CHAP. iv. J THE VASCULAR MECHANISM. 207 



ventricle, as measured by the length of the systolic plateau. 

 Hence, it being further assumed that the whole of the contents 

 of the ventricle were ejected at each systole, it was inferred 

 that the ventricle remained empty and yet contracted for 

 an appreciable period after the discharge of its contents. And 

 this led, in turn, to a great divergence of opinion as to the exact 

 time at which the semilunar valves were closed. 



But when we carefully explore the pressure in the aorta and 

 in the ventricle at the same time, making use .of the differential 

 manometer, we come upon facts which seem to disprove this view. 

 Examining Fig. 52 we find that, while during the systolic plateau 

 the pressure is falling in both aorta and ventricle, the curve 

 of difference of pressure D remains above the base line, though 

 not far above it and continually approaching it, up to the mark (3) 

 at the very end of the plateau. At this point, however, at the end 

 of the plateau, at the beginning of relaxation, a very great difference 

 of pressure is established ; while the ventricular pressure falls 

 suddenly and soon reaches or even passes the base line (becoming 

 in the latter case negative, i.e. below that of the atmosphere), the 

 pressure in the aorta undergoes relatively little change, indeed, 

 immediately afterwards receives an increase of which we shall 

 have to speak later on as the dicrotic crest of the pulse wave ; 

 and the curve of difference D falls with very great abruptness. 



The interpretation of this seems to be as follows. During 

 the whole of the systolic plateau up to the mark (3) the semi- 

 lunar valves are open, the cavity of the ventricle and the root 

 of the aorta form a common cavity which is occupied by a 

 continuous column of blood. Hence the curves of ventricular 

 and aortic pressure, of the pressure at the one end and at the 

 other end of this column, follow the same general course, and, 

 indeed, shew the same secondary variations ; this general course 

 is, in the case which we are studying, a -descending one by 

 reason, as we have said, of the relatively free escape of blood from 

 the arterial system through the peripheral resistance. But the 

 column of blood in question is a column in motion, the ventricular 

 pressure is driving the blood from the ventricle into the aorta ; to 

 effect this the pressure in the ventricle must continue to be higher 

 than that which it is itself generating in the aorta, the curve of 

 difference must remain above the base line. And, since the curve 

 of difference does remain above the base line right up to the mark 

 (3), we may infer that up to this point blood does pass from the 

 ventricle into the aorta. At (3), however, there is a sudden change. 

 The systole suddenly ceases, and with that the curve of difference 

 suddenly sinks below the base line ; the flow from ventricle ceases 

 not because there is no more blood to come, but because the pressure 

 in the ventricle now becomes lower than that in the aorta ; arid, 

 indeed, the blood would flow back from the aorta to the region of 

 lower pressure, to the ventricle, were it not that the very first effect 



