The Velocity of the Pulse Wave in Man. 



303 



an isolated artery lies in the fact that no considerable length of artery can be 

 obtained, and the time-interval available for measurement is therefore very 

 small. By replacing the blood with mercury, however, this interval can be 

 increased 3"58 times, and the utilisation of this fact makes it possible to 

 measure the velocity in an isolated artery with fair accuracy, and then to 

 obtain the velocity in an artery containiug blood by multiplication by 3 - 58. 

 The reason for this is as follows : the velocity of the pulse wave is inversely 

 proportional to the square root of the density of the fluid in the vessel, so that 

 replacing blood (p = T055) with mercury (p = 13 - 5) decreases the velocity in 

 the ratio ^/lS-o/l-OSS, i.e , of 3"58:1. This principle is embodied in the 



Fig. 1. 



apparatus shown in fig. 1. A length of artery is tied firmly at A and B on to 

 the copper pipes, which are clamped rigidly to a board. These copper pipes, 

 at their other ends, are firmly joined to two long rubber tubes, one of which 

 goes to a mercury reservoir capable of being raised and lowered, the other to 

 a mercury pressure gauge. Thus the pressure in the artery filled with 

 mercury (with bubbles carefully eliminated) can be adjusted to any required 

 value. At X and T in the copper pipes are two small windows, as shown in 

 the diagram on the left. Over each window a small thin piece of rubber tube 

 is carefully and firmly fixed, and on the rubber, over the edge of the window, 

 is glued a minute aluminium angle-piece carrying the finest possible bamboo 



