The Anatomy of the Ear

Essential for hearing and balance, each ear has an intricate structure of bones, nerves, and muscles. The ears can be affected by bacterial infections, viral infections, hearing loss, tinnitus (ringing in the ears), Meniere’s disease, and more.  

Anatomy

The ear is divided into three portions: the outer ear, the middle ear, and the inner ear.

Outer Ear

The outer ear includes the visible outer portion of the ear and the ear canal.

• Auricle: The outwardly visible part of the ear is composed of skin and cartilage, and attaches to the skull. It has an outer (lateral) and an inner (medial) aspect. The inner aspect serves as an attachment, and the outer is instrumental in hearing and has characteristic ridges and grooves. Notable among these is the outer rim or helix, which runs from the skull and bends around to terminate at the ear lobe. Parallel to this is another curved structure called the antihelix, which has a triangular upper fossa (space) bound by the borders of the helix and antihelix. The auricle also has, at its center, a space next to the opening of the external acoustic meatus (ear canal) called a concha, which is partially covered by a triangular flap of cartilage known as the tragus.
• External acoustic meatus: This is the bone and cartilage lined canal that leads from the outside to the inside of the ear. Its outer portion is surrounded by cartilage, and the inner part is surrounded by bones of the skull. This portion curves, slightly up and to the back, before bending forward and down. The inner portion—representing about two-thirds of its course—is surrounded by temporal bone and terminates at the tympanic membrane.
• Tympanic membrane (eardrum): This portion represents the border between the external and middle ear. It is composed of a membrane attached by fibrous cartilage to the surrounding bone. It has a more flaccid part (pars flaccida), and a more taut part (pars tensa). The inner, medial surface is convexed towards the middle ear and connects with the malleus bone of the middle ear.

Middle Ear

The middle ear (also known as the tympanum or tympanic cavity) is a complicated network of tunnels, chambers, openings, and canals mostly inside openings within the temporal bone on each side of the skull. The 2 largest chambers are called the middle ear space and mastoid.

The middle ear space, shaped like a narrow tube with concave walls, is separated from the external ear by the tympanic membrane and the inner ear by its labyrinthine (medial) wall.

Roughly speaking, it has three major compartments—the mesotympanum (directly to the side of the membrane), the epitympanum or attic (located at the top of the cavity), and six major walls—the tegmental wall (roof), jugular wall (floor), membranous (lateral) wall, labyrinthine (medial) wall, mastoid (posterior) wall, as well as the carotid (anterior) wall.

Three auditory ossicles in the middle ear, the three smallest bones in the human body, transmit sound to the labyrinth of the inner ear.

• Malleus (hammer): Attached to the tympanic membrane on its outer side, and the incuse via a joint called the incudomalleolar joint, it has a head connected to the tegmental wall of the middle ear, and a neck, which has two portions: the anterior and lateral processes. The anterior is linked to the carotid wall, and the lateral is attached to the middle surface of the tympanic membrane.
• Incus (anvil): This connects the malleus and stapes and consists of three portions: the body, and the long and short limbs. The body is connected to the malleus by way of the incudomalleolar joint and sits in a space called the epitympanic recess. The long limb runs parallel to the handle of the malleus and terminates as it accesses the tenticular process. Via the incudostapedial joint, it links to the stapes. The short limb runs towards the back of the body, attaching to the posterior wall of the tympanic cavity.
• Stapes (stirrup): This bone connects with the incus on the side via the incudostapedial joint. In its middle, it accesses the oval window as part of the mechanism that carries sound to the inner ear. This bone also has a head, which connects with the lenticular process, and two limbs that attach to the oval base, which connects with the oval window.    

The eustachian tube connects the middle ear to the nasopharynx, which is the upper throat and nasal cavity. It helps regulate pressure in this part of the ear, its bony part arises in the carotid wall before moving downward and forward about 30 to 35 degrees, narrowing as it progresses through an area called the pharyngeal space.

Inner Ear

The inner ear—known also as the labyrinth and cochlea—is intricate and the most complicated portion of the ear. It is the portion deepest in the skull, positioned in the petrous part of the temporal bone on the side of the skull. It has a bony labyrinth, which is filled with a fluid called perilymph, in which the membranous labyrinth, which contains a fluid called endolymph, is suspended.

