Imagini ale paginilor
[ocr errors]

of bodies, such for example as warmth or coldness. They | of the organs must accordingly be adapted in each particular also give warning of any action which threatens to be in- case to receive the impression made by these agents, and jurious to the part impressed or to the system at large, and must be modified in exact conforinity with the physical this is done by the sensation of pain which the nerves are laws they obey. The eye must be constructed with strict commissioned to excite on all these occasions, “ They reference to the laws of optics, and the ear to the reception act the part of sentinels,” says Dr. Roget, “placed at the of those minute impressions from the aërial vibrations which outposts to give signals of alarm on the approach of occasion sound. The extremities of the nerves in these and danger.”

other organs of sense are spread out into a delicate expanThe system of nerves, comprising those which are designed sion of surface, having a softer and more uniform texture to convey the impressions of touch, is universally present in than the rest of the nerve, whereby they are susceptible of al! classes of animals; and among the lowest orders they being affected by their own appropriate agents and by no appear to constitute the sole medium of communication with other. the external world. By ascending in the scale of organiza- In most cases the impression made on the sentient extretion, the faculties of perception are found to take a wider mity of the nerve appropriated to sensation, is not the direct range; and many qualities depending on the chemical action effect of the external body, but results from the agency of of bodies are rendered sensible, more especially those which some intervening medium. There is always a portion of belong to the substances employed as food. Ilence arises the the organ of sense interposed between the object and the sense of taste, which may be regarded as a new and more nerve on which the impression is to be made. The object refined species of touch. This difference in the nature of is never allowed to come into direct contact with the nerves; the impressions to be conveyed, renders it necessary that the not even in the case of touch, where the organ is defended structwe of the nerves, or, at least, of those parts of the by the cuticle, through which the impression is made, and nerves which are to receive the impression, should be modi- by which that impression is modified so as to produce the fied and adapted to this particular mode of action.

proper effect on the subjacent nerves. So also with the As the sphere of perception is enlarged, it is made to organs of taste and of smell, the nerves are not only comprehend, not merely those objects which are actually in sheathed with cuticle, but defended from too violent an contact with the body, but also those at a distance, the action by a secretion expressly provided for that purpose. existence and properties of which it is highly important the In the senses of hearing and of vision, the changes which individual should be apprised. It is also necessary that he take place in the organs interposed between the external acquire an accurate knowledge of the distances, situations, impressions and the nerves are still more remarkable and and motions of surrounding objects. He is, therefore, pro- | important. The objects of these senses, as well as those of vided with suitable organs for vision, for hearing, and for smell, being situated at a distance, produce their first imthe perception of odours; all of which senses establish pressions by the aid of some medium, exterior to our extensive relations between him and the external world, and bodies, through which their influence extends; thus the give him the command of various objects which are neces- air is the usual medium through which both light and sary to supply his wants, or procure him gratification; and sound are conveyed to our organs. Hence, in order to which also apprise him of danger while it is yet remote and understand the whole series of phenomena belonging to may be avoided. Endowed with the power of combining sensation, regard must be bad to the physical laws which all these perceptions, he commences his career of sensitive regulate the transmission of these agents. We propose, and intellectual existence; and though he soon learns that therefore, to consider these intermediate processes in the he is dependent for most of his sensations on the changes case of each of the senses, describing for each organ of sense, which take place in the external world, he is also con- first, its structure and the way in which its functions are scious of an internal power which gives him some kind of performed, and secondly, the laws which regulate the control over many of those changes, and that he moves physical phenomena by which the organ is affected". his limbs by his own voluntary act.

