THE BLOOD AND ITS FUNCTIONS 129 



is formed while the blood is clotting. Furthermore, it is known 

 that blood does not clot at all unless calcium is present in it. 

 The agents involved in the production of fibrin, and there- 

 fore of clotting, may be shown in relation to one another as 

 follows : 

 Thrombokinase (from tissue juice, white corpuscles or platelets) 



+ Thrombogen (from plasma) 

 + Calcium salts (from plasma) 



= Thrombin = Fibrin ferment 

 Thrombin + Fibrinogen (in solution in blood) 

 = Fibrin (insoluble) 



We have already noted that in the blood plasma there is a 

 proteid called fibrinogen (see page 124). This is dissolved in 

 the liquid, and is no more visible than the sugar in a cup of 

 coffee. It is from this fibrinogen that the fibrin is produced. 

 Fibrinogen, however, will not give rise to fibrin if left to 

 itself, but if a certain amount of material called thrombin, 

 or fibrin ferment, is present, it at once breaks up into two 

 substances; one of these remains in solution in the blood, but 

 the other is not soluble, and appears at once as fibres form- 

 ing the fibrin. Hence blood clotting is due to the formation 

 of fibrin out of fibrinogen under the influence of fibrin ferment. 



Whence comes this fibrin ferment? Since the blood will 

 not clot when flowing through the arteries and veins, we con- 

 clude that this ferment cannot be present in living blood. It 

 must be formed when the blood is drawn. Its source is not 

 certainly known but there are strong reasons for believing 

 that three different agents are involved: 



(a) thrombokinase (from white corpuscles, platelets, or 



the cut tissues) ; 



(b) thrombogen (from the plasma) and 



(c) calcium salts. 



When these three factors are present at the same time, blood 

 clots quickly, and thus we conclude that they form the sub- 

 stance which changes fibrinogen into fibrin. At all 



