chap, viii.] SCHEME OF ELECTROTONUS. 



81 



removing the plugs, resistance will be interposed in 

 the short circuit, the result of which will be that the 

 current will branch at screw e of plate a, part going the 

 short way and part the long. The intensity of the 

 current going to the nerve will be proportional to the 

 resistance thus thrown into the short circuit, and it 

 can, therefore, always be regulated and measured. 



This being understood, let us now see what further 

 is necessary for showing some- of the effects of 

 electrotonus upon a 

 nerve. A reference 

 to Fig. 43 will show 

 what is required. The 

 figure shows the ordi- 

 nary muscle-nerve pre- 

 paration. On the upper 

 side of the nerve is an 

 element connected by 

 its poles with the side 

 cups of a commutator, 

 provided ivith a cross. 

 From the end cups pass 



two electrodes to the Fig. 43. Scheme of Electrotonus. 



nerve. When the 



bridge of the commutator is inclined in the direction 

 of the continuous arrow, the current will traverse the 

 nerve between the two poles in a downward direction, 

 towards the muscle, as shown by the. arrow above the 

 nerve. When the bridge is reversed, as indicated by 

 the dotted arrow, the current will be up the nerve, in 

 the direction of the dotted arrow below the muscle. 

 In the former case, the pole next the muscle will be 

 negative, in the latter, positive. Now the positive 

 pole is called the anode, and the negative the katode, 

 and it is found that the electrotonic condition of the 

 nerve is not the same at the positive and negative 

 poles. The condition at the positive pole is therefore 

 G 7 



