40 EXPERIMENTAL PHYSIOLOGY 



graph; see Fig. 66) is fixed against the masseter muscle; when this 

 muscle is made to contract voluntarily, its movements are com- 

 municated to the air within the cardiograph, and the differences of 

 pressure produced are transmitted to the second or recording tambour, 

 which writes upon a revolving drum. 



Sound of a voluntary contracting muscle. Place the tips of the 

 middle fingers in the ears, and contract the muscles of the arm strongly. 

 A rumbling sound is heard, which is caused by the vibration of the 

 contracting fibres. The sound actually heard is modified by the 

 resonance of the drum of the ear, and cannot be taken to indicate the 

 rhythm of contraction. 



