184 THE VASCULAR MECHANISM. 



began some fraction of a second after the stroke of the pump, meaning that 

 time is required for the transmission of the wave. 



The velocity with which the pulse-wave travels depends chiefly on the 

 amount of rigidity possessed by the tubing. The more extensible (with cor- 

 responding 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 metres per second. In all proba- 

 bility the lower estimate is the more correct one ; but it must be remembered 

 that the rate may vary very considerably under different conditions. Accord- 

 ing to all observers the velocity of the wave in passing from the groin to the 

 foot is greater than in passing from the axilla to the wrist (6 metres against 

 5 metres). 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 India-rubber 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 wall with the actual onward move- 

 ment of the blood itself. The pulse-wave travels over the moving blood 

 somewhat as a rapidly moving natural wave travels along a sluggishly flow- 

 ing river. Thus while the velocity of the pulse-wave is 6 or possibly even 

 10 metres per second, that of the current of the blood is not more than 

 half a metre per second even in the large arteries, and is still less in the 

 smaller ones. 



133. 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 \ths second to 

 pass by the lever. Taking the velocity of the pulse- wave as 6 metres per 

 second the length of the wave will be T 8 oths of 6 metres or nearly 5 metres. 

 And even if we took a smaller estimate, by supposing that the real expan- 

 sion and return of the artery at any point took much less time, say T %ths 

 second, the length of the pulse-wave would still be more than 2 metres. But 

 even in the tallest man the capillaries furthest from the heart, those in the 

 tips of the toes, are not 2 metres distant from the heart. In other words, the 

 length of the pulse-wave is much greater than the whole length of the arte- 

 rial system, so that the beginning of each wave has become lost in the small 

 arteries and capillaries some time before the end of it has finally passed away 

 from the beginning of the aorta. 



We must now return to the consideration of certain special features in 



