144 THE SIGNS OF LIFE [LECT. 



of a membrane between sheath and axis. It would lead us too 

 far to discuss this theory in all its details. My own opinion of 

 the matter is that MacDonald has sufficiently proved the reality 

 of the concentration currents of nerve by the annulling effects of 

 strong salt solutions and by the recuperative effects of weak 

 salt solutions, but that he has not disproved the irritative factor. 

 I think both factors are concerned in the current, and if called 

 upon to put a figure upon their value, should guess that in a 

 fresh demarcation current of say 0.05 volt something like 0.04 

 is by concentration, and o.oi, or less, by irritation. 



To return to the first question, stated at the outset of this 

 paragraph, I should tentatively answer that, in so far as an 

 " injury current " is irritation current, blaze-current bears relation 

 to it, but that in the absence of irritation current, or in so far 

 as " injury current " is of mere physical origin, blaze-current is 

 quite an independent phenomenon. 



I think that, on this view, we may now reconcile with each 

 other these several facts, of which the first three are affirmative 

 as regards a relation between blaze-current and injury current, 

 the last negative. 



1st. That a blaze-current is in general antidrome to an 

 injury current. 



2nd. That it is always antidrome to a previous (maximal) 

 blaze-current. 



3rd. That a (polarising) constant current favours homodrome 

 and disfavours antidrome blaze-current. 



4th. That in some cases blaze-current may be homodrome 

 with injury current. 



5th. That in many cases eyeball, skin, and electrical organ 

 there is no discernible relation between the direction and mag- 

 nitude of normal current and of blaze-current. 



The fact that a blaze-current, antidrome to an injury current, 

 is aroused by both directions of excitation, proves of itself, what 

 is more clearly proved by the ABC method, viz., that the 

 response may be post-kathodic as well as post-anodic, or, in 

 Biedermann's phrase, negative-kathodic as well as positive- 

 anodic. In either case it is the analogue of the " negative 

 variation " of a demarcation current of muscle or of nerve, where 



