Bargen 1
The ear is the organ of the human body involved in converting sound waves into nerve
impulses by means of transduction and gives the brain a sense of acceleration. The scope of this
paper will be to establish knowledge of the anatomy of the ear, outer, middle, and inner, which
will aid in describing its functions.
The outer ear consists of the auricle, or pinna, the external auditory canal, and the
tympanic membrane.
The external auditory canal is the tube in the ear that allows sound to pass from the
floor of the concha to the eardrum where sounds are absorbed. Its wall is constructed of
cartilage in the first third, and bone the inner two-thirds and is lined with skin the whole 24 mm
(1 inch). Lined with hair which discourages insects, a special type of sweat glands produce
earwax. (Hawkins)
The tympanic membrane, or eardrum, located at the end of the external auditory canal,
separates the outer and middle ear. It is about 8-10 mm (0.3 – 0.4) wide, and shaped in a
concave manner such that the apex points inward. The skin is encircled by and affixed to a
partial ring of bone called the tympanic annulus. The tympanic membrane is stretched tightly
on the lower portion where it is attached, but lies loose along the gap in the tympanic annulus.
There are three layers in the eardrum. The external is the same as the external auditory canal.
the internal is consistent with the mucous lining of the middle ear, and between the two are _
fibers that give the tympanic membrane its toughness. It is well supplied with blood vessels and
nerves, making the eardrum extremely sensitive to loud noises. (Hawkins)
Bargen 2
The middle ear cavity is a narrow air-filled space. A Slight constriction divides it into two
parts: the tympanum above and the epitympanum below also known as the attic and the
atrium, respectively. The middle ear has four walls as well as a floor and top, much like a
normal room. The top, (superior wall) is made of a bony plate, whereas the bottom, (inferior
wall) also made of bone, partitions the middle ear with the carotid artery and jugular vein. The
outer (medial), wall is formed by the tympanic membrane, and the inner (medial) wall is
composed of the “bony otic capsule of the inner ear. It has two small openings, or fenestrae,
one above the other. The upper one is the oval window, which is closed by the footplate of the
stapes. The lower one is the round window, which is covered by a thin membrane.” (Hawkins)
Finally, the rear (posterior) wall partly separates the middle ear cavity with the mastoid antrum,
and the front (anterior) wall opens into the Eustachian tube, connecting the middle ear with the
nasopharynx. (Hawkins)
Crossing the middle ear cavity are the auditory ossicles. They are the bones that
transmit vibrations from the eardrum to the oval window of the cochlea. The first, the malleus,
or hammer, is joined with the tympanic membrane from the middle to the upper margin. The
malleus, resembling a club, is then firmly coupled with the incus, and attached the
epitympanum above via three small ligaments. The incus, or anvil appearing more like a
malformed tooth than an anvil, is attached to the stapes, or stirrup bone by means of a loose
ligament enclosed joint. The stapes, being the smallest bone in the body, is 3 mm or 1 inch long
and weighs scarcely 3 milligrams (0.0001 ounce). The stapes, sitting horizontally and almost at
right angles to the incus transmits vibrations to the cochlea. It sits in the oval window,
enveloped by elastic annular ligament. Three bones, the malleus, incus, and stapes, from an
Bargen 3
ossicular chain to convey vibrations from the eardrum to the oval window in the cochlea.
(Hawkins)
In order to prevent damage to the inner ear, two muscles, the tensor tympani, and the
stapedius, pull on the various bones in the middle ear. The stapedius pulls on the upper end of
the malleus to maintain tension on the tympanic membrane. It originates just above the
Eustachian tube, pulling laterally, allowing it to maintain such a tension. The other muscle, the
stapedius, arises from the back wall of the middle ear cavity, and attaches itself to the stapes. In
this way the muscle can pull the stapes back out of the oval window, selectively reducing
tremors entering the inner ear. Thus, the two muscles can reduce the damage that occurs over
periods of time. Unfortunately, however, these muscles cannot contract instantaneously and
thus are not effective with sudden, explosive, sounds, such as a gunshot or violent crack, and
often tire over long periods of loud sounds.
