FORM OF THE PULSE. 73 



sphygmograph of Marey, to whose researches on the circulation we have repeatedly 

 referred. The instrument simply amplifies the changes in the caliber of the vessel; and, 

 although its application is, perhaps, not so easy as to make it generally useful in practice, 

 in the hands of Marey it has given us a definite knowledge of the physiological character 

 of the pulse and its modifications in certain diseases, information which is exceedingly 

 desirable and which could not be arrived at by other means of investigation. In short 

 its mechanism is so accurate that, when skilfully used, it gives on paper the actual 

 " form of the pulse." This instrument, applied to the radial artery, gives a trace very 

 different from that obtained by Vierordt, which was simply a series of regular eleva- 

 tions and depressions. A comparison of the traces obtained by these two observers 

 gives an idea of the defects which have been remedied by Marey ; for it is evident that 

 the dilatation and contraction of the arteries cannot be so regular and simple as would 

 be inferred merely from the trace made by the instrument of Vierordt. 



Analyzing the traces of Marey, we see 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 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 amount 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 circumstance 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 dimin- 

 ishes 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 oft' 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 manifestation 

 of dicrotism is diminished tension, which is always found coexisting with a very marked 

 exhibition of this phenomenon. 



The delicate instrument employed by Marey enabled him to accurately determine and 

 register these various phenomena,- by observations on the arteries of the human subject 

 and the lower animals ; and, by means of an ingeniously constructed " schema," repre- 

 senting the arterial system by elastic tubes and the left ventricle by an elastic bag pro- 

 vided with valves and acting as a syringe, he satisfactorily established the conditions of 

 tension, etc., necessary to their production. In this schema, the registering apparatus, 

 simpler in construction than the sphygmograph, could be applied to the tubes with more 

 accuracy and ease. He demonstrated, by experiments with this system of tubes, that 

 the amplitude of the pulsations, the force of the central organ being the same, is greatest 



