ORIGIN AND CHARACTERS OF THE DICROTIC WAVE. 137 



stroke and the base of the next up-stroke corresponds to a beat of the 

 heart, so that this figure 

 shows five heart-beats and 

 part of a sixth. The part, 

 a, b = the ascent, 5, the 

 apex of the up-stroke, and 

 b to h, the descent, with a 

 curve, d, called the first 57 



tidal or predicrotic wave, Sphygmogram of rad 'al ariery-pressure 2 oz. 

 e, an angle or notch, the 



aortic notch, /, a second elevation, called the dicrotic wave, g, a slight 

 curve, sometimes called the second tidal wave. The descent is con- 

 tinued to h, where the ascent of the next heart-beat begins.] 



I. Origin and Characters of the Dicrotic Wave. 



The dicrotic or recoil wave, which is always present in a normal 

 pulse, is caused thus : During the ventricular systole a mass of blood 

 is propelled into the already full aorta, whereby a positive wave is 

 rapidly transmitted from the aorta throughout the arterial system, even 

 to the smallest arterioles, in which this primary wave is extinguished. As 

 soon as the semi-lunar valves are closed, and no more blood flows into 

 the arterial system, the arteries which were previously distended by the 

 mass of blood suddenly thrown into them, recoil or contract, so that 

 in virtue of the elasticity (and contractility) of their walls, they 

 exert a counter-pressure upon the column of blood, and thus the blood 

 is forced onwards. There is a free passage for it towards the periphery, 

 but towards the centre (heart) it impinges upon the already closed 

 semi-lunar valves. This developes a new positive wave, which is pro- 

 pagated peripherally through the arteries, where it disappears in their 

 finest branches. In those cases where there is sufficient time for the 

 complete development of the pulse-curve (as in the short course of the 

 carotids, and in the arteries of the upper arm, but not in those of the 

 lower extremity, on account of their length), a second wave of reflec- 

 tion may be caused in exactly the same way as the first. 



Just as the pulse occurs later in the more peripherally placed 

 arteries than in those near the heart, so the secondary wave reflected 

 from the closed aortic valves must appear later in the peripheral 

 arteries. Both kinds of waves, the primary pulse wave, the secondary, 

 and eventually even the tertiary reflected wave arise in the same place, 

 and take the same course, and the longer the course they have to 

 travel to any part of the arterial system, the later they arrive at their 

 destination. 



