PHYSIOLOGICAL PARADOXES. 



prevent the blood flowing back into the 

 veins, some wrinkling and folding of its inner 

 skin as it contracts being sufficient for this 

 purpose. 



But the ventricle, having to drive the blood 

 round the body, at a pressure equal to that of 

 nine feet of water, must exercise great force, 

 and consequently has a strong, thick muscular 

 wall, shown at H and K. 



The most remarkable thing about the whole 

 heart is the valve by which, during the con- 

 traction of the ventricle, the blood which is 

 being forced out into the aorta E, is prevented 

 from going back into the auricle B. 



This valve, F, is called on the left side of the 

 heart the mitral, or bicuspid valve ; the latter 

 name referring to the fact that it consists of 

 two large flaps, which are, in fact, helped out 

 by two small ones. The valve on the right 

 side is very similar, but has three large flaps 

 and is called the tricuspid valve. 



The flaps of the mitral valve are shown in 

 black at F, and it is plain how easily, when they 

 are relaxed, the blood can push them aside 

 and flow into the ventricle. But to allow this 

 flow to take place rapidly, the valve-opening 

 must be a large one. Partly for this reason, and 

 partly because the opening is not a tubular 

 passage with parallel walls, the arrangement of 

 pocket- valves strong enough to resist the great 

 pressure due to the contracting ventricle is not 



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