We are so much accustomed to the phenomena of sensation and perception,

SECTION 1. that, in order to understand them, they scarcely appear to require an elaborate investigation. To behold external The eye, which, from the wonderful power of its exterior objects nothing more seems necessary than to direct our eyes expression and the extreme beauty of its internal arrangetowards them; the sight of those objects seems a necessary ments is called “ Nature's master-piece," does not present to consequence of the motion of the eyeballs; and there seems our contemplation a more beautiful piece of apparatus than not to be anything marvellous in the function of the


Indeed, it is difficult, and even invidious, to or that any other organ is required in a simple act of vision. speak of the excellence of one of Nature's works in partiTo ascertain the solidity of an object we have but to stretch cular, since all are excellent, and evince such skill as belongs forth the hand, and to feel in what degree it resists the pres- only to Omnipotence. sure we give to it. Even this simple exertion is not required Hearing is only a refinement of the sense of touch: a for hearing the voices of our companions, or being apprised solid while in the act of emitting a sound will convey to by the increasing loudness of falling waters, that we are

the finger placed upon it a sense of rapid motion; it will approaching nearer and nearer to the cataract. Yet, how also communicate to the air a motion by which a series of much is really implied in all these apparently simple phe- undulations is excited; each undulation being formed by the nomena! Science teaches us that these perceptions, far from advance and recoil of the aërial particles, and communicatbeing direct or intuitive, are only the final results of a ing with the external ear, imparts motion to its internal long series of operations, produced by agents of a most mechanism. subtle nature, which act by curious and complicated laws It has been thought by physiologists that all animals upon a refined organization, disposed in particular situations hear,—that those which are not provided with a distinct in our bodies, and adjusted with admirable art to receive organ receive impressions of sound through their nervous their impressions, to modify and combine them in a certain system generally, but we find that, according as the animal order, and to convey them in regular succession, and with is low in the scale of creation, its organs are more simple, out confusion, to the immediate seat of sensation.

less is required of them, and accordingly the means furnished Yet this process, complicated as it may appear, constitutes to them are adequate only to the performance of their simple but the first stage of the entire function of perception; for parts in the scheme of creation. As an animal rises in the before the mind can acquire a distinct knowledge of the pre

scale of intelligence, so also does its frame become more sence and peculiar qualities of an external object, a long complex as its wants become multiplied :-we have in it series of mental changes must intervene, and many intel

the parts of the lower animal together with something in Lectual operations must be performed. All these take addition; and so, as we advance, each lower grade has the place in such rapid succession that the whole appears to germ of some additional provision which is developed in occupy but a single instant. There are, however, according the one next above it, until the organ is met with in its to Dr. Roget, no less than twelve distinguishable kinds most complex and perfect state in man. of changes, or rather processes, some of which imply many The organ of hearing consists essentially of the expansion changes, which must always intervene in regular succession of a peculiarly sensitive nerve over a delicate membrane. In between the action of the external object on the organ of the perfect ear there is much additional mechanism, of the sense, and the voluntary movement of that part of the body particular use of which we are ignorant, but there can be no which it excites.

doubt, since Nature effects her object by the most perfect The external agents which affect the different parts of the means, and generally in the shortest way, that the design nervous system so as to produce sensation, are of different * For a more extended view of this important sulject, we refer the kinds, and are governed by peculiar laws. The structure reader to Dr. Roger's Bridgewater Treatise.

the ear.

[ocr errors]


[ocr errors]
[ocr errors]

of the unexplained portions of the ear is to increase the would partially be the case if the tube were straight. The force and vividness of the impression, to indicate the direc- structure of this tube is partly bony and partly cartilagition, the difference in pitch, the continuance, and the minute nous; it is formed in the petrous or rocky portion of variations of those pulses of sound, which are to be finally the temporal bone ; its surface is covered with a cuticle communicated to the mind through the medium of the audi- furnished with short stiff hairs, and also with a set of small tory nerve. Other portions probably fit and adjust and re- glands under the cuticle, which secrete and pour into the tain in a state of fitness and adjustment, the more important meatus their cerumen, which is a wax-like substance of an parts of the organ, so as to render them, as it were, inde- orange yellow colour and bitter taste; its object is to guard pendent of the atmospheric changes which often so materi- the internal ear from insects, and also it is said to moisten ally influence our sensations.

