172 THE PULSE. [BOOK i. 



which similarly appears on the pulse-curve as an elevation succeed- 

 ing the notch. 



Then again it has been argued that in any section of the 

 arterial tract, the inertia of the walls and of the contained blood, 

 in each expansion of the section, carries them on in their movement 

 of expansion some little time after the actual expanding force has 

 ceased to act. This leads to a falling back or contraction, which 

 again by reason of the same inertia overshoots its mark, and thus 

 through a series of oscillations, of which the first is the most 

 conspicuous, the artery settles down to its normal calibre before the 

 next expansion reaches it. The extent of such oscillations is 

 determined, not only by the character of the walls but by the 

 specific gravity of the contained fluid. In the artificial scheme 

 with the same elastic tubing the secondary waves thus caused are 

 much greater with mercury than with water, and disappear almost 

 wholly when air is employed. Such waves of oscillation may be 

 supposed to be generated in different degrees, in each and every 

 section of the arterial tract ; the waves due to a cessation of the 

 flow are on the contrary generated at the point where the inter- 

 mittence is effected, and may be seen in rigid as well as in elastic 

 tubes ; but these latter waves also are profoundly modified by the 

 nature of the walls of the tubes along which they are transmitted. 



Lastly, it has been maintained that these secondary waves are 

 of active not passive origin; that is, that they are caused by a 

 rapid muscular contraction of the arterial walls following up so to 

 speak the arterial beat. 



We have dwelt at so great a length on these secondary waves 

 of the pulse-curve because of the importance attached to them in 

 clinical medicine ; but it would be hardly profitable to enter more 

 fully into the discussion of these several contending views. As an 

 instance of the difficulty of the subject and the insufficiency of our 

 knowledge, we may point out that observers are not yet agreed as 

 to which part of the curve corresponds to the closure of the 

 semilunar valves. Thus some maintain that this event corresponds 

 to and indeed is indicated by the dicrotic wave, the dicrotic notch 

 representing the reflux towards the ventricle, and the dicrotic 

 elevation a new forward movement reflected from the closed 

 valves. But under this view, though it seems the more probable, 

 the predicrotic wave presents a difficulty; and indeed others 

 maintain that the moment of closure of the semilunar valves is 

 indicated by this the predicrotic, and not by the dicrotic wave. 

 Until this and other points are finally settled, all interpretations of 

 modifications of the pulse-curve must remain uncertain and un- 

 satisfactory. 



The following facts however may be borne in mind as not only 

 of practical importance, but as necessary data for any judgments 

 concerning the pulse-curve. 



