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TEXT-BOOK OF PHYSIOLOGY. 



so that the entire mass retains its characteristic color. These fibrils 

 are collectively known as fibrin (Fig. 95). 



If the coagulated blood be allowed to remain undisturbed, a 

 clear, transparent, slightly yellowish fluid makes its appearance on 

 the surface of the mass, which as it accumulates forms a layer of 

 varying degrees of thickness. Within a few hours the blood-mass 

 detaches itself from the sides of the vessel in consequence of the re- 

 traction of the fibrils, while at the same time the clear fluid increases 

 in amount and accumulates along the sides and bottom of the vessel. 

 The shrinkage in the volume of the red coagulum and the increase 

 of the volume of the clear fluid which is expressed from its meshes 

 continue for a period varying from ten to fifteen hours, according 

 to certain external conditions. The blood has now become separated 

 into two distinct portions: viz., a solid contracted rpr| 



, and a clear fluid, termed serum. The clot consists of the fibrin 

 containing in its meshes the red and white corpuscles; the serum 



FIG. 95. DIAGRAM TO ILLUSTRATE THE PROCESS OF COAGULATION, i. Fresh blood, 

 plasma, and corpuscles. 2. Coagulating blood (birth of fibrin). 3. Coagulated 

 blood (clot and serum). (Waller.) 



consists of all the constituents of the plasma except the antecedents 

 of the fibrin. The stages of coagulation are shown in Fig. 95. 



s If the blood coagulates slowly the red corpuscles, owing to their 

 greater specific gravity, subside to the bottom of the blood-mass, 

 giving to it a deeper color; the white corpuscles, owing to their lesser 

 specific gravity, remain near the surface of the clot and give to it a 

 more or less whitish appearance, producing the so-called buffy coat. 

 In certain inflammatory conditions the coagulating power of the blood 

 is much diminished, and the corpuscles, having time to subside, a well- 

 developed buffy coat is- formed which at one time had much interest 

 for^pathologists. As the contraction of the fibrin takes place more 

 actively in the center, there being here less resistance than at the 

 sides of the coagulum, the upper surface usually becomes depressed 

 or cupped. 



Coagulation of Plasma. Clear plasma may be obtained by 

 means of the centrifuge from blood to which sufficient .magnesium 

 has been added to prevent coagulation, or fforrrttfrse*? 



