FLOW OF BLOOD THROUGH THE ARTERIES 925 



pressure becomes suddenly slower, may be marked in the aortic curve by one 

 or two secondary waves. It must be remembered however that all these 

 secondary waves shown on the aorta at 4 and 5^ may be absent, the one 

 at 5 being the one which is most frequently seen. From 6 to 7 the ventricle 

 is still contracting and forcing blood into the aorta. The curve of pressure 

 is generally rounded. It may present a flat top, the plateau, or the top 

 may be rounded with an inclination to fall or to rise (cf. Fig. 398). At 6 

 the ventricle relaxes, the intraventricular pressure 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. 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. That the dicrotic elevation is for the most part a 

 positive wave running in a centrifugal direction is shown by the fact that 

 the distance between it and the primary wave does not alter appreciably 

 from whatever part of the arterial system the tracing be taken. If it were 

 a reflected wave the distance between it and the primary elevation of the 

 pulse- curve ought to be less the nearer the periphery the pulse tracing is 

 taken. 



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. 416, the lever of 

 the sphygmograph is jerked up, so that it practically leaves 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 



