CHAP, iv.] THE VASCULAR MECHANISM. 285 



obstacle to the return of blood from the aorta to the ventricle, 

 and without such an obstacle the circulation could not be carried 

 on. 



147. Moreover it must be remembered that though we may 

 thus regard the closed valves as so to speak the determining cause 

 of the dicrotic wave, the wave itself is an oscillation of the arterial 

 walls, and the remarks made a little while back concerning the 

 inertia of the walls hold good for this explanation also. Hence the 

 conditions which determine the prominence or otherwise of the 

 dicrotic wave are conditions relating to the elasticity of the 

 arterial walls, and to the circumstances which call that elasticity 

 into play. For instance, the dicrotic wave 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 secondary waves. Again, 

 the dicrotic wave is, other things being equal, more marked when 

 the mean arterial pressure is low than when it is high ; indeed it 

 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 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 a diminished cardiac stroke 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 may even exhibit 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 arterial 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 diminish 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 



