xi ELECTRICAL FISHES 449 



carry on the same experiments on Torpedo, without having to 

 stint himself in materials (4 g-i). 



In order to understand what follows, it must be kept in mind 

 that current in the direction of the discharge is termed homo- 

 dromous, in the opposite direction heterodromous ; an after-current 

 opposed to the polarising current is relatively negative, its contrary 

 relatively positive ; an after-current in the direction of the discharge 

 (homodromous) is absolutely positive, its converse absolutely negative. 



Du Bois-Reymond employed prismatic pieces of organ, the 

 polarising current being led in at the end-surfaces covered with 

 skin, while a second pair of impolarisable electrodes led off the 

 polarisation current, the clay tips being applied to the preparation 

 between the clay shields that led in the polarising current. 

 Here, again, under certain conditions, i.e. with brief closure of 

 stronger currents, there may be positive polarisation, otherwise a 

 diphasic effect is seen first a negative, then a positive variation. 

 As in muscle, the positive polarisation is more dependent on vitality, 

 on the normal physiological state of the preparation. " With de- 

 pressed excitability (Leistungsfahigkeit) only negative polarisation 

 at last survives, but it is a long time before the positive effect 

 dies away completely. The relation between polarisation and 

 direction of polarising current is conspicuous." Sachs found in 

 G-ymnotus that " the negative polarisation current is invariably more 

 marked in the direction of the discharge," and du Bdis-Eeymond 

 also determined in Torpedo that both homodromous (in direction of 

 discharge) and heteroclromous currents, after longer closure, or with 

 less excitable preparations, yield (with reference to direction of 

 polarising current) relatively negative polarisation, but that this is 

 invariably stronger with a homodromous current. The apparent 

 contradiction between this discovery and the original observations 

 of du Bois-Eeymond, according to which negative polarisation in 

 the organ of Malapterurus is independent of direction of current, 

 is explained by the fact that the heterodromous current never 

 produces any relatively positive polarisation either in Malapterurus 

 or Torpedo. Diphasic polarisation first relatively negative and 

 then positive appears with homodromous currents only. If it 

 be admitted that " both currents yield relatively negative polarisa- 

 tion in the same degree, but that the homodromous current is very 

 much stronger than the heterodromous, or alone polarises positively, 

 so that heterodromous, relatively positive polarisation (when pre- 



VOL. II 2 G 



