THE CIRCULATION OF THE BLOOD AND LYMPH 93 



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, accompanied 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 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 semi- 

 lunar valves, the aorta expands again, and the expansion is 

 propagated once more along the arteries as the dicrotic 

 elevation. 



Of the origin and significance of the predicrotic wave we 

 know so little that it would not be profitable to discuss it. 

 It seems, however, to be a secondary wave of oscillation. The 

 so-called elastic oscillations (Landois) are probably due, in 

 large part at least, to vibrations of the recording apparatus. 



When the semilunar valve becomes incompetent in disease, or is 

 rendered insufficient in animals by the artificial rupture of one or 

 more of its segments, the dicrotic wave, as will be readily understood 

 from the manner in which it is produced, either disappears 

 altogether or is markedly enfeebled. But apart from any anatomical 

 lesion or functional defect in the aortic valves, the prominence 

 of the wave varies with a great number of circumstances, some of 

 which are in a measure understood, while others remain obscure. 

 It varies in particular with the abruptness of discharge of the 

 ventricle and the extensibility of the arteries. The conditions are 

 usually favourable when the arterial pressure is low, for the blood then 

 passes quickly from the ventricle into the arteries, which, already 

 only moderately tense, are easily dilated by the primary wave, then 

 sharply collapse, and are again abruptly distended when the dicrotic 

 wave arrives. And, in fact, an exaggeration of the dicrotic wavelet 

 may be artificially produced by nitrite of amyl (Fig. 70, p. 183), 

 a drug which lessens the blood-pressure by dilating the small 



