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I propose to make some remarks on the second part of the External affections, or Inlets of the Mind.— These are usually termed the FIVE SENSES.
The external senses must not be considered as the powers of the mind; but as the expressions or indications of its powers, and its instruments or agents in conveying information to it, and also of conveying information to others.
No machinery or mechanism was ever at once so transcendently simple and well-ordered, as a whole, and, at the same time, so complex in its several parts, as the external organization of the mind, or the separate and combined external senses. Were we more closely to study these phenomena, every one of us would find that he carried about with him a universe, as replete with the indications and proof of the existence of the Infinite Jehovah as the world in which we live, and the universe itself of which we form a part.
We need not scale the heights above, nor dive into the depths beneath; neither need we take our seat in some particular part of the heavens in order to have overwhelming views and proofs of the power, majesty, and glory of God. We carry them about with us; yea, from the cradle to the grave.
We must now proceed to make some remarks upon some of the sensations which are produced by the external senses or inlets of the mind.
I. THE SENSE OF HEARING.-And, first, we shall notice the organ of hearing; and, second, some of its sensations.
The ear, or auricle, is a most delicate and complicated organ, fully adapted to answer all the great purposes of social intercourse, and the beneficent intentions of the Almighty.
The ears, in human beings, consist, first of the external ear; second, an internal cavity of bone, which is furnished with a great number of winding passages, formed within the temporal bone; third, a strong transparent membrane, which is stretched across the passage just named, and so separates the two other parts from each other. This membrane is styled the tympanum, i.e., the drum of the ear. 'By this membrane the vibrations of the air are received from the external ear, and are transmitted through the canals or passages called the labyrinth, to the auditory nerve, which is formed into a beautiful expansion." The impressions thus made upon the tympanum are conveyed by the auditory nerve to the brain, and the sensation of hearing is the immediate result.
How are these, sensations produced? The answer is that the equilibrium of the air is disturbed whenever any one speaks, or a bell is rung, or a gun is fired, &c., just the same as when a boy throws a stone or other hard substance into a pool of water, the effect of which is the describing of a great number of concentric circles. Now, when the air is thrown into a tremulous or vibratory state, and the ear is situated within the reach or influence of these undulations, a vivid sensation of sound is produced. These atmospheric concentric
circles, so to speak, strike the tympanum, and proceed through the medium of the auditory nerve, to be conveyed to the brain.
It is this sense which constitutes us capable of deriving pleasure from music. By it we become acquainted with the thoughts and emotions of others when orally expressed.
II. SENSE AND SENSATION OF SIGHT.-We shall first briefly describe the visual organs; and secondly, make some remarks on some of the sensations of sight.
First, the organ itself. We are informed that "the eye is situated in a circular orbit, and composed of transparent substances, called humours, of various refractive densities, viz.-the aqueous, crystalline, and vitreous humours. The first refraction takes place on the surface of what is called the convex cornea of the eye, which receives the rays of light, converges and transmits them to the aqueous humour, a transparent fluid situated between the cornea and the crystalline humour. The pupil, or perforation in the centre of the iris, admits of the transmission of the rays from the aqueous humour to the crystalline lens, by which they are again refracted and transmitted to the vitreous humour, in which is placed the retina, or net-like expansion of the optic nerve." These several refractions of rays of light produce the image of the object from which they proceed, upon the retina.
What a display of the wisdom and beneficence of the Divine Being do we discover in the organs of vision!
Consider their situation and adaptation to the purposes which they are intended to serve. Paley, in his Natural Theology, remarks, that "Were there no example in the world of contrivance, except the eye, it would be alone sufficient to the conclusion which we draw from it, as to the necessity and existence of an intelligent Creator. Its coats and humours, constructed as the lenses of a telescope, are constructed for the refraction of the rays of light to a point, which forms the proper action of the organ: the provision, in its muscular tendons, for turning its pupil to the object, similar to that which is given to the telescope by screws, and upon which power of direction in the eye, the exercise of its office, as an optical instrument, depends: the further provision for its defence, for its constant lubricity and moisture, which we see in its socket and its lids, in its gland, for the secretion of the matter of tears, its outlet or communication with the nose for carrying off the liquid after the eye is washed with it; these provisions compose altogether an apparatus, a system of parts, a preparation of means, so manifest in their design, so exquisite in their contrivance, so successful in their issue, so precious and so infinitely beneficial in their use, as in my opinion, to bear down all doubt that can be raised upon the subject."*
III. SENSE OF TOUCH.-As in the former senses, we shall in this notice, first, the organ or sense itself; and secondly, some of its sensations.
First, The Organ. This covers the entire body. It
is thought that the nervous papillæ of the skin are the inlets of the sensations of touch or feeling.
Many and exceedingly curious are the opinions of such men as Reid, Brown, Stewart, &c., upon the different kinds of sensations which the sense of Touch excites or produces; but we cannot introduce them into a lecture like the present.
Suffice it to say, that it connects the whole of the body with the brain. It differs, in this respect, from the other senses. Whenever a part of the body comes in contact with any substance, communication immediately takes place with the brain, and we are informed of what has occurred by the peculiar sensation produced. Were the nervous membrane as extensive as the world, or did it reach the sun, an impression made upon it at the antipodes or at the sun would be instantly communicated to the mind. Just in the same way as information is conveyed by the electric telegraph from England to America, &c.
IV. SENSE OF SMELL.-For the purposes of distinguishing our food, enjoying the fragrance of flowers, and of avoiding many things which would otherwise be deleterious to us, the Great Creator has been pleased to furnish man, in common with other animals, though to a less extent than most, with the sense of smell.
The infinitesimal odour of the rose, musk, &c,, is borne along the agitated strata of the atmosphere until it comes in contact with the olfactory nerves, and thus the sensation of smell is produced. We are told there