FORM OF THE PULSE 55 



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 con- 

 traction 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, indi- 

 cated 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 on the degree of constant 

 tension of the vessels. Marey has found that the amplitude is in an 

 inverse ratio to the tension; which is 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 greater. 



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 



Fig. 24. Trace of Marey. 

 Portions of four traces taken in different conditions of the pulse. 



properties of the larger arteries render this inevitable. As they yield to 

 the distending force of the heart, reaction occurs after this force is taken 

 off, and if the distention is very great, gives a second impulse to the 

 blood. This is quite marked, unless the tension of the arterial system 

 is 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 marked exhibition of this phenomenon. 

 Marey accurately determined and registered these various phenomena 

 and demonstrated that an important and essential element in the produc- 

 tion of dicrotism is the tendency to oscillation of the fluid in the vessels 

 during the intervals between the contractions of the heart. This can 

 occur only in a fluid that has a certain weight and acquires a velocity 

 from the impulse ; and when air is introduced into the apparatus, dicro- 

 tism can not be produced under any conditions, as the fluid does not pos- 

 sess weight enough to oscillate between the impulses. Water showed a 

 well-marked dicrotic impulse under favorable conditions ; and with mer- 

 cury, the oscillations made two, three or more distinct impulses. By 

 these experiments, he proved that the blood oscillates in the vessels, if 

 this movement is not suppressed by too great pressure or tension. This 



