ELECTROTONIC VARIATION OF THE EXCITABILITY. 199 



intervening to one pair of the N.P. electrodes. Call this 

 the " polarising current " (P P). 



(c.) Arrange an induction machine for single shocks, 

 the other pair of N.P. electrodes being in the secondary 

 circuit, arranged for short-circuiting. Call this the " exciting 

 current " (E E). 



(d.) Make a nerve-muscle preparation with the nerve as 

 long as possible, and arrange it for recording its movements 

 on a drum. Arrange the nerve on the two pairs of electrodes 

 in the moist chamber, the "polarising" pair being next the 

 cut end of the nerve (P P), and about 1 centimetre apart. 

 Between the polarising pair and the muscle, apply the 

 " exciting " pair of electrodes to the nerve (E E). 



(e.) With the polarising current short-circuited, pull away 

 the secondary from the primary coil, and find the minimum 

 distance at which a feeble contraction of the muscle is 

 obtained. Push the secondary coil up a little to obtain just a 

 weak contraction, and take a tracing. Previously arrange the 

 commutator to send a descending current through the nerve. 

 While the muscle is contracting feebly throw in the 

 descending polarising current, at once the contraction be- 

 comes much stronger. Reverse the commutator to send 

 an ascending polarising current through the nerve, and the 

 contraction will cease. 



(f.) Repeat the experiment using Neef's hammer. 



In the first case, the area stimulated by the exciting electrodes 

 was affected by the negative pole i.e., was in the condition of 

 kathelectrotonus, and the tetanus was increased ; therefore, the 

 kathelectrotonic condition increases the excitability of a nerve. In 

 the second case, the nerve next the exciting electrodes was in the 

 condition of anelectrotonus, and the contractions ceased ; there- 

 fore, the anelectrotonic condition diminishes the excitability of a 

 nerve (Fig. 90). 



B. Another Method. (a.) Connect two small Grove's cells 

 or two Daniell's to a Pohl's commutator with cross-bars, 

 introducing a du Bois key to shori>circuit the battery. Fill 

 the cups with mercury, and place the commutator in a 