The major structures of the inner ear include:

• Vestibule: A cavity considered a part of the membranous labyrinth, this structure contains two sacs: the utricle and the saccule. Via a structure on its outside wall called the oval window, it (along with another structure called the round window) is able to communicate with the middle ear, and it accesses the cochlea on the other side, with the semicircular canals behind and above it.
• Cochlea: This spiral-shaped organ—its shape resembles a snail shell—consists of three compartments: the scala vestibuli, scala media (often called the cochlear duct), and scala tympani. Notably, this feature is split into a base and its spiral canal, which wraps two and a half times around a central bony column, known as the modiolus. Each of these structures serves an important role in hearing; the scala vestibuli and media contain perilymph, and surround the third, which is filled with endolymph.    
• Semicircular canals: These three semi-circular canals are arranged at different angles and loop around, with each tipped roughly 90 degrees from the other. The anterior semicircular canal emerges from the sagittal plane (the line that divides the body into left and right). The posterior semicircular canal emerges along the frontal plane (dividing the front and back of the body), and the lateral semicircular canal runs horizontally to the ground. One side of the anterior and posterior canals is merged.

Anatomical Variations

Ear anatomy can vary. In addition to normal and relatively minor differences, there are a number of more significant and impactful variants. For instance, on the auricle, attachment—or lack thereof—of the earlobe to the face is a frequently seen genetic variation, with attached earlobes seen in anywhere from 19% to 54% of the population. There’s also a great deal of variation in the size and shape of other structures, such as the helix, antihelix, tragus, and others.

Some specific malformations of the external ear:

• Prominent ear: This relatively common variant involves ears that protrude out from the head more than 2 centimeters (cm).
• Constricted ear: In this case, the helical rim folds over, is wrinkled, or abnormally tight.
• Cryptotia: Due to malformation of ear cartilage, this variant gives off the appearance that the upper portion of the ear is buried inside the head.
• Microtia: This is an underdeveloped ear.
• Anotia: In some cases, there is a complete absence of the ear.
• Stahl’s ear: This is when additional cartilage in the crus of the ear lends to a pointy, elf-like appearance.
• Cauliflower ear: This condition occurs when there is excessive and abnormal cartilage formation on the top of the normal ear cartilage, resulting in misshapen, often bulkier ears (usually caused by trauma to the auricle).

Some variations in the middle and inner parts of the ear:

• Anagenesis of the pyramidal eminence and stapedial tendon: This condition is characterized by failed development of the stapedial tendon that connects the stapes to the surrounding structure.
• Absence of the ponticulus: In rare cases, the ponticulus, a small bony structure of the posterior of the middle ear is under-formed, shaped irregularly, or completely absent.
• Absence of the subiculus: This is a partial or complete absence of the subiculus, a small bony structure near the oval window of the middle ear.
• Facial dehiscence: This is a lack of bone covering part of the facial nerve as it runs thru the middle ear.   

Function

Primarily, the ear serves two functions—hearing and regulation of balance.

Hearing

The outer ear is shaped to direct sound waves from the external environment to the ear canal. These are then directed towards the tympanic membrane (eardrum), causing it to vibrate. This vibration then causes the malleus, incus, and stapes to vibrate, which leads the perilymph within the cochlea to vibrate, stimulating a small portion called the organ of Corti.

As the fluid moves, tiny hairs on the surfaces of the organ of Corti are stimulated and this is translated into electrical signals that are delivered to the auditory nerve of the brain for processing.

Balance

Sense of balance and position is regulated by structures in the inner ear, most notably the semicircular canals and the utricle and saccule in the vestibule.

The three semicircular canals correspond to the three dimensions (x, y, and z), and connect to the utricle at an ampulla—a widening of the canal. Within the ampulla are special sensory cells called epithelia and hair cells underneath a substance called gelatinous copula. Each semicircular canal is filled with endolymph as well, and, as the head rotates, the endolymph is displaced, exciting the cells and generating a sense of balance. 

