THE CIRCULATION OF THE BLOOD AND LYMPH 97 



(puhus parvus) refer to the increase in the quantity of blood which 

 every pulse-wave causes in the vessel. 



The ' force of the pulse ' is a phrase which is often ambiguously 

 used, sometimes apparently as synonymous with * strength,' and 

 sometimes with * size,' as above defined. In fact, the quantitative 

 information obtained by feeling the pulse with the finger, although 

 more valuable for clinical purposes than anything that can 'be 

 deduced from an ordinary sphygmographic record, is far inferior in 

 precision to the qualitative notion which that time-honoured pro- 

 ceeding affords. The * force of the pulse ' does not necessarily 

 correspond with the force of the heart. It depends partly on the 

 suddenness with which the pulse-wave distends the artery, partly on 

 the amount of this distension in relation to the previous permanent 

 distension, and to some extent on the calibre of the vessel. Other 

 things being equal, the pulse in a large vessel will feel stronger than 

 chat in a smaller vessel. This last factor accounts for the inequality 

 in the force of the pulse which is not infrequently found between the 

 two radials even of a healthy person. 



Rate of Propagation of the Pulse- wave. When pulse -tracings 

 are taken simultaneously at two points of the arterial system 

 unequally distant from the heart, by two sphygmographs 

 whose writing-points move in the same vertical straight line, 

 it is found that the ascent of the curve begins later at the 

 more distant, than at the nearer point. Since waves like 

 the pulse-wave travel with approximately the same velocity 

 in different parts of an elastic system like the arterial ' tree,' 

 this ' delay ' must be due to the difference in the length of 

 the two paths. The difference in length can be measured ; 

 the time-value of the ' delay ' can be deduced from the rate 

 of movement of the recording surface ; dividing the length 

 by the time, we arrive at the rate of propagation of the 

 pulse-wave. The average rate has been found to be about 

 7 metres per second in man in the arteries of the upper limb, 

 and 8 metres in those of the lower limb, the difference being 

 due to the smaller distensibility of the latter. The mean 

 velocity of the pulse-wave is about the same as the speed of 

 a moderately fast Atlantic liner (say 17 miles an hour), but 

 less than that of a wave of the sea in a strong gale. The 

 velocity of the pulse-wave must not be confounded with that 

 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 



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