xi ELECTRICAL FISHES 455 



properly continuation, of the excitation so in the electrical 

 organ also the true discharge (shock) may pass off in a homo- 

 dromous current. 



We may admit, with du Bois-Eeymond, that " every absolute 

 positive effect is not a discharge" just as every excitation of the 

 muscle, even if demonstrable on the galvanometer, does not 

 produce a visible contraction (twitch) ; in the fresher preparations 

 du Bois-Eeymond often observed a very pronounced effect, which 

 drove the scale out of the field of vision, and in which we must 

 undoubtedly admit the after-effect of a discharge, if not the last 

 phase of it. " This phenomenon," he continues, " is, however, 

 quite distinct from ordinary absolute positive polarisation 

 (obtained after frequent repetition of the experiment on the 

 same preparation under identical circumstances), since it exhibits 

 no duration proportional with the original intensity." Yet this 

 is just as little the case in the mechanical and galvanic conse- 

 quences of excitation of the muscle. 



If the absolute positive (homodromous) after-current in these 

 experiments is to be viewed as the after-effect of exciting the 

 organ-preparation by the homodromous current, we may expect it 

 to be very marked after brief tetanisation with alternating currents. 

 In order to avoid the disturbances caused by the inequality of 

 time-distribution in the make and break shocks of the ordinary 

 sliding coil, du Bois-Eeymond used a Sexton's machine, which pro- 

 duces series of quite congruent alternating currents. The results 

 were uniform ; no matter how the ends of the rotating coils 

 were connected up with the dorsal and ventral surfaces of the 

 preparation (Torpedo), or what was the duration of the tetanus, 

 there was always an absolute positive after-current, " in fresh 

 preparations of such magnitude that the scale disappeared from 

 the field, the effect becoming gradually weaker." 



The striking difference in the relative strength of homo- 

 dromous and heterodromous exciting currents in the electrical 

 organ, seems to depend intimately upon positive homodromous 

 polarisation, as du Bois-Eeymond observed in his earliest polar- 

 isation experiments on the Malapterurus organ. " The descend- 

 ing homodromous current in Malapterurus was always stronger 

 in fresh strips than with the ascending (heterodromous) direction, 

 in the ratio of 100:112, 116, even 125. In boiled and in 

 moribund strips the difference vanished." The same fact was 



