CH. xxi.] SPHYGMOGRAPHS. 27 1 



the pulse, aortic pressure, apex beat, atid intraveutricular 

 pressure, as in the researches of Hiirthle. By this means it 

 is found that the primary and pre-dicrotic 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, other than the 

 dicrotic wave, are due to the elastic tension of the arteries, 

 and are increased in number Avhen the tension of the arteries 

 is greatest ; the tauter an elastic substance is, the more does 

 it tend to vibrate under the influence of any fresh force suddenly 

 applied to it. Some of the post-dicrotic waves are also doubtless 

 instrumental in origin. The dicrotic wave is of different origin. 

 It was at one time thought that this wave was 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 is always situated at the same distance from the 



Fig. 265. Anacrotic pulse. 



beginning of the primary elevation, showing that it is centri- 

 fugal, travelling in the same direction as the primary wave, 

 and having its origin in the commencement of the arterial 

 system. Moreover, a single reflected wave from the periphery 

 would be impossible, as the waves reflected from one part would 

 be interfered with by those from other parts; and a reflected 

 wave would be increased by high peripheral resistance, and not 

 diminished as the dicrotic wave is. 



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

 semilunar valves ; the inflow of the blood into the aorta sud- 

 denly ceases, and the blood is driven up against the closed aortic 

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

 there and is propagated through the arterial system as the 

 dicrotic elevation. 



The systolic secondary waves, namely, the pre-dicrotic and the 

 anacrotic when it is present, are due to elastic vibrations of 

 the aortic wall and perhaps of the heart wall itself ; they are 

 increased by an increase in the peripheral resistance. 



