« VorigeDoorgaan »
no resemblance to the bell or to the stroke on the bell that caused it to produce the sensation of hearing. The sensation that follows the presenting of light to the eye bears no resemblance to the luminous object or to the light flowing from it, or to the eye, or to the motion of light towards the eye, or to the impact of the light upon the eye. All that can be said in explanation of the phenomenon of the sensation is, that it instantly follows the presenting of the luminous object to the eye.
TOUCH.-The external extremities of the tactual nerves are spread over the whole surface of the body, with the exception of some portions which in themselves are incapable of conveying sensation-as the hair, the nails, the enamel of the teeth, the cuticle, &c. Sensations can be conveyed through these, just as they can be conveyed through any extraneous substance laid on any part of the body; but, in themselves, they are destitute of nerves, and therefore are incapable of causing sensation. The tactual nerves, therefore, have no apparatus at their external extremity, like the nerves of hearing, seeing, smelling, for bringing the material substance fitted to act on them into communication with them. The whole body is, to this sense, the external portion of the organ. The sensations derived from this sense are called feelings, although that word is often applied figuratively to emotions generally, as when we speak of a feeling of sympathy, of envy, of resentment, and many others.
The qualities in bodies that affect the sense of touch are motion, and heat and cold. Motion towards the body produces impact, or pressure, which excites the sensation of touch more or less strongly, according to the violence of the impact or the pressure. Liquids and gases, when in motion, occasion this sensation; but when they are at rest, they do not. Solids affect the sense variously, according to the nature and force of their contact with the tactual nerve. A sharp edge or point produces a different sensation from a blunt
instrument, and both are modified by the manner and force of contact, whether a sudden blow or gradual pressure. The difference felt between hardness and softness resolves itself into greater or less violence of impact or pressure; and the difference between roughness and smoothness is merely difference of pressure on neighbouring portions of the nerve,-the projections of a rough body pressing more strongly than the other parts of it. In these statements we entirely exclude, for the present, all muscular exertion as connected with the sense of touch, and suppose the body to be entirely passive. The reason of this will appear as we proceed.
The tactual nerves are also affected with heat or cold; that is, if any substance at a higher or lower temperature than that of the extremity of the nerve at the moment of touch be applied to it, the sensation of heat or cold is excited in strength proportioned to the difference of temperature between the nerve and the object affecting it. If the temperature of the object be higher, the sensation will be that of warmth or heat, up to burning heat; if the temperature be lower, the sensation will be coolness, or cold, to freezing cold. It is somewhat remarkable that the sensation arising from extreme cold cannot be distinguished from that occasioned by extreme heat, both being destructive of the nerve. The pressure, or impact, also, of a sharp edge or point cannot be distinguished, by this sense alone, from burning heat or intense cold, probably for the same reason. Some chemical qualities in bodies, as strong acids, alkalies, and some salts, also produce sensations so similar to those which are produced by heated substances, as not to be distinguished from them by the sense alone.
TASTE.—The external extremities of the nerves of taste are placed in papillæ, or little risings on the upper surface of the tongue, and the sensation is produced by the body exciting it being brought into contact with the
tongue. The qualities in bodies which affect these nerves are called flavours, or sometimes tastes; as when we say of one thing, that it has a sour, and of another that it has a sweet, or bitter taste. But this is inaccurate language, for the material substance has not the taste which is in the mind, but only that quality which excites the sensation of taste in the mind.
The bodies that possess qualities fitted to excite the sensations of touch and taste, must, in order to do so, be brought into contact with the organ of sensation. In the senses which follow, the sensation is produced by certain emanations from the object that excites it.
