168 



THE CIRCULATION OF THE BLOOD 



opening and closing of the semilunar valves has been determined. By 

 similar methods we have been able to fix synchronism between other events 

 occurring during the beat. These we will summarize in the following section. 

 Cardiac Cycle. The entire series of occurrences in a single heart- 

 beat is called the Cardiac Cycle. If the condition of the heart is considered 

 at that moment when its muscular walls are at rest it will be found that the 

 auriculo-ventricular valves are open, that the blood is flowing from the great 



PAUSE 



DIASTOLE 



AURICLE 



VENTRICLE 



IMPULSE 



FIG. 167. Diagrammatic Representation of the Events of the Cardiac Cycle. For events 

 which occur in sequence, read in the direction of the curved arrow; for synchronous events, read 

 from the center to the periphery in any direction. (Coleman.) 



veins into the auricle and ventricle, which form a continuous cavity, and 

 that the pressure is about that of the atmosphere, but slowly rising. Now a 

 wave of contraction begins on the great veins and extends toward the auri- 

 cles, which immediately contract and discharge their blood into the ventri- 

 cles, somewhat distending their walls. At this moment the ventricular 

 systole begins, the tricuspid (and mitral) valves are closed, the flow of blood 

 into the ventricles is checked, and the first heart sound is heard. The con- 

 traction of the ventricles produces a rapidly rising pressure on the enclosed 



