HEARING.[18]
[Illustration:
FIG. 17.--Semidiagrammatic section through the right ear (Czermak).
_M_, concha; _G_, external auditory meatus; _T_, tympanic membrane;
_P_, tympanic cavity; _o_, oval foramen; _r_, round foramen; _R_,
pharyngeal opening of Eustachian tube; _V_, vestibule; _B_, a
semicircular canal; _S_, the cochlea; _Vt_, scala vestibuli; _Pt_,
scala tympani; _A_, auditory nerve.
]
=The Ear.=--"The auditory organ in man consists of three portions, known
respectively as the _external ear_, the _middle ear_ or _tympanum_, and
the _internal ear_ or _labyrinth_; the latter contains the end-organs of
the auditory nerve. The external ear consists of the expansion seen on
the exterior of the head, called the _concha_, _M_, Fig. 17, and a
passage leading in from it, the _external auditory meatus_, _G_. This
passage is closed at its inner end by the _tympanic_ or _drum membrane_,
_T_. It is lined by skin, through which numerous small glands, secreting
the _wax_ of the ear, open.
[Illustration:
FIG. 18.--_Mcp_, _Mc_, _Ml_, and _Mm_ stand for different parts of
the malleus; _Jc_, _Jb_, _Jl_, _Jpl_, for different parts of the
incus. _S_ is the stapes.
]
"_The Tympanum_ (_P_, Fig. 17) is an irregular cavity in the temporal
bone, closed externally by the drum membrane. From its inner side the
_Eustachian tube_ (_R_) proceeds and opens into the pharynx. The inner
wall of the tympanum is bony except for two small apertures, the _oval_
and _round foramens_, _o_ and _r_, which lead into the labyrinth. During
life the round aperture is closed by the lining mucous membrane, and the
oval by the stirrup-bones. The _tympanic membrane_ _T_, stretched across
the outer side of the tympanum, forms a shallow funnel with its
concavity outwards. It is pressed by the external air on its exterior,
and by air entering the tympanic cavity through the Eustachian tube on
its inner side. If the tympanum were closed these pressures would not be
always equal when barometric pressure varied, and the membrane would be
bulged in or out according as the external or internal pressure on it
were the greater. On the other hand, were the Eustachian tube always
open the sounds of our own voices would be loud and disconcerting, so it
is usually closed; but every time we swallow it is opened, and thus the
air-pressure in the cavity is kept equal to that in the external
auditory meatus. On making a balloon ascent or going rapidly down a deep
mine, the sudden and great change of aërial pressure outside frequently
causes painful tension of the drum-membrane, which may be greatly
alleviated by frequent swallowing.
_The Auditory Ossicles._--Three small bones lie in the tympanum forming
a chain from the drum-membrane to the oval foramen. The external bone is
the _malleus_ or _hammer_; the middle one, the _incus_ or _anvil_; and
the internal one, the _stapes_ or _stirrup_. They are represented in
Fig. 18.[19]
=Accommodation= is provided for in the ear as well as in the eye. One
muscle an inch long, the _tensor tympani_, arises in the petrous portion
of the temporal bone (running in a canal parallel to the Eustachian
tube) and is inserted into the malleus below its head. When it
contracts, it makes the membrane of the tympanum more tense. Another
smaller muscle, the _stapedius_, goes to the head of the stirrup-bone.
These muscles are by many persons felt distinctly contracting when
certain notes are heard, and some can make them contract at will. In
spite of this, uncertainty still reigns as to their exact use in
hearing, though it is highly probable that they give to the membranes
which they influence the degree of tension best suited to take up
whatever rates of vibration may fall upon them at the time. In
listening, the head and ears in lower animals, and the head alone in
man, are turned so as best to receive the sound. This also is a part of
the reaction called 'adaptation' of the organ (see the chapter on
Attention).
=The Internal Ear.=--"The labyrinth consists primarily of chambers and
tubes hollowed out in the temporal bone and inclosed by it on all sides,
except for the oval and round foramens on its exterior, and certain
apertures for blood-vessels and the auditory nerve; during life all
these are closed water-tight in one way or another. Lying in the _bony
labyrinth_ thus constituted are membranous parts, of the same general
form but smaller, so that between the two a space is left; this is
filled with a watery fluid, called the _perilymph_; and the _membranous
internal ear_ is filled by a similar liquid, the _endolymph_.
