CHAP, iv.] THE VASCULAR MECHANISM. 235 



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 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 exten- 

 sible and elastic section ; the primary and secondary expansions, 

 in spite of the general damping 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 (we shall see 

 presently 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 123 that 

 the curve of expansion of an elastic tube is modified by the pres- 

 sure 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. 61 

 for instance the dicrotic wave is more evident in the middle than 

 in the upper tracing. 



130. Concerning the other secondary waves on the pulse-curve 

 such as that which has been called the ' predicrotic ' wave (B on Fig. 

 63 and on some of the other pulse-curves) it will not be desirable 

 to say much here. They have been the occasion of much discus- 

 sion, especially when considered under the view that the ventricle 

 rapidly emptied itself at the earlier part of the systole. We will 

 content ourselves with the following remark. The predicrotic 

 and the other secondary waves in question are, like the dicrotic 

 wave, propagated from the heart towards the periphery, they are 

 seen in sphygmograms taken from the root of the aorta as well as 

 from more peripheral arteries, and some are also seen in the curves 

 of ventricular pressure. Some of the features of these secondary 

 waves may be due to imperfections in the instruments used, to 

 inertia and the like, but the main features undoubtedly represent 

 events taking place in the vascular system itself. When we com- 

 pare the curve of pressure in the aorta with that in the ventricle, 

 we observe that up to the dicrotic notch, (in what may be called 

 the systolic part of the pulse-curve, the part which corresponds to 

 the systole of the ventricle, in contrast to the diastolic part which 

 follows and which includes the dicrotic wave) , the variations seen 

 in the aortic curve, the secondary waves of which we are speaking, 



