◊ 18. Connexion of the brain with sensation and perception. (I.) It may perhaps be asked, Whether these views are intended to exclude the brain, as having a connexion with the senses in the results which are here ascribed to them? And this inquiry leads us to observe, (what has been before alluded to,) that the brain is a prominent organ in the material part of the process of sensation and of external perception. The senses evidently cannot be separated from the nervous system. But the substance which is found in the nerves, excepting the coat in which it is enveloped, is the same as in the brain, being of the same soft and fibrous texture, and in continuity with it. As a general statement, when the brain has been in any way injured, the inward sensation, which would otherwise be distinct on the presence of an external body, is imperfect. Also, if the nerve be injured, or if its continuity be disturbed by the pressure of a tight ligature, the effect is the same; a circumstance which goes to confirm the alleged identity of substance in the two.

(II.) The brain, therefore, and whatever of the same substance is in continuity with it, particularly the nerves, constitutes the sensorial organ, which, in the subordinate organs of taste, smell, sight, touch, and hearing, presents itself under different modifications to external objects. On this organ, the sensorial, as thus explained, an impression must be made before there can be sensation and perception.

An impression, for instance, is made on that part of the sensorial organ called the auditory nerve, and a state of mind immediately succeeds which is variously termed, according to the view in which it is contemplated, either the sensation or the perception of sound.

An impression is made by the rays of light on that expansion of the optic nerve which forms what is called the RETINA of the eye, and the intellectual principle is immediately brought into that new position, which is termed visual perception or a perception of sight.

The hand is impressed on a body of an uneven and rough surface, and immediately consequent on this application and pressure is that state of mind which is termed a sensation or perception of roughness.

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 to 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. When 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. We say 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.

CHAPTER IV.

THE SENSE OF HEARING.

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 state of 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