iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 243 



the nerve ; when the nerve is tied, bruised, or its physiological 

 conductivity in any way interrupted ; and when the nerve is 

 degenerated, exhausted, or dead. 



Anelectrotonic are stronger than katelectrotonic currents ; the 

 former gradually increase during the passage of the polarising 

 current, while the latter gradually decline. On cooling, both 

 decline to the point of total disappearance. The maximum 

 intensity of the electrotonic currents may exceed that of the 

 demarcation currents by more than twenty-five times. 



Electrotonic currents alter in direction when the polarising 

 current is reversed ; they persist during the whole time of the 

 passage of the polarising current, and their intensity decreases 

 along the extrapolar tracts in proportion with the distance from 

 the poles. These characters distinguish the electrotonic currents 

 sharply from the action currents, which, as we have seen, are 

 constant in direction, and arise from the active state or excitation 



rdh 



LJ 



FIG. 156. Diagram of the electrotonic currents which summate algebraically with the 

 demarcation currents in a length <>t' rxcised nerve. (Luciani.) 



of the nerve, independently of the nature of the stimulus, and of 

 the direction of the exciting current when an electrical stimulus 

 is employed. 



When the polarising current is sent into an excised nerve, from 

 which demarcation currents can be led off to the galvanometer, 

 these summate algebraically with the electrotonic currents, which 

 are accordingly reinforced if in the same direction as the demarca- 

 tion currents, weakened or reversed, if the latter are in the 

 opposite direction (see Fig. 156). These phenomena were 

 formerly known as the positive and negative phases of electrotonus, 

 an unfortunate expression as the electrotonic phenomena are 

 entirely independent of the demarcation currents. 



The fundamental phenomena of electrotonus can be reproduced 

 on very simple models. As early as 1863 Matteucci observed 

 that the electrotonic currents in both intrapolar and extrapolar 

 portions of the nerve can be demonstrated in all essential 

 particulars if the galvanic current is led through a platinum wire 

 surrounded by a porous sheath saturated with fluid, instead of 

 through a nerve. Hermann, Griinhagen, Hering confirmed 

 Matteucci's observations by means of slightly different models. 



