1042 



PHYSIOLOGY 



Aorta 



Ventricle 



against the aortic valves, closing them tightly and putting them on the 

 stretch. Even in a rigid tube the sudden cessation of flow causes a nega- 

 tive wave, followed by a positive wave in the opposite direction in the 

 aorta ; this positive wave is increased by the elastic reaction of the 

 stretched aortic valves. The blood is driven up against them by 

 the wave of positive pressure and then rebounds, like a billiard ball 

 from the elastic cushion, and gives rise to the dicrotic elevation. 

 That the dicrotic elevation is for the most part a positive wave 



running in a centrifugal direction is shown by 

 the fact that the distance between it and the 

 primary wave does not alter appreciably from 

 whatever part of the arterial system the tracing 

 be taken. If it were a reflected wave the dis- 

 tance between it and the primary elevation of 

 the pulse -curve ought to be less the nearer the 

 periphery the pulse tracing is taken. 



The pre- dicrotic waves are to be ascribed to 

 elastic oscillations set up in the beginning of the 

 arterial system by the sudden rise of pressure. 

 Just as an india-rubber ball allowed to drop 

 on to the ground will bounce several times 

 before it comes to rest, so any sufficiently 

 sudden rise of pressure in an elastic fluid system 



Lll must give rise to a series of oscillations whether 



I \ the fluid is moving or not. There is no doubt 



"^1 ' I ^-' ' that the pre-dicrotic elevations as usually repre- 

 FIG. 41 /. iiitraventricu- sented are much distorted by the peculiarities 

 Srf?XHrS of the instrument used. In the tracing of the 

 means of the capillary aortic pressure by Frank there is only one 



manometer. (BAYLISS , , v , , 



and STARLING.) primary elevation on the ascending part of the 



curve where the ascent suddenly becomes 



less steep. In the same way the post-dicrotic elevations may be 

 regarded as a dying away of the dicrotic wave more or less distorted 

 by instrumental vibrations. 



The general form of the pulse-curve varies with changes in the 

 heart, in the arteries, and in the peripheral resistance. Thus some 

 curves may present secondary elevations on the ascending part, as in 

 Fig. 416, III, and are called cmacrotic, while in others all secondary 

 elevations occur on the descending part. This latter type is called 

 rftfacrotic, and is the tracing usually obtained from a normal radial 

 artery. By comparing these two types of curves with the correspond- 

 ing intraventricular pressures, we find that in both cases blood is 

 flowing into the aorta during the whole time from the beginning of the 

 primary elevation to the notch just before the dicrotic elevation, 



Ventricle 