Bargen 4
The Eustachian tube helps ventilate the middle ear and maintain equal pressure on both
sides of the ear. About 31-38 mm or 1.2-1.5 inches long, leads downward and inward from the
tympanum to the nasopharynx. Its lining is similar to that of the mucous lining of the middle
ear, and becomes wider and cartilaginous. Small hairs, called cilia aid with the process of
drainage of mucous secretions. The Eustachian tube ventilates the middle ear; however, it is
not always open, in fact, it is tightly shut most of the time. This explains why one might feel
discomfort during a rapid airplane descent or subterranean dive. The middle ear has a higher
pressure than the surrounding air, causing the interior air to press harder against the eardrum
than the exterior air. This can often be remedied by closing both the mouth and nose, and
performing a forced expiration. This is called the Valsalva maneuver after the Italian physiciananatomist Antonio Maria Valsalva (1666–1723). Consequently, this tube maintains uniform
pressure regardless of exterior pressure fluctuations when it is open. (Hawkins)
There are two parts of the inner ear: the bony labyrinth and the membranous labyrinth.
The bony labyrinth envelopes the entirety of the membranous labyrinth, which contains the
entirety of the vestibular labyrinth and cochlea. The vestibule (the central chamber) contains
the utricle and saccule, the semicircular canals and their semicircular ducts. The membranous
labyrinth is lined with epithelium (a sheet of specialized cells that covers internal and external
body surfaces). Between the bony labyrinth and the membranous labyrinth is a fluid called
perilymph. It is made from blood plasma and closely resembles the cerebrospinal fluid of the
brain and the aqueous humor of the eye. It has a high concentration if sodium ions (about 150
milliequivalents per liter) and a low concentration of potassium ions (about 5 milliequivalents
per liter) like most extracellular fluids. There are two labyrinths in the inner ear: the bony
Bargen 5
labyrinth (which contains the membranous labyrinth and perilymph) and the membranous
labyrinth (which contains endolymph and vestibule). (Hawkins)
The vestibular system is responsible for maintaining balance, determining the position
of the head relative to vertical, and sensing acceleration. The two main sacs in the system are
the utricle and saccule, known also as otolith organs or gravity receptors. Each sac contains a
patch of sensory cells called a macula, about 2 mm or 0.08 inch in diameter. The macula
determines the position of the head relative to vertical. Projecting from the macula are bundles
of hair, stiff, nonmobile stereocilia and mobile kinocilia. When the bundles are bent, (for
example because of a tilt of the head) the sensory cells alter the nerve signals that are
constantly sending to the brainstem. The entire macula is covered by the gelatinous otolithic, or
statolithic, membrane. The surface of this membrane is covered with a blanket of
rhombohedral crystals, and add considerable amounts of weight to it. Thus, the position of the
head can be monitored with these organs. (Hawkins)
Bargen 6
The semicircular canals are notated according to their position: superior, posterior, and
horizontal. Each canal attaches itself to the vestibule with an ampulla. Contained in each canal
are the semicircular duct of smaller size, and are filled with endolymph and perilymph in the
same way as the bony and membranous labyrinth. (Hawkins)
The cochlea is the sensory organ of hearing. Resembling a snail, it is coiled one and a
half turns and end blindly at the modiolus. If stretched out, it would be a cone 30 mm in length.
The modiolus contains the cochlear artery and cochlear vein as well as the cochlear nerve. The
nerve originates in the modiolus and enters the brain through an opening in the petrous
portion of the temporal bone called the internal meatus. A thin bony shelf, known as the
osseous spiral lamina, winds inside the cochlear canal. This separates the canal into two
compartments: the upper known as the scala vestibuli, or, vestibular ramp, and the upper
known as the scala tympani, or, tympanic ramp. The scala vestibuli and scala tympani
communicate with each other through an opening at the apex of the cochlea called the
helicotrema. This can be seen if one were to cut the cochlea longitudinally down the middle.
The scala vestibuli opens into the vestibule whereas the scala vestibuli ends below the oval
window. A smaller scala, the scala media, or, cochlear duct, lies between the scala tympani and
scala vestibuli. Closely resembling a right triangle when cut on half, it ends blindly at the basal
end and the apex. Its base is formed by the basilar membrane and the osseous spiral lamina.
