CHAP. VIII.] THE VASCULAR SYSTEM. 101 



surface of the jelly. Increasing in number and running together, 

 the drops after a while form a superficial layer of pale straw- 

 coloured fluid. Later on, similar layers of the same fluid are 

 seen at the sides, and finally at the bottom of the jelly, which, 

 shrunk to a smaller size and of firmer consistency, now forms a 

 clot or crassamentum, floating in a liquid. The upper surface 

 of the clot is generally slightly concave. If a portion of the 

 clot be examined under the microscope, it is seen to consist of a 

 network of fine fibrils in the meshes of which are entangled the 

 red and white corpuscles of the blood. The fibrils are composed 

 of the fibrin ; and the liquid in which the clot is suspended is 

 blood minus corpuscles and fibrin, and is called serum. The 

 clotting of the blood is entirely dependent upon the fibrin ; for 

 if fresh blood, before it has time to clot, be whipped with a 

 bundle of twigs, the fibrin will form on the twigs, and if the 

 whipping of the blood be continued until all the fibrin has been 

 deposited on the twigs, the blood left in the vessel will be found 

 to have lost all power of clotting. 



The coagulation of blood is hastened by high temperature, 

 and by contact with any rough surface or non-living material. 

 On the other hand, a low temperature retards, and the addition 

 of salt in sufficient quantity prevents, coagulation. After death, 

 the blood usually remains a long time fluid in the vessels, and 

 it never clots so firmly and completely as when shed. It clots 

 first in the larger vessels, but not until several hours after death 

 in the smaller vessels. 



The coagulability of the blood differs in different individuals, 

 and in rare cases is so slight that the most trivial operation in- 

 volving hemorrhage is attended with great danger. 



The quantity of blood contained in the body is a balance 

 struck between the tissues which give to, and those which take 

 away from, the blood. Thus the tissues of the alimentary canal 

 largely add to the blood water and the material derived from 

 food, while the tissues of the excretory organs largely take 

 away water, urea, and the other substances resulting from the 

 waste of the tissues. From the result of a few observations 

 on executed criminals, it has been concluded that the total 

 quantity of blood in the human body is about T a of the body 

 weight. 



General composition of the blood. Not only do the several tis- 



