290 



THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



than the hoart-beat. On the descent are several secondary (kata- 

 crotic) elevations. 



A is the primary, or percussion wave ; C is the prc-dicrotic, or 

 tidal wave; D is the dicrotic wave, and E the post-dicrotic wave, 

 and of these there may be several. In some rare cases there is a 

 secondary wave on the upstroke, which is called an anacrotic wave 

 (fig. 291). 



These various secondary waves have received different inter- 

 pretations, but the best way of explaining them is derived from 

 information obtained by taking simultaneous tracings of the pulse, 

 aortic pressure, apex beat, and intraventricular pressure, as in the 

 researches of Hurthle. By this means it is found that the percussion 

 and tidal waves occur during the systole of the heart, and the other 

 waves during the diastole. The closure of the aortic valves occurs 

 just before the dicrotic wave. The secondary waves on the down- 

 stroke other than the dicrotic are due to the elastic tension of the 

 arteries, and are increased in number when the tension of the arteries 



FIG 291. Anacrotic pulse. 



is greatest. Some of the post-dicrotic waves are also doubtless 

 instrumental in origin. The dicrotic wave has a different origin. It 

 was at one time thought that this wave was due to a wave of pressure 

 reflected from the periphery, but this view is at once excluded by the 

 fact that wherever we take the pulse-tracing, whether from the aorta, 

 carotid, radial, dorsalis pedis, or elsewhere, this secondary elevation 

 always follows the percussion wave after the same interval, showing 

 that it has its origin in the commencement of the arterial system. 

 Moreover, a single pressure-wave reflected from the periphery would 

 be impossible, as such a wave reflected from one part would be inter- 

 fered with by those from other parts ; moreover, a dicrotic elevation 

 produced by a pressure-wave reflected from the periphery, would be 

 increased by high peripheral resistance, and not diminished as is 

 actually the case. 



The primary cause of the dicrotic wave is the closure of the semi- 

 lunar valves; as already explained when we were considering the 

 velocity pulse (p. 285), the inflow of blood into the aorta suddenly 

 ceases, and the blood is driven back against the closed aortic doors 

 by the elastic recoil of the aorta; the wave rebounds from these 

 and is propagated through the arterial system as the dicrotic 



