A STUDY OF THE CIRCULATION OF BLOOD 139 



is much thinner than is the case in the walls of an artery 

 (Fig. 51). Veins, however, are provided with valves shaped 

 much like the semilunars of the aorta and pulmonary artery. 

 The blood can flow toward the heart, but as soon as it begins 

 to pass in the opposite direction, these valves are immedi- 

 ately filled and thus the passage is obstructed (Fig. 52). If 

 one ties a cord tightly about the wrist, one can see the veins 

 swell, and small hillocks appear which indicate the position 

 of the valves just described. 



Position of the Capillaries. As we trace the arteries far- 

 ther and farther from the heart, we see that they divide and 

 subdivide until very small branches are formed. That these 

 fine branches are still arteries is proved by the fact that 

 elastic and muscular tissue are present in their walls. Fi- 

 nally, however, these tiny blood vessels become continuous 

 with still smaller tubes, the capillaries. So numerous are 

 the capillaries that one cannot push the point of a needle 

 for any considerable distance into any organ of the body 

 without piercing a number of them. These smallest of 

 blood vessels communicate freely with one another and 

 form a complicated network of tubes that bring blood 

 close to all cells of the body. As the capillaries begin 

 where arteries end, so they end where the smallest veins 

 begin. Throughout the body, therefore, is a continuous 

 system of blood vessels. 



Importance of the Capillaries. If the blood were kept con- 

 stantly within this system of tubes, it would be entirely un- 

 able to help in the nutrition of the body. Each cell must 

 take from the blood the nutrients it needs for its special 

 work; likewise it must give off to the blood the wastes it 

 has formed by oxidation. It is through the thin-walled 

 capillaries tlrnt all these exchanges of materials occur. 

 Hence, the capillaries form the most important portion, of 

 the blood system. We may regard the arteries as the sup- 

 ply pipes for the capillaries, and the veins as the drain 

 pipes from them. 



