84 THE BLOOD 



are unknown. It is indispensable to the coagulation of the 

 blood. 



The extractives of the plasma include fats, sugar, urea, 

 lecithin, cholesterin, and gases. The inorganic salts are peculiar 

 in their distribution, inasmuch as the plasma contains an excess 

 of sodium salts, while the corpuscles contain an excess of potas- 

 sium salts. 



Coagulation or Clotting. After the blood has escaped from 

 the vessels of the body it exhibits its most peculiar property 

 that of clotting. If the blood is caught in a beaker, it is at 

 first perfectly fluid, but soon becomes viscous and sets into 

 a jelly. As the clot shrinks in size it presses out a clear, faint 

 yellow liquid called blood serum, which increases in quantity 

 until at the end of about an hour it is sufficient in amount 

 to float the clot. The latter becomes separated from the sides 

 of the vessel. The appearance of the clot or crassamentum is 

 due to the formation in the plasma of fine fibrils which extend 

 in every direction and which gradually contract and enclose in 

 their meshes the various corpuscles. The process may be 

 indicated by diagram as follows : 



Blood. 



__L_____ 



Plasma. Corpuscles 



Serum. Fibrin. 



Clot. 



I 



Clotted blood 



When coagulation has been retarded for some time, the 

 red corpuscles have time to sink from the surface, producing 

 after coagulation a yellow layer which is known as the buffy 

 coat. Many leukocytes, owing to their ameboid movements, 

 escape from the meshes of the clot. If the blood, while it is 

 coagulating, is agitated with a bundle of rods, the fibrin is 

 removed as quickly as it is formed, and appears as a stringy 

 white mass on the rods. After this the blood appears normal, 



