310 ELECTRO-PHYSIOLOGY CHAP. 



mm. long (two frog's sciatics with the opposite ends in contact), 

 when the leading -off tract was successively as near as possible to 

 the anode, and then to the kathode, finally determined that " the 

 homodromous after-phase of the current was regularly absent in 

 nerve, as in muscle, when the physiological anode coincided with 

 the artificial transverse section and was led off from there." Hence 

 there is no doubt that the homodromous after-current (" positive 

 polarisation ") is to be viewed exclusively as the galvanic expres- 

 sion of the opening excitation. 



The wide extrapolar diffusion of the polarising current in 

 medullated nerve makes it desirable to test the reaction of the 

 extrapolar after -current on breaking the circuit. The first 

 investigation was made by Tick (44), who found that a hetero- 

 dromous after-current appeared on both sides of the polarising 

 current, and quickly vanished again. A little later on Hermann 

 (45), followed by Tick, asserted that this occurred only on the 

 side of the anode, while a current, homodromous with the polar- 

 ising current, appeared beyond the kathode, its strength being 

 always less than that of the anodic after-current. In regard to 

 the latter, moreover, Hermann subsequently ascertained (46) that 

 it was preceded by a brief variation, homodromous with the polar- 

 ising current. 



Hermann explained all these manifestations by the " polar- 

 isation" after-currents (which he carefully investigated on the 

 " core-model "), in combination with the " irritative " after-currents 

 due to polar manifestations of excitation, and especially to the 

 opening excitation. We have already seen that these last are 

 alone sufficient to account for all secondary electromotive effects 

 in muscle, and the same is a priori probable for nerve also. 

 Further investigation is, however, desirable before coming to a 

 final decision. In any case, the extrapolar, anodic after-current 

 (heterodromous to the polarising current) depends upon the 

 negativity, which gradually diminishes from the pole outwards, 

 and is the galvanic consequence of the opening excitation ; while 

 the homodromous, extrapolar, kathodic after-current may equally 

 be defined as " irritative," if the negativity, which as in muscle 

 again declines from the pole outwards, is viewed as the after- 

 effect of the previous excitation, extending, of course, in a 

 medullated nerve as far as there are points of exit for the lines of 

 current. 



