366 THE HUMAN BODY 



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 more quickly to 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 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 in- 

 creased 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 pulsa- 

 tion not by flowing along the tube itself, but by giving 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 phe- 

 nomena observed would not be essentially 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 morej as for example whether it is a readily 

 compressible or "soft pulse" showing a low arterial pressure, or 

 tense and rigid (" a hard pulse ") indicative of high arterial pres- 

 sure, 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 sanie individual it is faster when standing than when 

 sitting, and when sitting than when lying down. Any exercise 

 increases its rate temporarily, 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 

 conclusions from it. In children the pulse is quicker than in 

 adults, and in old age slower than in middle life. 



