144 ELECTRO-PHYSIOLOGY CHAP. 



duration of the first phase of decline of the kathodic alteration 

 of excitability (negative modification) is shorter, other conditions 

 being equal, in proportion as the polarising current is strengthened, 

 so that it is sometimes difficult to demonstrate (Obernier, 21), 

 and can only be shown when the test-stimulus acts simultaneously 

 with or immediately after the opening of the polarising current. 

 Otherwise the intensity and duration of the after-effects depend 

 entirely upon the intensity of the original changes, including of 

 course strength of the modifying current. 



From these facts, therefore, we learn that, when a tract of 

 medullated nerve is continuously traversed by a constant current, 

 the nerve falls not merely at and between the electrodes, but also 

 beyond the poles, into an altered state (elcdrotonus), as manifested, 

 apart from other phenomena to be described below, in alterations 

 of excitability towards given stimuli, in the sense that there is, 

 while the current is passing, an increase of excitability in the 

 region of the kathode, a depression of excitability in the region 

 of the anode. The term katelectrotonus is commonly applied to 

 the former state, the term amlectrotonus to the latter, though it 

 should be remarked of both expressions that they cover not 

 merely the alterations of excitability, but also the modification of 

 the nervous substance produced by an electrical current in the 

 region of the two poles, which, as we shall see, may find other 

 modes of expression. 



The electrotonic alterations of excitability may be clearly shown 

 in a graphic representation. If the excitability of each point is 

 drawn as ordinate upon the nerve as abscissa, the line which 

 unites the tops of the individual ordinates usually (apart from the 

 increase of excitability near the cross-section) forms a line parallel 

 with the abscissa. If current is then led through the middle 

 portion, there will be increased excitability on the side of the 

 kathode, diminished excitability on that of the anode. If the 

 former is represented by an ordinate drawn upwards (positive), the 

 latter may be similarly drawn as a downward line (negative). 

 The excitability declines from either point, both between and 

 beyond the poles, and spreads over a larger tract of the nerve as 

 the polarising current becomes stronger. Let gi (Fig. 185) be 

 the nerve, to which are applied the electrodes ^4 and B ; the 

 excitability of single points during closure to weak, medium, and 

 strong currents may be expressed by the three curves abc, dcf, 



