CH. XXVI.] 



BLOOD COAGULATION. 



409 



none of its contents escape. The solid mass is the crassamentum, 

 or dot. If the clot is watched for a few minutes, drops of a 

 light straw-coloured fluid, the serum, may be seen to make their 

 appearance on the surface, and, as they become more and more 

 numerous, to run together, forming a complete superficial 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 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 it of a sub- 

 stance called fibrin, which appears as a meshwork (fig. 340) of 



Fig. 340. Reticulum of fibrin, from a drop of human blood, after treatment with 

 rosanilin. The entangled corpuscles are not seen. (Ranvier.) 



fine fibrils. This meshwork entangles and encloses within itself 

 the blood corpuscles. The first clot formed, therefore, includes 

 the whole of the constituents of the blood in an apparently solid 

 mass, but soon the fibrinous meshwork begins to contract, and the 

 serum which does not belong to the clot is squeezed out. When 

 the whole of the serum has transuded the clot is found to be 

 smaller, but firmer and harder, as it is now made up chiefly of 

 fibrin and blood corpuscles. Thus coagulation re-arranges the 

 constituents of the blood ; liquid blood is made up of plasma 

 and blood corpuscles, and clotted blood of serum and clot. 



Fibrin is formed from the plasma, and may be obtained free 

 from corpuscles when blood-plasma is allowed to clot, the corpuscles 

 having previously been removed. It may be also obtained from 

 blood by whipping it with a bunch of twigs ; the fibrin adheres 



