THE PULSE, 241 



( The pulse is then a wave of increased pressure started by 

 the ventricular systole, radiating from the semilunar valves 

 over the arterial system, and grflflp ft] ]y d i ftppea-rl " g in 

 the smaller branches. -fn the aorta the pulse is most marked, 

 for the^resistance thereto^Ehe transmission onwards of the 

 blood sentS 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 consider- 

 able. The aorta, however, gradually squeezes out the ex- 

 cess 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 difficulty in passing it on; conse- 

 quently 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 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 ca- 

 pillaries. 



The pulse-wave travels over the arterial system at the 

 rate of about 9 meters (29.5 feet) in a second, commencing 

 at the wrist 0.159 seconds, and in the posterior tibial artery 

 at the ankle 0.193 seconds, after the ventricular systole. 

 The blood itself does not of course travel as fast as the 

 pulse-wave, for that quantity sent into the aorta at each 

 heart-beat does not immediately rush on over the whole 

 arterial system, but by raising the local pressure causes the 

 vessel to squeeze out faster than before some of the blood it 

 already contains, and this entering its branches raises the 

 pressure in them and causes them to more quickly fill their 

 branches and raise the pressure in them; the pulse-wave or 

 wave of increased pressure is transmitted in this way much 

 faster than any given portion of the blood. How the 

 wave of increased pressure and the liquid travel at differ- 

 ent rates may be made clearer perhaps by picturing what 

 would happen if liquid were pumped into one end of an 

 already full elastic tube, closed at the other end. At the 

 closed end of the tube a dilatation and increased tension 