[Illustration:
FIG. 19.--Casts of the bony labyrinth. _A_, left labyrinth seen
from the outer side; _B_, right labyrinth from the inner side; _C_,
left labyrinth from above; _Co_, cochlea; _V_, vestibule; _Fc_,
round foramen; _Fv_, oval foramen; _h_, horizontal semicircular
canal; _ha_, its ampulla; _vaa_, ampulla of anterior vertical
semicircular canal; _vpa_, ampulla of posterior vertical
semicircular canal; _vc_, conjoined portion of the two vertical
canals.
]
=The Bony Labyrinth.=--"The bony labyrinth is described in three portions,
the _vestibule_, the _semicircular canals_, and the _cochlea_; casts of
its interior are represented from different aspects in Fig. 19. The
vestibule is the central part and has on its exterior the oval foramen
(_Fv_) into which the base of the stirrup-bone fits. Behind the
vestibule are three bony semicircular canals, communicating with the
back of the vestibule at each end, and dilated near one end to form an
_ampulla_. The bony cochlea is a tube coiled on itself somewhat like a
snail's shell, and lying in front of the vestibule.
=The Membranous Labyrinth.=--"The membranous vestibule, lying in the bony,
consists of two sacs communicating by a narrow aperture. The posterior
is called the _utriculus_, and into it the membranous semicircular
canals open. The anterior, called the _sacculus_, communicates by a tube
with the membranous cochlea. The membranous semicircular canals much
resemble the bony, and each has
[Illustration: FIG. 20.--A section through the cochlea in the line of
its axis.]
[Illustration:
FIG. 21.--Section of one coil of the cochlea, magnified. _SV_,
_scala vestibuli_; _R_, membrane of Reissner; _CC_, membranous
cochlea (_scala media_); _lls_, _limbus laminæ spiralis_; _t_,
tectorial membrane; _ST_, _scala tympani_; _lso_, spiral lamina;
_Co_, rods of Corti; _b_, basilar membrane.
]
an ampulla; in the ampulla one side of the membranous tube is closely
adherent to its bony protector; at this point nerves enter the former.
The relations of the membranous to the bony cochlea are more
complicated. A section through this part of the auditory apparatus (Fig.
20) shows that its osseous portion consists of a tube wound two and a
half times round a central bony axis, the _modiolus_. From the axis a
shelf, the _lamina spiralis_, projects and partially subdivides the
tube, extending farthest across in its lower coils. Attached to the
outer edge of this bony plate is the membranous cochlea (_scala media_),
a tube triangular in cross-section and attached by its base to the outer
side of the bony cochlear spiral. The spiral lamina and the membranous
cochlea thus subdivide the cavity of the bony tube (Fig. 21) into an
upper portion, the _scala vestibuli_, _SV_, and a lower, the _scala
tympani_, _ST_. Between these lie the lamina spiralis (_lso_) and the
membranous cochlea (_CC_), the latter being bounded above by the
membrane of Reissner (_R_) and below by the basilar membrane (_b_)."[20]
The membranous cochlea does not extend to the tip of the bony cochlea;
above its apex the scala vestibuli and scala tympani communicate. Both
are filled with perilymph, so that when the stapes is pushed into the
oval foramen, _o_, in Fig. 17, by the impact of an air-wave on the
tympanic membrane, a wave of perilymph runs up the scala vestibuli to
the top, where it turns into the scala tympani, down whose whorls it
runs and pushes out the round foramen _r_, ruffling probably the
membrane of Reissner and the basilar membrane on its way up and down.
[Illustration:
FIG. 22.--The rods of Corti. _A_, a pair of rods separated from the
rest; _B_, a bit of the basilar membrane with several rods on it,
showing how they cover in the _tunnel of Corti_; _i_, inner, and
_e_, outer rods; _b_, basilar membrane; _r_, reticular membrane.
