CH. XVI.] 



ELECTROTONUS. 



177 



invented by Poggendorf. The number of turns of wire is greater, 

 so that the resistance can be varied to a much greater extent 

 than in the simpler form of the instrument. 



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





fig. 187. Poggendorf B rheochord. (M'Kendrick.) 



nerve ; first an electrical change, and secondly changes in excita- 

 bility 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 



Anelectrotonic 

 Current 



=* 



Katelectrotonic 

 Current 



* Polarising 

 Current 



Fig. 188. Electrotonic currents. 



used ; it is called the polarising current. If portions of the nerve 

 beyond the electrodes are connected ("led off") as in the dia- 

 L'i-:im (fig. 1 88) by non-polarisable electrodes to galvanometers, a 

 current will in each case be indicated by the swing of the galvano- 

 ni'-tiT needles. The electrotonic current in the neighbourhood of 

 the negative pole or kathode is called the katelectrotonic current ; 

 and that in the neighbourhood 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 

 K.P. N 