Bargen 7
Arranged on the surface of the basilar membrane are rows of sensory hair cells, which, in
conjunction with their supporting cells, make up “a complex neuroepithelium called the basilar
papilla” (Hawkins) or, organ of Corti. The hair cells, supported by a cell of dieters which holds
the hair in a cup shaped depression, suspend a stiff membrane, called the reticular lamina in
the perilymph. Under the reticular lamina is the cortilymph, thought to be similar, if not
identical to the composition of the perilymph. The vestibulocochlear nerve originates in the
modiolus and passes through a canal called the canal of Rosenthal, near the root of the osseous
spiral lamina. This chain of nerves, covered with myelin, winds itself around the cochlea in this
Bargen 8
manner. There are two types of nerve endings in the cochlea. The larger and more abundant of
the two contain fluid sacs, which contain liquid and neurotransmitters. The smaller and less
numerous are terminations of the different fibres of the cochlear nerve, and transmit their
signals from hair cells to the brainstem. Not much is known about the distribution between hair
cells. However, we know that some cells share a “party line,” that is, multiple hair cells share
the same nerve ending. Likewise, some cells receive multiple nerve endings. Viewed from
above, the organ of Corti along with its reticular lamina form a mosaic pattern, with three rows.
However, in the inner hair cells there are scattered fourth or fifth rows. The spaces between
the rows are covered with supporting cells (phalangeal plates). When a hair cell dies due to
aging, noise, etc., these cells cover in its place to form a well-recognized “scar.” Thus, the
cochlea contains several scalae with hair cells and nerve endings which signal when sound
reaches the ear, moving the hair bundles. (Hawkins)
Physiology of hearing
Hearing is the process by which the ear perceives sound. Sound is created when objects
cause disturbances in the air, or sound waves. (Hawkins) These waves are longitudal, i.e., the air
has rarefaction (stretched out) in one place, while it is compressed in others. (Dincher) under
normal conditions, sound travels at about 370 yards per second. (Whitfield and Stoddart) The
ear can detect several different aspects of sound: pitch (frequency), loudness (amplitude). Pitch
is a perception of frequency, which is usually measured in cycles per second, or, Hertz. The
human ear is mostly sensitive to frequencies from 1,000 to 4,000 Hertz, although young ears
can detect 20 to 20,000 Hertz. Volume, or amplitude is a perception of magnitude in sound
waves. It is measured in bels or decibels. Bels and decibels are a logarithmic scale because a
Bargen 9
linear scale would mean working with very long cumbersome numbers. (Whitfield and
Stoddart) That is, decibels are used to compare the intensity of a given sound with a sound just
perceptible to the human ear at the frequency range most efficiently perceptible to the human
ear. On this scale, 0 decibels represent a sound that is barely audible and 130 is the threshold of
pain. (Hawkins)
In order for sound to reach the brain, it must undergo three transformations. First, the
waves are converted to mechanical vibrations via the eardrum. Then, with the auditory ossicles
and oval window, it is converted to fluid vibrations which stimulate the organ of Corti and nerve
cells. The outer ear (the auricle) collects sound waves and funnels them into the external
auditory canal. Because of its small size and near immobility, the auricle cannot detect sound
origin as efficiently as some animals. However, the auditory canal can enhance the sound
reaching the eardrum for frequencies between 2,000 and 7,000 Hertz for the use of
distinguishing consonants. Next, sound waves hit the eardrum. Some are reflected, and some
absorbed. The tendency for the ear to resist the passage of sound is called acoustic impedance.