the passage, in the same way that a flute, in order to produce The parts of the human ear are usually considered under sound, must have its interior surface moist. three divisions. 1st, The external ear, leading by means of Here we may offer a remark against the vulgar prejudice à curved tube to—2nd, the middle ear, consisting of the which condemns a harmless insect to the unmitigated horror tympanum or drum of the ear, a cavity containing air, a of the ignorant and the timid, and gives it a name calculated membrane, bones, and a muscular arrangement, all which to excite fear and disgust; we refer to the ear-wig. This increase the force or manner of vibration, and convey it to insect is a nocturnal one, and having a great dread of ex-3rd, the inner ear, which contains the labyrinth, a general posure to the light, one of its kind may possibly by chance name applied to the intricate canals which contain the have taken shelter in the ear, not for the purpose of resting portio mollis, or soft portion of the seventh pair of nerves; this in and feeding upon the brain, as has been supposed, but then is the proper seat of hearing. In fig. 1 are exhibited in simply as a refuge from the light. This, however, is a one view, the principal parts of this complicated organ, situation which no ear-wig in its senses would prefer.' The as they exist in man, in their relative situations, and of ear-wig, in common with all insects, has an especial dislike their natural size : thereby affording a scale by which the to the taste and odour of the bitter wax which lines the real dimensions of those portions which will afterwards be path of hearing, and an insect would at all times rather avoid explained by magnified representations, may be properly the ears of animals than seek them. Small insects have been appreciated. In all the figures similar parts are indicated known to get accidentally into the ear, and to produce uncomby the same letters.

fortable sensations by crawling over the membrane of the

tympanum, but they soon get out again ; and no instance of Fig. 1.

real injury to the ear by insects is on record.

It has been remarked by Smellie that infants hear bluntly, because the bones of their ears are soft and cartilaginous;

and therefore the vibrations excited in them comparatively S

weak. “Young children accordingly are extremely fond of noise : it rouses their attention, and conveys to them the agreeable sensation of sound; but feeble sounds are not perecived, which gives infants, like deaf



appearance of inattention, or rather of stupidity.

2. The tympanum, T, is an irregular cavity containing air, and communicating with the external air through a tube called the Eustachian tube, E, (from the name of the anatomist who first pointed out its use,) which runs by the side of the Fig. 2. labyrinth, gradually increasing in width, until it ends in a trumpetshaped opening behind the curtain which separates the nostrils from the mouth. This is the only means which the internal surface of the tym

panum has of communicating with the 1. The form of the shell (concha) or external ear, c, is, of external air, but by this tube a free accourse familiar to every one. It projects a little forward, cess of air is admitted into the cavity presenting five prominences and three cavities, the whole to equipoise the atmospheric pressure forming a sort of funnel, which by its concave shape and the on the external surface of the memsinuosities all leading into each other, is well adapted to brane of the tympanum, D, and to allow the reception and collection of sound. The substance of it free motion. This membrane is exwhich it is formed is admirably adapted to its office, being tended across the

inner end of the meatus auditorius, where a Fery sensitive, and composed of a firm though elastic carti- circular sort of osseous groove is adapted to its reception. lage, and provided with small muscles, which stretch or It consists externally of a continuation of the cuticle which relax is as occasion may require. It is covered by a thin lines the meatus; there is also an internal layer continuous dry skin attached to the cartilage by a strong tissue. Many with that of the cavity beyond it; and a middle layer comnerves and blood vessels run through it, whereby its sensi-posed of radiating muscular fibres proceeding from the cirbility is great, and it becomes red from a very small exciting cumference to the centre, where they are inserted into the cause. The lap of the ear, or that part usually pierced for extremity of a minute bony process, H, fig. 4. In fig. 1 the tar-rings, is peculiar to the human ear. Its texture is very inner surface of the tympanum is shown, the cavity T being different to the other part of the external ear. Instead of laid open. · The opposite side of the cavity of the tympanum being composed of firm cartilage it is soft and full of fat, a contains a rounded eminence, P, called the promontory; on provision made probably to prevent the escape of the sound each side of which is an opening in the bone, closed, howcollected in the windings of the concha when it has ever, by the membrane lining the internal surface of the reached the entrance of the passage. Analogy would lead cavity. At the upper edge of the promontory is an opening, 15 to suppose, from the free motions which other animalso called the fenestra ovalis, or oval window; and the other give to their ears, that man is also capable of moving situated near the under edge r, fenestra rotunda, or round his ear towards the direction of the sound without window. These are shown in fig. 2. moving his head; and accordingly we find that savages and Any sonorous impulse communicated to the air falling natives of the Oriental nations generally possess this power, upon the membrane of the tympanum, sets it in motion but that the bandages and head-dresses applied to the in a manner somewhat similar to the parchment head of a heads of European infants, prevent, in limine, the action of drum. This is a favourite comparison with most writers, the muscles destined to this motion, and they accordingly but here

