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the most part consist of capillary tubes through which distilled water and 

 blood are caused to flow under the influence of a constant positive or nega- 

 tive pressure. The distance the water flows in a unit of time, compared 

 with that of blood, represents the degree of viscosity. Among these ap- 

 paratus those of Hess and Determann are generally employed, descriptions 

 of which will be found in works on clinical methods. 



8. Coagulability. Within a few minutes after the blood is withdrawn 

 from the vessels of a living animal it begins to lose its fluidity, becomes some- 

 what viscid, and if left undisturbed passes rapidly into a semisolid or jelly- 

 like state. To this change in the physical condition of the blood the term 

 coagulation has been applied. The blood, during the progress of coagula- 

 tion, not only assumes the shape of the vessel in which it is contained, but 

 becomes so firmly adherent to its walls that it may be inverted without the 

 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. 90). 



FIG. 90. DIAGRAM TO ILLUSTRATE THE PROCESS or COAGULATION, i. Fresh blood, plasma 

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

 (Waller.) 



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 

 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. 90. 



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, pro- 

 ducing the so-called buffy coat. In certain inflammatory conditions the 



