2 CIRCULATION OF THE BLOOD 



this system of arteries the blood can move in only one 

 direction, always away from the heart. 



At length, however, the arteries that we are tracing 

 cease to get smaller. Instead of branching and getting 

 smaller they now remain the same size and unite into a 

 network of minute tubes. Nearly every part of the body 

 is traversed by this network of tubes containing blood. 

 They are known as the capillary blood-vessels. 



The pressure of the blood in the arteries rises with 

 each beat of the heart. As the pressure is raised, the 

 blood, as we have seen, is forced from the heart into 

 the aorta and onward into the other arteries. The blood 

 moves along them and escapes into the capillaries. The 

 effect of this escape is that the pressure in the arteries 

 falls, until it is raised again by the next beat of the heart 

 -and so on. If, therefore, an artery is cut or opened, 

 the blood will escape from it in spurts, each spurt corre- 

 sponding to a beat of the heart or a pulsation of the artery. 



In the capillaries, owing to the extent of their network, 

 owing to the great amount of friction due to the minute- 

 ness of the vessels, and owing also to certain other factors 

 that we need not discuss here, the pressure of the blood 

 is much more steady than in the arteries, and does not, 

 as in the case of the arteries, increase and decrease in 

 spurts. The general movement of the blood in the 

 capillaries nevertheless continues. If you prick or cut 

 your finger you will not usually sever an artery, but only 

 capillaries. The result is that the blood flows from the 

 prick with a steady ooze, and does not come out in jerks 

 or spurts as in the case of a cut artery. 



If the movement of the blood, passing always farther 

 and farther from the heart, is traced on and on, we find 

 that the network of minute capillaries at last unites again 

 into larger vessels. These vessels are called veins. The 

 veins in certain respects resemble, and in certain respects 



