PRODUCTION OF THE PULSE. 69 



In short, their mechanism is so accurate, that when skillfully used, they give 

 on paper the actual "form of the pulse." The modern instruments, 

 applied to the radial artery, give traces very different from those obtained 

 by Vierordt, which were simply series of regular elevations and depressions. 

 A comparison of these with the traces obtained by Vierordt gives an idea of 

 the defects which have been remedied by Marey ; for it is evident that the 

 dilatation and contraction of the arteries can not be so regular and simple 

 as would be inferred merely from the trace made by the instrument of 

 Vierordt. 



Analyzing the traces taken by Marey, it is seen that there is a dilatation 

 following the systole of the heart, marked by an elevation of the lever, 

 more or less sudden, as indicated by the angle of the trace, and of greater 

 or less amplitude. The dilatation having arrived at its maximum, is 

 followed by reaction, which may be slow and regular, or may be, and 

 generally is, interrupted by a second and slighter upward movement of the 

 lever. This second impulse varies very much in amplitude. In some rare 

 instances, it is nearly as marked as the first and may be appreciated by the 

 finger, giving the sensation of a double pulse following each contraction of 

 the heart. This is called the dicrotic pulse. As a rule, the first dilatation of 

 the vessel is sudden and is indicated by an almost vertical line. This is 

 followed by a comparatively slow reaction, indicated by a gradual descent of 

 the trace, which is not, however, absolutely regular, but is marked by a slight 

 elevation indicating a second impulse. The amplitude of the trace, or the 

 distance between the highest and the lowest points marked by the lever, 

 depends upon the degree of constant tension of the vessels. Marey has 

 found that the amplitude is in an inverse ratio to the tension ; which is very 

 easily understood, for when the arteries are but little distended, the force of 

 the heart must be more marked in its effects than when the pressure of 

 blood is very great. Any condition which facilitates the flow of blood 

 from the arteries into the capillaries will, of course, ' relieve the tension of 

 the arterial system, lessen the obstacle to the force of the heart, and increase 

 the amplitude of the pulsation, and vice versa. In support of this view, 

 Marey has found that cold applied to the surface of the body, contracting, as 

 it does, the smallest arteries, increases the arterial tension and diminishes the 

 amplitude of the pulsation, while a moderate elevation of temperature pro- 

 duces an opposite effect. 



In nearly all the traces given by Marey, the descent of the lever indicates 

 more or less oscillation of the mass of blood. The physical properties of the 

 larger arteries render this inevitable. As they yield to the distending 

 influence of the heart, reaction occurs after this force is taken off, and if the 

 distention be very great, gives a second impulse to the blood. This is quite 

 marked, unless the tension of the arterial system be so great as to offer too 

 much resistance. One of the most favorable conditions for the manifesta- 

 tion of dicrotism is diminished tension, which is always found coexisting 

 with a very marked exhibition of this phenomenon. 



Marey accurately determined and registered these various phenomena, by 