]
=The Terminal Organs.=--"The membranous cochlea contains certain solid
structures seated on the basilar membrane and forming the _organ of
Corti_. This contains the end-organs of the cochlear nerves. Lining the
sulcus spiralis, a groove in the edge of the bony lamina spiralis, are
cuboidal cells; on the inner margin of the basilar membrane they become
columnar, and then are succeeded by a row which bear on their upper ends
a set of short stiff hairs, and constitute the _inner hair-cells_, which
are fixed below by a narrow apex to the basilar membrane; nerve-fibres
enter them. To the inner hair-cells succeed the _rods of Corti_ (_Co_,
Fig. 21), which are represented highly magnified in Fig. 22. These rods
are stiff and arranged side by side in two rows, leaned against one
another by their upper ends so as to cover in a tunnel; they are known
respectively as the _inner_ and _outer rods_, the former being nearer
the _lamina spiralis_. The inner rods are more numerous than the outer,
the numbers being about 6000 and 4500 respectively. Attached to the
external sides of the heads of the outer rods is the _reticular
membrane_ (_r_, Fig. 22), which is stiff and perforated by holes.
External to the outer rods come four rows of _outer hair-cells_,
connected like the inner row with nerve-fibres; their bristles project
into the holes of the reticular membrane. Beyond the outer hair-cells is
ordinary columnar epithelium, which passes gradually into cuboidal cells
lining most of the membranous cochlea. From the upper lip of the sulcus
spiralis projects the _tectorial membrane_ (_t_, Fig. 21) which extends
over the rods of Corti and the hair-cells."[21]
[Illustration:
FIG. 23.--Sensory epithelium from ampulla or semicircular canal,
and saccule. At _n_ a nerve-fibre pierces the wall, and after
branching enters the two hair-cells, _c_. At _h_ a 'columnar cell'
with a long hair is shown, the nerve-fibre being broken away from
its base. The slender cells at _f_ seem unconnected with nerves.
]
The hair-cells would thus seem to be the terminal organs for 'picking
up' the vibrations which the air-waves communicate through all the
intervening apparatus, solid and liquid, to the basilar membrane.
Analogous hair-cells receive the terminal nerve-filaments in the walls
of the saccule, utricle, and ampullæ (see Fig. 23).
=The Various Qualities of Sound.=--Physically, sounds consist of
vibrations, and these are, generally speaking, _aërial waves_. When the
waves are non-periodic the result is a _noise_; when periodic it is
what is nowadays called a _tone_, or _note_. The _loudness_ of a sound
depends on the _force_ of the waves. When they recur periodically a
peculiar quality called _pitch_ is the effect of their _frequency_. In
addition to loudness and pitch tones have each their _voice_ or
_timbre_, which may differ widely in different instruments giving
equally loud tones of the same pitch. This voice depends on the _form_
of the aërial wave.
=Pitch.=--A single puff of air, set in motion by no matter what cause,
will give a sensation of sound, but it takes at least four or five
puffs, or more, to convey a sensation of pitch. The pitch of the note
_c_, for instance, is due to 132 vibrations a second, that of its octave
_c´_ is produced by twice as many, or 264 vibrations; but in neither
case is it necessary for the vibrations to go on during a full second
for the pitch to be discerned. "Sound vibrations may be too rapid or too
slow in succession to produce sonorous sensations, just as the
ultra-violet and ultra-red rays of the solar spectrum fail to excite the
retina. The highest-pitched audible note answers to about 38,016
vibrations in a second, but it differs in individuals; many persons
cannot hear the cry of a bat nor the chirp of a cricket, which lie near
this upper audible limit. On the other hand, sounds of vibrational rate
about 40 per second are not well heard, and a little below this they
produce rather a 'hum' than a true tone-sensation, and are only used
along with notes of higher octaves to which they give a character of
greater depth."[22]
The entire system of pitches forms _a continuum of one dimension_; that
is to say, you can pass from one pitch to another only by one set of
intermediaries, instead of by more than one, as in the case of colors.
