160 THE CIRCULATION. 



It was formerly supposed that the pulse was caused not 

 by the direct action of the ventricle, but by the propaga- 

 tion of a wave in consequence of the elastic recoil of the 

 large arteries, after their distension ; and successive acts of 

 dilatation and recoil, extending along the arteries in the 

 direction of the circulation, were supposed to account for 

 the later appearance of the pulse in the vessels most dis- 

 tant from the heart. The observation of Mr. Colt, however, 

 that the pulse is perceptible in every part of the arterial 

 system previous to the occurrence of the second sound of 

 the heart, that is, previous to the closure of the aortic 

 valves, is a very forcible objection to this theory. For, if 

 the pulse were the effect of a wave propagated by the 

 alternate dilatation 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 proposed by Mr. Colt, which seems to recon- 

 cile all the facts of the case, and especially those two which 

 appear most opposed, namely, that the pulse always pre- 

 cedes 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 

 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 slowly. 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 



