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 not by the auricles proper, but by an area 

 of specialized tissue in the right auricle lying between the open- 

 ings of the two cavse. This portion of the auricular wall corre- 

 sponds physiologically to a definite chamber, the venous sinus, 



Fig. 220. 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 right ventricle; Vent. G, tracing from left ventricle. Obtained from the heart of the 

 horse by means of tubes communicating with the cavities. 



in the heart of the lower vertebrates (see Fig. 224) . The contrac- 

 tion of any part of the heart is designated as its systole, its relaxa- 

 tion 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. 220). It will be 

 noted that the auricular systole is shorter and its diastole longer 

 than the similar conditions in the ventricles. 



The Musculature of the Auricles and Ventricles. Embryo- 



533 



