THE EXCITATION OF NERVE FIBRES 297 



other hand, if a strong ascending current be passed through the nerve 

 for a considerable time, the muscle when the current is broken may 

 go into continued contraction, which may last some time. Normally, 

 however, the muscle simply responds with a single twitch at the make 

 and break of the current, although, on investigating the condition of 

 the nerve during the passage of the current, we find that it is consider- 

 ably modified. This modification in the condition of the nerve is 

 spoken of as electrotonus, and includes changes in its irritability and 

 its electrical condition. 



To investigate these changes the following apparatus is necessary : 

 two constant batteries, induction-coil, a reverser and keys, a pair of 



FIG. 114. Arrangement of apparatus for showing electrotonic changes 



in irritability. 

 e, exciting current ; p, polarising current ; r, Pohl's reverser. 



non-polarisable electrodes, and a pair of ordinary platinum electrodes. 

 Fig. 114 represents roughly the arrangement of the experiment. A 

 constant current from the battery is led through a part of the nerve 

 by means of non-polarisable electrodes, which are about one inch 

 apart. In this circuit we put a reverser, by means of which the direc- 

 tion of the current of the nerve may be changed at will, and a key to 

 make or break the current. This is the polarising circuit. The other 

 battery is arranged in the primary circuit of the coil, a key being inter- 

 posed, so that we may use make or break induction shocks, which are 

 applied to the nerve by means of the small platinum electrodes. The 

 tendon of the muscle is connected by a thread with a lever, which is 

 arranged to write on a smoked surface, so that the height of the con- 

 traction can be recorded. 



We first find the position of the secondary coil, at which the break 

 induction shock is a submaximal stimulus, and we employ this 

 strength of stimulus throughout the experiment. The make induc- 

 tion shock is prevented from acting on the nerve by closing a short- 

 circuiting key in the circuit of the secondary coil. The nerve is now 



