CHAP, iv.] THE VASCULAR MECHANISM. 263 



pulse-wave, takes some time to travel along the tube. In the 

 same way it would be found that the rise of the near lever began 

 some fraction of a second after the stroke of the pump. 



The velocity with which the pulse-wave travels depends chiefly 

 on the amount of rigidity possessed by the tubing. The more 

 extensible (with corresponding elastic reaction) the tube, the slower 

 is the wave ; the more rigid the tube becomes, the faster the wave 

 travels ; in a perfectly rigid tube, what in the elastic tube would 

 be the pulse, becomes a mere shock travelling with very great 

 rapidity. The width of the tube is of much less influence, though 

 according to some observers the wave travels more slowly in the 

 wider tubes. 



The rate at which the normal pulse-wave travels in the human 

 body has been variously estimated at from 10 to 5 meters per 

 second. In all probability the lower estimate is the more correct 

 one ; but it must be remembered that the rate may vary very 

 considerably under different conditions. According to all observers 

 the velocity of the wave in passing from the groin to the foot is 

 greater than that in passing from the axilla to the wrist (6 m. 

 against 5 m.). This is probably due to the fact that the femoral 

 artery with its branches is more rigid than the axillary and its 

 branches. So also in the arteries of children, the wave travels 

 more slowly than in the more rigid arteries of the adult. The 

 velocity is also increased by circumstances which heighten and 

 decreased by those which lessen the mean arterial pressure, since 

 with increasing pressure the arterial walls become more and with 

 diminishing pressure less rigid. Probably also the velocity of the 

 pulse-wave depends on conditions of the arterial walls, which we 

 cannot adequately describe as mere differences in rigidity. In 

 experimenting with artificial tubes it is found that different 

 qualities of indiarubber give rise to very different results. 



Care must be taken not to confound the progress of the pulse- 

 wave, i.e. of the expansion of the arterial walls, with the actual 

 onward movement of the blood itself. The pulse-wave travels 

 over the moving blood somewhat as a rapidly moving natural 

 wave travels along a sluggishly flowing river. Thus while the 

 velocity of the pulse- wave is 6 or possibly even 10 meters per sec., 

 that of the current of blood is not more than half a meter per sec. 

 even in the large arteries, and is still less in the smaller ones. 



144. Referring again to the caution given above not to 

 regard the pulse-curve as a picture of the pulse-wave, we may now 

 add that the pulse-wave is of very considerable length. If we know 

 how long it takes for the pulse- wave to pass over any point in the 

 arteries and how fast it is travelling, we can easily calculate the 

 length of the wave. In an ordinary pulse-curve the artery, owing to 

 the slow return, is seen not to regain the calibre which it had before 

 the expansion, until just as the next expansion begins, that is to say, 

 the pulse-wave takes the whole time of a cardiac cycle, viz. T 8 ^ths 



