PULSE-TRACINGS. 127 



valves, is a fatal objection to this theory. For, if the pulse 

 were theVeffect of a wave propagated by the alternate dilata- 

 tion and contraction of successive portions of the arterial tube, 

 it ought, in all the arteries except those nearest to the heart, 

 to 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 supposes 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 ar- 

 teries, it displaces and propels that which they before con- 

 tained, 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 slowly. The slower stream offers resist- 

 ance to the more rapid one, till their velocities are equalized; 

 arid 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 main- 

 tained in its velocity by the continuous contraction of the ven- 

 tricle ; and it thus dilates in succession every portion of the 

 arterial system, and produces the pulse in all. At length, the 

 whole arterial system (wherein 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 

 simultaneously dilated ; then it begins to contract, and the 

 contractions of its several parts ensue in the same succession 

 as the dilatations, commencing at the heart. The contraction 

 of the first portion produces the closure of the valves and the 

 second sound of the heart ; and both it and the progressive 

 contractions of all the more distant parts maintain, as already 

 said, that pressure on the blood during the inaction of the 

 ventricle, by which the stream of the arterial blood is sustained 

 between the jets, and is finally equalized by the time it reaches 

 the capillaries. 



It may seem an objection to this theory, that it would prob- 

 ably require a larger quantity of blood to dilate all the ar- 



