MECHANICS OF THE CIRCULATION IN THE VESSELS 109 



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 the Arteries. 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 occu*- 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 Arterial 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. Poise uille, 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, supporting a style 

 which writes on a revolving drum, or kymograph. (For the method 

 of taking a blood-pressure tracing, see p. 208.) 



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. 93). 



A blood-pressure tracing taken from an artery with a manometer 

 of this sort yields the truest picture of the pulse -wave which it is 

 possible to obtain, because the reproduction of it is the most direct. 

 The fact that such a tracing shows a close agreement with the trace 

 of a good sphygmograph properly applied to the corresponding artery 

 on the other side is a striking proof of the general accuracy of the 

 sphygmographic method for physiological purposes, and enables us to 

 guide ourselves in transferring to man, in whom, of course, the sphyg- 

 mograph can alone be used, the information derived from direct 

 manometric observations in animals. 



