THE INJURY CURRENT OF NERVE 
289 
Before doing so, however, it is necessary to examine the limits within which 
such a study can be pursued, limits which have already made their appearance as 
modifying the action of water. The first and most important of which is that set by 
the nature of the diffusion obstacle separating the external and internal solution. 
It is not claimed for this ' membrane ' that it is an absolute barrier to processes 
of diffusion between the two solutions. Such a claim would place it in a unique 
position amongst all the other limiting cell surfaces of the body as a strictly defined 
' semi-permeable membrane ; ' would place it in fact, if logically maintained, in a unique 
position amongst the membranes studied by the physicists and called by them ' semi- 
permeable.' This membrane was called into theoretical being by necessities which 
arose in explanation of the polarization phenomena, and, once postulated, its existence 
was found, from general analogy and from other circumstances, to be not only not 
contra-indicated but even to be confirmed. Even if it be admitted, and it is not, that the 
necessities of the polarization phenomena form, after this subsequent examination, its 
only claim for existence ; still, it must be granted that these necessities are far from 
demanding a strictly semi-permeable membrane. All that is required by them is that 
the membrane shall be a ' partial ' barrier to the movement of dissolved electrolytes, 
and shall so give rise to the accumulation of charged particles on its surfaces, particles 
left behind by their fellows which have successfully traversed it, and so conducted the 
electrical current. In short, neither theoretical necessity nor the teachings of physio- 
logical analogy point to the presence of anything but an imperfect obstacle to processes 
of diffusion between the solutions of the nerve. 
Granted that this is so, and that as observed in the action of water (see page 275) 
the effect of immersing a nerve in a given solution is not merely to replace with this 
the external solution of the nerve, but also by penetration of the diffusion obstacle 
to modify the internal solution ; then here certainly we have a reason for accepting 
the formerly rejected standard of concentration, namely, that of the ' isotonic solution,' 
which is removed with the nerve from the body. 
Granted that an immersion affects both the solutions of the nerve, then immersion 
in a dilute solution 
(1) dilutes the external solution and so increases the injury current, 
(2) dilutes the internal solution and so diminishes the injury current; 
whereas immersion in a concentrated solution 
(1) concentrates the external solution and so diminishes the injury current, 
(2) concentrates the internal solution and so increases the injury current. 
It is fortunate that the second effect must in both cases be completed later than the 
first, and that so an opportunity is left for the study of the first : even if it is not as 
perfect as a physicist, with the advantages of determining his own conditions, would 
choose for the examination of the differences of potential caused by diffusion into 
contrasted solutions. To make the most of the opportunity it is necessary to accept 
