ioo A MANUAL OF PHYSIOLOGY 



of the blood-stream itself, which is not one-thirtieth as great. 

 A ripple passes over the surface of a river at its own rate a 

 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 par- 

 ticles. The mere fact that the ripple can pass upstream 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 

 (in aneurism), or greatly constricted (in endarteritis obliterans). 

 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 the blood itself 

 can be directly registered, and these changes may be spoken of 

 as the blood-pressure pulse. At bottom, as already pointed 

 out, the phenomenon is exactly the same as that we have been 

 dealing with in our study of the external pulse. We are only 

 now to follow, by a more direct, and in some respects a more 

 perfect method, the same wave of blood along the same channel. 



Measurement of the Blood-pressure. Hales was the first to 

 measure the blood-pressure. This he did by connecting a tall glass 

 tube with the crural artery of a horse. The height to which the blood 

 rose in the tube indicated the pressure in the vessel. Poiseuille, 

 nearly half a century later, applied the mercury manometer, which 

 had already been used in physics, to the measurement of blood- 

 pressure. Ludwig and others improved this method by making the 

 manometer self-registering by means of a float in the open limb, sup- 

 porting a style which writes on a revolving drum, or kymograph. 

 (For the method of taking a blood-pressure tracing, see p. 195.) 



For reasons already mentioned the mercurial manometer is better 

 suited for measuring the mean blood-pressure, or for recording 

 changes in the pressure which last for some time, than for following 

 the rapid variations of the pulse- wave. For the latter purpose, one 

 of the class of elastic manometers is required (p. 86). 

 ; ;A bipod-pressure tracing taken from an artery with a manometer 

 of tms; r Gc;rt yields' tiie truest picture of the pulse- wave which it is 



