THE BLOOD. 



65 



indeed, that the quantity of blood obtainable from a fasting animal barely 

 exceeds a half of that which is present soon after a full meal. 



Coagulation of the Blood. One of the most characteristic proper- 

 ties which the blood possesses is that of clotting or coagulating, when 

 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; 

 this change next taking place, in a minute or two, at the sides of the 

 vessel in which it is contained, and then extending 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 volume as the 

 previously liquid blood, and adheres so closely to the sides of the contain- 



FIG. 67. Reticulum of fibrin, from a drop of human blood, after treatment with rosanilin. 



(Ranvier.) 



ing 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 min- 

 utes, drops of a light straw-colored fluid, the serum, may be seen to make 

 their appearance on the surface, and, as they become more and more nu- 

 merous, 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 meshwork (Fig. 67) of fine fibrils. This 

 meshwork entangles and encloses within it the blood corpuscles, as clot- 

 ting 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 con- 

 VOL. I. 5. 



