224 HUMAN PHYSIOLOGY 



to death from slight wounds. Their condition is known 

 as hemophilia and it is inherited in certain lines of 

 descent. The clotting of blood upon injured surfaces has 

 a secondary function since it gives the basis of the crust 

 or scab beneath which the healing processes may go on. 



When blood coagulates in a beaker the whole mass 

 appears for a time as a solid which gradually becomes 

 more tenacious. In the course of some hours it con- 

 tracts and a clear or but slightly stained fluid exudes. 

 The distribution of the coloring shows that the cor- 

 puscles are in the clot and one might infer that the liquid 

 separating must be plasma. It is better to call it by the 

 special name of serum. Most of the materials which 

 were originally in the plasma remain in the serum, but 

 there is an important exception: something has been 

 taken from the plasma to knit the corpuscles together 

 and form the clot. 



The central fact in coagulation is the generation of a 

 gummy compound which is called fibrin. The absolute 

 quantity of this new substance is very small but its 

 physical nature is such that is capable of changing a 

 liquid to a solid much as gelatin might do. In the ab- 

 sence of corpuscles the clot would have little firmness; 

 they give it body and coherence. We have now to 

 inquire as to the source of the fibrin and why it is formed 

 at particular times rather than at others. 



Fibrin is a protein. It is derived from another protein, 

 fibrinogen, which exists in normal plasma. Fibrinogen 

 is soluble and therefore its presence does not attract 

 attention; fibrin is relatively insoluble and hence con- 

 spicuous in its effects. The formation of the insoluble 

 fibrin from the soluble fibrinogen is an instance of enzyme 

 action and may recall the curdling of milk by gastric 

 juice. The enzyme which causes the fibrin to form and so 

 brings about the coagulation of blood is called thrombin. 

 The enzyme comes into existence, or at least becomes 

 effective, under just such conditions as attend the 

 shedding of blood. 



