||j|!| 80 SCIENCE PRIMERS. [§ v. 



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i it in order to find room. As the ventricle squeezes 



i and squeezes, until its walls meet in the middle, 



8 all the blood that was in it finds its way out into the 



\ artery. But the beat of the ventricle soon ceases, 



^ the squeeze is over and gone, and back tumbles the 



I blood into the ventricle, or would tumble, only the 



^ first few drops that shoot backwards are caught by 



\ the watch-pocket semilunar valves. Back fly these 



valves with a sharp click (for the things of which we 



are speaking happen in a fraction of a second), and 



t all further return is cut off. The blood has been 



\ squeezed out of the ventricle, and is safely lodged 



in the pulmonary artery. ' ^^ » 



But the pulmonary artery is ever so much on the 



stretch. It was fairly full before it received this fresh 



% lot of blood ; now it is over-full — at least that part of 



it which is nearest to the heart is over-full. What 



happens next? What happens when you stretch a 



\ piece of india-rubber and then let it go ? It returns 



\- to its former size. The ventricle has stretched the 



piece of pulmonary artery near it, beyond the natural 



size, and then (when it ceased to contract) has let it 



Igo. Accordingly the piece of pulmonary artery tries 

 to Return to its former size, and since it cannot send 

 i the blood back to the ventricle, squeezes it on to the 

 i next piece of the artery nearer the capillaries, stretch- 

 I ing that in turn. 



1 This again in turn sends it on the next piece — and 

 I so on right to the capillaries. The over-full pulmonary 

 \ artery, stretched to hold more than it fairly can, 

 i empties itself through the capillaries into the pul- 

 1 monary veins until it is not more than comfortably full. 

 1 But the pulmonary veins also are already full, — what 



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