128 THE ELECTRICAL PHENOMENA OF MUSCLE [CH. XI. 



traction. It may be either a secondary twitch or secondary tetanus, 

 according as to whether the muscle B is made to contract singly or 

 tetanically. 



3. Secondary contraction from the heart. If an excised but still 

 beating frog's heart is used instead of muscle B, and the nerve of 

 A laid across it, each heart's beat, accompanied as it is by an electrical 

 variation, will stimulate the nerve and cause a twitch in the rheo- 

 scopic muscle A. 



