THE MUSCULAR SYSTEM. 



429 



stance, viz., the direction of the current. The direction of the current 

 may be ascending or descending; if ascending, the anode or positive 

 pole is nearer the muscle than the kathode or negative pole, and the 

 current to return to the battery has to pass up the nerve; if descend- 

 ing, the position of the electrodes is reversed. It will be necessary 

 before considering this question further to return to the apparent want 

 of effect of the constant current during the interval between the make 

 and break contraction: to all appearances no effect is produced at all, 

 but in reality a very important change is brought about in the nerve by 

 the passage of this constant (polarizing) 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 condi- 

 tion of the nerve by means of a pair of non-polarizable electrodes con- 

 nected with a galvanometer, while to the central portion a pair of elec- 

 trodes connected with a Daniell's battery be applied, it will be found that 

 natural nerve-currents are profoundly altered on the passage of the con- 



Fio. 301. Diagram illustrating the effects of curious intensities of the polarizing currents, n, n', 

 nerve; a. anode; fc, kathode; the curves above indicate increase, and those below decrease of 

 irritability, and when the current is small the increase and decrease are both small, with the neutral 

 point near a, and so on as the current is increased in strength. 



stant 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 opposite to it, the natural current is diminished. This change, 

 produced by the continual passage of the battery-current through a por- 

 tion of the nerve, is to be distinguished from the negative variation of 

 the natural current to which allusion has been already made, and which 

 is a momentary change occurring on the sudden application of the stim- 

 ulus. The condition produced 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 nerve 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 bat- 

 tery current may be reversed in direction. If the exact point be ascer- 

 tained to which the secondary coil should be moved from the primary 



