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.