iv THE BLOOD : FOKMED CONSTITUENTS 97 



The blood has the highly important property of coagulating 

 spontaneously. In a few moments (3-12 minutes for human 

 blood) after it has been taken from the blood-vessels it is trans- 

 formed into a gelatinous mass, which assumes the shape of the 

 vessel that receives it. It is the formation of this clot which 

 checks the continuation of haemorrhage in small injured vessels 

 which would otherwise lead to the death of the animal. Clotting- 

 depends on the formation and separation of a protein from the 

 plasma, i.e. fibrin (which, as we shall see, does not pre-exist as such) 

 in the form of a fibrillar reticulum of such excessive fineness, that 

 it encloses in its meshes not merely the whole of the corpuscles, 

 but also the entire liquid portion of the blood. This fact appears 

 the more marvellous when we consider that the amount of fibrin 

 formed during coagulation never exceeds 1 per cent of the mass 

 of blood, but is more often represented by a fraction, O4 per cent, 

 of this, and may even fall to the minimum of O'l per cent. The 

 separation of the fibrin from the mass of blood can be effected by 

 prolonged washing of the clot (Malpighi, 1666), or by whipping 

 the freshly-extracted blood (liuysch, 1707). In this last case the 

 fibrin clings to the rod used for whipping as a fibrous, elastic, 

 whitish mass ; and blood thus defibrinated is incapable of clotting. 



From the clot containing the whole mass of blood a yellowish 

 fluid gradually separates out in consequence of the physical 

 retraction of the fibrous reticulum, the so-called serum, which 

 represents that part of the plasma that remains liquid after 

 coagulation. When all or nearly all this serum has separated out 

 from the clot, the latter is seen to be considerably diminished in 

 volume, though it still keeps the form of the vevssel. The clot 

 thus reduced by the separation of the plasma is sometimes termed 

 the crassamentum. 



In blood which has been rendered incoagulable by defibrina- 

 tion, the red corpuscles, being heavier than the serum, tend to fall 

 to the bottom of the vessel, so that an almost transparent upper 

 layer is formed by degrees, consisting principally of serum, with 

 an opaque lower layer formed almost exclusively of the mass of 

 corpuscles. The separation of the serum from the corpuscles is 

 effected with maximal speed and perfection by the Centrifuge, 

 which can be performed with the elegant little model represented 

 in Fig. 25. 



If coagulation is delayed in blood newly drawn from the 

 veins (as is often observed in human blood during inflammatory 

 diseases, and normally in horses' blood) there is again a partial 

 separation of the plasma from the red corpuscles, and the clot 

 subsequently formed presents a greyish superficial layer of greater 

 or less density, known as the buffy coat, or crusta phlogistica, which 

 consists of coagulated plasma mixed with leucocytes, without any 

 red corpuscles. 



VOL. I H 



