THE EVENTS OF THE HUMAN CARDIAC CYCLE 79 



TABLE II 



The terms employed in Table I. require brief definition. It must 

 not be forgotten that the curves of intra-ventricular pressure cannot 

 be taken as indicative of the changes in the volume of the ventricular 

 blood content. When the ventricles contract and the pressures in 

 them augment, their blood content remains unchanged, for no blood 

 leaves them until the pressures rise to aortic and pulmonary pressure. 

 The interval which elapses in this way, prior to the raising of the 

 arterial valves, constitutes what is termed the " presphygmic " in- 

 terval. But the pressures in the arteries begin, and those of the 

 ventricles continue, to rise subsequent to this interval ; this continued 

 rise in the ventricles is consequently spoken of as the " period of rising 

 arterial pressure." Following upon it is the plateau, indicated in the 

 pressure curve by a wavy line, which runs horizontally, or slopes 

 slightly upwards or downwards. During the whole of this plateau 

 the volume of the ventricles is decreasing and blood is leaving them ; 

 but as it leaves them at a lessened rate, it does not, necessarily, further 

 raise the arterial pressures. 



At the end of the plateau, the pressures in the ventricles fall, and 

 when they are reduced below the arterial pressures, the pulmonary 

 and aortic valves close. It is, however, a little while before the pres- 

 sures fall sufficiently to allow the auriculo-ventricular valves to open, 

 and this interval is often spoken of as the " postsphygmic " interval. 

 Following upon it there is a pause, during the whole of which the 