all further analogy ends, for although the memdwindle into an insignificant size, with loss of all motive brane of the tympanum is tensely stretched, it is not a

plane surface like a drum-head, but it is drawn into a The aperture (m), or entrance into the ear from without, funnel shape by the adhesion of the long process of a bone is called the meatus auditorius externus, or the external | (called the malleus) to its centre. This is effected by path of hearing. This is a tube about three-fourths of an means of a muscle called the tensor, which pulls the inch in length, curved into the form of an Italic

letters, malleus inward, and so tightens the membrane as to preprobably to prevent the sound from being reflected into the vent too energetic a sonorous impression ; and there are two air again before the sensation of sound be produced, which other muscles which act in an opposite direction and relax

[ocr errors]









the membrane. So that, like the ciliary process in the | any contact 'with those sides. Fig. 6 shows the internal eye, whereby the pupil is made to contract according as the arrangements of the osseous labyrinth on a large scale. The flow of light is copious, and to expand when the abundant membranous labyrinth is seen floating in the perilymph p. stimulus of light is wanting, the membrane of the tympa- The form of the labyrinth is more distinctly seen in fig. 7, num is, by a spontaneous adjustment, adapted to the re- where it is represented in a position exactly corresponding ception of vibrations of various intensities. This membrane to the former figure, but wholly detached from the bony has a shining tendinous appearance, and is transparent, since labyrinth, and connected only with the nervous filaments the end or process of the malleus can be seen through it in which are proceeding to be distributed to its different parts. a strong light.

A simple inspection of these figures will show at once

the form and the connections of the three semicircular Fig. 3.

Fig. 4.

canals, x y z, each of which presents at its origin from the

vestibule, a considerable dilatation, termed an ampulla, M

A A A, while, at its other extremity, where it terminates in the vestibule, there is no enlargement of its diameter; and it will also be noticed that two of these canals, x and y,

unite into one before their termination. The same descripРУН

tion applies both to the osseous and, to the membranous canals contained within them; the space, P, which intervenes between the two, being filled with the perilymph. But the form of the membranous vestibule is not so exact an imitation of that of the osseous cavity; is composed of two

distinct sacs opening into each other; one is called the The vibrations of this membrane are transmitted to the utricle, u, and the other the sacculus, s. Each sac contains a fenestra ovalis, or oval hole, before alluded to, along a chain of small mass of white calcareous matter, 0, 0, resembling little moveable bones, (B, fig. 1,) articulated with the nicest powdered chalk, which seems to be suspended in the fluid care. The malleus, M, so called from its rude resemblance to a contained in the sacs by the intermedium of a number of hammer, being connected at n with the membrane of the nervous filaments, proceeding from the auditory nerves G and tympanum, receives its oscillations, and transmits them to N, as seen in fig. 7. “From the universal presence of these the incus or anvil, 1, in the upper part of which is a depres- cretaceous substances in the labyrinth of ali the mammalia," sion which receives the bulbous head of the malleus. The (says Dr. Roget, to whom we are indebted for this part of our longer leg, or process of the incus, is directed down into the anatomical description,) and from their much greater size cavity of the tympanum, and has attached to its point the and hardness in aquatic animals, there can be little doubt that os orbiculare, o*, or rounded bone, which in size resembles a they perform some office of great importance in the physiograin of sand, and is, indeed, the smallest bone in the body: logy of hearing.” it is the medium of articulation between the incus and the The cochlea is a convoluted canal, and derives its name from stapes, s, which is the last of the chain of bones now to be its resemblance to the shell of a snail. Its structure is described. The stapes accurately resembles a stirrup-iron, exceedingly curious, being formed of the spiral convolutions but delicately grooved within so as to give lightness to the of a tube separated into two compartments, by a partition, l bone: the base, which answers to that part of the stirrup called the lamina spiralis, which extends its whole length iron where the foot rests, is attached to the membrane over except at the very apex of the cone, where it suddenly terthe fenestra ovalis, in connexion with the vestibule of the minates in a curved point or hook, , leaving an aperture by labyrinth, at which we have now arrived.

