as follows : 



Thrombokinase 



Prothrombin Calcium Salts 



V" 



Thrombin->Fibrinogen 



4^ 



Fibrin 



