43G AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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

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

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

 dicrotic wave. 



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

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

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

 believed by 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 opinion and 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 same relative position as in a sphygmogram taken from an artery 

 near 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 be 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 

 already stated, however, the probabilities are in favor of the valvular origin 

 of the dicrotic wave. 



If it be true that the closure of the aortic valve causes the dicrotic wave, 

 the instant marked by the commencement of this wave, in the manometric 

 trace inscribed by the pressure within the first part of the arch of the aorta 

 itself, practically marks the instant of closure of the aortic valve. We have 

 seen (p. 422) that this doctrine has been made use of in the elucidation of the 

 curve of the pressure within the ventricle. 



The Diagnostic Limitations of the Sphygmogram. The feeling of 

 the pulse, imperfect as is the most skilled touch, cannot be replaced by the 

 use of the sphygmograph. The presence, between the cavity of the artery 

 and the surface of the body, of a quantity of tissue the amount and elasticity 

 of which differ in different people, and even differ over neighboring points of 

 the same artery, renders it impossible so to adjust the spring of the sphygmo- 



