412 



THE BLOOD 



[CII. XXVI. 



Coagulation of the Blood. 



One of the most characteristic properties which the blood pos- 

 sesses is that of clotting or coagulating. This phenomenon may be 

 observed under the most favourable 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-likt, 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 

 occupied in these changes is about eight or nine minutes. The solid 

 mass is of exactly the same volume as the previously liquid blood, 

 and adheres so closely to the sides of the containing vessel that if the 



FIG. 844. Beticulum of fibrin, from a drop of human blood, after treatment with rosanilin. The 

 entangled corpuscles are not seen. (Ranvier.) 



latter is inverted none of its contents escape. The solid mass is the 

 crassamentum, or clot. 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 mesh work (fig. 344) of fine 

 fibrils. This mesh work entangles and encloses within itself the blood 

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



