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$ 19. Order in which the senses are to be considered. In considering those ideas which we become possessed of by means of the senses, it is natural to begin with that sense which will cause us the least difficulty in the analysis of its results; and to proceed the others successively, as we find them increasing in importance. It may not be altogether easy to apply this principle
with strictness, but it will answer all the purpose for which it is here introduced, if we consider the senses in the following order, the smell, taste, hearing, touch, and sight.
The mind holds a communication with the material world by means of the sense of smelling. All animal and vegetable bodies (and the same will probably hold good of other bodies, though generally in a less degree) are continually sending out effluvia of great subtilty. These small particles are rapidly and widely scattered abroad in the neighbourhood of the body from which they proceed. No sentient being can come within the circumference occupied by these continually moving and volatile atoms, without experiencing effects from it.
$ 20. Of the sense and sensations of smell. The medium through which we have the sensations and perceptions of smell, is the organ which is termed the olfactory nerve, situated principally in the nostrils, but partly in some continuous cavities nen some odoriferous particles, sent from external objects, affect this organ, there is a certain state of mind produced which varies with the nature of the odoriferous bodies. But we can no more infer from the sensation itself merely, that there exists any necessary connexion between the smell and the external objects, than that there exists a connexion between the emotions of joy and sorrow and the same objects. It might indeed be suggested to us by the change in our mental states, that there must be some cause or antecedent to the change, but this suggestion would be far from implying the necessity of a corporeal cause.
(II.) How then does it happen, that we are not merely sensible of the particular sensation, but refer it at once to some external object, to the rose, or the honeysuckle ? In answer it may be remarked, if we had always been
destitute of the senses of sight and touch, this reference never could have been made; but, having been furnished with them by the beneficent Author of our being, we make this reference by experience. When we have seen the rose, when we
have been near to it and handled it, we have uniformly been conscious of that state of mind which we term a sensation of smell. When we have come into the neighbourhood of the honeysuckle, or when it has been gathered and presented to us, we have been reminded of its fragrance. And thus, having learned by experience that the presence of the odoriferous body is always attended with the sensations of smell, we form the habit of attributing the sensations to that body as their cause.
$ 21. Of perceptions of smell in distinction from sensations. The mental reference spoken of in the last section is made with almost as much promptness as if it were necessarily involved in the sensation itself. It is at least so rapid, that we find ourselves utterly unable to mark the mind's progress from the inward feeling to the conception of the outward cause. Nor is this inability surprising, when we consider that we have repeated this process, both in this and in analogous cases, from our earliest childhood. No object has ever been present to us capable of operating on the senses, where this process has not been gone through. The result of this long-continued and frequent repetition has been an astonishing quickness in the mental action; so much so that the mind leaps outward with the rapidity of lightning, to be present with, and to comprehend the causes of the feeling within.
This view, it will be seen, helps in illustrating the nature of PERCEPTION as distinguished from sensation. The outlines of that distinction have already been given ; and every one of the senses, as well as that now under consideration, will furnish proofs and illustrations of it. Accordingly, when we are said to perceive the smell, or to have perceptions of the smell of a body, the rapid process which has been described is gone through, and the three things which were involved in the definition of Perception, already given, are supposed to exist; (1.) The presence of the odoriferous body and the affection of its appropriate organ; (2.) The change or sensation in the mind; and, (3.) The reference of the sensation to the external body as its cause.
§ 22. Of the sense and the sensations of taste. The tongue, which is covered with numerous nervous papillæ, forms essentially the organ of taste, although the papillæ are found scattered in other parts of the cavity of the mouth. The application of any sapid body to this organ immediately causes in it a change or affection; and that is at once followed by a mental affection or a new state of the mind. In this way we have the sensations and perceptions, to which we give the names sweet, bitter, sour, acrid, &c.
