116 GENERAL PROPERTIES OF LIVING TISSUES 



secured on closing a weak ascending current and 

 yet cannot be obtained with a strong one. The 

 first step in the inquiry into the causes of the 

 phenomena is to determine whether the stimu- 

 lation is polar. That the nerve impulse really 

 starts at the cathode on closure and at the anode 

 on opening is shown (1) by the fact that the 

 interval between stimulation and contraction, 

 with the ascending current, in which the anode 

 is next the muscle, is longer at closure than on 

 opening, while the opposite is the case when the 

 current is descending. (2) With descending 

 currents, it sometimes happens that opening 

 produces tetanus instead of a simple twitch. 

 If this tetanus appears, the student should sever 

 the nerve between the electrodes. Immediately 

 the contractions will cease. They must there- 

 fore have arisen at the anode, for the cathode 

 still remains in full connection with the muscle. 



Changes in Irritability. The second step in 

 this inquiry is to determine the nature of the 

 changes at the poles. For this purpose the 

 nerve should be stimulated in the cathodal and 

 anodal regions during the passage of the constant 

 current. 



1. Pass two needles through a cork placed in 

 the rubber holder next the muscle in the moist 

 chamber. Connect them with the secondary coil 



