THE PHENOMENON OF CONDUCTION. 83 



fact is true for muscle as well as nerve, and possibly for all irritable 

 tissues capable of stimulation by the galvanic current. This 

 important generalization may be demonstrated for motor nerves 

 by separating the anode and cathode as far as possible and re- 

 cording the latent period for the contractions caused respect- 

 ively by the making and the breaking of the current in the nerve. 

 If the cathode is nearer to the muscle the latent period of the mak- 

 ing contraction of the muscle will be shorter than that of the break- 

 ing contraction by a time equal to that necessary for a nerve impulse 

 to travel the distance between anode and cathode. If the position 

 of the electrodes is reversed the latent period of the making con- 

 traction will be correspondingly longer than that of the breaking 

 contraction. It is very evident from these facts that when a 

 current is passed into a nerve or muscle the changes at the two 

 poles are different, as shown by the differences in reactions and 

 properties of the nerve at these points. It is interesting to add 

 that Bethe has shown recently that this difference may be demon- 

 strated histologically. After the passage of a current through a 

 nerve for some time the axis cylinders stain more deeply than 

 normal at the cathode with certain dyes (toluidin blue), while at 

 the anode they stain less deeply. 



Electrotonus. The altered condition of the nerve at the poles 

 during the passage of the galvanic current is designated as electro- 

 tonus, the condition at the anode being known as anelectrotonus, 

 on the cathodal side as catelectrotonus. The electrotonus expresses 

 itself as a change in the electrical condition of the nerve which gives 

 rise to currents known as the electrotonic currents, a brief descrip- 

 tion of these currents will be given in the next chapter, and also 

 by a change in irritability and conductivity. The latter changes 

 were first carefully investigated by Pfluger, who showed that when 

 the galvanic current, or, as it is usually called in this connection, 

 the polarizing current, is not too strong there is an increase in 

 irritability and conductivity in the neighborhood of the cathode, 

 the so-called catelectrotonic increase of irritability, while in the 

 region of the anode there is an anelectrotonic decrease in irrita- 

 bility and conductivity. These opposite variations in the state 

 of the nerve are represented in the accompanying diagram. Be- 

 tween the two poles that is, in the intrapolar region there is, 

 of course, an indifferent point, on one side of which the irritability 

 of the nerve is above normal and on the other side below normal. 

 The position of this indifferent point shifts toward the cathode 

 as the strength of the polarizing current is increased. In other 

 words, as the current increases the anelectrotonus spreads more 

 rapidly and becomes more intense, and the conductivity in this 

 region soon becomes so depressed as to block entirely the passage 



