226 THE BODY AT WORK 



through the auricles into the ventricles. In a sense, the auricles 

 are not necessary parts of the double pump. They collect 

 blood while the ventricle is contracting, thus preventing it 

 from heading up in the veins. They save time. Their con- 

 traction completes the filling of the ventricle, so that the 

 instant the ventricular contraction begins blood enters the 

 aorta and pulmonary artery. 



The Valves. If ever expressions of admiration were appro- 

 priate in a treatise on the animal body, such preface might be 

 permitted to a description of the cardiac valves. Which means 

 no more than this : Men make pumps. Therefore they are in 

 a position to appreciate the mechanism of the heart. We 

 cannot admire what we do not understand. If we made secret- 

 ing organs or self-contracting springs, glands and muscles 

 would evoke our commendation. We should recognize that 

 Nature's apparatus is even better adapted to its work than 

 any that men can make. This is the admission which is forced 

 from us when we study the heart. 



The apertures connecting auricles and ventricles are ex- 

 tremely wide, allowing the contents of the former to be emptied 

 into the latter almost instantaneously. If we attempted to 

 make a pump fulfilling this condition, we should find that it 

 failed in several respects. In the first place, the rush of fluid 

 from the one chamber into the other would press the flaps of 

 the valves back against the wall of the second chamber. They 

 would cling to the wall, and would not float up quickly into 

 place when the second chamber was squeezed. Let us call the 

 two chambers A and V for brevity's sake. When V contracted, 

 some of the fluid would be thrown back into A, because, the 

 resistance in that direction being lower than the resistance 

 offered by the column of fluid above the pump (the resistance 

 in the aorta is very high), the contents of V would rush past the 

 margins of the A-V valve. This would happen even though its 

 flaps were not pressed back against the wall. Further, at 

 the height of contraction the membranous valve would bulge 

 backwards into A, making a cup towards V which V could 

 not empty. In the heart these difficulties have been overcome. 



The tricuspid valve, which separates the right auricle from 

 the right ventricle, has three flaps. The mitral valve, on the 

 left side of the heart, has but two. The flaps are composed of 



