THE CIRCULATION OF THE BLOOD. 141 



radial, the upstroke is usually single. In this case the percussion-im- 

 pulse is not sufficiently strong to jerk up the lever and produce an effect 

 distinct from that of the systolic wave which immediately follows it, 

 and which continues and completes the distention. In cases of feeble 

 arterial tension, however, the percussion-impulse may be traced by the 



FIG. 128.~Pulse-tracing of radial artery, with double apex. (Sanderson.) 



sphygmograph, not only in the carotid pulse, but to a less extent in the 

 radial also (Fig. 128). 



The interruptions in the downstroke are called the katacrotic waves, 

 to distinguish them from an interruption in the upstroke, called the 



FIG. 129. Anacrotic pulse from a case of aortic aneurism. A, anacrotic wave (or percussion 

 wave). B, tidal or pre-dicrotic wave, continued rise in tension (or higher tidal wave). 



anacrotic wave, which is occasionally met with in cases in which the 

 pre-dicrotic or tidal wave is higher than the percussion wave. 



There is considerable difference of opinion as to whether the dicrotic. 

 wave is generally present in health, and also as to its cause. The 

 balance of opinion, however, appears to be in favor of the belief that 

 the dicrotic wave is present in health, although it may be very faint;, 

 while in certain conditions not necessarily diseased, it becomes so marked 

 as to be quite plain to the unaided finger. Such a pulse is called dicro- 

 tic. Sometimes the dicrotic rise exceeds the initial upstroke, and the 

 pulse is then called liy per dicrotic. 



As to the cause of dicrotism, one opinion (1) is that it is due to a re- 

 covery of pressure during the elastic recoil, in consequence of a rebound 

 from the periphery. It may indeed be produced on a schema by ob- 

 structing the tube at a little distance beyond the spot where the sphygmo- 

 graph is placed. Against this view, however, is the fact that the notch 

 appears at about the same point in the downstroke in tracings from the 

 carotid and from the radial, and not first in the radial tracing, as it should 

 do, if this theory was correct, since that artery is nearer the periphery 

 than the carotid, and as it does in the corresponding experiment with 

 the arterial schema when the tube is obstructed. (2) The generally ac- 



