THEORY OF THE PULSE. 667 



ever, is the momentary distension and recoil of the coats of the artery, 

 propagated along the vessel from the heart onwards, after the manner 

 of a ivave-motion, and produced by the propulsion of successive quan- 

 tities of blood into the arterial system of the left ventricle ; and com- 

 mencing at the instant of closure of the mitral valve. The force trans- 

 ferred to those successive quantities of blood, is partly exhausted in 

 urging on the blood already in the vessels ; but the resistance thus 

 met with, as we have seen, diverts the force partly on to the elastic 

 sides of the arteries, and so distends them. 



This distension of the arteries occurs first in the aorta, close to the 

 heart, but rapidly follows along the entire arterial system. It con- 

 sists, not only of a lateral dilatation of the vessels, but also of an 

 elongation. The former change is but slight in arteries which can be 

 subjected to examination, and is too quick to be followed readily by 

 the eye ; whilst the latter is much more evident, as in the case of 

 superficial and tortuous arteries, such as the temporals, which may be 

 seen to become more curved during the passage of the pulse-wave 

 along them. 



The total amount of dilatation observed during the passage of a 

 pulse-wave along a given length of the carotid artery of a dog, has 

 been measured, by placing the artery in a tube filled with water, and 

 having another fine upright glass tube fitted into it ; the elevation of 

 the water in the latter, at each pulse, shows an increase of ^ s d of the 

 bulk of the piece of artery so inclosed. (Poiseuille.) According to 

 Vierordt, the increase is as much as y^th. The mechanical effect of 

 this combined dilatation and elongation, but especially of the elonga- 

 tion, of a living artery, and of its subsequent contraction and short- 

 ening, and particularly of the latter, is a movement of the vessel in its 

 bed, a motion which is visible in superficial arteries, especially in thin 

 and aged persons, and which can be rendered more perceptible by plac- 

 ing a small bristle across it. It is this change of place, or locomotion, 

 of the artery, which is the chief cause of the pulse felt on placing the 

 finger upon the vessel. The blood itself being practically incompress- 

 ible, the shock of the heart's stroke upon it is communicated, almost 

 instantly, throughout the whole blood in the arterial system ; but the ef- 

 fect of the distension, or distension-wave, which begins in the aorta, near 

 the heart, apparently takes a certain time to be continued onwards, for 

 reasons to be presently explained ; hence there is a certain measurable 

 rate in the propagation of the pulse to the distant arteries. This is 

 the theory of Marey. But the rate of motion of the distending pulse- 

 wave is much more rapid than the motion of the blood particles them- 

 selves within the vessels, being about 30 feet per second ; whilst, as 

 already stated, the velocity of the blood is only about 10J inches per 

 second in the carotid, and about 2J inches per second in the distant 

 arteries. This comparison will serve to impress on the mind the fact, 

 that the pulse-wave is not caused by the onward motion of the blood, 

 but by a wave-motion induced in the entire column of blood, which 

 operates in its passage, laterally, as well as longitudinally, on the coats 

 of the arteries. 



The impulse of the heart nearly coincides with the systole of the 