which the two portions of the tube communicate together. 3. The labyrinth, (so called from those cavities and tubes In fig. 6 a bristle, BB, is passed through this aperture. The leading into each other in so intricate a manner as to be central pillar round which these tubes take two and a half traced with much difficulty,) or internal ear, is, as we have circular turns, is called the modiolus. Its apex is seen at said, the proper seat of hearing; it contains the vestibule

M. One of these passages is called the vestibular tube, in or middle cavity, v, the semicircular canals, x y z, and the consequence of its arising from the cavity of the vestibule ; cochlea, K. (Fig. 5.) It is seen s, V, K, in fig. 1, p. 83. and the other the tympanic tube, because it begins from the Fig. 5.

inner side of the membrane which closes the fenestra rotunda,

and forms the only separation between the interior of that Y

the tube and the cavity of the tympanum. The trunk of the
auditory nerve occupies a hollow space immediately behind
the ventricle; and its branehes pass through minute holes in
the bony plate which fornis the wall of that cavity ; being
finally expanded on the different parts of the membranous


In fig. 7 the anterior trunk of the auditory nerve is seen

at a, distributing branches of the ampullæ, as, the utricle, v, and the calcareous body it contains: while the posterior trunk, n, divides into a branch, which supplies the sacculus, s, and its calcareous body, o, and a second branch, k, which is distributed over the cochlea. D is the nerve called the portio dura, which merely accompanies the auditory nerve,

but has no relation to the sense of hearing. In fig. 1 the K

auditory nerve, n, is seen entering at the back of the vestibule.

Such is the complicated mechanism of this wonderful structure that mere descr ion fails, even when illus

trated by copious drawings. He who would study the The vestibule is a chamber leading into the semicircular anatomy of the ear in all its intricate and mysterious canals: these latter are constructed of hard brittle bone,

minuteness, must receive his instructions from the hand of forming tubes of small bore, their diameter being such as

nature. not to admit the head of a common pin. They are three in We have seen that the chambers of the external ear number, and from their positions, vertical, oblique, and

and of the tympanum, are filled with air, and the recesses horizontal, they severally receive their distinctive names.

of the labyrinth with water, or at least a thin gelatinous The cavity of the osseous labyrinth contains membranes

fluid resembling water ; .called the perilymph. When of nearly the shape of the vestibule and semicircular canals;

we find any variation in the adaptation of nature, so but these membranes do not extend into the cochlea. They

sure it is that this variation is connected with some compose what has been called the membranous labyrinth,

admirable design whereby the operation of a particular (fig. 7,) and form one continuous but closed sac, containing organ is rendered more efficacious, or more easy in its huid, perfectly similar in appearance to the perilymph action; accordingly we find that the essential seat of which surrounds it on the outer side, and intervenes hearing contains a fluid which transmits sonorous pulses between it and the sides of the osseous labyrinth, preventing much better than air, and it is exceedingly probable, that * Some anatomists do not allow this to be a separate bone, but only a

did these cavities contain air instead of water, the mind process of the incus.

would receive its impressions of sound, compared with its




[ocr errors]


[ocr errors]

present activity, with painful slowness ; 'but the fact must 5. The base of the stirrup, s, is drawn outwards by the awaken in our minds emotions of admiration and of grati- impulses of the air, and performs the action of a piston. tude, that we are furnished with means, the very best that 6. The inuscles of the tympanum antagonize the impulses can be contrived, for effecting the purposes for which we and restore the membranes and bones to their quiescent are placed in others, whether as they minister to our natural wants, or atmosphere through the Eustachian tube, E, so as to enable assist the operations of the higher powers of the mind. the stapes to be raised. Fig. 6.

