CONDUCTIVITY CHANGES. 499 



many considerations which render it impossible to regard this as the 

 sole cause. In the first place, it has been already indicated that the 

 excitability after effects on cessation do not correspond in their time 

 relations with those during closure ; the anodic after effect is prolonged, 

 unlike the cathodic closing effect. Secondly, the opening anodic excita- 

 tion may be produced by merely diminishing the intensity of the 

 polarising current to such a point that the reduced value of this is still 

 in excess of the polarisation current, so that the latter does not come 

 into play. Thirdly, as will be seen in the section dealing with the 

 electromotive effects produced by currents, the anodic electrical response 

 may be so pronounced and prolonged, on the cessation of the current, as 

 to overpower the actual polarisation after effect, through its being of 

 opposite sign, in which case no such reversal of poles can occur. 

 Finally, in non-medullated nerves, such as those of the crayfish, etc., 

 the amount of polarisation is relatively small, but the excitability 

 changes both on closure and opening are extremely pronounced. There 

 can be no question that the electrolytic phenomena, in so far as these 

 are displayed by polarisation, profoundly modify the nerve conditions, 

 but several facts render it difficult to assume that the polar changes 

 of excitability are entirely dependent upon their presence. In many 

 instances the two effects appear to be concomitant tissue reactions, 

 but even in these cases the excitability changes, although intimately 

 associated with the formation of ions, are presumably based upon a 

 more subtle chemical rearrangement. As regards the nature of such 

 rearrangement, the utmost that can be said is, that it presents itself in 

 two phases, one of augmented, the other of lessened molecular stability. 

 If an agent, such as a current, enforces by its continued passage the 

 predominance of either phase, its cessation is followed by a rebound 

 towards that of the other. 



The alterations of excitability, due to the passage of electrical currents, 

 may be caused by muscle or nerve demarcation currents. The latter 

 is developed in the neighbourhood of a cut section or injury, the tissue 

 here becoming galvanometrically negative to its surroundings ; a current 

 thus flows along the nerve fibres from the cut section, it then leaves 

 the fibres to return by the juices of the tissue to the seat of its 

 causation. The neighbourhood of a cut section is thus thrown into the 

 condition of catelectrotonic rise of excitability, and in this way the 

 increased excitability due to injury is explained. 



Conductivity changes. Nerve conductivity is also altered by the 

 passage of electrical currents ; this shows itself as an alteration, both 

 in the magnitude of the transmitted excitatory change, and in the 

 rate of propagation. The simplest demonstration of the former is that 

 afforded by the following experiment. If the central part of the frog's 

 sciatic be excited by a stimulus of minimal intensity, the resulting 

 muscular response fails or is much diminished when a galvanic current 

 is closed in either direction through the peripheral portion of the 

 nerve. A more or less effective block for transmitted excitatory states is 

 thus shown to be produced on and during closure at the anode ; i.e., the 

 nerve in this region is rendered less responsive to the internal stimulus 

 which is the basis of the transmission. The effect lasts for a little time 

 after the cessation of the current, especially if this has been moderately 

 intense and of short duration, but this after block is now localised at 

 the cathode. 



