266 DICEOTISM. [BOOK i. 



artery under different conditions of the body, these secondary 

 waves are found to vary very considerably, giving rise to many 

 characteristic forms of pulse-curve. Were we able with certainty 

 to trace back the several features of the curves to their respective 

 causes, an adequate examination of sphygmographic tracings 

 would undoubtedly disclose much valuable information concerning 

 the condition of the body presenting them. Unfortunately the 

 problem of the origin of these secondary waves is a most difficult 

 and complex one ; so much so that the detailed interpretation 

 of a sphygmographic tracing is still in most cases extremely un- 

 certain. 



146. The chief interest attaches to the nature and meaning 

 of the dicrotic wave. In general the main conditions favouring 

 dicrotism are (1) a highly extensible and elastic arterial wall, 



(2) a comparatively low mean pressure, leaving the extensible 

 and elastic reaction of the arterial wall free scope to act, and 



(3) a sufficiently vigorous and sufficiently rapid stroke of the 

 ventricle. The development of the dicrotic wave may probably 

 be explained as follows. 



At each beat the time during which the contents of the left 

 ventricle are injected into the aorta is as we have seen ( 136) very 

 brief. The expansion of the aorta is very sudden, and the cessation 

 of that expansion is also very sudden. 



Now when fluid is being driven with even a steady pressure 

 through an elastic tube or a system of elastic tubes, levers placed 

 on the tube will describe curves indicating variations in the 

 diameter of the tube, if the inflow into the tube be suddenly 

 stopped, as by sharply turning a stop-cock ; and a comparison of 

 levers placed at different distances from the stop-cock will shew 

 that these variations of diameter travel down the tube from the 

 stop-cock in the form of waves. The lever near the stop-cock will 

 first of all fall, but speedily begin to rise again, and this subsequent 

 rise will be followed by another fall, after which there may be one 

 or more succeeding rises and falls, that is oscillations, with decreasing 

 amplitudes, until the fluid comes to rest. The levers farther from 

 the stop-cock will describe curves, similar to the above in form but 

 of less amplitude, and it will be found that these occur somewhat 

 later in time, the more so the farther the lever is from the stop- 

 cock. Obviously these waves are generated at or near the stop-cock 

 and travel thence along the tubing. 



We may infer that at each beat of the heart similar waves 

 would be generated at the root of the aorta, upon the sudden 

 cessation of the flow from the ventricle, and would travel thence 

 along the elastic arteries. The facts that each beat is rapidly 

 succeeded by another, and that the flow which suddenly ceases is 

 also, by the nature of the ventricular stroke, suddenly generated, 

 may render the waves more complicated, l?ut will not change their 

 essential nature. 



