CHAP. VIL] THE VASCULAR SYSTEM. 79 



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 remains a long time fluid, 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 excretory organs largely take away water and 

 the other substances constituting the excretions. 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 Jg- of the body weight. 



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

 sues take up from the blood and give up to the blood different 

 things at different rates and at different times, but all the 

 tissues take up oxygen and give up carbonic acid in varying 

 quantities. From this it follows, on the one hand, that the 

 composition and character of the blood must be forever varying 

 in different parts of the body ; and, on the other hand, that the 

 united action of all the tissues must tend to establish and main- 

 tain an average uniform composition of the whole mass of blood. 