8. The water of the labyrinth (fig. 6,) receives the full momentum.of the impulses of the air on the membrane of the tympanum without loss from condensation, because the difference of the areas of the membrane of the tympanum and fenestræ ovales, combined with the difference of range of motion of the point of the malleus m and base of the stapes s, is equal to the difference of specific gravity between air and water,

9. The perilymph oscillates between the two fenestræ, and its alternate fluxes and refluxes over the membranous labyrinth (fig. 7,) excite the sensation of hearing:

10. The cochlea, K, regulates the extent of the oscillations of the perilymph by the expansion of its spiral laminæ.

11. The petrous bone deafens the internal. ear, so as to prevent any vibrations from acting on the perilymph, except those which have been previously adjusted for creating accurate oscillations by being transmitted by the chain of little bones.

SECTION 2. We have said that the seat of hearing is to be found in the expanded portions of a nerve found within the cavities of the inner ear, in the same way that the expanded portion of the optic nerve, the retina, is the seat of vision-all the other apparatus, then, of the ear bears the same relation to sound as the membranes, lenses, and humours of the eye bear to light, that is, as so much auxiliary apparatus

placed for the purpose of perfecting the final perception Fig. 7.

communicated to the mind through the agency of the nerve. But we can understand how it is that light passing through

transparent media of varying shape and density is refracted X

and brought to a focus upon the very nerve wherein we suppose resides the seat of vision, but respecting the auxili

ary apparatus of the ear our knowledge is so small as to А.

lead us to no certain decision. Nay, we are met with startling facts which would tend to disturb our notions of the simplicity and perfect adaptations of nature, were it not that science teaches us the useful lesson to let our decisions

keep pace only with our knowledge: that when the latter A

is limited in amount, uncertain in its kind, or wanting
altogether, we must never be guided by the uncertain
gleams of conjecture, but wait for the dawn of the powerful

and steady light of well-supported theory.

Were the juices and lenses of the eye to be extruded from

their coverlids, irremediable blindness would ensue, although, G -N

as far we know, the optic nerve be uninjured; but when the tympanum and chain of bones are removed by design, by accident, or by disease, the mind does not cease to discrimi

nate sounds in the same useful manner, it is said, as before. We must now be considered as having traced a single The cases are not quite parallel, since it is admitted that vibration through the meatus auditorius to the membrane of if the liquid in the labyrinth flow out, deafness ensues, as the tympanum, which, being set in motion, communicates its irremediable as blindness in the other case. The question impulse to the little chain of bones. They convey it to the then has been asked, without ever eliciting a satisfactory vestibule, whence it passes into the canals and cochlea, reply,“ What purpose does this apparatus serve in the ecowhose numerous cavities it probably permeates : it is then nomy of the ear, since it may be removed, and hearing not taken up by the auditory nerve, conveyed to a particular be destroyed ?”' Now it cannot be contended that this part of the brain, and constitutes one of those mental im- deprivation does not diminish the power of hearing, any pressions which we call a perception. Here, then, is the more than that the loss of one leg, or of many teeth, does limit to our knowledge; for how the mind receives its not limit the power of locomotion or of eating. But the impression through the medium of the auditory nerve is a amount of diminution is not known, and cannot be known question which has never been answered, and probably until we arrive at a more perfect knowledge of the funcnever will be answered.

tions of these parts. Sir Astley Cooper communicated to the The following summary of Dr. Sym's views of the me- Royal Society, more than thirty years ago, some interesting chanical functions of the ear, may be found a useful facts in relation to this subject. A gentleman had been appendix to the descriptions given above.

attacked with inflammation and suppuration in both ears, 1. The external ear, c, protects the membrane of the tym- became totally deaf, and continued so for three months : panum, D, and contributes to a knowledge of the direction the hearing then began to return, and was restored to the of sounds.

state at which Sir Astley made his observations. The patient 2. The membrane of the tympanum is a passive medium having filled his mouth with air, he closed the nostrils and of communication of vibrations, and the impulses of the air contracted the cheeks: the air thus compressed was heard draw its apex outwards.

