204 THE CIRCULATION OF THE BLOOD 



That the dicrotic is not a reflected wave is clearly established by the 

 fact that if the distance between the dicrotic wave and the main pulse 

 wave is measured at different points of the arterial stream, it will al- 

 ways be found to be the same, which obviously would not be the case 

 were the dicrotic wave reflected. If, for example, we were to examine 

 the contour of the wave produced by throwing a stone into a tub of 

 water, we should find that near the edge the secondary wave was very 

 close to the main wave, whereas near the center the secondary wave 

 would occur much later. 



Our problem therefore narrows itself down to an investigation of 

 the cause for the dicrotic wave at the central end of the circulation. It 

 occurs, as we have seen, immediately after the beginning of diastole. 

 That it can not be due to anything taking place in the ventricle itself is 

 evidenced by the fact that such a wave is absent from an intracardiac 

 pressure curve (see page 151), although it is present in the very begin- 

 ning of the aorta. Now, the only structures existing between those two 

 points which could be held responsible for this wave are the semilunar 

 valves a conclusion which is sustained by the fact that, if the aortic 

 valves are rendered incompetent by hooking them back, or if the pulse 

 beat is examined in patients suffering from an aortic insufficiency, it 

 will be found that the dicrotic wave is not nearly so evident as usual. 



To understand how the valves are responsible for the production of the 

 wave, the mechanical changes occurring at the root of the aorta must 

 be clearly understood (see page 155). The stretching of the elastic walls 

 of the -aorta which occurs with each systolic outrush of blood is fol- 

 lowed by a powerful and sudden contraction of the stretched walls, 

 and the pressure thus brought to bear on the column of blood in the aorta 

 tends to impel it both forward and backward. The forward movement 

 adds itself to the wave of increased pressure already produced by the 

 ventricular contraction. The backward component travels as far as the 

 semilunar valve, from which it is reflected, and now proceeds peripher- 

 ally along the blood stream during the time at which the original pres- 

 sure pulse is declining. It therefore imprints itself on the pulse trac- 

 ing as a separate wave, and does so all the -more markedly when the 

 decline in the main pulse wave is rapid, as in cases in which the periph- 

 eral resistance is low, but fails to be prominent when, on account of 

 a high peripheral resistance, the decline in the main pulse wave is tardy. 

 This explanation coincides exactly with the well-known clinical fact 

 that the dicrotic wave is conspicuous in pulses of low tension, but ill 

 marked or absent in pulses of high tension. 



One point remains to be considered, and that is the cause for the 

 sudden decline in the main wave at the cessation of the ventricular out- 



