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the membrane. So that, like the ciliary process in the eye, whereby the pupil is made to contract according as the flow of light is copious, and to expand when the abundant stimulus of light is wanting, the membrane of the tympanum is, by a spontaneous adjustment, adapted to the reception of vibrations of various intensities. This membrane has a shining tendinous appearance, and is transparent, since the end or process of the malleus can be seen through it in a strong light.
SMALL BONES OF THE TYMPANUM.
The vibrations of this membrane are transmitted to the fenestra ovalis, or oval hole, before alluded to, along a chain of little moveable bones, (B, fig. 1,) articulated with the nicest care. The malleus, м, so called from its rude resemblance to a hammer, being connected at H with the membrane of the tympanum, receives its oscillations, and transmits them to the incus or anvil, 1, in the upper part of which is a depression which receives the bulbous head of the malleus. The longer leg, or process of the incus, is directed down into the cavity of the tympanum, and has attached to its point the os orbiculare, o*, or rounded bone, which in size resembles a grain of sand, and is, indeed, the smallest bone in the body: it is the medium of articulation between the incus and the stapes, s, which is the last of the chain of bones now to be described. The stapes accurately resembles a stirrup-iron, but delicately grooved within so as to give lightness to the bone: the base, which answers to that part of the stirrupiron where the foot rests, is attached to the membrane over the fenestra ovalis, in connexion with the vestibule of the labyrinth, at which we have now arrived.
3. The labyrinth, (so called from those cavities and tubes leading into each other in so intricate a manner as to be traced with much difficulty,) or internal ear, is, as we have said, the proper seat of hearing; it contains the vestibule or middle cavity, v, the semicircular canals, X Y Z, and the cochlea, K. (Fig. 5.) It is seen s, V, K, in fig. 1, p. 83.
POSITION OF THE SMALL BONES IN THE TYMPANUM.
any contact with those sides. Fig. 6 shows the internal arrangements of the osseous labyrinth on a large scale. The membranous labyrinth is seen floating in the perilymph P. The form of the labyrinth is more distinctly seen in fig. 7, where it is represented in a position exactly corresponding to the former figure, but wholly detached from the bony labyrinth, and connected only with the nervous filaments which are proceeding to be distributed to its different parts.
A simple inspection of these figures will show at once the form and the connections of the three semicircular canals, x y z, each of which presents at its origin from the vestibule, a considerable dilatation, termed an ampulla, AAA, 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 description 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; it is composed of two distinct sacs opening into each other; one is called the utricle, u, and the other the sacculus, s. Each sac contains a small mass of white calcareous matter, o, o, resembling powdered chalk, which seems to be suspended in the fluid contained in the sacs by the intermedium of a number of nervous filaments, proceeding from the auditory nerves G and N, as seen in fig. 7. "From the universal presence of these cretaceous substances in the labyrinth of all the mammalia," (says Dr. Roget, to whom we are indebted for this part of our anatomical description,) " and from their much greater size and hardness in aquatic animals, there can be little doubt that they perform some office of great importance in the physiology of hearing."
The cochlea is a convoluted canal, and derives its name from its resemblance to the shell of a snail. Its structure is exceedingly curious, being formed of the spiral convolutions of a tube separated into two compartments, by a partition, L, called the lamina spiralis, which extends its whole length except at the very apex of the cone, where it suddenly terminates in a curved point or hook, H, leaving an aperture by which the two portions of the tube communicate together. In fig. 6 a bristle, BB, is passed through this aperture. The central pillar round which these tubes take two and a half circular turns, is called the modiolus. Its apex is seen at
One of these passages is called the vestibular tube, in consequence of its arising from the cavity of the vestibule; and the other the tympanic tube, because it begins from the inner side of the membrane which closes the fenestra rotunda, and forms the only separation between the interior of that the tube and the cavity of the tympanum. The trunk of the auditory nerve occupies a hollow space immediately behind the ventricle; and its branches pass through minute holes in the bony plate which forms the wall of that cavity; being finally expanded on the different parts of the membranous labyrinth.
MAGNIFIED VIEW OF THE LABYRINTH DETACHED FROM THE
The vestibule is a chamber leading into the semicircular
The cavity of the osseous labyrinth contains membranes of nearly the shape of the vestibule and semicircular canals; but these membranes do not extend into the cochlea. They compose what has been called the membranous labyrinth, (fig. 7,) and form one continuous but closed sac, containing Huid, perfectly similar in appearance to the perilymph which surrounds it on the outer side, and intervenes between it and the sides of the osseous labyrinth, preventing Some anatomists do not allow this to be a separate bone, but only a process of the incus,
In fig. 7 the anterior trunk of the auditory nerve is seen at G, distributing branches of the ampullæ, AA, the utricle, u, 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 auditory nerve, N, is seen entering at the back of the vestibule.
