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CIRCULATION.] PHYSIOLOGY. 8i 



are they to do ? To empty the surplus into the left 

 auricle. Oftener than every second there will come a 

 time when they can do so. 



For at the same time that the right ventricle 

 pumped a quantity of blood into the pulmonary artery 

 and safely lodged it there, the left ventricle pumped a 

 like quantity into the aorta, safely lodged it there, and 

 was left empty itself. But just at that moment the 

 left auricle began to contract and to squeeze the blood 

 that was in it. 



Where could that blood go ? It could not go back 

 into the pulmonary veins, for they were already full, 

 and the blood in them was being pressed behind by the 

 over-full pulmonary arteries. But it could pass easily 

 into the empty ventricle — and in it tumbled, the mitral 

 flaps readily flying back and opening up a wide way. 

 And so the auricle emptied itself into the ventricle. 

 But now the auricle ceases to contract — its walls no 

 longer squeeze — it is empty and wants filling, and so 

 comes the moment when the pulmonary veins can 

 pour into it the blood which has been driven into 

 them by the over-full pulmonary artery. 



Thus the right ventricle drives the blood into tl^ 

 over-full pulmonary artery, the pulmonary artery over- 

 flows into the pulmonary veins, the pulmonary veins 

 carry the surplus to the empty left auricle, the left 

 auricle presses it into the empty left ventricle, the left 

 ventricle pumps it into the aorta — (the stretching of the 

 aorta and of its branches is what we call the pulse) — • 

 the over-full aorta overflows just as did the pulmonary 

 artery, through the capillaries of the body into the 

 great venae cavae — through these the blood falls into 

 the empty right auricle, the right auricle drives it into 



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