THE BLOOD. 



59 



removed from the body. This phenomenon may be observed under the 

 most favorable conditions in blood which has been drawn into an open 

 vessel. In about two or three minutes, at the ordinary temperature of 

 the air, the surface of the fluid is seen to become semi-solid or jelly-like, 

 and this change takes place, in a minute or two afterwards, at the sides 

 of the vessel in which it is contained, and then extends throughout the 

 entire mass. 



The time which is required for the blood to become solid is about 

 eight or nine minutes. The solid mass occupies exactly the same vol- 

 ume as the previously liquid blood, and adheres so closely to the sides 

 of the containing vessel that if it be inverted none of its contents escape. 

 The solid mass is the crassamentum or clot. If the clot be watched for 

 a few minutes, drops of a light, straw-colored fluid, the serum, may be 

 seen to make their appearan-ce on the surface and, as they become more 



FIG. 65. Reticulum of fibrin, from a drop of human blood, after treatment with rosaniluu 

 (Ranvier. > 



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 substance 

 called fibrin, which appears as a mesh work (Fig. 65) of fine fibrils. 

 This meshwork entangles and incloses within it the blood-corpuscles, as 

 clotting takes place too quickly to allow them to sink to the bottom of 

 the plasma. The first clot formed, therefore, includes the whole of the 

 constituents of the blood in an apparently solid mass, but soon the fibrin- 

 ous meshwork begins to contract, and the serum which does not belong 



