THE CIRCULATION OF THE BLOOD 365 



required by the aorta to pass on the. blood sent in during systole), 

 so the increased tension in the aorta immediately after the cardiac 

 contraction drives on some of its contents into its branches, and 

 fills these faster than they are emptying, and so causes a dilatation 

 of them also, which only gradually disappears as the aortic tension 

 falls before the next systole. Hence after each beat of the heart 

 there is a sensible dilatation of all the larger arteries, known as 

 the pulse, which becomes less and less marked at points on the 

 smaller branches farther from the heart, but which in health can 

 readily be recognized on any artery large enough to be felt by 

 the finger through the skin. The radial artery near the wrist, 

 for example, will always be felt tense by the finger, since it is 

 kept overfilled by the heart in the way already described. But 

 after each heart-beat it becomes more rigid and dilates a little, 

 the increased distension and rigidity gradually disappearing as 

 the artery passes on the excess of blood before the next heart- 

 beat. The pulse is then a wave of increased pressure started by 

 the ventricular systole, radiating from the semilunar valves over 

 the arterial system, and gradually disappearing in the smaller 

 branches. In the aorta the pulse is most marked, for the resist- 

 ance there to the transmission onwards of the blood sent in by 

 the heart is greatest, and the elastic tube in which it consequently 

 accumulates is shortest, and so the increase of pressure and the 

 dilatation caused are considerable. The aorta, however, gradually 

 squeezes out the excess blood into its branches, and so this be- 

 comes distributed over a wider area, and these branches having 

 less resistance in front find less and less difficulty in passing it on; 

 consequently the pulse-wave becomes less and less conspicuous 

 and finally altogether disappears before the capillaries are reached, 

 the excess of liquid in the whole arterial system after a ventricular 

 systole being too small to raise the mean pressure sensibly once it 

 has been widely distributed over the elastic vessels, which is the 

 case by the time the wave has reached the small branches which 

 supply the capillaries. 



The pulse-wave travels over the arterial system at the rate of 

 about 9 jneters (29.5 feet) in a second, commencing at the wrist 

 0.159 second, and in the posterior tibial artery at the ankle 0.193 

 second, after the ventricular systole. The blood itself does not 

 of course travel as fast as the pulse-wave, for that quantity sent 



