THE CIRCULATION OF THE BLOOD. 273 



THE MECHANICS OF THE HEART. 



The immediate cause of the movement of the blood through the 

 vessels is the contraction and relaxation of the muscle -walls of the 

 heart, and more particularly of the walls of the ventricles, each of 

 which plays alternately the part of a force-pump, and to a slight 

 extent of a suction-pump. The motive power is furnished by the 

 heart itself, by the transformation of potential energy, stored up 

 during the period of rest, into kinetic energy i. e., heat and mechanic 

 motion. 



The contraction of any part of the heart is termed the systole; 

 the relaxation, the diastole. As each side of the heart has two cavities 

 the walls of which contract and relax in succession, it is customary to 

 speak of an^auricular systole and diastole, and a ventricular systole 

 and diastole.' As the two sides of the heart are in the same anatomic 

 relation to eacli other, they contract and relax in the same periods 

 of p'me. 



The movements of the heart, as well as many phenomena con- 

 nected with the flow of blood through its cavities, have been deter- 

 mined by observation of, and experiment on, the exposed heart of a 

 mammal, e. g., dog, cat, rabbit, supplemented and corrected by 

 experiments on the heart in its normal relations. Valuable informa- 

 tion as to the heart-beat and the influences which modify it has been 

 obtained from experiments made on the isolated heart of the turtle, 

 frog, and allied animals. 



If the thorax of a dog completely anesthetized is opened and 

 artificial respiration established, the heart will be observed in active 

 movement inside the pericardium. If this sac is divided and turned 

 aside, the heart will be fully exposed to view. At the normal rate 

 of movement characteristic of the dog it will be almost impossible 

 to determine either the succession of events or their duration. But 

 by observing the heart under different conditions at different rates of 

 movement and with instrumental aids physiologists have succeeded 

 not only in analyzing the movements, but in describing their sequence 

 and in estimating their time duration. 



Thus it has been determined that the heart presents two distinct 

 movements which alternate with each other in quick succession. One 

 is the movement of contraction, or the systole, by which the blood 

 contained within its cavities is ejected into the arteries pulmonary 

 artery and aorta; the other is the movement of relaxation, or the 

 diastole, followed by a pause during which the cavities again fill up 

 with blood from the venae cavae and pulmonary veins. 



Sequence of Events. It has been ascertained that though occur- 

 ring with extreme rapidity neither the contraction nor the relaxation 

 of either the auricle or the ventricle is a simultaneous process; 

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