164 



EXPERIMENTAL PHYSIOLOGY 



is greatly increased, and so, too, is the wave length. Moreover, as 

 the velocity increases, the secondary waves, due to reflection at the 

 peripheral resistance, occur at an earlier point upon the descending 

 part of the record. 



From a consideration of the pulse wave as studied in a schema, 

 we may now pass to an examination of the pulse as occurring in an 

 artery during life. The instruments by which this is effected are 

 termed sphygmographs, and their aim is to record changes in diameter 

 of the artery with each heart beat. These changes in diameter are 

 due to changes of the pressure of the blood in the artery, and con- 

 sequently in taking a pulse tracing we are in reality recording the 

 variations of blood pressure in that artery. 



Experiment 3. — Take a pulse tracing by means of Marey's Sphyg- 

 mograph, fig. 126. 



It consists of a recording lever which writes on a blackened surface moved 

 by clockwork. Fig. 127 shows the principle of the instrument. It is supported 



"Pr" 



Fig. 126. — Marey's Sphygmograph. 



Fig. 127. — Diagram to Show the Arrangement of the Levers in 

 Marey's Sphygmograph. 



upon the arm by two runners, between which lies a button, k, placed at the 

 end of a spring, s' 2 . The button is placed over the radial artery at the wrist, 





