PULSE-TRACIXGS. 149 



follow or coincide with, but could never precede, the second 

 sound of the heart ; for the first effect of the elastic recoil 

 of the arteries first dilated is the closure of the aortic 

 valves ; and their closure produces the second sound. 



The theory which seems to reconcile all the facts of the 

 case, and especially those two which appear most opposed, 

 namely, that the pulse always precedes the second sound 

 of the heart, and yet is later in the arteries far from the 

 heart than in those near it, may be thus stated : It sup- 

 poses that the blood which is impelled onwards by the left 

 ventricle does not so impart its pressure to that which the 

 arteries already contain, as to dilate the whole arterial 

 system at once ; but that it enters the arteries, it displaces 

 and propels that which they before contained, and flows on 

 with what may be called a head-wave, like that which is 

 formed when a rapid stream of water overtakes another 

 moving more slowlj*. The slower stream offers resistance 

 to the more rapid one, till their velocities are equalized : 

 and, because of such resistance, some of the force of the 

 more rapid stream of blood just expelled from the ventricle, 

 is diverted laterally, and with the rising- of the wave the 

 arteries nearest the heart are dilated and elongated. They 

 do not at once recoil, but continue to be distended so long 

 as blood is entering them from the ventricle. The wave 

 at the head of the more rapid stream of blood runs on, 

 propelled and maintained in its velocity by the continuous 

 contraction of the ventricle : and it thus dilates in succes- 

 sion every portion of the arterial system, and produces the 

 pulse in all. At length, the whole arterial system (where- 

 in a pulse can be felt) is dilated ; and at this time, when 

 the wave we have supposed has reached all the smaller 

 arteries, the entire system may be said to be simulta- 

 neously dilated ; then it begins to contract, and the con- 

 tractions of its several parts ensue in the same succession 

 as the dilatations, commencing at the heart. The contrac- 

 tion of the first portion produces the closure of the valves 