Such is the complicated mechanism of this wonderful structure that mere description fails, even when illustrated by copious drawings. He who would study the anatomy of the ear in all its intricate and mysterious minuteness, must receive his instructions from the hand of
We have seen that the chambers of the external ear and of the tympanum, are filled with air, and the recesses of the labyrinth with water, or at least a thin gelatinous fluid resembling water; called the perilymph. When we find any variation in the adaptation of nature, so sure it is that this variation is connected with some
admirable design whereby the operation of a particular organ is rendered more efficacious, or more easy in its action; accordingly we find that the essential seat of hearing contains a fluid which transmits sonorous pulses much better than air, and it is exceedingly probable, that did these cavities contain air instead of water, the mind would receive its impressions of sound, compared with its
MEMBRANOUS LABYRINTH AND ITS NERVES.
We must now be considered as having traced a single vibration through the meatus auditorius to the membrane of the tympanum, which, being set in motion, communicates its impulse to the little chain of bones. They convey it to the vestibule, whence it passes into the canals and cochlea, whose numerous cavities it probably permeates: it is then taken up by the auditory nerve, conveyed to a particular part of the brain, and constitutes one of those mental impressions which we call a perception. Here, then, is the limit to our knowledge; for how the mind receives its impression through the medium of the auditory nerve is a question which has never been answered, and probably never will be answered.
The following summary of Dr. Sym's views of the mechanical functions of the ear, may be found a useful appendix to the descriptions given above.
1. The external ear, c, protects the membrane of the tympanum, D, and contributes to a knowledge of the direction
2. The membrane of the tympanum is a passive medium of communication of vibrations, and the impulses of the air draw its apex outwards.
3. The ossicula, or little bones (figs. 3 and 4,) of the tympanum, form a system of levers by which the extent of the undulations falling on the membrane of the tympanum is diminished, while their momentum is preserved. 4. Vibrations communicated from the larynx to the hammer and anvil, м and I, have their extent increased whilst their momentum remains the same.
5. The base of the stirrup, s, is drawn outwards by the impulses of the air, and performs the action of a piston. 6. The muscles of the tympanum antagonize the impulses and restore the membranes and bones to their quiescent positions.
7. The membrana rotunda, R, receives the pressure of the atmosphere through the Eustachian tube, E, so as to enable the stapes to be raised.
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 м 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.
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 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 and brought to a focus upon the very nerve wherein we suppose resides the seat of vision, but respecting the auxiliary apparatus of the ear our knowledge is so small as to 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 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, 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 discriminate sounds in the same useful manner, it is said, as before. The cases are not quite parallel, since it is admitted that if the liquid in the labyrinth flow out, deafness ensues, as irremediable as blindness in the other case. The question then has been asked, without ever eliciting a satisfactory reply, "What purpose does this apparatus serve in the economy of the ear, since it may be removed, and hearing not be destroyed?" Now it cannot be contended that this deprivation does not diminish the power of hearing, any more than that the loss of one leg, or of many teeth, does not limit the power of locomotion or of eating. But the amount of diminution is not known, and cannot be known until we arrive at a more perfect knowledge of the functions of these parts. Sir Astley Cooper communicated to the Royal Society, more than thirty years ago, some interesting facts in relation to this subject. A gentleman had been attacked with inflammation and suppuration in both ears, became totally deaf, and continued so for three months: the hearing then began to return, and was restored to the state at which Sir Astley made his observations. The patient "having filled his mouth with air, he closed the nostrils and contracted the cheeks: the air thus compressed was heard to rush through the meatus auditorius, with a whistling noise, and the hair hanging from the temples became agitated by the current of air which issued from the ear. When a candle was applied the flame was agitated in a similar manner." Sir Astley Cooper then passed a probe into each ear, and he thought the membrane on the left side was entirely destroyed, since the probe struck against the petrous portion of the temporal bone. The space usually occupied
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
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
nearly at the same time, that the mind is conscious of them at the same precise moment; we think that the two notes are heard in succession, however rapid and however minute the point of time between them, still in succession. When we hear an orchestra play a piece of music, we are conscious of a general effect resulting from a general impulse given to the ear, but we are not conscious of one particular performer unless we direct our particular attention to him, and in so doing we abstract our attention from the rest; and although we get a vague impression of what the rest are doing, it is only by alternately attending to the one favourite performer and to his associates. Again, when a chord is struck upon any instrument or instruments we are not conscious at the moment of the individual notes which compose that chord; the general effect is harmonious and pleasing, resulting from an harmonious impulse given to the air, and it is by an after process of mental analysis that we decompose the chord into its component parts. So rapidly does the mind receive general impressions, and resolve them into particular ones, that we are apt to blend the first impression and the after decomposing process into one; as when we hear an echo, if too near the reflecting surface, the real and the reflected sound become incorporated into one; we hear the echo, but are not conscious of it.
