208 



THE MECHANISM OF THE BEAT. [BOOK i. 



of the reflux is to close the semilunar valves. So soon as these 

 are closed, the pressures in the ventricle and the aorta, which were 

 previously following similar courses, now take separate courses; the 

 latter falls suddenly, the former decreases gradually, and continues 

 to decrease until the next systole once more opens the semilunar 

 valves. We may add that this view is consistent with the conclu- 

 sion mentioned in 114, that not only the pulse-volume may vary, 

 but also, at times at least, the whole contents are not driven out 

 at the systole, some blood remaining behind. 



Moreover, the pressure does not always gradually decline 

 during the systolic plateau ; sometimes it gradually rises during 

 the whole of the period of the plateau, reaching its highest point 

 just before the final sudden fall. This is shewn in Fig. 53. 



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FIG. 53. CURVE OF AORTIC AND VENTRICULAR PRESSURE, WITH AN 



ASCENDING SYSTOLIC PLATEAU. HiJRTHLE. 



In this figure the general features are the same as in Fig. 52, 

 save that the curve of ventricular pressure rises during the whole 

 of the systolic plateau. But the curve of aortic pressure also rises 

 in a corresponding manner, and the curve of difference, if shewn, 

 would be the same as in Fig. 52. The explanation of the difference 

 between the two cases is that in Fig. 52 the peripheral resistance 

 in the arterial flow ( 99) is not very great, and the ventricular 

 systole soon overcomes it to such an extent as to lead at once to 

 some fall of pressure in the aorta (and in the ventricle). In Fig. 

 53 the peripheral resistance is very great ; it is not overcome at 

 first, the ventricle does its best working against it, and produces 

 the most effect, raising the pressure to the highest point, just 

 before its systole comes to an end. We may add that a similar 

 course of the curve may be seen even when the pressure in the 

 aorta is not very high, provided that the pulse-volume, the quantity 

 discharged at the systole is very great; the form of the curve 

 depends on the relative amounts which are entering the arterial 

 system from the heart, and leaving it by the peripheral vessels. 



It is possible that under some circumstances the whole of the 



