24:8 THE HUMAN BODY. 



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 press- 

 ure 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 different 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 would be felt immediately after each stroke 

 of the pump, although the liquid pumped in at the other end 

 would have remained about its point of entry; it would cause 

 the pulsation not by flowing along the tube itself, but by giv- 

 ing a push to the liquid already in it. If instead of absolutely 

 closing the distal end of the tube one brought about a state 

 of things more nearly resembling that found in the arteries 

 by allowing it to empty itself against a resistance, say through 

 a narrow opening, the phenomena observed would not be es- 

 sentially altered; the increase of pressure would travel along 

 the distended tube far faster than the liquid itself. 



The pulse being dependent on the heart's systole, " feeling 

 the pulse" of course primarily gives a convenient means of 

 counting the rate of beat of that organ. To the skilled touch, 

 however, it may tell a great deal more, as for example whether 

 it is a readily compressible or " soft pulse" showing a low ar- 

 terial pressure, or tense and rigid ("a hard pulse") indicative 

 of high arterial pressure, and so on. In adults the normal 

 pulse rate may vary from sixty-five to seventy-five, the most 

 common number being seventy-two. In the same individual 

 it is faster when standing than when sitting, and when sitting 

 than when lying down. Any exercise increases its rate tem- 

 porarily, and so does excitement ; a sick person's pulse should 

 not therefore be felt when he is nervous or excited (as the 

 physician knows when he tries first to get his patient calm 

 and confident), as it is then difficult to draw correct conclu- 

 sions from it. In children the pulse is quicker than in adults, 

 and in old age slower than in middle life. 



The Bate of the Blood-Flow. As the vascular system 



