XLVIII.] ELECTROTONUS. 245 



{/'.) Connect two Daniell's cells with a Pohl's commutator with cross-bars 

 (C) ; connect the commutator — a short-circuiting key intervening— to one 

 pair of the N.P. electrodes. This is the "polarising current" (P, P). 



(f.) Arrange an induction coil for tetanising shocks ; use N.P. electrodes 

 and short-circuit the secondary circuit. This is the "exciting current" 

 (E, E). 



{d. ) Make a nerve-muscle preparation with the nerve as long as possible, 

 and arrange it to write on a drum. Place 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 i centimetre apart. Between the polarising 

 })air and the muscle apply the "exciting" pair of electrodes to the nerve 

 (E, E). 



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

 from the primary coil, and find the minimum distance at which a feeble con- 

 traction of the muscle is obtained. Push the secondary coii up until a weak 

 contraction is obtained, and take a tracing. Previously arrange the com- 

 mutator to send a descending current through the nerve. While the muscle 

 is contracting feebly, throw in the descending polarising current ; at once the 

 contraction becomes much stronger. Reverse the commutator to send an 

 ascending polarising current through the nerve, and the contraction will 

 cease. 



FlO. 170. — Tracinoj showing effect of Anode and Kathode on Excitability of Nerve, the 

 latter stimulated with repeated shocks. T. Time in seconds. 



(/.) Repeat the experiment, using NeePs hammer, selecting a strength of 

 stimulus just insufficient to give tetanic response when the + pole of the polar- 

 ising current is next the muscle. Reverse the commutator, and at once the 

 previously inadequate shocks become adequate and tetanus results as shown 

 in lig. 170, where the effect of + and - poles are shown alternately. 



In tlie first case, the area influenced by the exciting electrodes was affected 

 by tlie negative pole, i.e., was in the condition of kathelectrotonus, and the 

 tetanus was increased ; therefore, the kathelccfrotonic condition increascH the 

 excitability of a nerve. In the second, the nerve next the exciting electrodes 

 yas in the condition of anelectrotonus, and the contractions ceased ; therefore, 

 tlie anelectrotonic condition diminishes the excitability of a nerve (fig. 171). 



3. Rheochord —use salt as stimulus. — The experiment may also be done by 

 using a rheochord to graduate the polarising current, salt again being used as 

 the stimulus. 



{a.) Arrange two N.P. electrodes in a moist chamber, provided with a 

 recording lever, placing the N.P.'s about i cm. apart. 



(b.) Connect the terminals of two Daniell's cells (arranged in circuit) to the 

 central screws of a Pohl's commutator (with cross-bars) as in tig. 172, ]»lacing 

 a mercury key in the circuit. Connect th'e wires, x, y, to the two blocks on 



