PHYSIOLOGICAL PARADOXES. 



itself by squeezing itself together till its opposite 

 sides touch and there is no space left to hold 

 any blood. 



It is a duplicated pump, the work of the 

 left side being to drive the blood, when aerated, 

 all over the body, that of the right side to 

 drive it through the lungs to be aerated again. 

 Each side is similar in the principle of its action 

 to the other, so it will be sufficient to explain 

 that principle on one side, say the left. 



The left side of the heart, then, contains two 

 chambers, one the ventricle, the strong mus- 

 cular chamber whose powerful contractions 

 drive the blood all through the body ; the 

 other, the auricle, not so strong, and used 

 merely as a collecting chamber. If the heart, 

 after each contraction, had to be refilled from 

 the veins directly, this would take a long time, 

 which is saved by having the auricle, or collect- 

 ing chamber, B (Fig. 63), which receives blood 

 from the veins C, D, during the time when the 

 ventricle A is expelling an earlier charge into 

 the large artery E, called the aorta. (All the 

 parts in this illustration are much simplified for 

 diagrammatic purposes.) 



By the time that the auricle B is filled, the 

 ventricle A has squeezed out all the previous 

 charge of blood into the aorta E, from which 

 it is prevented from returning by the valve G. 

 This valve consists of flaps forming cups or 

 pouches with the wall of the blood-vessel, 



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