188 THE VASCULAR MECHANISM. 



the elastic walls to expand again after their primary shrinking, produces 

 tho dicrotic wave C. On this view, it is the blood driven back from the 

 valves which expands the artery ; on the view given above, it is the ex- 

 panding artery which draws the blood back toward the valves. 



Moreover, quite other views have been or are held concerning this 

 dicrotic wave. According to many authors, it is what is called a " re- 

 flected " wave. Thus, when the tube of the artificial model bearing two 

 levers is blocked just beyond the far lever, the primary wave is seen to be 

 accompanied by a second wave, which at the far lever is seen close to, 

 and often fused into, the primary wave (Fig. 64, VI. a f ), but at the near 

 lever is at some distance from it (Fig. 64, I. a'), being the further from 

 it the longer the interval between the lever and the block in the tube. The 

 second wave is evidently the primary wave reflected at the block and travel- 

 ling backward toward the pump. It thus, of course, passes the far lever be- 

 fore the near one. And it has been argued that the dicrotic wave of the 

 pulse is really such a reflected wave, started either at the minute arteries 

 and capillaries, or at the points of bifurcation of the larger arteries, and 

 travelling backward to the aorta. But if this were the case, the distance 

 between the primary crest and the dicrotic crest ought to be less in arteries 

 more distant from than in those nearer to the heart, just as in the artificial 

 scheme the reflected wave is fused with a primary wave near the block (Fig. 

 64, VI. 6 a. a'), but becomes more and more separated from it the further 

 back toward the pump we trace it (Fig. 64, I. 1 a. a'}. Now this is not the 

 case with the dicrotic wave. Careful measurements show that the distance 

 between the primary and dicrotic crests is either greater, or certainly not 

 less, in the smaller "or more distant arteries than in the larger or nearer 

 ones. This feature indeed proves that the dicrotic wave cannot be due to 

 reflection at the periphery, or, indeed, in any way a retrograde wave. Be- 

 sides, the multitudinous peripheral division would render one large periph- 

 erally reflected wave impossible. Again, the more rapidly the primary 

 wave, is obliterated, or at least diminished, on its way to the periphery, the 

 less conspicuous should be the dicrotic wave. Hence increased extensibility 

 and increased elastic reaction of the arterial walls which tend to use up 

 rapidly the primary wave, should also lessen the dicrotic wave. But as a 

 matter of fact these conditions, as we have said, are favorable to the promi- 

 nence of the dicrotic wave. 



On the other hand, these and the other conditions- which favor dicrotism 

 in the pulse are exactly those which would favor such a development of 

 secondary waves as has been described above, and their absence would be 

 unfavorable to the occurrence of such waves. Thus dicrotism is less marked 

 in rigid arteries (such as those of old people) than in healthy elastic ones ; 

 the rigid wall neither expands so readily nor shrinks so readily, and hence 

 does not so readily give rise to such secondary waves. Again, dicrotism is 

 more marked when the mean arterial pressure is low than when it is high ; 

 indeed, dicrotism may be induced when absent, or increased when slightly 

 marked by diminishing, in one way or another, the mean pressure. Now, 

 when the pressure is high, the arteries are kept continually much expanded, 

 and are therefore the less capable of further expansion ; that is to say, are, 

 so far, more rigid. Hence the additional expansion due to the systole is not 

 very great ; there is a less tendency for the arterial walls to swing backward 

 and forward, so to speak, and hence a less tendency to the development of 

 secondary waves. When the mean pressure is low, the opposite state of 

 things exists ; supposing, of course, that the ventricular stroke is adequately 

 vigorous (the low pressure being due, not to diminished cardiac force, but to 

 diminished peripheral resistance), the relatively empty but highly distensible 



