CHAPTER XX 



THE ACTION QF THE HEART. THE REGULATION OF THE 



HEART-BEAT 



The Beat of the Heart. It is possible with some little skill and 

 care to open the chest of a living narcotized animal, such as a 

 rabbit, and see its heart at work, alternately contracting and re- 

 laxing. As observed under ordinary conditions these phases fol- 

 low one another so rapidly as seemingly to defy analysis. When 

 Harvey, the discoverer of the circulation, first looked upon the 

 beating heart of a mammal he was so impressed by the complex- 

 ity and rapidity of its action as to believe for the moment that 

 the human mind could never fathom it. 



By proper treatment the beat of the heart can be much slowed. 

 When this has been done it is observed that each beat commences 

 at the mouths of the great veins; from there runs over the rest 

 of the auricles, and then over the ventricles; the auricles dilating 

 the moment the ventricles commence to contract. Having fin- 

 ished their contraction the ventricles also dilate, and so for some 

 time neither they nor the auricles are contracting, but the whole 

 heart is at rest. The contraction of any part of the heart is 

 known as its systole and the relaxation as its diastole. 



The average heart-rate in man is 72 beats per minute, giving 

 for each beat 0.8 second. The two sides of the heart work syn- 

 chronously, the auricles together and the ventricles together. In 

 describing the " cardiac cycle," therefore, the auricles are treated 

 as one organ and the ventricles as one. The auricular systole 

 occupies about 0.1 second, its diastole lasts 0.7 second. The 

 ventricular systole begins at the end of the auricular contraction; 

 it occupies about 0.3 second; the diastole of the ventricle lasts 

 about 0.5 second. During fully half of each cardiac cycle, then, 

 there is no muscular activity going on in any part of the heart. 

 During diastole the heart if taken between the finger and thumb 

 feels soft and flabby, but during systole it (especially its ventric- 

 ular portion) becomes hard and rigid. 



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