When viewing a sheet of white paper, my mind informs me that the eye receives at least three impressions, one of red, one of yellow, and a third of blue, that each is the result of a rapidity of vibration different from that of the other two, which in their turn differ from each other; this is evidently a mental process, an exercise of reason over perception, for my eye tells me it is white. Again, when the eye is pleased with a skilful combination of colours, the pleasure is afforded by the tendency those colours have to combine in order to form white light; other colours which interfere with this tendency stand out and offend the eye in the same way that discordant notes stand out and assume that isolated character which offends the ear.
But however we regard the membrana tympani, if it be considered as a stretched elastic membrane considerable perplexity will attend our efforts to explain its facility of yielding to vibrations of varying velocities: it is only within a few years that experiment has thrown anything like a clear light on the subject. This has been done by the researches of M. Savart, the most successful cultivator of the science of sound of the present age. The labours also of M. Flourens and of M. Magendie have not a little contributed to our knowledge of the functions of the various parts of the ear.
M. Savart submitted the ears of animals to experiment. Having removed the temporal bone, he made with a saw a section parallel to the external surface of the membrane, so as to lay it open, and to be enabled to cover it with sand; when a vibrating plate was brought parallel with the membrane, and very near its surface, a slight motion was observed in the sand; but, from the limited extent of surface, and particularly from its form, no nodal lines could be observed. This experiment was with the human tympanum; but when that of a calf's ear was substituted, the motions of the sand were very distinct.
He observed that when the malleus acted and its tensor muscle tightened the membrane, it was more difficult to produce motions in the grains of sand; hence the use of this muscle appeared evident. The malleus also has an important influence on the motions of the membrana tympaní. If a small wooden rod reduced at its edges, be fixed on a stretched membrane, extending from the centre to the circumference or even beyond, and the surface of the membrane be covered with sand, the figures produced upon its surface will be modified by the presence of the little rod, which will be made to vibrate so strongly, that very distinct nodal lines will be produced in it.
It appears then, that the membrane of the tympanum may be considered as destitute of all elasticity, and so fitted to receive impressions of varying velocity": "that the office
There are a few peculiarities connected with the vibration of a stretched membrane which may be noticed here, in order to render the experiments
detailed in the text intelligible. One of the best modes of obtaining a stretched membrane is by wetting very thin paper, stretching it over a goblet glass, such as a soda water glass, folding it down, and securing the edges with a solution of gum. This when perfectly dry will present a dense surface, inclosing a volume of air, and susceptible of small vibrating impulses, which may be detected by sifting upon the surface a thin layer of dry lycopodium. If now we hold over and parallel to this membrane a vibrating plate of glass the figure which the powder would have assumed had it been upon the glass, will be assumed by the powder on the membrane. In this way a large variety of figures may be obtained by employing various plates, vibrating in different ways. By breathing upon
of the malleus is, first, to modify by means of its muscles the tension of the membrane, in order to protect the organ from too intense impressions, and to dispose it to the reception or such as are very weak. Secondly, the malleus reciprocates the motions of the membrane, and communicates them along the chain of bones to the membranes of the fenestra ovalis. Now it is probable that were it not for the membrane of the tympanum, the membranes which close the entrances of the labyrinth being constantly exposed to the external atmospheric air, their elastic state would be continually influenced by changes in temperature and other causes. It is therefore imagined, that the use of the membrana tympani is to prevent the contact of the external air, and that the cavity of the tympanum is to preserve a volume of air at the constant temperature of the body; for coming as it does along the Eustachian tube it is warmed in its progress, and an atmosphere is established before the labyrinth, whose properties are invariable; a portion of the great temporal artery is separated from the cavity of the tympanum only by a very thin bony partition, and this it is supposed is of some importance in the process of audition.