Having experienced the inward sensation, the affections of the mind are then referred by us to something external as their cause. We do not, however, always, nor even generally, distinguish the qualities which constitute this cause by separate and appropriate designations; but express them by the names that are employed for the internal feeling, viz., sweetness, bitterness, sourness, &c. This reference of what is internally experienced to its external cause is very rapidly made; so that we at once say of one apple it is sweet, and of another it is sour. Still it is to be kept in mind, that, in point of fact, it is subsequent, both in the order of nature and of time, to the mere sensation ; although we may not be able, in consequence of its rapidity, to mark distinctly the progress of the mental action from the one to the other. As in the case of smells, which have already been remarked upon, the reference is the result of our former experience. of one body it is sweet, and of another it is sour, because we have ever observed that the mental states indicated by those terms have always existed in connexion with the presence of those bodies.
Whenever, therefore, we say of any bodies that they are sweet, bitter, sour, or apply any other epithets expressive of sapid qualities, we mean to be understood to say that such bodies are fitted, in the constitution of things, to cause in the mind the sensations of sweetness, bitterness, and sourness, or other sensations expressed by
denominations of taste. Or, in other words, that they are the established antecedents of such mental states, as there is, further than this, no necessary connexion between them.
THE SENSE OF HEARING. 4.
$ 23. Organ of the sense of hearing. FOLLOWING the order which has been proposed, we are next to consider the sense of HEARING. And, in proceeding to the consideration of this subject, the remark is a very obvious one, that we should be unable to hear if we had not a sense designed for, and appropriate to, that result. The air, when put strongly in motion, is distinctly perceived by the touch; but no impression which it could make on that sense would cause that internal feeling, which is termed a sensation of sound. Our Creator, therefore, has taken care that these sensations shall have their own organ; and it is obviously one of precise and elaborate workmanship.
The ear is designedly planted in a position where, with the greatest ease, it takes cognizance of whatever is going on in the contiguous atmosphere. When we examine it externally, we not only find it thus favourably situated, but presenting a hollowed and capacious surface, so formed as to grasp and gather in the undulations of air, continually floating and in motion around it. Without, however, delaying to give a minute description of the internal construction of the ear, which belongs rather to the physiologist, it will answer our present purpose merely to add, that these undulations are conducted by it through various windings, till they are brought in a
concentration, as it were, against the membrane called the TYMPANUM.—It is worthy of notice, that on the internal surface of this membrane (the drum, as it is popularly called) there is a nerve spread out in a manner analogous to the expansion of the optic nerve at the bot
tom of the eye. Whether this nervous expansion be indispensably necessary to the result or not, it is certain that a pressure upon or affection of the tympanum by the external air, is followed by a new state of the mind, known as the sensation or perception of sound.
Ø 24. Varieties of the sensation of sound. The sensations which we thus become possessed of by the hearing are far more numerous than the words and the forms of speech, having relation to them in different languages, would lead us to suppose. It will help to illustrate this subject if we recur a moment to the sense of TASTE. The remark has somewhere been made to this effect, and probably with much truth, that if a person were to examine five hundred different wines, he would hardly find two of them of precisely the same flavour. The diversity is almost endless, although there is no language which distinguishes each variety of taste by a separate name. It is the same in respect to the sensations of sound. These sensations exhibit the greatest variety, although their differences are too minute to be separately and distinctly represented by language.
These views will appear the less objectionable when it is remembered that sounds differ from each other both in the tone and in the strength of the tone. It is remarked by Dr. Reid, that five hundred variations of tone may be perceived by the ear, also an equal number of variations in the strength of the tone; making, as he expressly informs us, by a combination of the tones and of the degrees of strength, more than twenty thousand simple sounds, differing either in tone or strength.
In a perfect tone, a great many undulations of elastic air are required, which must be of equal duration and extent, and follow each other with perfect regularity. Each undulation is made up of the advance and retreat of innumerable particles, whose motions are all uniform in direction, force, and time. Accordingly, there will be varieties also and shades of difference in the same tone, arising from the position and manner of striking the sonorous body, from the constitution of the elastic media um, and from the state of the organ of hearing,