THE FLOW OF BLOOD THROUGH THE ARTERIES 967 



The velocity of the pulse can be measured by taking simultaneous 

 tracings from two arteries separated by some distance from one another, 

 such as the femoral artery and the dorsalis pedis, or from the carotid and 

 radial arteries. In a healthy individual 

 the velocity varies between 7 and 10 

 metres per second. The more rigid 

 the arteries the greater will be the 

 velocity, so that the velocity of 

 propagation gradually increases with 

 advancing age, and is higher in the 

 arteries of the lower extremities 

 than in the more distensible arteries 

 of the arm. 



The length of the pulse wave can 

 be found by multiplying the velocity Circling 

 of transmission by the time occupied graph, 

 by the wave in passing any given 

 point. The duration of the wave at 

 any point corresponds to the time 

 of a cardiac cycle, viz. 0*8 sec., so 

 that if the velocity of transmission 

 be taken as 7 metres per second, the FlG - 425 - Pulse curve from T&di&l arter y- 

 length of the wave is about 5-6 metres. 



The pulse wave thus reaches the periphery long before it has been com- 

 pleted in the aorta. Fig. 425 represents a pulse curve taken from the radial 

 artery. The elevation due to the expansion of the artery is rapid and 

 uninterrupted. We have already explained that this is due to the sudden 

 pumping of blood into the first part of the aorta, whence the impulse is 

 transmitted as a wave along the arteries. The curve descends gradually 

 till the next beat occurs, since the elastic reaction of the arteries, which 

 tends to keep up the pressure, acts more constantly and steadily than 

 the heart beat. On this descending part of the curve occur two or three 

 secondary elevations : b is the primary or ' percussion ' wave, c the pre- 

 dicrotic or ' tidal ' wave, and e the dicrotic wave. Elevations, which are 

 called post-dicrotic waves, may occur on the curve after e. It is better to 

 class the elevations before the dicrotic notch d as systolic elevations, and 

 those afterwards, including the dicrotic elevation itself, as diastolic. 



For the exact understanding of these elevations it is necessary to com- 

 pare the pulse tracings taken from a small artery with the variations in 

 pressure which occur at the same time in the aorta and in the left ventricle 

 (Fig. 426). We are enabled in this way to dissociate the waves caused by 

 the ventricular systole from those which have their origin in the arterial 

 system. In Fig. 426 are given somewhat diagrammatically typical tracings 

 of the intra-auricular, intra ventricular, and aortic pressures during one 

 heart beat. The dotted lines represent approximately the sort of curve 

 which would be given by a sphygmograph applied to the aorta, taking 