When sound waves strike the tympanic membrane, it vibrates in such a way that the middle
(called the umbo) goes farther than the other portions, forming a cone. Sounds of higher
frequency make the eardrum vibrate faster. However, at higher frequencies, the motion of the
eardrum becomes erratic, and the quality of the sound transmitted to the ossicular chain, and
thus, inner ear is lowered. Sounds then are transmitted to the ossicular chain. The malleus and
incus are suspended by elastic ligaments, and finely balanced about their common axis of
rotation. They are loosely bound, resulting in loss of sound energy to some degree. The stapes
Bargen 10
does not move in or out of the oval window, but moves about its lower pole of its footplate,
impinging on the membrane covering the oval window. (Hawkins)
In order to transmit sound to the middle ear, it must be changed from mechanical
vibrations, i.e., vibrations in the auditory ossicles, to vibrations in the cochlear fluids. However,
there is quite a bit of acoustic impedance between the stapes and the cochlear fluids.
Ordinarily, when a sound strikes the surface of water, only about 10% passes into the water. In
the ear this would be a loss of 30 decibels, enough to seriously impact the efficacy of the ear.
But there are two things that prevent this. First, the stapes footplate is dramatically smaller
than the eardrum. Secondly, the mechanical advantage that the two levers (the malleus and the
incus) form. The area of the tympanic membrane is 69 square mm (0.1 square inches). It has
been estimated that the portion of the tympanic membrane that actually moves when sound
strikes it is 43 square mm (0.07 square inches). The area of the stapes footplate is 3.2 square
mm. Thus, the pressure is increased 13 times! Further, the mechanical advantage presented by
the ossicular chain is about 1.3. Therefore, the total pressure increase from the eardrum to the
stapes footplate is not less than about 17-fold, depending on how much of the tympanic
membrane actually vibrates. These features allow the threshold of hearing to be as low as 1
angstrom (Å; 1 Å = 0.0000001 mm) (Hawkins)
After sound vibrations pass through the ossicular chain and are amplified, they pass into
the cochlea. Stereocilia under the reticular lamina transform vibrations into nerve impulses.
Not much is known about how they do this; however, certain key features are known. One
aspect is the endocochlear potential, which exists between the endolymph and perilymph. The
Bargen 11
electrical difference of about +80 millivolts is sustained by the constant transfer of potassium
ions from the perilymph to the cochlear duct. (Hawkins)
A German physicist and physiologist, Hermann von Helmholtz, theorized that pitch is
distinguished by the varying lengths of resonators, fibres in the cochlea. He thought that the
cochlea was a multi-resonant structure, meaning different portions of the cochlea vibrated
more intensely than others based on the frequency that passed through. However, Helmholtz’s
theory is no longer accepted. Subsequent experiments carried out by Georg von Békésy showed
that the cochlea does not distinguish pitch because of separately tuned resonators, as
Helmholtz had theorized. Rather, pitch is distinguished because of the changes in the length of
the basilar membrane. High frequencies resonate near the basal end of the cochlea, and lower
frequencies near the apex. Each frequency has a portion of the cochlea that it stimulates most,
Bargen 12
and thus determines which nerve cells are activated. In this manner, pitch is distinguished.
(Hawkins)
The technical terms used to describe waveforms aid in the understanding of the
following paragraphs. A wave’s period is the distance between two apexes of a sinusoidal
waveform (A measure of time). It is reciprocally related to the wave’s frequency, (the number
of cycles in a given amount of time) meaning that they are inversely related. This means
f = 1/t and t = 1/f
f being frequency, t as time to complete one cycle. (Whitfield and Stoddart) The
wavelength of a sound is the distance between two points of maximum air compression (A
measure of distance.
 = v/f
Where  is wavelength, v as velocity, and f as frequencies (Whitfield and Stoddart)
The vestibular system is the apparatus designed to detect acceleration, maintaining the
body’s equilibrium and posture. It is the natural equivalent to what we would call an
accelerometer, and is much like it in structure. Nevertheless, it is still dissimilar. The three
semicircular canals detect angular momentum, that is, the acceleration produced by the tilt of
the head. In 1824 Jean-Pierre Flourens, a neurologist, performed a series of experiments where
he observed the role of the vestibular canals. Flourens cut the vestibular canals of pigeons. This
resulted in the pigeon being uncoordinated in the same plane of the injured canal. When,
however, he removed the cochlea, it was observed that the pigeon had lost its sense of hearing.
