iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 245 



influence the electrical reaction of nerve, are also capable of 

 temporarily suppressing electrotonus. 



Besides the electrotonic phenomena strictly so-called, polarising 

 currents evoke other parallel specific changes of excitability and 

 conductivity in both the intrapolar and the extrapolar portions of 

 the nerve. We owe our knowledge of the principal phenomena 

 of this subject to Pfliiger, who followed up the earlier researches 

 of Ritter, Nobili, Matteucci, Valentin, and Eckhard. The main 

 facts in regard to the electrotonic changes of the excitability of 

 nerve are as follows : 



(a) The passage of a constant current through a nerve causes a 



s.c. 



FIG. 157. Diagram to show electrotonic modifications of excitability, according to ascending or 

 descending direction of polarising current. (Waller.) p.c., polarising current ; s.c., exciting 

 current ; m, muscle. In the upper diagram the direction of the polarising current is ascending, 

 and excitability is therefore lowered in the anelectrotonic region ; in the lower diagram the 

 direction of the polarising current is descending, and excitability is therefore raised in the 

 katelectrotonic region of the nerve. 



rise of excitability at the kathode and a fall of excitability at the 

 anode. 



(&) These changes in excitability are most marked at the poles, 

 but they also spread into the intra- and extra-polar regions, 

 growing weaker. There is in the intrapolar portion an indifferent 

 point, at which excitability remains unaltered. 



(c) When the current that sets up electrotonus ceases the 

 alterations of excitability are reversed; the kathodic region 

 becomes less excitable, the anodic region more so. 



The nerve -muscle preparation of the frog is generally used 

 in experimental demonstration of these electrotonic changes in 

 excitability. According as the polarising current is passed in an 

 ascending or a descending direction through the nerve, the 

 anelectrotonic or katelectrotonic region will be found nearer the 

 muscle. In order to show that excitability is depressed in the 

 former and raised in the latter, the nerve is excited near the anode 

 or kathode respectively, either by an induction current (as in Fig. 





