POLAR STIMULATION 37 



each direction of the current. Note that there is no con- 

 traction at break. If the currents are further increased 

 in strength a contraction will also be obtained at break. 

 Unpin the joint, hold the muscle by the tendon, and 

 dip the upper third to a half of the nerve for 2 3 

 seconds in water at 70 80 C. Replace the nerve in 

 the electrodes so that one electrode is on the dead part 

 and the other on the living part. Repeat the stimulation 

 as before. It will be found that the first effect of the 

 ascending current is to cause contraction at the break 

 instead of at the make. 



ASC. 

 B 



-f 



DESC. 



Fig. 17. 



The explanation of the change in response is as fol- 

 lows. A stimulus is only set up where a current enters or 

 leaves the axis cylinder; on sending a current length- 

 ways through a nerve the greater part of the current 

 enters and leaves the axis cylinders at the poles, part 

 enters and leaves in other parts of the course of the 

 nerve, but this is insufficient to stimulate. A stimulus 

 cannot of course be set up in a dead nerve, thus when 

 one electrode is on the dead part of the nerve and the 

 other one on the living part, a stimulus is only set up at 