The following experiment, due also to Savart, shews, that the outer ear and its meatus auditorius serve to render the aerial vibrations more intense, to reciprocate the vibrations of the air, and to transmit them to the membrane or the tympanum with the same or nearly the same degree of force, whatever be their direction. A conical tube made of thin card-board, and of large diameter at the base, received at its smaller truncated end a membrane which was firmly fixed: on bringing a vibrating plate parallel to the upper external surface of this membrane, which was covered with sand, the grains of sand were only slightly agitated: but on holding the plate near the large orifice of the tube they vibrated strongly. Another conical tube was opposed by its summit to the preceding one, but without touching the membrane; the vibrating plate was afterwards held at the orifice of each of the tubes, when it was observed that the motions communicated to the membrane were far more energetic when the aërial undulations arrived through the tube which was in immediate contact with the membrane, than when they arrived through the other, with which it was not in contact,
Flourens performed some most extraordinary experiments on the ears of birds, by which it appears that the nerves in the canals of the labyrinth have other uses in addition to their more direct object of conveying the sensation of sound to the brain: they serve in some mysterious manner to give the animals the power of balancing themselves upon their feet, and directing their motions. This provision seems to be especially necessary to animals which roost and sleep upon their perches: any noise falling upon their ear and tending to awaken them suddenly, would probably also cause them to drop from off their perches; it is said then that any motion of their tympanum serves to contract their claws, and so when suddenly roused they have a firm hold on the perch upon which they
SECTION 4. The powers of the human ear to discriminate sounds vary with different individuals. The functions of the ear vary, of health, as also with age. It would also seem to vary at in common with other functions of the body, with the states different times of the day, since it has been observed by Savart, that small steel rods producing upwards of 24,000 vibrations per second, yielded a sound which he could sometimes hear and sometimes could not hear; he supposes either that his ear was more sensible at one time than at another; or that his mode of inducing vibration was not always precisely the same,
Dr. Wollaston made the curious discovery that many persons having a distinct and perfect perception of all common sounds, are at the same time completely insensible to such as are at one or other extremity of the scale of musical notes, the hearing or not hearing of which seems to depend wholly on the pitch, or frequency of vibration constituting the note, and not upon the intensity or loudness of the noise.
Although persons labouring under common deafness have an imperfect perception of all sounds, the degree of indisthe paper the moisture varies its tension, and the consequent figure is also varied, which gradually reassumes its first form as the moisture evaporates, provided the plate near it be kept vibrating. The tones of a musical instrument held near the membrane will arrange the powder into figures, which change as often as the tones which produce them are varied.
tinctness of different sounds is commonly not the same; for it will be found that they usually hear sharp sounds much better than low ones; they distinguish the voices of women and children better than the deeper tones of men; and the generality of persons accustomed to speak to those who are deaf employ a shriller tone of voice, by which they are heard better than by merely speaking louder.
In persons not so afflicted, when the mouth and nose are shut, the tympanum may be so exhausted by a forcible attempt to take breath by expansion of the chest, that the pressure of the external air is strongly felt upon the membrana tympani, and that in this state of tension, from external pressure, the ear becomes insensible to grave tones, without losing in any degree the perception of sharper
The state to which the ear is thus reduced by exhaustion may even be preserved for a certain time without the continued effort of inspiration, and without even stopping the breath, since, by sudden cessation of the effort, the internal passage to the ear becomes closed by the flexibility of the Eustachian tube, which acts as a valve, and prevents the return of air into the tympanum. This state may be removed by the simple act of swallowing, which opens the tube and restores equilibrium by letting in the air.
Dr. Wollaston found that he could thus render his own ears insensible to all sounds below F marked by the bass cleff. He compares the effect to the mechanical separation of larger and smaller bodies by a sieve. "If," says he, "I strike the table before me with the end of my finger, the whole board sounds with a deep dull note. If I strike it with my nail, there is also at the same time a sharp sound produced by quicker vibrations of parts around the point of contact. When the car is exhausted, it hears only the latter sound, without perceiving in any degree the deeper note of the whole table. In the same manner in listening to the sound of a carriage, the deeper rumbling noise of the body is no longer heard by an exhausted ear, but the rattle of a chain or loose screw remains at least as audible as before exhaustion."
The effect at a concert is amusing when the great mass of louder sounds is suppressed, and the shriller ones more distinctly heard even to the rattling of the keys of a bad instrument, or scraping of catgut unskilfully touched.
In the natural healthy state of the human ear there does not seem to be any strict limit to our power of discerning low sounds. In listening to those pulsatory vibrations of the air of which sound consists, if they become less and less frequent, we may doubt at what point tones suited to produce any musical effect terminate. Yet all persons but those whose organs are palpably defective continue sensible of vibratory motion, until it becomes a mere tremor, which may be felt, and even almost counted.
