ELEMENTARY EXPERIMENTAL PHYSIOLOGY 



123 



is convenient, but Mackenzie's polygraph allows long continuous 

 tracings to be taken in ink on white paper, and is a far more valuable 

 clinical instrument. Apply the sphygmograph to the radial artery, as 

 in Fig. 121. The right position of the button may be found by marking 



Fio. 121. Dudgeon's sphygmograph. 



the position of the pulse with ink. The pressure of the instrument can 

 be varied both by the straps and by the dial which regulates the 

 pressure of the spring. The instrument should be applied with a 

 pressure sufficient to flatten the artery, and then the pressure should 



Fio. 122. Marey's sphygmograph. 



be diminished until the maximal excursion is obtained. We have no 

 means of accurately reading the pressure of the spring or the changes 

 of pressure indicated by the pulse curve. The instrument gives us the 

 form of the pulse curve only. When the smoked paper is in position, 

 and the writing style placed upon it, and the maximum excursion 

 obtained, the clock is started and the record taken. The pulse curve 

 consists of a primary and several secondary waves. The primary wave 

 is the wave of expansion produced by the systolic output of the heart 



