96 A MANUAL OF PHYSIOLOGY 



travels with the same velocity and in the same direction as the 

 primary wave. It is not, then, a wave reflected directly from the 

 peripheral distribution of the artery from which the pulse-tracing 

 is taken. 



Some writers have contended that it is a centrifugal twice-reflected 

 wave, and, indeed, traces of such waves may be detected in the 

 vessels of newly-killed animals when changes of pressure of the same 

 order of magnitude as the arterial pulse are artificially produced by 

 a pump and recorded by elastic manometers connected with the 

 interior of an artery. It has been supposed that these secondary 

 waves are reflected first from peripheral points at which the blood- 

 flow is particularly obstructed (the bifurcations of the larger arteries, 

 and the small arteries and capillaries in general), and that running 

 towards the heart, they are again reflected outwards from the semi- 

 lunar valves. It has been urged in support of this view that in very 

 small animals (guinea-pigs) no dicrotic elevation occurs on the pulse- 

 tracing, since the path which the reflected wave has to follow is so 

 short that it arrives at the root of the aorta before the primary 

 elevation is over. But this argument is by no means conclusive, 

 and, indeed, the great difference in the distance from the heart of 

 the numerous points at which reflection must take place is one of 

 the chief difficulties of the hypothesis. For it is not easy to under- 

 stand how the reflected fragments of the primary wave, arriving at 

 different intervals at the heart, can be integrated into the single 

 considerable dicrotic elevation. 



The explanation that best takes account of the facts and 

 renders most clear the role of the semilunar valves is some- 

 what as follows : When the systole abruptly comes to an end and 

 the outflow from the ventricle ceases, the column of blood 

 in the aorta tends still to move on in virtue of its inertia, and a 

 diminution of pressure, accompanied by a corresponding con- 

 traction of the aorta, takes place behind it, just as a negative 

 wave is set up in the central end of the elastic tube when the 

 stroke of the pump is over. At the same moment, and while 

 the semilunar valves are still for an instant incompletely closed, 

 the diminution of pressure in the beginning of the aorta is 

 intensified by the aspiration of the relaxing ventricle, which sucks 

 the blood back against the valves, and draws them a little 

 way into its cavity. A negative wave, therefore a wave of 

 diminished pressure, represented in the pulse - curve by the 

 ' aortic notch ' - travels out towards the periphery. The 

 diminution of pressure is quickly followed by a rebound, as 

 always happens in an elastic system. The recoiling blood meets 

 the closed semilunar valves. The aorta expands again, and the 

 expansion is propagated once more along the arteries as the 

 dicrotic elevation. It is possible that this elevation may be re- 

 inforced by a reflected wave produced in the manner described. 



When the semilunar valve becomes incompetent in disease, or is 

 rendered insufficient in animals by the artificial rupture of one or 