to rush through the meatus auditorius, with a whistling 3. The ossicula, or little bones (figs. 3 and 4,) of the noise, and the hair hanging from the temples became agitympanum, form a system of levers by which the extent tated' by the current of air which issued from the ear. When of the undulations falling on the membrane of the tym- a candle was applied the flame was agitated in a similar patum is diminished, while their momentum is preserved. manner.". Sir Astley Cooper then passed a probe into each

4. Vibrations communicated from the larynx to the ear, and he thought the membrane on the left side was hammer and anvil, m and 1, have their extent increased entirely destroyed, since the probe struck against the petrouş whilst their momentum remains the same.

portion of the temporal bone. The space usually occupied



[ocr errors]

by the membrana tympani was found to be an aperture, more grave, and dissonant from that received by the left ear. without one trace of membrane remaining. On the right Having recovered from the catarrh, the distinct hearing of side also a probe could be passed into the cavity of the tym- his ear was restored." panum, but here by producing it along the sides of the

SECTION 3. meatus, some remains of the circumference of the membrane could be discovered, with a circular opening in the centre,

There are several considerations respecting the use and about the fourth of an inch in diameter. Now, instead of action of the membrane of the tympanum, which, as they the total annihilation of the powers of the organ, this gentle involve a seeming anomaly, demand some attention. man was capable of hearing whatever was said in company, This membrane has been considered analogous to the although the membrane of both ears was destroyed. He could even hear better in the ear in which no traces of the parchment head of a drum. Now, we know, both from membrane remained. This gentleman was only in a smali theory and from experience, that so long as the tension

of the parchment remains the same, the tone elicited by degree deaf from the loss of the membrane, but his ear

its vibration remains constant, whether the impulse which remained nicely susceptible of musical tones, for he played

causes it to vibrate be energetic or weak. If therefore the well on the flute, and had frequently borne a part in a con

analogy between the two membranes be worth entertaining, cert, and sung with much taste, and perfectly in tune.”

the question naturally arises-how does the membrane of The tympanum being destroyed, and the chain of bones

the tympanum accommodate itself to the ever-varying broken, it is absolutely necessary that the stapes retain its place

, for if this bone he destroyed, the membrane of the velocity of the impulses to which it is submitted during fenestra ovalis will be destroyed, and the fluids of the laby its vibrations in equal times, so long as its tension remains

the process of hearing? The head of a drum perforins ali rinth being allowed to flow out, incurable deafness is the

unaltered. If the sensation, therefore, of hearing be concertain result. This is a species of deafness which is said to

nected with the vibrations of the membrane of the tymbe very common. It is distinguished from that disease which affects the middle and external ear by a very simple membrane vibrate isochronously, the mind would be totally

panum, it is almost a necessary conclusion that if the test: if the ticking of a watch, held between the teeth, or placed on the mastoid process behind the ear, be not heard ignorant of difference of pitch in different sounds: all sounds

would be the same except in intensity, and the various at all, or heard very obscurely, it is a proof that the inter

beautiful modulations of tone which we associate with the nal ear is diseased: when this is not the case the sound

term melody would be unknown. can be heard distinctly, because the bones, &c., placed

It becomes therefore a legitimate source of inquiry, whether between the sounding body and the nerve are good conduc

the muscular apparatus of the membrana tympani, or of tors of sound. Musical instruments such as the drum, violin, guitar, &c. the tension of the membrane, so as to adjust it to the vary

the malleus, &c., is not endowed with the power of varying are furnished with apertures which serve to connect the resonant volume of air within the instrument with the ing, velocities of vibration; and if so, in what degree does

volition bear a part in the adjustment. We may here external air, so that the vibrations of the former are partici- perhaps add another to the list of analogies observable in the pated in and diffused by the latter. But for these aper- phenomena presented by light and sound, and make it availtures the sounds on the drum would always be mufjled, | able to the present inquiry. There are two opinions respecand those of stringed instruments faint and ineffective. In ting the adjusting powers of the eye to great distances : the drum of the ear this aperture is provided for by means

