COAGULATION OF THE BLOOD 103 



ficial stratum above the solid clot. At the same time the fluid begins to 

 transude at the sides and at the under surface of the clot, which in the course 

 of an hour or two floats in the liquid. The appearance of the serum is due 

 to the fact that the clot contracts, thus squeezing the fluid out of its mass. 

 The first drops of serum appear on the surface about eleven or twelve minutes 

 after the blood has been drawn; and the fluid continues to transude for from 

 thirty-six to forty-eight hours. 



The clotting of blood is due to the development in the plasma of an in- 

 soluble substance called -fibrin. This fibrin forms threads or strands through 

 the mass in every direction. The strands adhere to each other wherever 

 they come in contact, thus forming a very dense tangle and meshwork which 

 incloses within itself the blood-corpuscles. The clot when first formed, 

 therefore, includes the whole of the blood in an apparently solid mass, but 

 soon the fibrinous meshwork begins to contract and the serum is squeezed 

 out. When a large part of the serum has been squeezed out the clot is found 

 to be smaller, but firmer and harder, as it is now made up largely of fibrin 

 and blood-corpuscles. Thus in coagulation there is a rearrangement of the 

 constituents of the blood; liquid blood being made up of plasma and blood- 

 corpuscles, and clotted blood of serum and clot. 



Liquid Blood. 



Plasma. Corpuscles. 



Serum. Fibrin. 



I 



Clot. 



Clotted Blood. 



The rapidity with which coagulation takes place varies greatly in different 

 animals and at different times in the same animal. Where coagulation is 

 very slow the red corpuscles, which are somewhat heavier than plasma, 

 often have time to settle considerably before the fibrin is formed. If the 

 blood is rapidly cooled to something approaching o C. then the clot is very 

 greatly delayed. Horse's blood is particularly favorable for demonstrating 

 this point. In it clotting occurs so slowly that very often the red corpuscles 

 will completely settle out, and when the blood is again warmed and the clotting 

 takes place there is a superficial stratum differing in appearance from the 

 rest of the clot, and is of a grayish-yellow color. This is known as the buffy 

 coat or crusla phlogistica. The buffy coat, produced in the manner just 

 described, commonly contracts more than the rest of the clot, on account of 

 the absence of colored corpuscles from its meshes. When the clot is allowed 

 to stand the white corpuscles migrate to the surface by ameboid movement, 



