178 ELECTROTONUS. [CH. XVI. 



dependent on the physical integrity of medullated nerve ; they 

 are not found in muscle, tendon, or non-medullated nerve ; they 

 are absent or diminished in dead or degenerated nerve. They 

 can, however, be very successfully imitated in a model made of 

 zinc wire encased in cotton soaked with salt solution. The 

 electrotonic currents must be carefully distinguished from the 

 normal current of action, which is a momentary change rapidly 

 propagated with a nervous impulse which may be produced by 

 any method of stimulation. The electrotonic currents are pro- 

 duced only by an electrical (polarising) current ; they vary in 

 intensity with the polarising current, and last as long as the 

 polarising current passes through the nerve. 



After the polarising current is removed, after-electrotonic currents occur 

 in different directions in the three regions tested. 



() In the intrapolar region, the after-current is opposite in direction 

 to the original polarising current ; unless the polarising current is 

 strong and of short duration, when it is in the same direction. 

 (J) In the katelectrotonic 'region, the after-current has the same 



direction as the katelectrotonic current. 



(c) In the anelectrotonic region, the after-current has at first the 

 same, then the opposite direction to the anelectrotonic current. 



The experiment known as the paradoxical contraction depends 

 upon electrotonic currents. The sciatic nerve of the frog divides 

 in the lower part of the thigh into two parts. If one division is 

 cut across, and its central end stimulated electrically (the spinal 

 cord having been previously destroyed), the muscles supplied by 

 the other branch contract ; the nerve fibres in this branch having 

 been stimulated by the electrotonic variation in the divided 

 branch.* 



Electrotonic alterations of excitability. When a constant 

 current is passed through a nerve, the excitability of the nerve 

 is increased in the region of the kathode, and diminished in 

 the region of the anode. When the current is taken out the 

 excitability is temporarily increased in the neighbourhood of the 

 anode, and diminished in that pf the kathode. 



This may be shown in the case of a motor nerve by the fol- 

 lowing experiment. The next diagram represents the apparatus 

 used. 



An exciting circuit for single induction shocks is arranged in 



* This experiment must be carefully distinguished from Kiihne's gracilis 

 experiment described on p. 173. In the gracilis experiment the nerve 

 fibres themselves branch, and any form of stimulation applied to one branch 

 will cause contraction of both halves of the muscle. In the paradoxical con- 

 traction, the bundles of nerve fibres are merely bound side by side in the 

 sciatic trunk ; there is therefore no possibility of conduction of a nerve 

 impulse in both directions ; the stimulus, moreover, must be an electrical ono>. 