SMELLING. The nerves of smell, called the olfactory nerves, have their external terminations in the nostrils. The qualities in bodies that act upon them are called odours, perfumes, fragrance, stench, or, improperly, smells. The odours consist of an effluvia which the odoriferous body throws off, which mingles with the air, and thus enters the nostrils, where it meets the extremities of the olfactory nerves, and produces sensation. If an odoriferous body be present, but covered with a glass receiver, so as to prevent the odour from mingling with the external air, no sensation of smell will be experienced. This sense gives no intimation of the distance of the odoriferous substance, or the direction in which it lies, and no experience enables us to determine these points by this sense alone.
HEARING. The nerves of hearing, called also the acoustic, or auditory nerves, have their outer extremity in the ear, where a beautiful apparatus is provided for collecting and conveying sound to them. The material substance that affects these nerves is the atmospheric air thrown into a tremulous, or vibratory motion by the vibrations of a sonorous body; therefore there is no sound in a vacuum. If a bell be hung in the receiver of an air pump, and the receiver exhausted, and the bell be shaken, so as to make the tongue strike the sides of
it, no sound will be produced. If a little water be put into a glass tumbler, and a moist finger passed rapidly round the edge of the tumbler, a sound will be produced, and the vibrations of the glass producing the sound will at the same time be visible on the surface of the water. When the deeper-toned strings of a harp, or violoncello, or pianoforte, are twanged or struck, the vibrations that produce the sound are visible. The vibrations of the air produced by sounding bodies, and causing the sensation of sound, have been compared to the concentric circles produced on the surface of a pool of still water when a stone is thrown into it, except that the concentric vibrations in the air, causing sound, travel with much greater rapidity. The vibrations causing the sensation of sound, it has been ascertained, travel at the rate of a mile in eleven seconds.
This sense of itself cannot discern either the direction or the distance of the object from whence sound proceeds; nevertheless, direction, distance, the intervention of other objects, and many other circumstances, so modify the sensation caused by sound, that the mind, by accurately observing these modifications, learns to make good guesses at these particulars, but never learns to determine them absolutely. The most practised ear may mistake a faint sound proceeding from a body at hand for a loud sound proceeding from a distant body, or it may be deceived in regard to the direction from whence the sound proceeds. There is one peculiarity in the sense of hearing which deserves notice,-namely, that the acoustic nerves may be acted on through other parts of the body than the ear. A deaf man has been known to derive much pleasure from music, by making the nails of his fingers a medium of communication between himself and the musical instrument, and most persons are familiar with the experiment of suspending a bar of iron (a poker is often made use of for the purpose) with a string, holding the string in the teeth, and then striking
the bar of iron. The sensation of sound produced is much stronger than it is when heard through the ear. Some substances have a singular power of conveying sound. If the ear be applied to a log or deal of timber near the one end, the slightest scratch at the other end will be distinctly heard. Soldiers in the field often discover the tramp of cavalry at a considerable distance by applying their ear to the ground. The teeth and other bones of the body may communicate the vibrations of sound to the acoustic nerves.
SIGHT. The nerves of sight, called also the visual or optic nerves, have their outer extremity in the inner surface of the back part of the eye, called the retina. The material substance that acts upon these nerves is light. Light proceeds from every luminous body with extreme velocity, having been ascertained to travel at the rate of 192,000, say, for the sake of round numbers, 200,000 miles in a second. It has been supposed to consist of particles of matter of inconceivable minuteness, sent off in right lines from the luminous body. But the opinion seems to be gaining ground that it consists rather of the vibrations of an extremely subtle fluid everywhere dispersed, somewhat like the vibrations of the air which cause sound, but generated incomparably more rapidly in consequence of the incomparably greater subtlety of the fluid. The great objection to this theory is, that rays of white light are found to be composed of rays of differently coloured light, refrangible in different degrees, and that the rays of the sun consist not merely of light but of heat, and of other rays that produce a chemical effect on some objects on which they fall; the rays of light and heat and the chemical rays being also refrangible in different degrees. Whether the difficulties connected with the compound nature of rays of light, and the different powers which the same medium exercises upon different rays in refracting or bending them out of their straight course, shall ever be satisfactorily answered,