294 THE CIRCULATION IN THE BLOOD-VESSELS [dl. XXII. 



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. 288), 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; tha 

 FIG. 26i. Dicrotic pulse wave rebounds f rom these and is 



propagated through the arterial 



system as the dicrotic elevation. The production of the dicrotic 

 wave is favoured by a low blood-pressure when the heart is beating 

 forcibly, as in fever. Such a pulse is called a dicrotic pulse (fig. 261), 

 and the second beat can be easily felt by the finger on the radial 

 artery. 



The percussion wave is produced by the ventricular systole 

 expanding the artery. The sharp top at its summit is due to the 

 sudden upward spring of the light lever of the 

 sphygmograph. If it were possible to obtain a 

 true record of what really occurs, we should 

 doubtless have a tracing as shown by the con- 

 tinuous line in the accompanying figure (fig. 259). 

 The apex of the tidal wave, B, marks the end of 

 the ventricular systole. 



In our study of intracardiac pressure, we 

 saw that the systolic plateau sometimes has an 

 ascending, sometimes a descending, slope (see dicrotic waves. 

 p. 243) ; we now come to the explanation of this 

 fact. If after the first sudden rise of pressure in the aorta the peri- 

 pheral resistance is low, and the blood can be driven on from the 

 aorta more rapidly than it is thrown in, the plateau will sink. If, 

 on the other hand, the peripheral resistance is high, the aortic 

 pressure will rise as long as the blood is flowing in, and we get an 

 ascending systolic plateau and an anacrotic pulse. Thus an anacrotic 

 pulse is seen in Blight's disease, where the peripheral resistance is 

 very high. 



If a long pulse-tracing is taken, the effect of the respiration can 

 be seen causing an increase of pressure, and a slight acceleration of 

 the heart's beats during inspiration. 



The main waves of the pulse can be demonstrated without the 



