THE PHENOMENON OF CONDUCTION. 87 



place: the muscle remains relaxed. We may express this fact 

 by saying that the motor nerve fibers are stimulated by the mak- 

 ing and the breaking of the current or by any sudden change 

 in its intensity, but remain unstimulated during the passage of cur- 

 rents whose intensity does not vary. 



The Anodal and Cathodal Stimuli. It has been shown quite con- 

 clusively that the nerve impulse started by the making of the current 

 arises at the cathode, while hat at the breaking of the current 

 begins at the anode, or, in other words, the making shock or 

 stimulus is cathodal, while the breaking stimulus is anodal. This 

 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. Bethe has shown that a 

 difference may be demonstrated even by histological means. After 

 the passage of a current through a nerve for some time the axis 

 cylinders stain more deeply than normal at the cathode with cer- 

 tain dyes (toluidin blue), while at the anode they stain less deeply. 



Electrotonus. The altered physiological condition of the nerve 

 at the poles during the passage of the galvanic current is designated 

 as electrotonus, the condition round the anode being known as 

 anelectrotonus, that round the cathode as catelectrotonus. Elec- 

 trotonus expresses itself as a change in the electrical condition of 

 the nerve which gives rise to currents known as the electrotonic 

 currents, a brief description of these currents will be given in 

 the next chapter, and also by a change in irritability and con- 

 ductivity. The latter changes were first carefully investigated 

 by Pfliiger, 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 anelec- 

 trotonic decrease in irritability and conductivity. These opposite 

 variations in the state of the nerve are represented in the accom- 



