54 CIRCULATION OF THE BLOOD 



It is evident from what is known of the modifications that occur in 

 the force of the heart and the quantity of blood in the vessels, and from 

 the changes which may take place in the calibre of the arteries, that the 

 characters of the pulse must be subject to great variations. Many of 

 these may be appreciated simply by the sense of touch. Writers treat 

 of the soft and compressible pulse, the hard pulse, the wiry pulse, the 

 thready pulse, etc., as indicating various conditions of the arterial system. 

 The character of the pulse in disease, aside from its frequency, has 

 always been regarded as of great importance. 



Form of the Pulse. But few of the characters of a pulsation, oc- 

 cupying as it does only one-seventieth part of a minute, can be ascer- 

 tained by the sense of touch alone. This fact has been appreciated by 

 physiologists; and within the last sixty years, instruments for registering 

 the pulse have been devised, with the view of analyzing the dilatation 

 and movements of the vessels. The instrument now used for this pur- 

 pose is called the sphygmograph. Vierordt (1855) constructed a complex 



apparatus, so arranged 

 that the impulse from 

 an accessible artery, like 

 the radial, was conveyed 

 to a lever, which marked 

 the movement on a re- 

 volving cylinder. These 



Fig. 23. Trace of Vierordt. ' ' 



traces, however, were 

 perfectly regular and simply marked the extremes of dilatation exag- 

 gerated, of course, by the length of the lever and the number of 

 pulsations in a given time. The instruments now in use differ from 

 each other mainly in the convenience with which they are applied, the 

 principle in all being substantially that of the sphygmograph of Marey. 

 The modern instruments, applied to the radial artery, give traces quite 

 different from those obtained by Vierordt, which were simply series of 

 regular elevations and depressions. A comparison of these with the 

 traces first obtained gives an idea of the defects that were remedied by 

 Marey ; for it is evident that the dilatation and contraction of the arte- 

 ries 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 dilata- 

 tion 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 

 ordinarily is, interrupted by a second and slighter upward movement of 



