THE INJURY CURRENT OF NERVE 
267 
The argument is not as good a one as might at first sight appear, since this 
property of the core model structure is necessitated by a characteristic which may be 
peculiar to it, and may not adequately represent a condition present in the nerve. tor 
the successful imitation of the physical characteristics of the nerve by the core model 
is due to the acknowledged fact that, although commonly only composed of two 
materials, it opposes resistances of three kinds to the passage of an electrical current 
through it : 
(1) the surface resistance of the mantle ; 
(2) a high resistance at the surface of separation of core from mantle, due 
to internal polarization, and therefore only existing during the passage 
of the current ; 
(3) the internal resistance of the core. 
Three conditions are, therefore, obtained in the core model by the presence of 
two materials : but in order to obtain these three conditions from two materials, it is 
necessary that an extraordinary difference should be found between them. The con- 
ditions are only adequately so obtained by using a metallic conductor as the core, a 
dilute moist conductor as the mantle. The high specific conductivity of the core sub- 
stance of the model is therefore possibly only an accidental attribute, due to the fact 
that a metallic conductor is necessarily chosen to obtain the polarization resistance. 
The nerve fibre is, however, throughout a moist conductor. It seems an absurd, 
but is a necessary, statement that no metallic conductor exists within its core. The 
nerve, therefore, may indeed imitate the three conditions present within the core model 
carrying a current, it cannot, however, imitate them so successfully as to- be composed 
of two analogous sets of materials. It is necessary to consider how the three conditions 
can be obtained by the use of moist conducting material alone without the assistance 
of the metallic core. The result of such a consideration is of extreme importance, for 
it has been found impossible to represent the three conditions by the use of moist con- 
ducting material without making use of three materials of different specific conductivity, 
(1) of fair conductivity . . . mantle. 
(2) of bad conductivity . . . intervening structure. 
(3) of conductivity better than (1) . core. 
To carry the lesson learned from the core model to the structure of the nerve 
fibre we must, therefore, seek in the nerve fibre and the solution covering its surface 
for the analogues of these three different materials of different specific conductivity. 
It is to be noted that in such a core' model polarization resistance developed during 
the presence of the current (and only present then as in the case of the two material 
core model) is now of secondary importance. A pre-existing resistance now partially 
at least occupies its place, and the polarization which occurs is only a secondary 
addition to this pre-existing resistance. 
