144 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



duces the middle wave of each of the pulsations of Figure 26, is so constant 

 in its occurrence that it is undoubtedly a normal and important phenomenon, 

 although, in different sphygmograms, the height, and position in the trace, of 

 the wave inscribed by this oscillation may vary. Occasionally this oscillation 

 is morbidly exaggerated, so thai it may be not only recorded by the sphygmo- 

 graph, but even felt by the finger, as a second usually smaller up-stroke of 

 the pulse. In such a ease the artery is felt to beat twice at each single beat 

 of the ventricle, and is said, technically, to show a "dicrotic" 1 pulse. Where 

 a dicrotic pulse cau be detected by the finger, it is apt to accompany a mark- 

 edly low menu tension of the arterial wall. The dicrotic pulse was known, 

 and named, long before the sphygmograph revealed the fact that the pulse is 

 always dicrotic, although to a degree normally too slight for the linger to 

 appreciate. The sphygmographic wave which records the slight "dicrotism " 

 of the normal pulse is called the " dicrotic wave." Where dicrotism can be 

 felt bv the finger, the sphygmogram naturally exhibits a very conspicuous 

 dicrotic wave. 



The origin of the dicrotic oscillation has been much discussed, and is not 

 vet thoroughly settled, important as a complete settlement of it would be to 

 the true interpretation and clinical usefulness of the sphygmogram. It is 

 believed bv some that this fluctuation of pressure is produced at the smaller 

 arterial branches, as a reflection of the main pulse-wave, and that the dicrotic 

 wave, thus reflected, travels toward the heart, and, naturally, reaches a given 

 artery after the main wave of the pulse has passed over it, travelling in the 

 opposite direction. The weight of probability, however, is in favor of the 

 view that the dicrotic wave essentially depends upon a slight rise of the 

 arterial pressure, or slackening of its decline, due to the closing of the semi- 

 lunar valve ; and that, therefore, this wave follows the main wave of arterial 

 expansion outward from the heart, instead of being reflected inward from the 

 periphery. If the dicrotic wave be caused solely by reflection from the 

 periphery, it ought, in a sphygmogram from a peripheral artery, to begin at 

 a point nearer to the highest point of each pulsation than in the case of an 

 artery near the heart, in which latter vessel, naturally, a reflected wave would 

 undergo postponement. On the other hand, if the dicrotic wave be trans- 

 mitted toward the periphery, and caused solely by the closure of the aortic 

 valve, it ought, in a sphygmogram from a peripheral artery, to occupy very 



nearly the sa relative position as in a sphygmogram taken from an artery 



Dear the heart. But a wave running toward the periphery may be modified 

 by a reflected wave in the same vessel, and a reflected wave may undergo a 

 second reflection at the closed aortic valve, or even elsewhere, and thus give 

 rise to an oscillation which will he transmitted toward the periphery. These 

 statements show with what technical difficulties the subject is beset, whether 

 the sphygmograph be employed, or, in the case of animals, the elastic man- 

 ometer, the traces recorded by which also exhibit the dicrotic wave. As 



1 From iViKporcic, double-beating. 



