198 



EXPERIMENTAL PHYSIOLOGY. 



LESSON XLI. 



ELECTROTONUS ELECTROTONIC 

 VARIATION OF THE EXCITABILITY. 



1. Electrotonus. When a nerve is traversed by a constant 

 current, its vital properties are altered i.e., its excitability, 

 conductivity, and electromotivity. The region of the nerve 

 affected by the positive pole is said to be in the anelectrotonic, 

 and that by the negative in the kathelectrotonic condition. 

 Therefore we have to study the 



I. Electro-motive alteration of the excitability and conductivity. 

 II. Electro-motive alteration of the electromotivity. 



2. Electro-tonic Variation of the Excitability Apparatus re- 

 quired. Three Daniell's cells, two pairs of N.P. electrodes, two 

 du Bois keys, a Morse or spring key, commutator with cross-bars, 

 or Thomson's reverser, induction machine, wires, moist chamber, 

 drum, frogs, and the usual instruments. 



A. (a.) Arrange the apparatus according to the scheme 

 (Fig. 89) introducing the Morse key in the primary circuit. 

 Prepare two pairs of KP. electrodes for the nerve. 



Fig. 89. Scheme of Electrotonic Variation of Excitability. P P, 

 Polarising, and E E, stimulating current. 



(b.) Take two Daniell's cells and connect them with a 

 Pohl's commutator with cross-bars (C), or Thomson's re- 

 verser; connect the commutator a short-circuiting key 



