CHAP, iv.] THE VASCULAR MECHANISM. 



231 



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. 63 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 ; 

 at other times it may be so marked as FIG. 64. ANACROTIC SPHYG- 



to give rise to a really double pulse MO RAPH TR ^ING 



. & _ r , . , /. , 1*1, THE ASCENDING AORTA 



(Fig. 65), i.e. a pulse which can be felt (Aneurism). 



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." 



FlG. 65. TWO GRADES OF MARKED DICROTISM IN RADIAL PULSE OF MAN. 



(Typhoid Fever.) 



Neither it nor any other secondary elevations can be recognized 

 in the tracings of blood pressure taken with a mercury manometer. 

 This may be explained, as we have said 121, by the fact that 

 the movements of the mercury column are too sluggish to repro- 

 duce these finer variations. 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, sometimes to a marked 

 extent, sometimes to a less extent, not only in sphygmograms and 

 in curves of arterial pressure taken by adequate instruments, but 

 also and 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. 63, 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. 63, 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 



