CAUSES AND PHENOMENA OF MOTION. 47 



to return to the want of apparent effect of the constant current during 

 the interval between the make and break contraction: to all appearance, 

 indeed, no effect is produced at all, but in reality a very important change 

 is brought about in the nerve by the passage of the current. This may 

 be shown in two ways, first of all by the galvanometer. If a piece of 

 nerve be taken, and if at either end an arrangement be made to test the 

 electrical condition of the nerve by means of a pair of non-polarizable 

 electrodes connected with a galvanometer, while to the central portion 

 a pair of electrodes connected with a Daniell's battery be applied, it will 

 be found that the natural nerve-currents are profoundly altered on the 

 passage of the constant current (which is called the polarizing current) 

 in the neighborhood. If the polarizing current be in the same direction 

 as the latter the natural current is increased, but if in the direction oppo- 

 site to it, the natural current is diminished. This change, produced by 

 the continual passage of the battery-current through a portion of the 

 nerve is to be distinguished from the negative variation of the natural cur- 

 rent to which allusion has been already made, and which is a momentary 

 change occurring on the sudden application of the stimulus. The con- 

 dition produced in a nerve by the passage of a constant current is known 

 by the name of electrotonus. 



The other way of showing the effect of the same polarizing current is 

 by taking a nerve-muscle preparation and applying to the nerves a pair 

 of electrodes from an induction coil whilst at a point further removed from 

 the muscle, electrodes from a Daniell's battery are arranged with a key 

 for short circuiting and an apparatus (reverser) by which the battery cur- 

 rent may be reversed in direction. If the exact point be ascertained to 

 which the secondary coil should be moved from the primary coil in order 

 that a minimum contraction be obtained by the induction shock, and 

 the secondary coil be removed slightly further from the primary, the in- 

 duction current cannot now produce a contraction; but if the polarizing 

 current be sent in a descending direction, that is to say, with the kathode 

 nearest the other electrodes, the induction current, which was before in- 

 sufficient, will prove sufficient to cause a contraction; whereby indicating 

 that with a descending current the irritability of the nerve is increased. 

 By means of a somewhat similar experiment it may be shown that an 

 ascending current will diminish the irritability of a nerve. Similarly, if 

 instead of applying the induction electrodes below the other electrodes they 

 are applied between them, like effects are demonstrated, indicating that 

 in the neighborhood of the kathode the irritability of the nerve is in- 

 creased by a constant current, and in the neighborhood of the anode 

 diminished. This increase in irritability is called katelectrotonus, and 

 similarly the decrease is called anelectrotonus. As there is between the 

 electrodes both an increase and a decrease of irritability on the passage of 

 a polarizing current it must be evident that the increase must shade off 



