36 EXPERIMENTAL PHYSIOLOGY. 



to vibrations produced by the rapidly contracting muscle. The 

 sound heard is usually an overtone because the actual vibration 

 rate is near the lower limit of audibility. 



Third, a magnified record obtained by means of a lever, of a 

 muscle, under sustained voluntary contraction, is not perfectly 

 smooth, but contains many fine regularly occurring contractions 

 superimposed upon the mam contraction (Fig. 10). 



FIG. 11. The effects of successive stimuli on skeletal muscle and 

 cardiac muscle. The vertical marks show where stimuli were introduced. 

 Tracings at bottom are the result of stimuli sufficiently close together to pro- 

 duce tetanus. Notice that the heart shows neither summation nor tetanus. 

 (Compiled from tracings published by T. G. Brodie and Leonard Hill). 



Finally, in order to produce a sustained artificially stimulated 

 contraction resembling a voluntary contraction, it is necessary to 

 send into the muscle a rapid succession of stimuli. Such a con- 

 traction is called tetanus. 



