ELECTROTONUS. 505 



nerve be placed in the circuit of a constant current from a battery, 

 when the circuit of the constant (now called polarizing} current 

 is completed, a change is found to take place in the natural cur- 

 rent ; this is called electrotonus. Instead of the natural currents 

 from the equator to the pole of the nerve, a current is found to 

 pass through the entire length of the nerve in the same direction 

 as the polarizing current from the battery. This electrotonic cur- 

 rent is not proportional to the strength of the natural currents, 

 and is to be recognized when the latter are no longer to be found. 



FIG. 202. 



Diagram to illustrate Electrotonus. N N'. Portion of nerve. G G x . 

 Galvanometers. D. Battery from which polarizing current can be sent into 

 nerve by closing key K. The direction of the polarizing and electrotonic 

 currents is indicated by the arrows, and is seen to be the same. 



The electrotonic current is stronger with a strong polarizing cur- 

 rent, and is most marked in the immediate neighborhood of the 

 poles, fading gradually away as one passes to the remoter parts of 

 the nerve. The electrotonic state is not to be attributed to an 

 escape of the constant polarizing current, because it decreases 

 gradually with the waning of the physiological activity of the 

 nerve, and ceases at the death of the nerve long before the tissue 

 has lost its power of conducting electric currents. Moreover, it 

 has been shown that a ligature applied to the nerve so as to destroy 



