ACTION OF ELECTRICITY UPON THE NERVES. 531 



direction as the primary current. The direction of the primary current is 

 uniform, but the direction of the induced current alternates with every in- 

 terruption of the primary current. These induced currents are of momen- 

 tary duration, being produced only when the primary current is closed and 

 opened. A rapid interruption of the primary current is produced by what 

 is called a rheotome, or current-interrupter, which is attached to all induc- 

 tion-batteries. 



The points or surfaces used in closing a circuit in which a portion of 

 nerve or muscle is included are called electrodes. They are usually desig- 

 nated as the copper, or positive electrode or pole, and the zinc, or negative 

 electrode or pole. The positive pole is also called the anode, and the nega- 

 tive pole, the cathode. The direction of the current, when the circuit is 

 closed, is from the anode to the cathode. 



When a galvanic current is passed through a liquid or a moist, animal 

 tissue, decomposition occurs, by what is known as electrolysis or internal 

 polarization. The results of this decomposition, called ions, are of course 

 different in different liquids or moist tissues. These accumulate at the poles 

 and after a time disturb the currents and the phenomena produced. In ani- 

 mal tissues, acids accumulate at the anode, and alkalies, at the cathode. The 

 ions which go to the anode are called anions, and those which accumulate at 

 the cathode are called cations. In physiological experiments, it is often de- 

 sirable to eliminate electrolysis, or internal polarization, and this is done by 

 using the non-polarizable electrodes devised by Du Bois-Reymond. These 

 may be described as follows : " The researches of Regnault, Matteucci and 

 Du Bois-Reymond have proved that such electrodes can be made by taking 

 two pieces of carefully amalgamated pure zinc wire, and dipping these in 

 a saturated solution of zinc sulphate contained in tubes, their lower ends 

 being closed by means of modeller's clay, moistened with a 0*6 per cent, 

 normal saline solution. The contact of the tissues with these electrodes 

 does not give rise to polarity." (Landois and Stirling.) 



It is evident that the galvanic current may be applied to a nerve so that 

 the direction may in the one case follow the course of the nerve, that is, 

 from the centre to the periphery, and in the other, be opposite to the course 

 of the nerve. These have been called respectively descending and ascending 

 currents. When the positive pole (copper) is placed nearer the origin of the 

 nerve, and the negative pole (zinc), below this point in the course of the 

 nerve, the galvanic current follows the normal direction of the motor con- 

 duction, and this is called the descending current. When the poles are re- 

 versed and the direction is from the periphery toward the centre, it is called 

 the ascending current. It will be convenient to speak of these two currents 

 respectively as descending and ascending, in detailing experiments upon the 

 action of electricity upon the nerves. 



The points to be noted with regard to the effects of the application of 

 electricity to an exposed nerve are the action of constant currents, the phe- 

 nomena observed on closing and opening the circuit, and the effects of an 

 interrupted current. 