Balance related to the forward and back as well as upward and downward motion of the head and body is regulated by the utricle and saccule. These structures contain cells called macula, which are the primary sensory apparatus for this type of balance. They contain hair cells. Macula in the utricle are associated with forward and back mobility, whereas those in the saccule are involved in detecting vertical or downward movement. The motion of the head displaces these hairs and provides signaling for the sensation of motion.

Associated Conditions

Many diseases and health conditions can affect the functioning of the ear.

The most common include:

• Tinnitus: Persistent ringing in the ear can result from abnormal activity in the auditory nerve of the brain or muscle spasm or another process in the middle ear. Tinnitus can also result from age-related hearing loss, overexposure to loud noises, physical injury, Meniere’s disease, or neurological disorders. Treatment may include correcting the hearing loss with hearing aids, modifying lifestyle, or cognitive behavioral therapy (CBT).
• Vertigo: This is a perception of dizziness, which can be so severe that it makes it hard to stand or walk. It can result from Meniere’s disease, certain types of migraine headaches, infections, stroke, multiple sclerosis, or other neurological conditions. Treatment varies based on the underlying cause and can include medications or lifestyle changes.
• Meniere’s disease: Also known as idiopathic endolymphatic hydrops, this disorder of the inner ear can cause vertigo, tinnitus, fluctuations in hearing ability, pain, headaches, nausea, and other symptoms. Not entirely understood by doctors, this condition is thought to be related to changes in fluid levels within the inner ear. It's incurable and managed by symptomatic treatment. High blood pressure can contribute to Meniere’s disease. Medications may be prescribed to control blood pressure or combat headaches and nausea. Vestibular rehabilitation may help between attacks.
• Inflammation: Common infections of the ear include otitis media, an infection of the middle ear, and swimmer’s ear, an infection of the outer ear. Symptoms include ear pain, fever, and pressure in the ear. Bacterial infections can be treated with oral antibiotics or otic drops. If untreated, these conditions can cause lasting ear damage.
• Deafness: Hearing loss up to and including deafness include high tone deafness (sensorineural hearing loss), which occurs due to damage caused by overexposure to loud sounds. This type can be managed with the use of hearing aids or potentially cochlear implants for certain patients.
• Impacted cerumen: Excessive wax build-up (cerumen), can affect hearing and block passages between the outer and middle ear. This wax can be physically removed to treat the condition.
• Auricular hematoma: Bleeding within the ear can lead to a collection of blood that may impact the blood supply to the ear. This is often the result of trauma or injury, and it’s usually treated by careful draining, which should happen as soon as an injury occurs. Disruption of blood to the cartilage can lead to a cauliflower ear deformity.

Tests

A range of medical tests and examinations can be used to assess the ear and its functions.

Common tests:

• Otoscopy: This is the most commonly administered test. The healthcare provider examines the ear canal using a special tool called an otoscope. Infection of the middle and outer ear, as well as a host of other problems, can be seen visually.
• Pure-tone testing: Administered to assess overall audition, this test involves patients wearing headphones and having to raise a hand if and when they hear certain tones. The healthcare provider notes the sounds that a person can hear at different pitches.
• Speech testing: Hearing loss can also be tested by having patients repeat certain words or phrases played at specific volumes.
• Tympanometry: To test the motion and health of the tympanic membrane and middle ear pressure, healthcare providers insert a small probe into each ear, which will push air into each one.
• Acoustic reflex measure: Among the tests to assess the extent of hearing loss, the acoustic reflex measure seeks to stimulate some of the musculature in the middle ear. Low activity (or complete absence of response) is a sign of deafness or sensory loss.
• Static acoustic impedance: Rupture, holes, build-up of fluid, blockage, or other issues with the tympanic membrane are measured using this test, which looks at how much air there is in the ear canal.
• Auditory brainstem response (ABR) test: A test of inner ear function (as well as neural pathways from there), this examination involves using electrodes placed on the skin to measure brain activity in response to stimuli.
• Otoacoustic emissions (OAE) test: During this test, sounds are emitted by the vibrations of hair cells in response to a stimulus. The level of OAE is a reliable test of hearing ability. This test is performed by inserting a small, specialized probe into the ear that emits sounds and measures the response.