66 Physiology. 



amount of work to do, as when one is using a large num- 

 ber 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 and Capillaries. The 

 blood flows rapidly in the arteries, slowly 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. Hence as the blood flows on it is continu- 

 ally entering a wider and wider channel ; for if all the cap- 

 illaries fed by the aorta were united they would make a 

 tube several hundred times as large as the aorta. 



The Flow of the Blood compared with the Current of a 

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

 channel keeps changing 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 

 comparatively narrow artery is swift. In the capillaries, 

 although any single channel 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 num- 

 ber of channels running past many islands. 



The Flow of Blood in the Veins. When two veins unite, 

 the one vein they form is not quite equal to the sum of the 

 two ; so when the blood gathers in the veins it is really en- 

 tering a narrower channel, and it flows faster. And it 

 keeps gaining in speed till it reaches the heart. 



Flow in Arteries and Veins compared. Although the 

 blood keeps flowing fast as it gets nearer the heart in the 