On the contrary, if we turn our attention to the opposite extremity of the scale of audible sounds, and with a series of pipes exceeding each other in sharpness, if we examine the effects of them successively upon the ears of any considerable number of persons, we shall find (even within the range of those tones which are produced for their musical effects,) a very distinct and striking difference between the powers of different individuals, whose organs of hearing are in other respects perfect, and shall have reason to infer that human hearing in general is more confined than has been supposed, with regard to its perception of very acute sounds, and has, probably, in every instance, some definite limit, at no great distance beyond the sounds ordinarily heard.
Dr. Wollaston gives the case of a gentleman whose power of hearing sharp sounds terminated at a note four octaves above the middle E of the pianoforte; also two other cases of persons incapable of distinguishing the chirping of the grasshopper (Gryllus campestris,) in the hedges during a summer's evening; also that of a gentleman who could never hear the chirping of a common house-sparrow. "This latter," he says, "is the lowest limit to acute hearing that I have ever met with, and I believe it to be extremely rare. Deafness, even to the chirping of the house-cricket, which is several notes higher, is not common. Inability to hear the piercing squeak of the bat seems not very rare, as I have met with several instances of persons not aware of such a sound."
Dr. Wollaston is inclined to think that at the limit of hearing, the interval of a single note between two sounds may be sufficient to render the higher note inaudible, although the lower note is heard distinctly.
"The suddenness of the transition from perfect hearing to total want of perception occasions a degree of surprise,
which renders an experiment on this subject with a series of small pipes among several persons rather amusing. It is curious to observe the change of feeling manifested by various individuals of the party, in succession, as the sounds approach and pass the limits of their hearing. Those who enjoy a temporary triumph, are often compelled, in their turn, to acknowledge to how short a distance their little superiority extends.
Though it has not yet occurred to me to observe a limit to the hearing of sharp sound in any persons under twenty years of age, I am persuaded, by the account that I have received from others, that the youngest ears are liable to the same kind of insensibility. I have conversed with more than one person who never heard the cricket or the bat, and it appears far more likely that such sounds were always beyond their powers of perception, than that they never had been uttered in their presence.
"The range of human hearing comprised between the lowest notes of the organ and the highest known cry of insects, includes more than nine octaves, the whole of which are distinctly perceptible by most ears, although the vibrations of a note at the higher extreme are six or seven hundredfold more frequent than those which constitute the greatest audible sound.
"Since there is nothing in the constitution of the atmosphere to prevent the existence of vibrations incomparably more frequent than any of which we are conscious, we may imagine that animals like the grylli, whose powers appear to commence nearly where ours' terminate, may have the faculty of hearing still sharper sounds, which at present we do not know to exist, and that there may be other insects hearing nothing in common with us, but endued with a power of exciting, and a sense that perceives vibrations, of the same nature, indeed, as those which constitute our ordinary sounds, but so remote, that the animals who perceive them may be said to possess another sense, agreeing with our own solely in the medium by which it is excited, and possibly wholly unaffected by those slower vibrations of which we are sensible."
It has been remarked that the ear is capable of perceiving four or five hundred variations of tone, and probably as many different degrees of intensity; by combining these, we have above 20,000 simple sounds that differ either in tone or strength, supposing every tone to be perfect. How wonderful then is that mechanism which enables the mind to discriminate so nicely, and to be able to decide.by its aid alone the infinity of sources which produce sound. The tones of musical instruments are as peculiar as the voices of animals. Natural sounds are well known from artificial, and we often need not the assistance of another sense to decide upon one individual out of a thousand varieties of sounds around us. We are even capable of appreciating the minute shade of difference between two notes, one due to a velocity of 400 and the other of 405 vibrations in a second, a rapidity of motion and a minuteness of difference which we might almost be pardoned for thinking beyond the range of mental perception.
In concluding this short account of the ear and of some of its functions, we may remark that there is none other of the perceptive faculties, which, considered with reference to the organic means which connect them with external nature, more amply repays a philosophical inquiry into its modes of operation. The mind turns from the contemplation of the exquisite and delicate mechanism of this organ, with admiration; not with the admiration of knowledge, but rather with that which the aspect of a sublime and difficult problem induces on the mind of the accomplished student of nature: he may be unable to afford a full solution of it, he looks therefore to advancing science for more perfect and more powerful means for effecting the solution, future," he is content to apply the amount of his present and although such means may be in "the dim and misty knowledge to explain, as far it goes, the works of God, so as to prepare himself and his fellow-creatures the better to appreciate the infinite wisdom and goodness of Our Father in Heaven.
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