Were there a distinct fluid belonging to the nerves of sensation, and
insulated, it could not be affected by external circumstances, nor
its cerebral excitement be productive in the least of any knowledge,
relative or inferential of external bodies. Were the fluid not
insulated, it should be subject to waste like the lachrymal fluid, and
must excite the brain differently at different times, even under equal
circumstances; which must make it impossible to identify the same body
after its removal out of the axis-of-vision direction.
A distinct fluid, not insulated, has to be in contact with the line of
medium of space which the external object terminates, which adds to the
difficulty of waste, in the possibility of the nerves becoming flooded
with an abnormal fluid, medium of space. Much more likely is it,
that, _the cerebral exciting fluid, of the nerves generally, consists
in medium of space_, received from without through the cuticular
insertions and orifices of the nerves as streamlets from the great
ocean of space, subject to neither ebb nor flow, and liable to change
of pressure occasioned by external agency. According to this idea, the
object and brain are the terms of the visual line; and medium of space,
continuous from the object through the nerves to the brain, is the
connecting link.
Further; although medium of space is the nervous fluid and immediate
cerebral exciting cause, (which entitles it to be named the TRUE
_nervous fluid_,) there are strong grounds for concluding that, with
the true fluid, the nerves include a pair of correlative elements.
Because of the mesmeric effected polarities being without the comatose
flow, which leaves nothing to look to for the polarizing means but the
contents of the nerves. Next, as clairvoyance is a cerebral effect,
something connected with the nervous fluid must be concerned in its
production, or why not clairvoyance take place without the magnetic
passes. Finally, the true fluid, or any single fluid, is incapable
of being polarized; and the true fluid might be rendered immovable
at times, were there no electric or minus-pressure matter within
the nerves, also to prevent its increase, and to retain the normal
quantity of the true fluid. All extremes being prevented, and the
polarities of the extremities productive of increased lucidity, are
consistent with idea of the nerves including magnetising correlatives,
which, beside, serve as an elastic break against the fluid exciting the
brain indistinctly, irregularly, or exquisitely; and only, as it were,
muffled, to prevent the sensibility of the cerebral organs being worn
out prematurely.
Another object may be attained by the included electric correlatives,
namely, restricting the exciting pressure to certain degrees, so that
the sensation shall be defined and directing, but otherwise useless
and misleading. Another may be, that of regulating the degrees of
pressure on such a scale, as that, by the same senses, sensations shall
be excited as different from each other as those of red, yellow, and
blue by the optic sense, heat and cold by the feeling sense, sweet and
bitter by the gustory sense. To which the conjecture may be added, for
the purpose of anatomic and physiologic inquiry, that, as not even an
elementary interstice is without design, so may the orifices of the
retina be of regulated diameters, to ensure such definite degrees of
pressure on the brain as shall excite the sensations recognised as
primitive colours.
On the principle that the nervous fluid is derived from without, the
question is decided as to the cuticular termination of the nerves,
which is objected to by some, in consequence of a few of the nerves
being observed to have "inward bending." And is it not a matter of
common observation, that "feeling is most sensible at the tips of the
fingers" or apparent place of the sensation.