THE BLOOD. 



231 



coagulum becoming dislodged. If a portion of such a jelly-like mass be 

 examined microscopically, it will be found to be penetrated in all directions 

 by a felt- work of extremely fine delicate fibrils, which, having made their 

 appearance before the corpuscles have had time to settle to the bottom of 

 the fluid, have entangled them in the meshes so that the entire mass 

 retains its characteristic color. These fibrils are collectively known as fibrin 

 (Fig. 96). 



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 thick- 

 ness. Within a few hours the blood-mass detaches itself from the sides of 

 the vessel in consequence of the retraction of the fibrils, while at the same 



FIG. 96. DIAGRAM TO ILLUSTRATE THE PROCESS or COAGULATION, i. Fresh blood, plasma 

 and corpuscles. 2. Coagulating blood (birth of fibrin). 3. Coaglulated blood (clot and serum). 

 (Waller.) 



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 red mass, termed clot, and a 

 clear fluid, termed serum. The clot consists of the fibrin containing in its 

 meshes the red and white corpuscles; the serum consists of all the constitu- 

 ents of the plasma except the antecedents of the fibrin. The stages of coagu- 

 lation are shown in Fig. 96. 



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

 specific gravity, subside to the bottom of thebloodmass, 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, pro- 

 ducing the so-called bu/y 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 sulphate has been 

 added to prevent coagulation, or from horse's blood which has been allowed 

 to flow into a tall vessel surrounded by a cooling mixture so as to prevent 

 coagulation and thus permit the red corpuscles to subside. If such plasma 



