164 THE PULSE. [BOOK i. 



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 axil- 

 lary. So also in the arteries of children, the wave travels more 

 slowly than in the more rigid arteries of the adult ; and the velocity 

 appears to be increased by circumstances which heighten and 

 decreased by those which lessen the mean arterial pressure, since 

 with increasing or diminishing pressure the arterial walls become 

 more or less rigid. 



4. When two curves taken at different distances from the 

 pump are compared with each other, the far curve will be found to 

 be shallower, with a less sudden rise, and with a more rounded 

 summit than the near curve : compare 5a with la, Fig. 27. In 

 other words, the pulse-wave as it travels onward becomes diminished 

 and flattened out. If a series of levers, otherwise alike, were 

 placed at intervals on a piece of tubing sufficiently long to convert 

 the intermittent stream into a continuous flow, the pulse-wave 

 might be observed to gradually flatten out and grow less until it 

 ceased to be visible. 



Care must be taken not to confound the progression of the 

 pulse-wave with the progression of the fluid itself. The pulse- 

 wave travels over the moving blood somewhat as a rapidly moving 

 natural wave travels along a sluggishly flowing river, the velocity 

 of the pulse-wave being 9 metres per sec., while that of the current 

 of blood is not more than half a metre per sec. even in the large 

 arteries, and diminishes rapidly in the smaller ones. 



Taking the duration of the pulse-wave, that is the time taken 

 by any point in the arterial tract, in expanding and returning to 

 its former calibre, so low as -^ of a second, it is evident that the 

 pulse-wave started by any one systole, even if it travels so slowly 

 as 5 m. per sec., will before it is completed have reached a 

 point y^ of 5m. = 2 in. distant from the ventricle. But even in the 

 tallest man the tips of the toes are not 2 m. distant from the heart. 

 In other words, the length of the pulse-wave is much greater 

 than the whole length of the arterial 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. 



The general causation of the pulse may then be summed up 

 somewhat as follows. The systole of the ventricle drives a 

 quantity of blood into the already full aorta. The sudden injection 

 of this quantity of blood expands the portion of the aorta next 

 to the heart, and thus gives rise to the sudden up-stroke of the 

 pulse-curve. The rapidity of the flow from the ventricle being 

 greatest at its beginning, the maximum of expansion is soon 

 reached, and the aortic walls, even while for a short time blood is 

 still, with diminishing rapidity, issuing from the ventricle, tend by 

 virtue of their elasticity to return to their former calibre. This 



