84 EATE OF BLOOD-FLOW. 



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 ven- 

 tricle 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 an 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, we would say that the sum of the area 

 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 capil- 

 laries fed by the aorta is several hundred times that of the 

 aorta itself. 



If we walk along a stream we see that the channel varies 

 considerably in width and depth. Where the channel is large, 

 whether from increased width or depth, there the current is 

 slower; but wherever the channel is reduced, the current is- 

 more rapid. So the stream in the relatively narrow artery 

 is swift. In the capillaries, although any individual chan- 

 nel is small, these channels all together are wide; the result 

 is the same whether a river widens out into a single lake, or 

 divides into a great number of channels running past innu- 

 merable islands. All the tissues of the body may be regarded 

 as so many islands lying between the capillary streams. 