Bargen 13
Thus, it was realized half a century later that the semicircular canals were apparatus for
balance, and didn’t affect hearing. (Hawkins)
The inertia contained in the canals stimulates the cupula. There are two concepts
concerning this operation. First, the “hydrostatic concept” establishes that the cupula is
stimulated by the weight they contain, and the pressure exerted on the cupula. Second, the
“hydrodynamic concept” postulates that the cupula is stimulated by the motion of endolymph.
This theory was verified when investigators injected a drop of oil into a fish’s semicircular canal.
they then observed that when the fish accelerated in one direction, the cupula was deflected
momentarily, and then returned to its resting position when the fluid stopped moving relative
to the canal. That is, the endolymph-oil solution was rotating with the semicircular canal.
When, however, the angular movement stopped, the cupula was deflected again momentarily,
and returned to its position again. This proved that the inertia of the endolymph caused the
stimulation of the cupula, consistent with the “hydrodynamic concept.” This stimulation
depolarizes the cupula when endolymph flows toward the utricle in horizontal canals, and
hyperpolarizes it when endolymph flows away in order to stimulate the nerves. The effects are
reversed with the superior canals. (Hawkins)
Likewise, the utricle and saccule inside the vestibule detect linear motion with respect
to gravity. Their maculae are stimulated by the “shearing forces between the otolithic
membrane and the cilia of the hair cells beneath it.” (Hawkins) These sensory organs help the
body maintain an upright position and assist corrective reflexes. (Hawkins)
Bargen 14
If, however, one of the vestibular apparatus becomes damaged, then the uncontrolled
activity of the other would cause the brain to have a false sense of turning, also known as
vertigo, because the brain cannot get an accurate sense of acceleration. This also causes
rhythmical, jerky movements of the eye toward the uninjured side, known as nystagmus. If,
however, both ears are damaged because of medication such as gentamicin or streptomycin,
severe vertigo and disorientation, as well as trouble walking, known as ataxia, can result. In
younger persons these effects can subside, with reliance placed on vision, proprioceptive
impulses from muscles and joints, as well as nerves on the soles of feet. Recovery of some
injured cells may occur. (Hawkins)
The “abnormal” movement of a car, ship, boat, or plane may cause uncomfortable
symptoms including nausea, discomfort, or in exacerbated cases, vomiting. The effects come
from a combination of an inability to perceive high speeds such as those experienced on an
airplane, and apparently contradictory signals because of this. For example, looking out the side
of a vehicle while traveling but not accelerating would cause the brain to perceive that the
subject is moving. However, the lack of acceleration and/or “deceleration” for a period of time
would cause the brain to believe no motion was present. Thus, two contradictory signals cause
“confusion” in the brain, and producing the above symptoms. (Britannica)
The Ear is the organ of hearing. Sounds pass through the external auditory canal, collide
with the tympanic membrane, and are converted to mechanical motion, then to vibrations in
the cochlear fluid, and finally to changes in nerve impulses constantly sending to the brain. The
ear also assists the body in maintaining an upright position, using the semicircular canals and
Bargen 15
vestibular organs. The ear is an essential part to the body’s senses of hearing, balance,
acceleration, and awareness of its surroundings.
Bargen 16
Works Cited
Britannica, The Editors of Encyclopaedia. Motion Sickness. Invalid Date Invalid Date Invalid Date. 12 April
2021. <https://www.britannica.com/science/motion-sickness>.
Dincher, Vicki. Exploring Creation With Physical Science, 3rd ed. 3rd. Anderson: Apologia Educational
Ministries, Inc., 2020. 16 March 2021.
Hawkins, Joseph E. Human Ear. Ed. Gloria Lotha, Richard Pallardy, Dutta Promeet, Emily Rodriguez, Kara
Rogers, Marco Sampaolo, Shiveta Singh, Surabhi Sinha, Grace Young, The Editors of
Encyclopaedia Britannica John Higgins. Encyclopedia Britannica, invalid year. Web. 2020.
<https://www.britannica.com/science/ear>.
Whitfield, Dr. Philip and Dr. D. M. Stoddart. Hearing, Taste and Smell. New York; Toronto: Torstar Books
Inc., 1984. Book.