OF THE SENSES IN GENERAL.
The animal body is composed of different matters, of which some are insensible, as the bones, the fat, the blood, &c. and others, as the membranes and the nerves, appear to be active substances, on which depend the action of every member. The nerves are the immediate organs of the mind, but which may be said to diversify from a difference in disposition, insomuch, that according to their position, arrangement, and quality, they transmit to the mind different kinds of sentiment, which have been distinguished by the name of sensations, and which appear, in effect, to have no resemblance to each other. Nevertheless, if we consider that all external senses are only nervous membranes, differently placed and disposed; that the nerves are the general organs of feeling, and that in the animal body, no other substance is possessed of this property, we shall be led to believe that the senses, having all one common principle, and the nerves proceeding from the same substance, though in various forms, the sensations which result from them are not so essentially different as they at first appear.
The eye ought to be regarded as an expansion of the optic nerve, whose position being more exterior than that of any other nerve, has the most quick and the most delicate sensation. It will be moved, therefore, by the smallest particles of matter, as those of light; and will consequently give us sensations of distant bodies, provided they produce or reflect those small particles. The ear is not placed so exteriorly as the eye, and in which there not being so great an expansion of nerves, will not be possessed of the like degree of sensibility, nor will it be affected by particles more gross, as those which form sounds, and will give us sensations of such distant objects as can put those particles in motion. As they are much grosser than those of light, and have less quickness, they cannot extend themselves so far; and consequently the ear will not give us sensations of objects so distant as those which the eye communicates. The membrane, which is the seat of smell, being still less furnished with nerves than the ear, it will only give us sensations of particles of matter which are more gross and less remote, such as the odour from bodies, which may be said to be the essential oils which exhale and float in the air, as light bodies swim upon the water. As the nerves are also in less quantity, and more divided over the tongue, and palate, and the odoriferous parts are not strong enough to affect them, the oily or saline parts must detach themselves from other bodies, and lodge upon the tongue to produce the sensation of taste. This sense differs materially from that of smelling, because the last brings to us sensations of things at a certain distance, but the former requires a kind of contact, which operates by the means of the fusion of certain parts of matter, such as salts, oils, &c. In short, as the nerves are minutely divided, and as the skin affords them but a very thin covering, no particles of matter so small as those which form light, sound, or odours, can affect them; and the sense of feeling gives us no sensation of distant objects, but of those only whose contact is immediate.
It appears, therefore, that the difference between our senses is occasioned by the more or less exterior position of the nerves, and of their greater or smaller quantity in the different organs. It is for this reason that a nerve, when irritated by a stroke, or uncovered by a wound, gives us often the sensation of light, without the assistance, of the eye; and from the same cause we often experience sensations of sound, though the ear be not affected by any thing exterior.
When the particles of luminous or sonorous matters are re-united in great quantities, they form a kind of solid body, that produces different kinds of sensations, which appear not to have any relation with the first. The particles which compose light being collected in great quantities, affect not only the eyes but also the nervous parts of the skin, and produce the sensation of heat, which is a sentiment, different from the first, though originating from the same cause. Heat, then, is a sensation arising from a contact with light, which acts as a solid body, or as a mass of matter in motion. The action of light, like other matters in motion, is evident when we expose, light bodies to the focus of a burning glass; the action of the light communicates before even it heats them, a motion by which they are disturbed and displaced. Heat, then, acts as solid bodies act upon each other, since it is capable of displacing light matters, and communicating to them a movement of impulsion.
The like happens when the sonorous particles are collected in great quantities; they produce sensible agitation, which is very different from the action of sound upon the ear. Any violent explosion, as a loud clap of thunder, shakes us, and communicates a kind of trembling to all the neighbouring bodies. Sound then also acts as a solid body, for it is not the agitation of the air which causes this tremulous motion, since even at that time we do not remark that it is accompanied with the wind; besides, however strong the wind may be, it never produces such violent agitations. It is by this action of the sonorous particles that a cord in vibration sets the next in motion; and we ourselves feel, when the noise is violent, a kind of fluttering very different from the sensation of sound by the ear, although it be an effect of the same cause.
All the difference in our sensations are produced by the greater or smaller number, and by the more or less exterior position of the nerves, which is the cause that some of our senses, as the eye, ear, and smell, may be affected by the small particles which exhale from particular bodies; others, as tasting and feeling, require actual contact, or more gross emanations, so as to form a solid mass; and it is this feeling which gives us the sensation of solidity, or fluidity, and of the heat of bodies.
A fluid differs from a solid, because it has not any particles gross enough to admit us to grasp it on different sides at one time. The particles which compose fluids cannot touch each other but at one point, or so few points, that no part can have any considerable adliesion with another. Solid bodies, reduced even into an impalpable powder, do not absolutely lose their solidity, because the parts, touching each other by many sides, preserve a degree of cohesion; and this is the reason why we can make them up in masses, and squeeze them together.