80 PHYSIOLOGY 



Through its activity fibrinogen, one of the proteids of the 

 blood, is transformed into fibrin. As the change takes 

 place, the small particles of the proteid which are held 

 in suspension separate themselves from the liquid and 

 assume a fibrous character. The fibers gather together 

 in a larger and larger mass and in so doing enmesh the 

 red corpuscles. These are in no way responsible for the 

 process, for if the blood is whipped with a bunch of 

 wires the fibrin may be obtained quite clear of them; 



The power of the blood to coagulate is of great service 

 to the body, because in case of serious injury to a blood 

 vessel, the contraction of its wall tends to close the 

 wound and the coagulation of the blood prevents con- 

 tinued bleeding. 



The Serum. — When the clot begins to form, the blood 

 changes from a liquid to a red jelly ; later the jelly con- 

 tracts and forces out a yellow liquid. This liquid is 

 the serum. It represents the blood with the fibrin and 

 the red corpuscles removed. Under the influence of heat 

 it also is able to coagulate because of the presence in it 

 of proteids other than fibrin. Out of serum from the 

 blood of horses is made the remedy used so wonder- 

 fully in fighting diphtheria. 



A glance at the following table will show at once the 

 relation of coagulated blood to the substances present in 

 liquid blood. 



( water 

 ( serum •< I 



Coagulated blood \ ) pr ° teid9 > P ' aSma 



fibrin 

 clot 



corpuscles 



I 



liquid blood 



