186 THE VASCULAR MECHANISM. 



as we have said ( 128) by the fact that the movements of the mercury 

 column are too sluggish to reproduce these finer variations ; but dicrotism 

 is also conspicuous by its absence in the tracings given by more delicately 

 responsive instruments. Moreover, when the normal pulse is felt by the 

 finger, most persons find themselves unable to detect any dicrotism. But 

 that it does really exist in the normal pulse is shown by the fact that it 

 appears in a most unmistakable manner in the tracing obtained by allow- 

 ing the blood to spurt directly from an opened small artery, such as the 

 dorsalis pedis, upon a recording surface. 



Less constant and conspicuous than the dicrotic wave, but yet appearing 

 in most sphygmograms, is an elevation which appears higher up on the 

 descending limb of the main wave ; it is marked B in Fig. 68, and on several 

 of the other curves, and is frequently called the pre-dicrotie wave ; it may 

 become very prominent. Sometimes other secondary waves, often called 

 " post-dicrotic," are seen following the dicrotic wave, as at D in Fig. 68, and 

 some other curves ; but these are not often present, and usually, even when 

 present, inconspicuous. 



When tracings are taken from several arteries, or from the same 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 sphygmo- 

 graphic 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 com- 

 plex one; so much so, that the detailed interpretation of a sphygmographic 

 tracing is still in most cases extremely uncertain. 



135. The chief interest attaches to the nature and meaning of the di- 

 crotic wave. In general, the main conditions favoring 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, and the discharge of a large quantity of blood into the aorta. 

 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 ( 125), very brief. The expan- 

 sion 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 wall 

 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 

 show 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 further 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 further 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 foot of the aorta upon the sudden cessation of the flow from 