(See p. 41.) The whole series of pitches is embraced in and between the
terms of what is called the musical scale. The adoption of certain
arbitrary points in this scale as 'notes' has an explanation partly
historic and partly æsthetic, but too complex for exposition here.
=The 'timbre'= of a note is due to its _wave-form_. Waves are either
simple ('pendular') or compound. Thus if a tuning-fork (which gives
waves nearly simple) vibrate 132 times a second, we shall hear the note
_c_. If simultaneously a fork of 264 vibrations be struck, giving the
next higher octave, _c´_, the aërial movement at any time will be the
algebraic sum of the movements due to both forks; whenever both drive
the air one way they reinforce one another; when on the contrary the
recoil of one fork coincides with the forward stroke of another, they
detract from each other's effect. The result is a movement which is
still periodic, repeating itself at equal intervals of time, but no
longer _pendular_, since it is not alike on the ascending and descending
limbs of the curves. We thus get at the fact that non-pendular
vibrations may be produced by the fusion of pendular, or, in technical
phrase, by their _composition_.
Suppose several musical instruments, as those of an orchestra, to be
sounded together. Each produces its own effect on the air-particles,
whose movements, being an algebraical sum, must at any given instant be
very complex; yet the ear can pick out at will and follow the tones of
any one instrument. Now in most musical instruments it is susceptible of
physical proof that with every single note that is sounded many upper
octaves and other 'harmonics' sound simultaneously in fainter form. On
the relative strength of this or that one or more of these Helmholtz has
shown that the instrument's peculiar voice depends. The several
vowel-sounds in the human voice also depend on the predominance of
diverse upper harmonics accompanying the note on which the vowel is
sung. When the two tuning-forks of the last paragraph are sounded
together the new form of vibration has the same _period_ as the
lower-pitched fork; yet the ear can clearly distinguish the resultant
sound from that of the lower fork alone, as a note of the same pitch but
of different timbre; and within the compound sound the two components
can by a trained ear be severally heard. Now how can one resultant
wave-form make us hear so many sounds at once?
=The analysis of compound wave-forms= is supposed (after Helmholtz) to be
effected through the different rates of sympathetic resonance of the
different parts of the membranous cochlea. The basilar membrane is some
twelve times broader at the apex of the cochlea than at the base where
it begins, and is largely composed of radiating fibres which may be
likened to stretched strings. Now the physical principle of sympathetic
resonance says that when stretched strings are near a source of
vibration those whose own rate agrees with that of the source also
vibrate, the others remaining at rest. On this principle, waves of
perilymph running down the scala tympani at a certain rate of frequency
ought to set certain particular fibres of the basilar membrane
vibrating, and ought to leave others unaffected. If then each vibrating
fibre stimulated the hair-cell above it, and no others, and each such
hair-cell, sending a current to the auditory brain-centre, awakened
therein a specific process to which the sensation of one particular
pitch was correlated, the physiological condition of our several
pitch-sensations would be explained. Suppose now a chord to be struck in
which perhaps twenty different physical rates of vibration are found: at
least twenty different hair-cells or end-organs will receive the jar;
and if the power of mental discrimination be at its maximum, twenty
different 'objects' of hearing, in the shape of as many distinct pitches
of sound, may appear before the mind.
The rods of Corti are supposed to be _dampers_ of the fibres of the
basilar membrane, just as the malleus, incus, and stapes are dampers of
the tympanic membrane, as well as transmitters of its oscillations to
the inner ear. There must be, in fact, an instantaneous _damping_ of the
physiological vibrations, for there are no such positive after-images,
and no such blendings of rapidly successive tones, as the retina shows
us in the case of light. Helmholtz's theory of the analysis of sounds
is plausible and ingenious. One objection to it is that the keyboard of
the cochlea does not seem extensive enough for the number of distinct
resonances required. We can discriminate many more degrees of pitch than
the 20,000 hair-cells, more or less, will allow for.
