270 DICROTISM. [BOOK i. 



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

 extensible and elastic section ; the primary and secondary expan- 

 sions, 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 

 (and 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 141 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. 47 

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

 in the upper tracing. 



147. The predicrotic wave, (marked B on Fig. 49, and 

 on several other of the pulse curves), which precedes the dicrotic 

 wave and is still more variable than that wave, being some- 

 times slight or even invisible and sometimes conspicuous, has 

 given rise to much controversy. In the interpretation 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 towards the ventricle ; and it has been suggested that 

 this reflux meeting and closing the semilunar valves starts a 

 small wave of expansion before the larger dicrotic wave has had 

 time to develope 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 begin- 

 ning, not during the development of the dicrotic wave ; it is still 

 possible however, even on this view as on the other, to suppose 

 that an antecedent reflux due to the negative pressure succeeding 

 the cessation of flow from the ventricle closes the valves and starts 

 the predicrotic wave. But the matter is one not yet beyond the 

 stage of controversy. 



