CIRCULATION' OF THE BLOOD. 59 



The valves are most numerous in the superficial veins and 

 those of the muscles. The pressure of the muscles during 

 their action (thickening while shortening) produces pres- 

 sure on the veins ; and as the muscles act for a short time 

 only, and then relax, this alternate compression and release 

 aids very considerably in moving the blood on toward the 

 heart. It is worthy of remark that this effect is more 

 pronounced at the time the muscles need the most active 

 circulation ; namely when they are in action, and are using 

 the most blood. The heart has power enough to pump 

 the blood clear around from each ventricle to the auricle 

 of the other side of the heart ; but this outside aid comes 

 in good play to relieve the heart at a time when it has ah 

 unusual amount of work to do, as when one is using a 

 large number of muscles vigorously. 



" Every active muscle is a throbbing heart, squeezing 

 its blood tubes empty while in motion, and relaxing so 

 as to allow them to fill up anew." 



Rate of Blood Flow in the Arteries, Capillaries, and 

 Veins. The blood flows most rapidly in the arteries, 

 slowest in the capillaries. Why is this ? 



When an artery divides, the two branches taken together 

 are larger than the one artery that divided to form them. 

 Stated more exactly, the sum of the areas of the cross- 

 sections of the branches is greater than the area of the 

 cross-section before branching. Hence as the blood flows 

 on it is continually entering wider and wider channels ; 

 and we are told that the united cross-section of all the 

 capillaries fed by the aorta is several hundred times that 

 of the aorta itself. 



The Flow of the Blood compared with the Current of 

 a Stream. If we walk along a stream, we see that the 



