92 A MANUAL OF PHYSIOLOGY 



be closed or open. But if it is closed, or sufficiently obstructed 

 without being actually closed, secondary waves of another kind may 

 also be generated, the primary wave on arriving at the distal end 

 being reflected there. The reflected wave running back towards the 

 central end may there again undergo reflexion, and pass out once 

 more towards the distal end as a centrifugal, twice-reflected wave. 

 When the liquid ceases to enter the tube at the end of the stroke, a 

 wave of diminished pressure a negative wave is generated at the 

 central end, and is propagated to the distal end, where it may be 

 reflected just like the positive wave. 



Although under certain conditions the dicrotic wave is so 

 marked that the double beat of the pulse was discovered 

 and named by physicians long before the invention of any 

 sphygmograph, perhaps no physiological question has been 

 more discussed or is less understood than the mechanism of 

 its production. Two points, however, seem to be clear : 

 (i) That it is a centrifugal, and not a centripetal, wave 

 that is to say, it travels away from, and not towards, the 

 heart ; (2) that the aortic semilunar valves have something 

 to do with its origin. 



It is not a centripetal wave, for in tracings taken at all 

 parts of the arterial path, no matter what the distance from 

 the heart and the capillaries (e.g., the origin of the carotid 

 and the radial at the wrist), the dicrotic wave is separated by 

 the same interval from beginning of the primary elevation. 

 This can only be explained by supposing that it has the same 

 point of origin, and travels with the same velocity and in the 

 same direction as the primary wave. It is not, then, a wave 

 reflected directly from the peripheral distribution of the artery 

 from which the pulse-tracing is taken. Nor does the conten- 

 tion of v. Frey and v. Kries, that it is a centrifrugal twice-re- 

 flected wave, seem more likely, although they have indeed by 

 experiments on newly-killed animals been able to detect the 

 traces of such waves. They suppose that these, reflected first 

 from peripheral points at which the blood-flow is particularly 

 obstructed (the bifurcations of the larger arteries, and the 

 small arteries and capillaries in general), run towards the 

 heart, and are then again reflected outwards from the semi- 

 lunar valves. 



Perhaps the explanation that best takes account of the 

 facts and renders most clear the role of the semilunar valves 



