io 4 



tracings, and it is important to recognize their cause, so that no 

 weight may be given to them. 



In the explanation of the pulse-tracing, a fundamental fact to be 

 borne in mind is the elasticity of the vessels. When an incompres- 

 sible fluid like water is injected by an intermittent pump into one end 

 of an elastic tube a wave is set up, which is transmitted to the other 

 end of the tube. It is a positive wave that 

 is, it causes an increase of pressure and an 

 expansion of the tube wherever it arrives; 

 and if a series of levers be placed in contact 

 with the tube, they will rise and sink in 

 succession as the wave passes them. After 

 the passage of this primary wave the walls of 

 the tube, instead of coming instantly to rest 

 in their original position, regain it by a series 

 of oscillations, first shrinking too much, then 

 expanding too much, but at each movement 

 coming nearer to the position of equilibrium. 

 Each vibration of the elastic wall is of course 

 accompanied by a change of pressure in the 

 contents of the tube. This change of pressure 

 runs along the tube as a wave; and such 

 waves, succeeding the primary one, may be 

 called secondary waves of oscillation. These 

 secondary waves will be set up in an elastic 

 system whether the distal end of the system 

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

 sufficiently obstructed without being actu- 

 ally 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 posi- 

 tive 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 in- 

 vention 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 



Fig. 39. Pulse - Tracings 

 from Different Arteries 

 (v. Frey). T, temporal; 

 R, radial ; P, artery of foot. 



Fig. 40. Pulse-Curve from 

 Human Aorta (after Tiger - 

 stedt). 