=The so-called Fusion of Sensations in Hearing.=--A very common way of
explaining the fact that waves which singly give no feeling of pitch
give one when recurrent, is to say that their several sensations _fuse
into a compound sensation_. A preferable explanation is that which
follows the analogy of muscular contraction. If electric shocks are sent
into a frog's sciatic nerve at slow intervals, the muscle which the
nerve supplies will give a series of distinct twitches, one for each
shock. But if they follow each other at the rate of as many as thirty a
second, no distinct twitches are observed, but a steady state of
contraction instead. This steady contraction is known as _tetanus_. The
experiment proves that there is a physiological cumulation or
overlapping of processes in the muscular tissue. It takes a twentieth of
a second or more for the latter to relax after the twitch due to the
first shock. But the second shock comes in before the relaxation can
occur, then the third again, and so on; so that continuous tetanus takes
the place of discrete twitching. Similarly in the auditory nerve. One
shock of air starts in it a current to the auditory brain-centre, and
affects the latter, so that a dry stroke of sound is heard. If other
shocks follow slowly, the brain-centre recovers its equilibrium after
each, to be again upset in the same way by the next, and the result is
that for each shock of air a distinct sensation of sound occurs. But if
the shock comes in too quick succession, the later ones reach the brain
before the effects of the earlier ones on that organ have died away.
There is thus an overlapping of processes in the auditory centre, a
physiological condition analogous to the muscle's tetanus, to which new
condition a new quality of feeling, that of pitch, directly corresponds.
This latter feeling is a new kind of sensation altogether, not a mere
'appearance' due to many sensations of dry stroke being compounded into
one. No sensations of dry stroke can exist under these circumstances,
for their physiological conditions have been replaced by others. What
'compounding' there is has already taken place in the brain-cells before
the threshold of sensation was reached. Just so red light and green
light beating on the retina in rapid enough alternation, arouse the
central process to which the sensation _yellow_ directly corresponds.
The sensations of red and of green get no chance, under such conditions,
to be born. Just so if the muscle could feel, it would have a certain
sort of feeling when it gave a single twitch, but it would undoubtedly
have a distinct sort of feeling altogether, when it contracted
tetanically; and this feeling of the tetanic contraction would by no
means be identical with a multitude of the feelings of twitching.
=Harmony and Discord.=--When several tones sound together we may get
peculiar feelings of pleasure or displeasure designated as consonance
and dissonance respectively. A note sounds most consonant with its
octave. When with the octave the 'third' and the 'fifth' of the note are
sounded, for instance _c--e--g--c´_, we get the 'full chord' or maximum
of consonance. The ratios of vibration here are as 4:5:6:8, so that one
might think simple ratios were the ground of harmony. But the interval
_c--d_ is discordant, with the comparatively simple ratio 8:9. Helmholtz
explains discord by the overtones making 'beats' together. This gives a
subtle grating which is unpleasant. Where the overtones make no 'beats',
or beats too rapid for their effect to be perceptible, there is
consonance, according to Helmholtz, which is thus a negative rather than
a positive thing. Wundt explains consonance by the presence of strong
identical overtones in the notes which harmonize. No one of these
explanations of musical harmony can be called quite satisfactory; and
the subject is too intricate to be treated farther in this place.
=Discriminative Sensibility of the Ear.=--Weber's law holds fairly well
for the intensity of sounds. If ivory or metal balls are dropped on an
ebony or iron plate, they make a sound which is the louder as they are
heavier or dropped from a greater height. Experimenting in this way
(after others) Merkel found that the just perceptible increment of
loudness required an increase of 3/10 of the original stimulus
everywhere between the intensities marked 20 and 5000 of his arbitrary
scale. Below this the fractional increment of stimulus must be larger;
above it, no measurements were made.
Discrimination of differences of _pitch_ varies in different parts of
the scale. In the neighborhood of 1000 vibrations per second, one fifth
of a vibration more or less can make the sound sharp or flat for a good
ear. It takes a much greater _relative_ alteration to sound sharp or
flat elsewhere on the scale. The chromatic scale itself has been used as
an illustration of Weber's law. The notes seem to differ equally from
each other, yet their vibration-numbers form a series of which each is a
certain multiple of the last. This, however, has nothing to do with
intensities or just perceptible differences; so the peculiar parallelism
between the sensation series and the outer-stimulus series forms here a
case all by itself, rather than an instance under Weber's more general
law.