FLOW OF BLOOD THROUGH THE ARTERIES 



923 



FIG. 414. Diagram of arrangement of re- 

 cording lever in Dudgeon's sphygmo- 

 graph. 



Either form of sphygmograph is generally applied to the radial artery, 

 since this is near the surface and is supported by bone, and the arm is well 

 adapted for the application of the sphymograph. The pulse-curve obtained 

 by means of a sphymograph varies according to the artery employed and 

 the force with which the lever presses on the artery, but all the curves 

 present the same general features. 



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 

 of transmission by the time occupied 

 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 

 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. 415 represents a pulse-curve, taken from the radial 

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

 uninterrupted. We have alreadv 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 may occur 

 on the curve after e which are called post-dicrotic waves. 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 

 compare 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 



FIG. 415. Pulse-curve from radial artery. 



