THE CIRCULATION 



6l 



Fig. 62. — Blood Clot 

 separated from serum. 



Some impurities and a large amount of water escape 

 from the blood as it passes through the skin. 



Coagulation. — So long as blood is in an uninjured blood 

 vessel it remains a liquid. In a few minutes after it flows 

 from a blood vessel, it forms into a 

 stiff, jelly like mass called a clot (Fig. 

 62). The process of forming the clot 

 is called coagulation, and it is brought 

 about by the albuminous substance 

 called fibrin, which is always in the 

 plasma of healthy blood. On expos- 

 ure to air the fibrin forms into a net- 

 work of fine threads throughout the 

 mass (Fig. 63) and the corpuscles become entangled in the 

 meshes. The clot consists of the fibrin of the plasma and 

 corpuscles ; the watery portion of the plasma, called the 

 serum, separates from the clot (Fig. 62). The property of 



coagulating is a great safe- 

 guard, as a clot often plugs 

 up a cut blood vessel. What 

 is the difference between se- 

 rum and plasma? 



Veins and Arteries com- 

 pared. — The veins have thin, 

 soft zvalls and the arteries 

 have thick, tough, elastic zvalls. 

 When a vein is cut, it may 

 usually be closed by pinching 

 the walls of the end together. 

 If an artery is cut, the walls zvill not readily stick together, 

 but often stand open until the end of the artery is tied. 

 For this reason, and because an artery is subject to the 

 direct pressure of the heart, a cut artery is more dangerous 



Fig. 63. — Network of Fibrin in 

 Human Blood (enlarged). 



