THE CIRCULATION OF THE BLOOD AND LYMPH 95 



predicrotic wave. Oscillations, due to vibrations of the record- 

 ing apparatus, appear on many pulse-tracings, and it is important 

 to recognise 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 move- 

 ment 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 suffi- 

 ciently obstructed without being actually 

 closed, secondary waves of another kind 

 may also be generated, the primary wave 

 on arriving at the distal end being re- 

 flected 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 FIG. 33. PULSE-CURVE FROM 

 end of the stroke, a wave of diminished HUMAN AORTA (AFTER 



pressure a negative wave is generated TIGERSTEDT). 



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 sphygmo- 

 graph, perhaps no physiological question has been more dis- 

 cussed or is less understood than the mechanism of its production. 

 Two points, however, seem to be clear : (i) That it is a centri- 

 fugal, 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 the beginning of the primary elevation. This can only be 

 explained by supposing that it has the same point of origin, and 



