THE PULSE. 189 



artery is rapidly expanded, and, falling rapidly back, enters upon a second- 

 ary (dicrotic) expansion, and even a third. 



Moreover, the same principles may be applied to explain why sometimes 

 dicrotisrn 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 toward 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 show neither primary nor secondary ex- 

 pansion, 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 

 waves 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, that is, 

 less extensible and elastic section, to a less rigid, more extensible and elastic 

 section ; the primary and secondary expansions, in spite of the general damp- 

 ing effect, would suddenly increase. Similarly in the living body a pulse- 

 curve which, so long as it is travelling along arteries in which the mean 

 pressure is high, and which are therefore practically somewhat rigid, is not 

 markedly dicrotic, may become very markedly dicrotic when it comes to a 

 particular artery in which the mean pressure is low (and we shall see pres- 

 ently that such a case may occur), and the walls of which are therefore for 

 the time being relatively more distensible than the rest. 



Lastly, we may recall the observation made above ( 130), that the curve 

 of expansion of an elastic tube is modified by the pressure exerted by the 

 lever employed to record it, and that hence, in the same artery and with the 

 same instrument, the size, form, and even the special features of the curve 

 vary according to the amount of pressure with which the lever is pressed 

 upon the artery. Accordingly the amount of dicrotism apparent in a pulse 

 may be modified by the pressure exerted by the lever. In Fig. 64, for 

 instance, the dicrotic wave is more evident in the middle than in the upper 

 tracing. 



136. The pre-dicrotic wave (marked B on Fig. 68, and on several 

 other of the pulse-curves), which precedes the dicrotic wave and is still 

 more variable than that wave, being sometimes slight or even invisible and 

 sometimes conspicuous, has given rise to much controversy. In the inter- 

 pretation of the dicrotic wave given in the preceding paragraph it was 

 stated that the negative pressure developed on the cessation of the flow in the 

 rear of the column of blood, led by itself to a reflux toward the ventricle ; 

 and it has been suggested that this reflux meeting and closing the semi- 

 lunar valves starts a small wave of expansion before the larger dicrotic 

 wave has had time to develop itself. On this view the semilunar valves 

 would be actually closed before the occurrence of the secondary dicrotic 

 expansion of the arterial walls, though the larger, more powerful reflux of 

 this later event must render the closure more complete, and in doing so 

 possibly gives rise to the second sound. According, however, to the second 

 view given in the same paragraph, which regards the reflux due to the 

 shrinking of the artery in face of the great pressure in front as firmly 

 closing the semilunar valves, and as thus starting the secondary dicrotic 

 wave of expansion, the firm closing of the semilunar valves must take place 

 before the beginning, riot during the development of the dicrotic wave ; it 



