CH. XV.] ELECTROTONIC CURRENTS 171 



The term " electrotonus " includes two sets of changes in the 

 nerve ; first an electrical change, and secondly changes in excitability 

 and conductivity. We will take the electrical change first. 



Electrotonic currents. The constant current is passed through 

 the nerve from a battery, non-polarisable electrodes being used ; it is 

 called the polarising current. If portions of the nerve beyond the 

 electrodes are connected ("led off") as in the diagram (fig. 168) by 

 non-polarisable electrodes to galvanometers, a current will in each 

 case be indicated by the swing of the galvanometer needles. The 

 electrotonic current in the neighbourhood of the negative pole or 

 kathode is called the Teat 'electrotonic current ; and that in the neighbour- 

 hood of the anode is called the anelectrotonic current. In both cases the 

 electrotonic current has the same direction as the polarising current. 

 These currents are dependent on the physical integrity of medullated 



FIG. 168. Electrotonic currents. 



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 electro- 

 tonic 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 produced 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. 



(b) 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. 



