THE FLOW OF BLOOD THROUGH THE ARTERIES 969 



or to rise (cf. Fig. 408). At 6 the ventricle relaxes, the intra ventricular pres- 

 sure falls rapidly, and at 7 falls below the aortic pressure. The aortic valves 

 must now close since the pressure is greater on their aortic side. The pressure 

 in the ventricle now continues to fall until it becomes zero. In the aorta 

 however there is a sharp elevation immediately after 7, i. e. immediately 

 after the closure of the aortic valves. This is known as the dicrotic elevation, 

 the previous depression being the dicrotic notch or incisure. It is at this 

 point that the second sound of the heart is heard and is evidently due to the 

 vibrations which are represented graphically in the record of intra-aortic 

 pressure. 



There are several factors at work tending to produce a secondary wave 

 at this point. With the sudden cessation of the inflow of blood from the 

 ventricles at the end of the ventricular contraction, a negative wave must 

 be produced at the beginning of the aorta which, transmitted along the 

 arterial system, will tend to produce a reflux of blood towards the heart. 

 The movement so caused is reinforced by the elastic reaction of the arterial 

 wall so that the returning blood is driven up against the aortic valves, 

 closing them tightly and putting them on the stretch. Even in a rigid 

 tube the sudden cessation of flow causes a negative wave, followed by a 

 positive wave in the opposite direction in the aorta; this positive wave 

 is increased by the elastic reaction of the stretched aortic valves. The 

 blood is driven up against them by the wave of positive pressure and then 

 rebounds, like a billiard ball from the elastic cushion, and gives rise to the 

 dicrotic elevation. 



The predicrotic waves in the pulse tracing are evidently due to the 

 instrumental exaggeration of the wave, which may occasionally be seen 

 even in a perfect pressure tracing at 5. The rapid rise of pressure in the 

 aorta or in the more peripheral artery, which follows the opening of the 

 aortic valves, sets up a tendency to secondary oscillations at this point. 

 The greater the inertia of the instrument, the greater is the exaggeration 

 of these waves. As is shown by the dotted line in Fig. 426, the lever of 

 the sphygmograph is jerked up, practically leaving the artery, and then 

 falls and rebounds again, so that the simple rounded top becomes resolved 

 by instrumental error into a curve with two waves, which have been called 

 the percussion wave and the predicrotic wave. In the same way the inertia 



I of the instrument will tend to exaggerate the dicrotic elevation and 

 possibly to give rise to slight post-dicrotic waves. 

 It would seem that the pulse curve, as recorded in the aorta and the 

 arterial trunks given off from the aortic arch, can be referred entirely to 

 events taking place in the heart during systole or at the beginning of the 

 aorta at the commencement of diastole ; and there seems no reason to assume 

 the co-operation of waves reflected from the periphery to explain the pro- 

 duction of any of the secondary waves observed on the pulse. A considerable 

 difference is however noticeable between the pulse as recorded in the aorta 

 and that recorded in the brachial or femoral arteries, or in the radial at the 

 wrist. The effect of the propagation along an endless system of elastic tubes 



