and on the probable Nature of the Nerve Current. 175 
nerve force can hardly be of this nature. If it were electricity, surely 
the conductor ought to be metallic ! The nerve force, therefore, they 
conclude, is not electricity. Having affirmed this, all further doubt 
and hesitation vanish, and we are confidently assured that nerve 
force is unquestionably some other mode of force not yet discovered, 
hut undoubtedly correlated with heat, light, electricity, motion, and 
the other modes of energy. This, according to the teaching of the pre- 
sent day, is certain. Nerve force must he a mode of ordinary force, 
because there is nothing else that it can he. And yet one has a 
misgiving that the advocates of positive-fact knowledge may be exhi- 
biting some little weakness and inconsistency in trusting to this sort 
of argument. For upon what science principles do they even dare 
to talk about new modes of force they know nothing about ? How 
can they bring the observation and experiment in which only they 
believe, to bear upon a new mode of force which they admit exists 
at present only in their own imaginations ? Of course we can 
imagine many things which we cannot investigate ; but imagination 
is not observation and experiment, and imagination has no place in 
the positive science of our day. New modes of force which mag 
exist are but philosophic phantoms — mere fictions of the imagination 
and of the inventive faculty. Is it reasonable to teach the existence 
of undiscovered forces correlated with ordinary force ? It has been 
affirmed to be unreasonable, fanciful, frivolous, absurd, Ac., &c., to 
suggest that of undiscovered forces, some there may be which are 
distinct altogether from ordinary forces, and not correlated with 
these. 
Some consider that the axis cylinder is a soft pulpy material, 
the relative position of whose ultimate particles is much altered 
during its active state. Others look upon it as a peculiarly vital 
form of tissue, although it behaves towards reagents very much as 
certain forms of fibrous tissue. It certainly is not allied to any 
form of bioplasm or living matter. Generally the tissue of the axis 
cylinder has been regarded as very peculiar, and it has been in- 
ferred that its peculiar function must be in some manner deter- 
mined by its peculiar structure or its peculiar chemical composition. 
Not that there are any new facts in favour of this notion ; but it 
has been laid down most authoritatively that the properties of this 
particular tissue, as of everything else in nature, are due to the 
properties of the material of which it is composed, and of course to 
the properties of the elements entering into its composition. 
Now when we come to examine this mysterious force-conveying 
axis cylinder, what do we find? A filament possessing an ex- 
ceedingly simple structure, which, at least in many instances, looks 
very like ordinary fibrous tissue. Indeed, if we were shown only 
a very small piece of an axis cylinder of a frog’s nerve fibre, and 
some pieces of fibrous tissue of the same shape and size, we should 
