THE PULSE. 



181 



in the form of a wave. At one moment the lever is at rest ; the tube 

 beneath it is simply distended to the normal amount indicative of the mean 

 pressure which at the time obtains in the arterial tubes of the model ; at the 

 next moment the pulse expansion reaches the lever, and the lever begins to 

 rise ; it continues to rise until the top of the wave reaches it, after which 

 it falls again until finally it comes to rest, the wave having completely 

 passed by. 



It may perhaps be as well at once to warn the reader that the figure which 

 we call the pulse-curve is not a representation of the pulse-wave itself; it is 

 simply a representation of the movements, up and down, of the piece of the 

 wall of the tubing at the spot on which the lever rests during the time that 

 the wave is passing over that spot. We may roughly represent the wave 

 in the diagram Fig. 65 in which the wave shown by the dotted line is 

 passing over the tube (shown in a condition of rest by the thick double 

 line) in the direction from H to C. It must, however, be remembered 



FIG. 65. 



A rough Diagrammatic Representation of a Pulse-wave passing over an Artery. 



that the wave thus figured is a much shorter wave than is the pulse-wave 

 in reality (that being, as we shall see, about 6 metres long), i. e., occu- 

 pies a smaller length of the arterial system from the heart H toward the 

 capillaries C. 



The curves below, X, Y, Z, represent, in a similarly diagrammatic fash- 

 ion, the curves described, during the passage of the wave, by levers placed 

 on the points x, y, z. At Z the greater part of the wave has already passed 

 under the lever, which during its passage has already described the greater 

 part of its curve, shown by the thick line, and has only now to describe the 

 small part, shown by the dotted line, corresponding to the remainder of the 

 wave from Z to H. At Fthe lever is at the summit of the wave. At X 

 the lever has only described a small part of the beginning of the wave, viz., 

 from Cto x, the rest of the curve, as shown by the dotted line, having yet 

 to be described. 



But to return to the consideration of Fig. 64. 



130. The rise of each lever is somewhat sudden, but the fall is more 

 gradual, and is generally marked with some irregularities which we shall 

 study presently. The rise is sudden because the sharp stroke of the pump 

 suddenly drives a quantity of fluid into the tubing and so suddenly expands 

 the tube ; the fall is more gradual because the elastic reaction of the walls 

 of the tube, which brings about the return of the tube to its former calibre 



