ARTERIAL PRESSURE. THE PULSE. 247 



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, etc. 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 de- 

 scribed. jfBut 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, ra- 

 diating 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 resistance there to 

 the transmission onwards of the blood sent in by the heart is 

 greatest, and the elastic tube in which it consequently accu- 

 mulates 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 becomes distributed over a wider area, and these 

 branches having less resistance in front find less and less diffi- 

 culty in passing it on ; consequently the pulse-wave becomes 

 less and less conspicuous and finally altogether disappears be- 

 fore the capillaries are reached, the excess of liquid in the 

 whole arterial system after a ventricular systole being too 

 small to sensibly raise the mean pressure 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 metres (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 



