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mind is enabled to take a review of any introduced impression, or to exercise its thought upon any introduced idea, empowers it to combine such impressions or ideas into every possible modification and variety. And hence arises an entirely new source of knowledge, far more exalted in its nature, and infinitely more extensive in its range: hence memory and the mental passions; hence reason, judgment, consciousness, and imagination, which have been correctly and elegantly termed the internal senses, in contradistinction to those by which we obtain a knowledge of things exterior to the sensorial region.
Thus far we can proceed safely, and feel our way before us; but darkness hangs over all beyond, and a gulf unfathomable to the plummet of mortals. Of the sensory, or mind itself, we know nothing; we have no chemical test that can reach its essence, no optical instruments that can trace its mode of union with the brain, no abstract principles that can determine the laws of its control. We see, however, enough to convince us that its powers are of a very different description from those of the body, and Revelation informs us that its nature is so too. Let us receive the information with gratitude, and never lose sight of the duties it involves.
But this subject would lead us astray even at our outset; it is important, and it is enticing; and the very shades in which much of it is enwrapped prove an additional incitement to our curiosity. It shall form the basis of some subsequent investigation*, but our present concern is with the external senses alone.
* Vol. III. Series III. Lect. I. II. III. IV.
These, for the most part, issue from the brain, which, in all the more perfect animals, is an organ approaching to an oval figure; and consists of three distinct parts: the cerebrum, or brain properly so called; the cerebel, or little brain, and the oblongated marrow. The first constitutes the largest and uppermost part; the second lies below and behind; the third, level with the second, and in front of itit appears to issue equally out of the two other parts, and gives birth to the spinal marrow, which may hence be regarded as a continuation of the brain, extended through the whole chain of the spine or back-bone.
From this general organ arises a certain number of long, whitish, pulpy strings or bundles of fibres, capable of being divided and subdivided into minuter bundles of filaments or still smaller fibres, as far as the power of microscopes can carry the eye. These strings are denominated nerves; and by their different ramifications convey different kinds or modifications of sensation to different parts of the body, keep up a perpetual communication with its remotest organs, and give activity to the muscles. They have been supposed by earlier physiologists to be tubular or hollow, and a few experiments have been tried to establish this doctrine in the present day, but none that have proved satisfactory.
As the brain consists of three general divisions, it might, at first sight, be supposed that each of them is allotted to some distinct and ascertainable purpose: as, for example, that of forming the seat of intellect, or thinking; the seat of the local senses of sight, sound, taste, and smell; and the seat of general feeling or motivity. But the experiments
of anatomists upon this abstruse subject, numerous and diversified as they have been of late years, and, unhappily, upon living as well as upon dead animals, have arived at nothing conclusive in respect to it; and have rather given rise to contending than to concurrent opinions. So that we are nearly or altogether unacquainted with the reason of this conformation, and of the respective share which each division takes in producing the general effect. The nerves uniformly issue in pairs, one for each side of the body, and the number of the pairs is thirty-nine; of which nine rise immediately from the great divisions of the brain, under which we have just contemplated it, and are chiefly appropriated to the four local senses; and thirty from the spinal marrow, through different apertures in the bone that encases it, and are altogether distributed over the body, to produce the fifth or general sense of touch and feeling, as also irritability to the muscles.
That these nervous or pulpy fibres are the organs by which the various sensations are produced or maintained, is demonstrable from the following facts. If we divide, or tie, or merely compress a nerve of any kind, the muscle with which it communicates becomes almost instantly palsied; but upon untying or removing the compression, the muscle recovers its feeling and mobility. If the compression be made on any particular portion of the brain, that part of the body becomes motionless which derives nerves from the portion compressed. And if the cerebrum, cerebel, or oblongated marrow, be ir ritated, excruciating pain or convulsions, or both, take place all over the body, though chiefly where,
the irritation is applied to the last of these three parts.
The matter of sensation, or nervous fluid, as for want of a more precise knowledge upon this subject we must still continue to call it, is probably as homogeneous in its first formation as the fluid of the blood; but, like the blood, it appears to be changed by particular actions, either of particular parts of the brain, or of the particular nervous fibres themselves, into fluids of very different properties, and producing very different results. And it is probably in consequence of such changes alone that it is capable of exciting one set of organs to communicate to the brain the sensation of sound alone, another set that of sight alone, and so of the rest. While branches from the spinal marrow, or fountainnerve of touch, are diffused over every portion of the body, sometimes in conjunction with the local nerves, as in the organs of local sense, and sometimes alone, as in every other part of the system. *
Such an idea leads us naturally to a very curious and recondite subject, which has never, that I know of, been attended to by physiologists, and will at the same time throw no small degree of light upon it:-I mean the production of other senses and sensorial powers than are common to the more perfect animals, or such a modification of some one of them as may give the semblance of an additional
What, for example, is that wonderful power by which migratory birds and fishes are capable of steering with the precision of the expertest mariner
* See Hunter's Anim. Economy, pp. 261, 262.
from climate to climate, and from coast to coast; and which, if possessed by man, might, perhaps, render superfluous the use of the magnet, and considerably infringe upon the science of logarithms? Whence comes it that the field-fare and red-wing, that pass their summers in Norway, or the wildduck and merganser, that in like manner summer in the woods and lakes of Lapland, are able to track the pathless void of the atmosphere with the utmost nicety, and arrive on our own coasts uniformly in the beginning of October? or that the cod, the whiting, and the herring, should visit us in innumerable shoals from quarters equally remote, and with an equal exactness of calculation? the cod pursuing the whiting, which flies before it, from the banks of Newfoundland to the southern coasts of Spain; and the cachalot, or spermaceti whale, driving vast armies of herrings from the arctic regions, and devouring thousands of those that are in the rear every hour.
We know nothing of this sense, or the means by which all this is produced: and knowing nothing of it, and feeling nothing of it, we have no terms by which to reason concerning it.
Yet it is a sense not limited to migratory animals. A carrier-pigeon has been brought in a bag from Norwich to this metropolis, constituting a distance of 120 miles; and having been let off with a letter tied round its neck, from the top of St. Paul's, has returned home through the air in a straight line, in three or four hours: and we have many examples of their travelling much greater distances, as from London to Antwerp, with equal certainty and rapidity.