668 SPECIAL PHYSIOLOGY. 



ventricles, or, rather, it happens somewhat later than the commence- 

 ment of the ventricular contraction. Now, the pulse-wave passes 

 along the larger arteries, at the termination of the ventricular con- 

 traction, i. e., after the impulse of the heart is felt on the side of the 

 chest; and it takes Jth of a second to reach the radial artery at the 

 wrist. Nevertheless, the pulse is felt even in the most distant parts 

 of the arterial system, before the second sound of the heart is heard, 

 whilst the cause of this sound is the sudden closure of the semilunar 

 valves across the mouth of the aorta and the pulmonary artery. This 

 fact, as first pointed out by Colt, refutes the following theory of We- 

 ber, once so generally adopted, as to the cause of the propagation of 

 the pulse-wave. That physiologist supposed that the aortic semilunar 

 valves, being closed by the backward movement of the blood near 

 them, owing to the recoil of the walls of the aorta nearest to the 

 heart, acted as a fulcrum, from which the blood was propelled onwards 

 by the yet unused resilient force of the aorta, into more and more dis- 

 tant portions of the arterial system, so as to produce the successive 

 wave-like distension of their coats. 



This theory of Weber assumes a minor cause, in place of the greater 

 and true one, viz., only a residual portion of the force originating in 

 a ventricular stroke, instead of the whole ventricular impulse. The 

 closure of the aortic valves is not essential to the phenomenon of the 

 pulse, which occurs before the second sound, when these valves are 

 open. But if the pulse-wave be essentially due to the direct force of 

 the heart, communicated through the arterial blood-column acting at 

 the closure of the mitral valve, so the closure of the aortic semilunar 

 valves is not without effect on that blood-column, and on the arteries 

 which contain it. When the pulse is very accurately examined, a sub- 

 sidiary wave occurs after the principal one, producing the phenomena 

 named dichrotism or the dichrotal pulse, and this, as will be soon ex- 

 plained, has been referred by some to the effects of the closure of 

 these valves. 



For the investigation of this and other phenomena of the pulse, 

 instruments named sphygmographs have been devised. The original 

 apparatus of Vierordt consists of a long, slender, well-balanced, hori- 

 zontal lever, or measuring rod, supported, near one end, on a proper 

 fulcrum, and having a short vertical stem projecting downwards from 

 near the fulcrum, and ending in a little button, which rests upon the 

 artery. To some part of the lever, near the button, are attached cer- 

 tain contrivances, to secure a true vertical motion ; and, at its free 

 end, it carries a short pencil, the point of which rests against an up- 

 right cylinder or drum covered with paper. To this cylinder, when 

 the instrument is in action, a known rate of motion is imparted by 

 clockwork ; at each pulsation of the artery against the button, the 

 lever rises and falls, and so the pencil describes an up and down line 

 on the revolving drum of paper. In this way a series of pulsations 

 are recorded by an up and down waved line of a peculiar character. 

 In Marey's improved instrument, and in other still later ones, the 

 delicacy of record is more perfect, the lever is longer and lighter, its 

 motions are steadied by the addition of a slight spring, and the pencil 



