But the field opens before us to an unbounded extent, and we should lose ourselves in the subject if we were to proceed much farther. It is obvious, that in organic, as in inorganic nature, every thing is accurately arranged upon a principle of mutual adaptation, and regulated by an harmonious antagonism, a system of opposite yet accordant powers, that balance each other with most marvellous nicety; that increase and diminution, life and death, proceed with equal pace; that foods are poisons, and poisons foods; and finally, that there is good enough in the world, if rightly improved, to make us happy in our respective stations so long as they are allotted to us, and evil enough to wean us from them by the time the grant of life is usually recalled.

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LECTURE XV.

ON THE EXTERNAL SENSES OF ANIMALS.

THE

HE subject of study for the present lecture is the organs of external sense in animals: their origin, structure, position, and powers; and the diversities they exhibit in different kinds and species.

The external senses vary in their number: in all the more perfect animals they are five; and consist in the faculties of sight, smell, hearing, taste, and touch.

It is by these conveyances that the mind or sensory receives a knowledge of whatever is passing within or without the system; and the knowledge it thus gets possession of is called perception.

The different kinds of perception, therefore, are as numerous as the different channels through which they are received, and they produce an effect upon the sensory which usually remains for a long time after the exciting cause has ceased to operate. This effect, for want of a better term, we call impressions; and the particular facts, or things impressed, and of which

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the impressions retain, as it were, the print or picture, ideas.

The sensory has a power of suffering this effect or these ideas to remain latent or unobserved, and of calling them into observation at its option: it is the active exercise of this power that constitutes thought.

The same constitution, moreover, by which the 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 clouds and darkness hang 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 glasses 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 wrapped up 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.

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 it-it 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 glasses can carry

Vol. 111. Series 111. Lect. 1. II, III, IV.

the eye. These strings are denominated nerves; and by their different ramifications convey different kinds or modifications of sensation to dif ferent 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 arrived 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,