CIRCULATION.] PHYSIOLOGY. 79 



go on until the moment comes when friends gathering 

 round your bedside will say that you are " gone/' 



34. But how does this beat of the heart 

 make the blood move ? Let us see. 



Remember that you have, or when you are grown 

 up will have, bottled up in the closed blood-vessels of 

 your body about 1 2 lbs. of blood. You have seen 

 that the heart and the blood-vessels form a system of 

 closed tubes; the walls are in some places, in the 

 capillaries for instance, very thin, but they are sound 

 and whole — and though the road is quite open from 

 the capillaries through the veins, heart, and arteries to 

 the capillaries again, there is no way out of the tubes 

 except by making a breach somewhere in the walls. 



This closed system of heart and tubes is pretty well 

 filled by the 1 2 lbs. of blood. 



What then must happen each time the heart con- 

 tracts? 



Let us begin with the right ventricle. Suppose it is 

 full of blood. It contracts. The blood in it, squeezed 

 on all sides, tries to go back into the right auricle, but 

 the tricuspid flaps have been driven back and block 

 the way. The more the blood presses on them, the 

 tighter they become, and the more completely t^ey 

 shut out all possibility of getting into the auricle. 



The way into the pulmonary artery is open, the 

 blood can go there. But stay, the artery is already 

 full of blood, and so are the capillaries and veins in 

 the lung. Yes, but the artery will stretch ever so 

 much. Take a piece of pulmonary artery, ^>tA 

 having tied one end, pump or pour water into the 

 other ; you will see how much it will stretch. Into 

 the pulmonary artery, then, goes the blood, stretching 



