226 THE BLOOD. 



so that it will not run over the edge of the vessel, when slightly 

 inclined ; and its surface may be gently depressed with the end of 

 the finger or a glass rod. It then becomes rapidly thicker, and at 

 last solidifies into a uniformly red, opaque, consistent, gelatinous 

 mass, which takes the form of the vessel in which the blood was 

 received. Its coagulation is then complete. The process usually 

 commences, in the case of the human subject, in about fifteen min- 

 utes after the blood has been drawn, and is completed in about 

 twenty minutes. 



The coagulation of the blood is dependent entirely upon the 

 presence of the fibrin. This fact has been demonstrated in various 

 ways. In the first place, if frog's blood be filtered, so as to separate 

 the globules and leave them upon the filter, while the plasma is 

 allowed to run through, the colorless filtered fluid which contains 

 the fibrin soon coagulates ; while no coagulation takes place in the 

 moist globules remaining on the filter. Again, if the freshly drawn 

 blood be stirred with a bundle of rods, as we have already de- 

 scribed above, the fibrin coagulates upon them by itself, while the 

 rest of the plasma, mixed with the globules, remains perfectly fluid. 

 It is the fibrin, therefore, which, by its own coagulation, induces 

 the solidification of the entire blood. As the fibrin is uniformly 

 distributed throughout the blood, when its coagulation takes place 

 the minute filaments which make their appearance in it entangle 

 in their meshes the globules and the albuminous fluids of the 

 plasma. A very small quantity of fibrin, therefore, is sufficient to 

 entangle by its coagulation all the fluid and semi-fluid parts of the 

 blood, and convert the whole into a volumi- 

 Fig. 63. nous, trembling, jelly-like mass, which is 



known by the name of the "erassamenturn," 

 or "clot." (Fig. 63.) 



As soon as the clot has fairly formed, it 

 begins to contract and diminish in size. Ex- 

 actly how this contraction of the clot is pro- 

 duced, we are unable to say; but it is proba- 

 bly a continuation of the same process by 

 of recently Co AOU- which its solidificationis atfirst accomplished, 

 BLOOD showing the Qr t j t similar to it. As the 



whole mass uniformly sohdi- * 



fied contraction proceeds, the albuminous fluids 



begin to be pressed out from the meshes in 



which they were entangled. A few isolated drops first appear on 

 the surface of the clot. These drops soon increase in size and be- 



