98 A MANUAL OF PHYSIOLOGY 



rate that is independent of the velocity of the current. The 

 passage of the ripple is not a bodily transference of the 

 particles of water of which at any given moment the wave is 

 composed, but the propagation of a change of relative position 

 of the particles. The mere fact that the ripple can pass up 

 stream as well as down is sufficient to .illustrate this. The 

 pulse-wave does not, however, correspond in every respect 

 to a ripple on a stream, for the bodily transfer of the blood 

 depends upon the series of blood-waves which the heart 

 sets travelling along the arteries. Every particle of blood 

 is advanced, on the whole, by a certain distance with every 

 pulse-wave in which for the time it takes its place. But 

 no particle continues in the front of the pulse-wave from 

 beginning to end of the arterial system. The * delay ' or 

 * retardation ' of the pulse (the interval, say, between the 

 beginning of the ascent of the carotid and radial curves) is 

 practically constant in the same individual, not only in 

 health, but also in most diseases. But the retardation is 

 markedly increased when the pulse-wave has to pass 

 through a portion of an artery whose lumen is either greatly 

 widened (aneurism), or greatly constricted (endarteritis 

 obliterans). 



The velocity of the pulse-wave has sometimes been de- 

 duced by comparing a tracing of the cardiac impulse with 

 a pulse-tracing taken at the same time from a distant artery. 

 But, as we have seen in dealing with the action of the heart, 

 the ventricle does not at the very beginning of its contraction- 

 acquire sufficient force to cause the opening of the semi- 

 lunar valves. The pulse, therefore, even in the aorta, must 

 lag behind the ventricular pulse ; and the amount of this 

 4 lag ' must be subtracted from the total retardation. But 

 since the aortic 'lag,' unlike the retardation between two 

 arteries, varies greatly even in health, depending as it does 

 on the arterial blood-pressure, this method of determining 

 the velocity of the pulse-wave is not satisfactory. 



The Blood-pressure Pulse. In man it is only possible to 

 trace the pulse-wave along the arteries by movements of 

 the walls of the vessels transmitted through the overlying 

 tissues. In animals the changes of pressure that occur in 



