in THE CLOTTING OF BLOOD 111 



becomes, through clotting, a jelly-like mass, so solid that 

 the basin may be turned upside down without any of the 

 blood being spilt. At first the clot is a uniform red jelly, 

 but very soon drops of a clear yellowish watery-looking 

 fluid make their appearance on the surface of the clot, and 

 between it and the sides of the basin. These drops in- 

 crease in number, and run together, and after a while it 

 has become apparent that the originally uniform jelly has 

 separated into two very different constituents — the one a 

 clear, yellowish liquid ; the other a red, semi-solid, slightly 

 shrunken mass, which lies in the liquid. The liquid 

 exudes from the coloured mass because the latter shrinks 

 and so squeezes it out. 



The liquid is called the serum ; the semi-solid mass 

 the clot. Now the clot obviously contains the cor- 

 puscles of the blood, bound together by some other 

 substance ; and this last, if a small part of the clot be 

 examined microscopically, will be found to be that 

 fibrous-looking matter, fibrin, which has been seen form- 

 ing in the drop of blood. Thus the clot is made up 

 of the corpuscles plus the fibrin of the plasma, while the 

 serum is the plasma minus the fibrinous elements which 

 it contained. 



^ The corpuscles of the blood are slightly heavier than 

 the plasma, and thei-efore, when the blood is drawn, they 

 tend to sink very slowly towards the bottom, but as a 

 rule clotting is complete before the corpuscles have had 

 time to sink appreciably. When, on the other hand, the 

 blood clots slowly, the corpuscles have so much time to 

 sink that the upper .stratum of i)lasma becomes quite free 

 from red corpuscles before the fibrin forms in it ; and, 

 consequently, the uppermost layer of the clot is nearly 

 white : it then receives the name of the buffy coat. This 

 is frequently well seen in the blood of the horse, which often 

 clots slowly, and wliose corpuscle? are unusually heavy. 



If the blood is " whipped " with a bunch of twigs as soon 

 as it is di'awn from the body, fibrin is formed as before. 



