212 THE BLOOD 



the color of which varies between yellowish gray and reddish gray. 

 It is composed in the main of fibrin and colorless corpuscles and seems 

 to originate in any blood in consequence of certain peculiarities in 

 its manner of coagulating. The name of "buffy coat" or crusta 

 inflammatoria, has been applied to this region. 



The liquid which separates from the clot in constantly increasing 

 quantity, is known as the serum. While its immediate source is the 

 plasma, it differs from it materially, because it contains no corpuscular 

 nor larger nutritive elements. The separation of the blood into the 

 clot and the serum begins as soon as coagulation sets in, but is not 

 completed as a rule until about 24 hours later. During this time the 



FIGS. 115 AND 116. THE FIBRIN NEEDLES FORMED IN THE CLOTTING OF BLOOD. 

 PLASMA OF OXALATED DOG'S BLOOD CLOTTED BY THROMBIN. THE PHOTOGRAPHS SHOW 

 THE NEEDLES AS SEEN WITH THE ULTRAMICROSCOPE. 



A, photographed by sun-light; B, by arc-light. Only the needles lying in the focal 

 plane are seen distinctly. (Howett.) 

 i 



fibrin shreds contract more and more and squeeze additional amounts 

 of serum out of the clot. If the vessel into which the blood is with- 

 drawn is kept in a cool place and is not disturbed, the serum separates 

 as a clear, straw-colored fluid. It frequently happens, however, that 

 the clot adheres to the walls of the receptacle and is torn, releasing 

 varying numbers of red and white corpuscles. The serum then 

 assumes a reddish color and acquires a specific gravity which is 

 much greater than that of clear serum. 



Chemical Changes in Coagulating Blood. While the final and most 

 important change effected during coagulation is the formation of fibrin, 

 this body cannot be obtained unless several preliminary reactions have 

 first been completed. Indeed, the process of clotting may be divided 



