88 



EXPERIMENTAL PHYSIOLOGY 



being stimulated the whole of the time the current is passing, still 

 we can show that its condition is altered. This altered state, due to a 

 constant current, is spoken of as clectrotonus, and its effect is 

 expressed as an alteration in the two physiological properties of a 

 nerve — its conductivity and its excitability. The alteration is different 

 in the parts of the nerve in the neighbourhood of the two points at 

 which the current is sent in and taken out of the nerve, i.e. the anode 

 and kathode respectively. The condition of the nerve in the neigh- 

 bourhood of the anode is termed anelectrotonus, that in the neigh- 

 bourhood of the kathode katelectrotonus. 



1. Changes in excitability. 



Experiment 7. — Test the changes in excitability in the following 

 manner. Arrange the primary and secondary coils for giving single induced 

 shocks (left half of fig. 75), using a pair of fine wire electrodes, E r Fit up a 

 '■polarising ' circuit with two batteries, a mercury key, k.-,, and a mercury 

 commutator with cross wires, K t . To the commutator attach a pair of un- 

 polarisable electrodes, E 2 (right half of fig. 75). Dissect out a nerve muscle pre- 



<&^ 



Fu 



K. 

 75.- 



-Plan of Apparatus for Studying the Changes of Excitability 

 of Electrotonus. 



paration, taking the whole length of the nerve, and being careful not to injure 

 it during the dissection. Fix the gastrocnemius in the crank myograph and 

 place the nerve across the unpolarisable electrodes. Place the wire elec- 

 trodes under the nerve at a point just below the unpolarisable electrodes 

 nearest the muscle. With the key it, still open, move the secondary coil to 

 such a position that the muscle gives a small twitch on break of the 

 primary circuit. Record the height of this twitch on a stationary drum. 

 Move the drum a little by hand and close the polarising circuit. Note that 

 the muscle gives a twitch. Again stimulate the nerve with a break shock 

 and record the twitch. It will be found that either the twitch is of less 

 height or even absent, or that it is of greater height. Now examine the 

 direction of the constant current through the nerve. If the twitch has been 

 prevented or diminished, it will be found that the current ascends along the 

 nerve from muscle towards vertebral end, and therefore the part of the nerve 

 stimulated is hi anelectrotonus. If, on the other hand, the twitch has been 

 increased in height, it will be found that the electrode nearest the nerve is 

 the kathode. After obtaining one of these two results, reverse the direction 

 of the polarising current by the commutator K 4 , when the second result will 

 be obtained 



