40 THE BLOOD 



to one cubic millimeter of blood. As to their function little 

 is known. Some claim they play an important part in the 

 coagulation of the blood. Nothing definite is known of their 

 origin. 



The Coagulation of the Blood. 



When blood is allowed to stand after being shed it rapidly 

 becomes more viscous and later sets into a firm jelly. 

 Later, as the fibrin contracts, a clear straw colored fluid, the 

 serum, is set free. The formation of fibrin is the essential 

 factor in coagulation. It is contained in the plasma in the 

 form of fibrinogen. 



The relation of plasma, serum and clot is shown by the fol- 

 lowing table : 



Plasma 



Blood \ dot 



Corpuscles 



