i88 



THE CIRCULATION OF THE BLOOD 



show, finally, that the minimum pressure in the ventricle may fall below 

 that of the atmosphere, but that the amount varies considerably. 



On the whole, the most satisfactory recording instrument for the measure- 

 ment of endocardiac pressures is the membrane manometer devised by 

 Hiirthle. This instrument avoids mechanical errors in a most satisfactory 

 manner. By simultaneous tracings of the pressure in the ventricle and in 

 the aorta by Hiirthle's differential manometer, the exact moment of the 



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.) 



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

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

 occuring 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 tricuspid 

 and mitral valves are open, that the blood is flowing from the great veins 

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



