CHAP, iv.] THE VASCULAR MECHANISM. 



265 



portion of the main curve: such a curve is spoken of as "anacrotic" 

 Fig. 50. 



Of these secondary elevations, the most frequent, conspicuous 

 and important is the one which appears 

 some way down on the descending limb 

 and is marked C on Fig. 49 and on most 

 of the curves here given. It is more or 

 less distinctly visible on all sphygmograms, 

 and may be seen in those of the aorta 

 as well as of other arteries. Sometimes 



it is so slight as to be hardly discernible ; KA 



J 3 . . . n J FIG. 50. ANACEOTIC SPHYG- 



at other times it may be so marked as 



to give rise to a really double pulse 



(Fig. 51), i.e. a pulse which can be felt 



as double by the finger ; hence it has been 



called the dicrotic elevation or the dicrotic wave, the notch 



preceding the elevation being spoken of as the " dicrotic notch." 



MOGRAPH TRACING FROM 

 THE ASCENDING AORTA 



(Aneurism). 



FlG. 51. TWO GRADES OF MARKED DICROTISM IN RADIAL PULSE OP MAN. 



(Typhoid Fever.) 



Neither it nor any other secondary elevations can be recognised 

 in the tracings of blood-pressure taken with a manometer. This 

 may be explained, as we have said 139, 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 in- 

 struments. 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 shewn by the fact 

 that it appears in a most unmistakeable manner in the tracing 

 obtained by allowing the blood to spirt 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. 49, and on several of the other curves, and is frequently 

 called the predicrotic wave ; it may become very prominent. Some- 

 times other secondary waves, often called ' post-dicrotic ', are seen 

 following the dicrotic wave, as at D in Fig. 49, 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 



