MUSCLE-NERVE PHYSIOLOGY. 529 



the battery has to pass up the nerve; if descending, the position of thb 

 electrodes is reversed. It will be necessary before considering this ques- 

 tion 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 change is produced, but in reality a very important 

 alteration of the irritability 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 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 con- 

 nected with a Daniell's battery be applied, it will be found that the 

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

 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 portion of the nerve, is to be distinguished from the negative varia- 

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

 which is a momentary change occurring on the sudden application of 

 the stimulus. The condition produced by the passage of a constant 

 current is known by the name of Electrotonus. 



A second way of showing the effect of the polarizing current is by 

 taking a nerve-muscle preparation and applying to the nerve a pair of 

 electrodes from an induction coil, while 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- 

 current 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 induction current cannot now produce a contraction; but if the 

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

 the cathode nearest the other electrodes, the induction current, which 

 was before insufficient, 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 cathode 

 the irritability of the nerve is increased by the passage of a constant 

 current, and in the neighborhood of the anode diminished. This in- 

 34 



