16 PHENOMENA OF CLOTTING. [BOOK i. 



to several hours. The times however will be found to vary accord- 

 ing to circumstances. Among animals the rapidity of clotting 

 varies exceedingly in different species. The blood of the horse 

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

 red and also some of the white corpuscles (both these being speci- 

 fically heavier than the plasma) have time to sink before viscidity 

 sets in. In consequence there appears on the surface of the blood 

 an upper layer of colourless plasma, containing in its deeper por- 

 tions many colourless corpuscles (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 certain abnormal conditions of the body. 



If a portion of horse's blood be surrounded by a cooling 

 mixture of ice and salt, and thus kept at about C., clotting 

 may be almost indefinitely postponed. Under these circumstances 

 a more complete descent of the corpuscles takes place, and a 

 considerable quantity of colourless transparent plasma free from 

 blood-corpuscles may be obtained. A portion of this plasma 

 removed from the freezing mixture clots in the same manner 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 this colourless plasma, or of a 

 similar plasma which may be obtained from almost any blood by 

 means which we will presently describe, be diluted with many 

 times its bulk of a 0'6 p.c. solution of sodium chloride 1 clotting 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 clotting 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 clot is stirred or 

 whipped with a bundle of rods (or anything presenting a large 

 amount of rough surface), no jelly-like clotting takes place, but 

 the rods become covered with a mass of shrunken fibrin. Blood 

 thus whipped until fibrin ceases to be deposited, is found to have 

 entirely lost its power of clotting. 



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

 ' normal saline solution.' 



