586 



THE PHYSIOLOGY OF ELECTRICAL ORGANS. 



ing voltaic current is not merely augmented but shows self-excitation, 

 whilst that due to excitation by the ascending current is a single small 

 effect. Since self-excitation occurs when the excitability of the nerve- 

 endings is high, it appears most probable that we have in this experiment 

 a demonstration of the establishment of extrapolar changes in excit- 

 ability along the nerve trunk to its ending in the organ. 



The second great feature of the changes caused in nerve by the 

 passage of currents is the production of secondary electromotive effects. 

 The simplest experiment upon this subject is the familiar fact that the 

 passage of a galvanic current is succeeded in polarisable media by the 

 establishment of counter-electromotive changes. In strips of organ this 

 is readily demonstrated, provided the galvanic current used be not 

 intense enough to excite. If, however, it should excite the strip, then 

 the intense current due to the response must obviously sum algebraically 

 with any polarisation after-effect. For this reason, homodromous polaris- 

 ing currents, though followed by opposite, i.e. negative, polarisation effects 

 when weak, are, when stronger, followed by a marked positive or 

 similarly directed change, that of the response itself. On the other 

 hand, heterodromous currents, which when weak are followed by the 

 counter-current of negative polarisation, are, when stronger, succeeded 

 by counter-effects of very great intensity, since the excitatory change, 

 being of like sign with that of polarisation, now sums directly with this. 



The presence of the response appears to have been overlooked by 

 du Bois-Eeymond, who considered that two sorts of polarisation effects, 

 negative and positive, could be produced in the electrical organ. 1 There 

 seems little doubt that when experiments are so arranged as to exclude 

 the strong excitatory effect which immediately follows the cessation of 

 a strong current, but little evidence of such doubly directed polarisation 

 exists. 



Excitatory and Polarisation After-Effects in Torpedo Organ. 



The particular positive after-effect, or positive polarisation, was 

 admitted by du Bois-Reymond to occur, as a secondary phenomenon, 

 only in the case of homodromous polarising currents. This in itself 

 suggests that the change is due to an excitation, since the response 

 is always in one direction, however aroused. It will be remembered 

 that a similar positive polarisation, declared by the same authority to 

 exist in muscle and nerve, has been shown to be due to local polar 



1 du Bois-Reymond, Arch. f. PhysioL, Leipzig, 

 'c. AJcad. d. Wissensch., Berlin, 1888, Bd. xxii. S. 



1885, S. 86 ; 1887, S. 51 ; Sitzungsb. d. 

 531. 