one, that it is adjusted once for all to objects at varying disof the Eustachian tube : by opening the mouth the orifice

tances from the observer, and consequently that it has a of this tube is enlarged, and consequently sounds are appreciated with greater ease. One of the characteristics

clear view of these objects at different distances without of what the poets call “breathless attention," is the open

a new adjustment for each: the other opinion is that the mouth, a circumstance which did not escape the sagacity of object, to another objeet less remote, without undergoing a

eye cannot be removed from the contemplation of a remote Shakspeare:

corresponding adjusting change. But it is admitted that I saw a smith stand with his hammer, thus,

for near objects the eye is adjusted for the reception of rays The whilst his iron did on the anvil cool,

of light proceeding from a certain distance, and that before With open mouth swallowing a tailor's news,

another object at a different distance can be clearly seen, King John.

that the refracting apparatus of the eye undergoes a slight

and rapid modification. In what this modification consists, Any cause, therefore, tending to stop up this channel must

whether in a change of position of the crystalline lens; in prevent the membrana tympani from vibrating, since the cavity will contain a volume of air incapable, from confine

an elongation or contraction of the axial diameter of the eye;

or a change in diameter of the iris, or in any other change ment, of exerting its elasticity; and probably its expansion, by animal heat, is such as to press upon the air-tight membrane

of condition, is not yet decided; we, however, are assured of the tympanum, and so also prevent any tendency in the

that an adjustment does take place, but whether the mind latter to vibrate. Accordingly we find that in sore throats,

is cognizant of that adjustment is not so clear; it would whereby this channel is sometimes closed upon itself, in con

seem that rays from an object at a given distance produce sequence of inflammation, or by cold, where it is stopped medium of the will or wholly independent of it, assumes

an in

a certain physical effect on the eye, which, either through the by an accumulation of mucus, deafness ensues, more or less complete according as the disease is violent or slight. A

ternal arrangement fitted for focalizing the rays at the retina. remedy has sometimes been found by dilating the canal by the action of the ear, the difference between the parchment

If now it be admissible to extend the same principle to means of a syringe passed through the nostrils, or to the back of the throat.

head of a drum and the membrana tympani of the ear In cases where it is permanently would be analogous to the difference between an assemblage closed, Sir Astley Cooper introduced the practice of puncturing the membrana tympani, so as to give elasticity to

of glass lenses and the refracting apparatus of the eye: in the confined air. Many obstinate cases of deafness have each case the natural instrument acts on the same principle instantly yielded to this treatment, although it is said that changes in its interior mechanism, minute causes producing

as the artificial, but is susceptible of minute spontaneous such punctures heal up very soon.

Sir Charles Bell quotes a curious case from Sauvage of mighty effects in the use to us of the instrument, which is disordered action of the ear, resulting from cold.-"A cer

observed in no work of art, and which brings us to the tain eminent musician, when he blew the German flute,

irresistible conclusion that perceived at the same time the proper sound of it, and

The Hand that made it is divine. another sound of the same rhythm or measure, but of a If we are correct in this view we must assume that when different tone. His hearing seemed thus to be doubled. It an aërial impulse is communicated to the membrane of was not an echo, for he heard both sounds at one and the the tympanum the muscular apparatus by which the same moment: neither were the sounds accordant and har- latter is stretched becomes (either voluntarily or involunmonious, for that would have been sweet and pleasant to his tarily) impelled to the production of such a tension in the ear. Having for several days persisted in his attempts, membrane as will fit it for vibrating isochronously with the and always been shocked with this grating sound, he at last aërial molecules from which the impulse was derived. It threw his flute aside. The day before he first became may be objected to this opinion that it involves the necessensible of this strange affection he had imprudently walked sary consequence that during the simultaneous production in a very cold and damp evening, and was seized with a of two or more musical sounds, the same tension of the catarrh in the right side, whence probably it arose that the membrane must be available for all; but it by no means natural tone of that ear was altered: the sound appeared follows that when two notes are sounded at the same, or

« ÎnapoiContinuați »