CHAP, iv.] THE VASCULAR MECHANISM. 171 



the dicrotic wave. Besides the multitudinous peripheral division 

 would render one large peripherically reflected wave impossible. 



But in addition to reflected waves, other waves which may be 

 called "waves of oscillation," make their appearance when a fluid is 

 driven through a system of tubes, by means of an intermittent 

 force. And different origins have been assigned to secondary waves 

 of this description. 



FIG. 37. DICROTIC PULSE-CURVE DUE TO LOSS OF BLOOD. 



From carotid of rabbit with, extra-vascular pressures in a of 50 mm., & of 40 mm. 

 c of 20 mm., and d of 10mm. of mercury. 



Thus when the rapid flow of a fluid along a tube is suddenly 

 checked at a point of its course the inertia of the fluid will carry the 

 column of fluid still forwards so as to leave behind it a diminution 

 of pressure. This diminution will appear on a graphic record of 

 the pressure as a depression or notch ; and will be followed by a 

 secondary rise as a reflux of fluid takes place towards the point 

 where the pressure has become diminished. Both the depression 

 and the secondary rise will travel as a wave along the tube, being 

 frequently followed by other smaller waves of similar character and 

 similar origin. Waves thus originating have been appealed to as 

 explaining the secondary waves of the pulse-curve. Thus at the 

 moment when the ventricle, having emptied itself, ceases to throw 

 any more blood into the aorta, the blood which was last ejected 

 being carried forward by its inertia gives rise to a diminution of 

 pressure in the ventricle and at the root of the aorta. The aortic 

 walls forthwith contract upon this diminished pressure, and a reflux 

 of blood towards the semilunar valves takes place, leading to the 

 appearance of a depression or notch in the pulse-curve, which is 

 propagated forwards along the aorta. This reflux closes the semi- 

 lunar valves and at the same time leads to a recovery of pressure 



