THE INJURY CURRENT OF NERVE 275 
is not that of the nerve cell process, the myelin and the axis cyclinder being broken 
up and discontinuous. The usual explanation of the presence of a small injury current 
in such nerves is a 'vital ' one, and in terms of the discontinuous fragments of the axis 
cylinder. On such lines, if it is possible to explain a small current, it is quite 
impossible to explain a phenomenon as great as that obtainable from the intact 
nerve. 
The explanation which is offered now in the terms of the statement given above, 
is baseci upon the fact that the value of a diffusion process (or of a potential difference 
caused by one) is dependent upon the ratio between the two solutions, the same 
value being obtainable by the contact of several sets of solutions occupying different 
places in the range of possible concentrations. 
If it were possible to obtain pure water free from disolved matter, the greatest 
possible difference between two solutions of approximately similar concentration would 
be between such pure water and a water in which there was only a slight trace of added 
electrolyte. The difference given by such a combination would be greater than that 
obtained by 'contrasting' impure water with any obtainable solution, for in the first 
case the ratio is infinitely great, its denominator being zero. Such a combination 
could, however, only exist for an infinitely short space of time, since contact with the 
impure water would rapidly soil its theoretically pure neighbour. Similarly, when 
there is any great difference such as this between the two solutions in contact, the 
difference is rapidly diminished by what we may term the soiling of the standard of 
comparison — the more dilute solution. Considerations such as this render a satisfactory 
explanation feasible for the practical impossibility of obtaining infinitely great differences 
of potential between solutions in contact. 
In the extreme case when water is offered as the standard of comparison to any 
solution, the actual result obtained is always smaller by reason of this error, and the 
greater the concentration of the contrasted solution the greater the diminution due 
to the error. 
Granted in the experiments quoted that the tubular membranes are intact or still 
serviceable. Granted, also, the small current found is to be explained as due to the 
existence of only a small difference between the internal and external solution. There 
is, then, every reason to conclude that it might be possible to obtain as great a 
current from such a nerve after an immersion in water as from a normal nerve. 
For in neither case, when the observation is taken, is the outer solution actually 
replaced and maintained as replaced by water ; and the depreciation in the value of 
this standard of comparison (water) is likely to be greater when the internal solution 
is of considerable concentration. 
It is worthy of note that, once granted the schematic structure which is claimed 
for the nerve, every difference between its internal and external solutions is liable 
to lead to an injury current, the direction and amount of which is determined by the 
