CHAPTER XXVIII. 

 THE HEART BEAT, 



General Statement. We divide the heart into four chambers, 

 the two auricles and the two ventricles. What we designate as a 

 heart beat begins with the simultaneous contraction of the two 

 auricles, immediately followed by the simultaneous contraction of 

 the two ventricles; then there is a pause, during which the whole 

 heart is at rest and is filling with blood. As a matter of fact, the 

 heart beat is initiated, according to usual belief, not by the auricles 

 proper, but by a contraction of the mouths of the large veins 

 venae cavse where they open into the auricle. This portion of these 

 veins corresponds physiologically to u definite chamber, the venous 



Fig. 207. To show the time relations of the auricular systole and diastole, and ven- 

 tricular systole and diastole (Marey) : Or. D, Tracing from right auricle ; Vent. D, tracing 

 from nght ventricle ; Vent. G, tracing from left ventricle. Obtained from the heart of the 

 horse by means of tubes communicating with the cavities. 



sinus, in the heart of the lower vertebrates. In the description of 

 the heart beat the contraction of the veins is usually neglected, 

 although in a fundamental consideration of the cause of the normal 

 sequence it is of great importance. The contraction of any part of 

 the heart is designated as its systole, its relaxation and period of rest 

 as its diastole. In the heart beat we have, therefore, the auricular 

 systole, the ventricular systole, and the heart pause during which 

 both chambers are in diastole. The general relations of systole, 

 diastole, and pause are represented graphically in the accompanying 

 figure (Fig. 207). It will be noted that the auricular systole is 



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