MECHANICS OF THE CIRCULATION IN THE VESSELS 105 



capillaries (e.g., the origin of the carotid and the radial at the wrist), 

 the dicrotic wave is separated by the same interval from the begin- 

 ning of the primary elevation. This can only be explained by 

 supposing that it has the same point of origin, and 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^he 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 particu- 

 larly 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 semilunar 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 understand 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 somewhat 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, accom- 

 panied by a corresponding contraction 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 in- 

 completely 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 reinforced by a reflected wave produced in the 

 manner described. 



