CHAP, iv.] THE VASCULAR MECHANISM. 269 



bility and increased elastic reaction of the arterial walls which 

 ti-nd 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 favourable to the prominence of the dicrotic wave. 



On the other hand these, and the other conditions which 

 favour dicrotism in the pulse, are exactly those which would favour 

 such a development of secondary waves as has been described 

 above, and their absence would be unfavourable 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 pres- 

 sure 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 backwards and forwards, 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 artery is rapidly expanded, and falling 

 rapidly back enters upon a secondary (dicrotic) expansion and 

 even a third. 



Moreover the same principles may be applied to explain 

 why sometimes dicrotism will appear marked in a particular 

 artery while it remains little marked in the rest of the system. 

 In experimenting with an artificial tubing such as the arte- 

 rial model, the physical characters of which remain the same 

 throughout, both the primary and the secondary waves retain 

 the same characters as they travel along the tubing save only 

 that both gradually dimmish towards the periphery; and in 

 the natural circulation, when the vascular conditions are fairly 

 uniform throughout, the pulse curve, as a rule, possesses the same 

 general characters throughout, save that it is gradually ' damped 

 off.' But suppose we were to substitute for the first section of the 

 tubing a piece of perfectly rigid tubing ; this at the stroke of the 

 pump on account of its being rigid would shew neither primary 

 nor secondary expansion, but the expanding force of the pump's 

 stroke would be transmitted through it to the second, elastic 

 section, and here the primary and secondary Avaves would at once 

 become evident. This is an extreme case, but the same thing 

 would be seen to a less degree in passing from a more rigid, 



