300 



Messrs. J. C. Bramwell and A. V. Hill. 



centimetres per second. Expressing p in millimetres of Hg, and v in metres 

 per second, and substituting p = T055, this equation becomes, 



v = 0-357 <s/(V/[dV/dpl). 



But (dY/dp)/Y is the relative increase in the volume of the artery, per 

 millimetre of Hg increase of pressure. Working in percentages, therefore, the 

 equation finally becomes 



v = 3'57/y/ (percentage increase in volume per millimetre of 

 Hg increase of pressure). . 



This is the form most intelligible and convenient in use. It requires no 

 knowledge of the elastic coefficient as such, nor of the radius and the thick - 

 ness of the arterial wall, but only of one simple and directly observable 

 function of these, the rate of increase of volume with pressure. Thus an 

 observation of the velocity of the pulse wave in any particular vessel tells 

 us at once, in absolute units, the degree of extensibility of that vessel. 



The energy expended by the heart, per beat, has been shown by Ehode (2) 

 and others, to depend (other things being equal) on the pressure developed by 

 it. Thus, if the heart is to work efficiently, the output for a given pressure 

 should be as large as possible, which implies a large increase in the volume of 

 the arteries per millimetre of pressure developed, and — from the formula — a 

 low velocity of the pulse wave. Another sign of an efficient circulation is 

 that the flow through the capillaries should remain as high and as constant 

 as possible during diastole, which implies a large diminution of volume of 

 the arteries for a given fall of pressure, and again a low velocity of the pulse 

 wave. Hence a low velocity of the pulse wave is a sign, both of an efficient 

 and continuous circulation and of an economical functioning of the heart. 

 Thus the velocity of the pulse wave is one important criterion of the general 

 efficiency of the circulation. 



In a paper by Roy (3), in 1880, is given a series of curves showing the 

 relation between volume and pressure, in the case of arteries and veins, made 

 by an ingenious method, commanding every confidence in its accuracy. 

 Eeplotting these curves in rectangular co-ordinates, and measuring their 

 slopes at various points, it is possible to deduce the percentage increase in 

 volume per millimetre of Hg, and so to calculate the velocity of the 

 pulse wave at various pressures. The following results are obtained by 

 so doing : — 



