266 SUMMARY OF HEART BEAT. [BOOK i. 



As soon as by the rapidly increasing shortening of the ventricular 

 fibres the pressure within the ventricle becomes greater than 

 that in the pulmonary artery, the semilunar valves open and the 

 still continuing systole discharges the contents of the ventricle 

 into that vessel. 



During the whole of this time the left side has with still 

 greater energy been executing the same manoeuvre. At the same 

 time that the venae cavae are filling the right auricle, the pulmonary 

 veins are filling the left auricle. At the same time that the right 

 auricle is contracting, the left auricle is contracting too. The 

 systole of the left ventricle is synchronous with that of the right 

 ventricle, but executed with greater force ; and the flow of blood 

 is guided on the left side by the mitral and aortic valves in the 

 same way that it is on the right by the tricuspid valves and the 

 valves of the pulmonary artery. 



As the ventricles become filled with blood, and so increased 

 in volume, the apex begins to press steadily on the chest- wall, 

 as may be often seen in the cardiogram, the curve of the 

 cardiac impulse. The fuller distension due to the auricular 

 systole is more obvious in the same curve ; but both these 

 changes are insignificant compared to the effect of the change of 

 form, and of the position of the apex during the ventricular 

 systole, by which the lever of the cardiograph is rapidly and 

 forcibly moved. 



With this systole of the ventricles the first sound is heard. 



We may more conveniently follow the remaining events in the 

 left ventricle. 



The effect of the discharge of the contents of the left ventricle 

 is to raise the pressure at the root of the aorta to nearly the same 

 height as that in the ventricle itself. The ventricular pressure 

 continues for some time, giving rise to the " systolic plateau " of 

 the various cardiac curves. In some cases this pressure soon 

 reaches a maximum, after which it gradually declines, the curve of 

 pressure sloping, with some secondary undulations, gently down- 

 wards. In other cases where there is great resistance to the 

 outflow along the arterial system, the pressure may continue to 

 rise during the whole of the ventricular systole. In both cases 

 the curves of the ventricular pressure and of the aortic pressure 

 are similar. 



Then comes the sudden cessation of contraction, the sudden 

 relaxation of the ventricular fibres. The pressure in the ventricle 

 becomes less than that which it itself has generated in the aorta, 

 and the semilunar valves suddenly close as the blood flows back 

 from the region of high pressure, the aorta, towards the region of 

 low pressure, the ventricle. At this moment the second sound is 

 heard. 



Owing to the semilunar valves being closed, the pressures in 

 the ventricle and in the aorta, which before were following the 



