172 



ELECTROTONUS 



[CH. XV. 



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, because the nerve-fibres in this branch have been 

 stimulated by the electrotonic variation in the divided branch. 



This experiment must be carefully distinguished from Kuhne's gracilis 

 experiment described on p. 160. 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 contraction, 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 one. 



Electrotonic alterations of excitability and conductivity. 



When a constant current is passed through a nerve, the excitability 

 and conductivity of the nerve are increased in the region of the 

 kathode, and diminished in the region of the anode. When the 

 current is taken out these properties are temporarily increased in 

 the neighbourhood of the anode, and diminished in that of the 

 kathode. 



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

 experiment. The next diagram represents the apparatus used. 



Cell 



Key 



EXCITING 



FIG. 169. Diagram of apparatus used in testing electrotonic alterations of excitability. 



An exciting circuit for single induction shocks is arranged in the 

 usual way, the exciting electrodes being placed on the nerve near the 

 muscle. A polarising circuit is also arranged, and includes a battery, 

 key, and reverser ; the current is passed into the nerve by means of 

 non-polarisable electrodes. When the polarising current is thrown 

 into the nerve, or taken out, a contraction of the muscle occurs, but 

 these contractions may be disregarded for the present. 



The exciting circuit is arranged with the secondary coil so far from 

 the primary that the muscle responds to break only, and the tracing 



