256 THE MECHANISM OF THE BEAT. [BOOK i. 



semilunar valves shelter the blood in the aorta from the ventricu- 

 lar pressure. But immediately after (1) the pressure in the aorta, 

 also begins to rise ; this shews that the semilunar valves are now 

 open, the blood in the ventricle and that in the aorta now forming 

 a continuous column and allowing the pressure of the ventricle to 

 be felt in the aorta. A very slight excess of pressure on the 

 ventricular side of the valves is sufficient to push aside the flaps 

 of the valve ; so that we may fairly say that the valves open 

 immediately after (1) which marks the point at which the curve 

 of difference of pressure between the ventricle and the aorta has 

 reached the base line x x, that is to say at which the difference 

 between the two has become nil. 



It will be observed however that the mark (1) cuts the ventri- 

 cular curve not at the summit of its rise but short of this ; the 

 pressure in the ventricle continues to rise after the valves are 

 open, the curve continues after this to ascend rapidly up to (2) 

 which marks the beginning of the systolic plateau. During the 

 interval between (1) and (2) the pressure is rising in the aorta also. 

 During this interval the pressure in the ventricle, continuing to 

 rise, becomes greater than that in the aorta, the curve of difference 

 rises above the base line ; but the excess of pressure in the ventricle 

 does not become very great, the curve of difference does not rise to 

 any great height, because that very excess of pressure is used up 

 in driving the contents of the ventricle into the aorta through the 

 open semilunar valves. 



During this interval the pressure in the aorta continues to 

 rise because, until the height of pressure at (2) is reached, the 

 pressure is not yet sufficient to drive the blood on along the 

 arterial system with adequate rapidity. 



With the point (2) the systolic plateau begins. During this 

 plateau the exact course taken by the curve of ventricular pressure 

 differs in different cases. We will take first the perhaps more 

 ordinary case in which the curve with intermediate variations which 

 we may at present pass over gradually declines until the point 

 (3) is reached, when the plateau comes to an end by reason of the 

 sudden fall of the ventricular pressure. 



There can be no doubt that the sudden fall after (3) is due to 

 the sudden cessation of the contraction of the ventricular walls, to 

 their sudden relaxation. But what is taking place during the 

 systolic plateau before this point is reached ? 



It used to be argued, taking count of the distension only of 

 the aorta as indicated by the sphygmograph, an instrument of 

 which we shall speak later on, that the ventricular contents 

 escape into the aorta during the period of the distension of the 

 aorta and during this only, having ceased to flow by the time that 

 this distension passes away giving place to a sequent shrinking 

 of the aorta. Now when this period of distension is carefully 

 measured it is found to be much shorter than the systole of the 



