38 EXPERIMENTAL PHYSIOLOGY 



It must not be concluded that this represents the rate of the successive 

 individual contractions which fuse to form the voluntary contractions, for it 

 has been shown by photographic records of the capillary electrometer (see 

 p. 49) that in a voluntary contraction there are not less than fifty electrical 

 changes per second, and this probably also represents the number of mechanical 

 changes -which succeed one another in a voluntary contraction. The causation 

 of the waves of ten or twelve per second is not fully understood. 



Another method of obtaining the curve of a voluntary contraction 

 is by the use of a transmission myograph, which consists of two tam- 

 bours connected by india rubber tubing. The first or receiving 

 tambour (which may be represented by an ordinary Marey's cardio- 

 graph ; see fig. 63) 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 against a revolving drum. 



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

 fingsrs in the ears, and contract the muscles of the arm strongly. A rumbling 

 sound is heard,j,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. 



