14 COAGULATION OF BLOOD. [BOOK i. 



In man, blood when shed becomes viscid in about two or 

 three minutes, and enters the jelly stage in about five or ten 

 minutes. After the lapse of another few minutes the first drops 



Jof serum are seen, and coagulation is generally complete in from 

 one to several hours. The times however will be found to vary 

 according to the condition of the individual, the temperature of 

 the air, and the size and form of the vessel into which the blood 

 is shed. Among animals the rapidity of coagulation varies ex- 

 ceedingly in different species. The blood of the horse coagulates 

 with remarkable slowness ; so slowly indeed that many of the red 

 corpuscles (these being specifically heavier than the plasma) have 

 time to sink before viscidity sets in. In consequence there ap- 

 pears on the surface of the blood an upper layer of colourless 

 plasma, containing in its deeper portions many colourless cor- 

 puscles (which are lighter than the red). This layer clots like 

 the other parts of the blood, forming the so-called 'buffy coat.' 

 A similar buffy coat is sometimes seen in the blood of man, in 

 inflammatory conditions of the body. 



This buffy coat makes its appearance in horse's blood even at 

 the ordinary temperature of the air. If a portion of horse's blood 

 be surrounded by a cooling mixture of ice and salt, and thus kept 

 at about C., coagulation may be almost indefinitely postponed. 

 Under these circumstances a more complete descent of the cor- 

 puscles takes place, and a considerable quantity of colourless 

 transparent plasma free from blood-corpuscles may be obtained. 



portion of this plasma removed from the freezing mixture 

 clots exactly as does the entire blood. It first becomes viscid and 

 then forms a jelly, which subsequently separates into a colourless 

 shrunken clot and serum. This shews that the corpuscles are not 

 an essential part of the clot. 



If a few cubic centimetres of the same plasma be diluted with 

 50 times its bulk of a 0'6 p.c. solution of sodium chloride 1 coagu- 

 lation is much retarded, and the various stages may be more 

 easily watched. As the fluid is becoming viscid, fine fibrils of 

 fibrin will be seen to be developed in it, especially at the sides 

 of the containing vessel. As these fibrils multiply in number, the 

 fluid becomes more and more of the consistence of a jelly ., and at 

 the same time somewhat opaque. Stirred or pulled about with 

 a needle, the fibrils shrink up into a small opaque stringy mass ; 

 and a very considerable bulk of the jelly may by agitation be 

 resolved into a minute fragment of shrunken fibrin floating in a 

 quantity of what is really diluted serum. If a specimen of such 

 diluted plasma be stirred from time to time, as soon as coagulation 

 begins, with a needle or glass rod, the fibrin may be removed 

 piecemeal as it forms, and the jelly stage may be altogether done 

 away with. When fresh blood which has not yet had time to 



1 A solution of sodium chloride of this strength will hereafter be spoken of as 

 ' normal saline solution.' 



