THE INJURY CURRENT OF NERVE 
269 
The fact is, however, capable of extension to other tissue elements such as nerve 
fibres, which are only finely-drawn processes of cells also containing inorganic salts 
within them differing, in proportional amount at least, from those contained in the 
solutions without. The extension of the fact enables the enunciation of an apparently 
unobjectionable general statement, that all cell walls (walls of nerve fibres included) 
are possessed of this property of limited permeability to the particles in motion in the 
solutions surrounding them, and this no matter what be the force under the action of 
which these particles are moving. The general statement therefore includes the move- 
ment of ions carrying an electrical current. 
From such or similar consideration G. N. Stewart 1 comes to the following 
conclusion as to the value of the polarization resistance stated by Hermann to be 
found on the surface of nerve fibres during the passage of an electrical current : — 
' But it nerve fibres are surrounded by an envelope whose specific resistance is much greater 
than that of the contents of the fibre, there must be a very abrupt change of potential as we pass 
along current lines that cut the envelope, and the surface of the envelope may therefore become 
strongly polarized. The fact discovered long ago by Hermann, that the apparent conductivity of 
nerve across the fibres is many times less than its conductivity in the longitudinal direction, although 
explained by him as due to the relatively great capacity for polarization of the nerve when the 
polarizing current passes transversely across it, receives a more natural explanation if we suppose 
that the nerve fibres are surrounded by badly-conducting envelopes. Of course if this is the case, 
a part of the apparent excess of transverse resistance may still be due to polarization, but not the 
whole of it, nor probably any large proportion of it.' 
Such a view is a repetition of Grunhagen's original position, that the major por- 
tion of the transverse resistance was due to a pre-existing high resistance envelope, 
separating the core of the nerve fibre from the outer ' nutritive fluid.' It is based 
like it upon a knowledge of the histological structure of the nerve, but whereas 
Grunhagen's envelope was peculiar to the nerve as its fatty sheath, and was, therefore, 
rejected on appeal to the similar existence of a property by muscle where such a sheath is 
non-existent ; Stewart's envelope is the common property of the nerve fibre and of all 
cells. The envelope may include the neurilemma and the myelin sheath or both. 
Its thickness is not of such importance as its quality, and its quality is the common 
property of all limits to cellular structures no matter how microscopically minute they 
may be. 
Such opinions based upon the one hand upon an intimate acquaintance with the 
properties, and the limits to the properties, of moist conductors (Nernst), and on the 
other hand, upon an intimate acquaintance with the physical value of histological 
structures, come to the same conclusion. The nerve fibre, covered with its surrounding 
solution, can only be successfully imitated by a concentric arrangement of three 
materials of different specific conductivity, and the theoretical considerations indicate 
amply that no simpler arrangement is possible. 
1. G. N. Stewart, Journal of Physiology. XXIV, p. 212-5 
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