Ear

• 1. Introduction

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  The stapes is the smallest named bone in the human body, and it sits deep inside the ear, passing along vibrations that the brain will eventually register as sound. The ear is the organ that enables hearing, and in mammals it also governs body balance through the vestibular system. In humans it is described as having three parts: the outer ear, the middle ear, and the inner ear. Each of those parts hides a different kind of machinery. How does a vibration in the air become a feeling of music in the temporal lobe of the brain? How does the same organ that lets a person hear also let them stand upright without falling? And why have humans, for thousands of years, hung jewelry from a structure this delicate?

• 2. Three Chambers

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  The outer ear consists of the auricle, the visible fleshy portion, and the ear canal, which stretches for about 1 inch, or 2.5 cm. The first part of that canal is surrounded by cartilage, while the section nearer the eardrum is surrounded by bone called the auditory bulla, formed by the tympanic part of the temporal bone. Because the auricle is the only visible portion, the word ear often refers to it alone. Its curving outer rim is the helix, its inner curved rim the antihelix, and the tragus protrudes to partially obscure the canal. A small bump called Darwin's tubercle is sometimes present in the descending part of the helix, corresponding to the ear-tip of mammals.

  The air-filled tympanic cavity of the middle ear is separated from the ear canal by the eardrum, also called the tympanic membrane. Inside it sit the three ossicles: the malleus, or hammer, the incus, or anvil, and the stapes, or stirrup. These bones receive, amplify, and transmit sound from the eardrum toward the inner ear. The middle ear connects to the upper throat at the nasopharynx through the pharyngeal opening of the Eustachian tube.

  The inner ear sits within the temporal bone in a complex cavity called the bony labyrinth. A central area, the vestibule, holds two small fluid-filled recesses, the utricle and the saccule. These connect to three semicircular canals angled at right angles to each other, and to the cochlea, a spiral shell-shaped organ. Together these structures form the membranous labyrinth, the wet interior where the senses of hearing and balance actually begin.

• 3. From Air To Signal

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  Sound waves striking the eardrum cause it to vibrate, and the malleus, resting on the membrane, picks that vibration up. The vibration travels along the incus to the stapes, whose wide base rests on the oval window. As the stapes moves, the oval window stirs fluid within the cochlea. The round window then bulges outward into the middle ear by a corresponding amount, giving the inner ear fluid somewhere to go. Across this chain, the ossicles amplify sound waves by nearly 15 to 20 times.

  The organ of Corti inside the cochlea is studded with hair cells, mechanoreceptors whose microscopic protein filaments project into the fluid. When sound moving through that fluid pushes the filaments, the cells open to the potassium-rich endolymph and depolarise. This creates an action potential that travels along the spiral ganglion and the auditory portion of the vestibulocochlear nerve to the temporal lobe of the brain, where it is registered as sound. Two small muscles, the tensor tympani and the stapedius, reflexively contract to dampen excessive vibration.

  The human ear can generally hear frequencies between 20 Hz and 20 kHz, a span called the audio range. Sound below 20 Hz is infrasound, and sound above 20 kHz is ultrasound. Locating where a sound comes from is a separate trick. The brain compares the arrival-times and intensities reaching each ear, working in circuits within the superior olivary complex and the trapezoid bodies.

• 4. Standing Upright

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  Providing balance, whether a person is moving or standing still, is also a central function of the ear. The ear handles two kinds: static balance, which lets a person feel gravity, and dynamic balance, which lets them sense acceleration. The two systems use different parts of the same labyrinth.

  Static balance comes from the utricle and the saccule. Cells lining their walls carry 50 to 70 small filaments each, plus one large filament called the kinocilium, all covered by a fine gelatinous layer. Embedded in that layer are otoliths, tiny formations of calcium carbonate. When a person moves, the otoliths shift, bending the filaments, opening ion channels, and sending an action potential along the vestibulocochlear nerve.

  Dynamic balance comes from the three semicircular canals, set orthogonal to each other. At the end of each canal a slight enlargement called the ampulla holds cells whose filaments gather in a central cupula. When the head changes acceleration, the inertia of the fluid changes, the pressure on the cupula shifts, and ion channels open. Dynamic balance also keeps the eyes steady during movement through the vestibulo-ocular reflex, the same circuit that lets the semicircular canals enable eye tracking when a person is on the move.

• 5. Building The Ear

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  Around the 22nd day of embryonic development, the inner ear is the first part to take shape, forming from the ectoderm out of two thickenings called otic placodes on either side of the head. Each placode sinks below the ectoderm, becomes an otic pit, then an otic vesicle, and is eventually wrapped in mesenchyme to build the bony labyrinth. Around the 28th day, parts of the otic vesicle begin forming the vestibulocochlear nerve as bipolar neurons. By roughly the 33rd day the vesicles differentiate, with posterior regions becoming the utricle and semicircular canals and anterior regions becoming a rudimentary saccule that gives rise to the cochlea around the sixth week.

  The middle ear and its components develop from the first and second pharyngeal arches. The malleus and incus derive from the first arch and the stapes from the second, though all three ossicles develop from the neural crest. Many of the genes guiding inner ear formation belong to the homeobox family, including Pax, Msx, and Otx genes, with structures like the cochlea regulated by Dlx5/Dlx6, Otx1/Otx2, and Pax2 under the master gene Shh, which is secreted by the notochord.

  The ear canal, unlike the inner and middle ear, originates from the dorsal portion of the first pharyngeal cleft and is fully expanded by the end of the 18th week. The auricle forms as a fusion of six hillocks, three from the lower part of the first pharyngeal arch building the tragus, crus of the helix, and helix, and three from the upper part of the second arch building the antihelix, antitragus, and earlobe. These outer ears begin in the lower neck and migrate upward as the mandible forms, settling level with the eyes. After birth the ears grow quickly until about age nine, then continue widening in circumference by about 0.5 millimeters a year for life.

• 6. When Hearing Fails

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  Normal hearing levels sit at 20 decibels, and hearing worse than that counts as partial or total hearing loss. Damage to the outer ear or the ossicles produces conductive hearing loss, which can stem from a canal blocked by earwax, ossicles that are fixed together or absent, or holes in the eardrum. Tympanoplasty repairs the eardrum and ossicles, often using grafts from muscle fascia, and ossiculoplasty rebuilds a disrupted ossicular chain. Damage to inner ear structures such as the cochlea or auditory nerve instead produces sensorineural hearing loss.

  A hearing aid can help when inner ear hair cells are damaged, amplifying the surrounding sound. For more severe loss a cochlear implant becomes an option, while a bone-anchored hearing aid, or BAHA, conducts sound directly to the cochlea. Vertigo, the inappropriate perception of motion, traces back to vestibular dysfunction. In benign paroxysmal positional vertigo an otolith is displaced into a semicircular canal, where it rests on the cupula and signals movement that is not happening.

  The inner ear faces two main damage mechanisms in industrialised society, both injuring hair cells: noise trauma and ototoxicity from drugs and other substances. The National Institute for Occupational Safety and Health has reported that an estimated 11% of people have hearing difficulty, with 24% of that attributable to occupational noise. The National Health and Nutrition Examination Survey found roughly twenty-two million US workers, about 17%, reported exposure to hazardous workplace noise. Tinnitus, the hearing of sound when none is present, is itself not a disease but a symptom, one of whose most common causes is that same noise-induced loss.

• 7. Marked And Imagined

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  Earlobes have been pierced and hung with jewelry for thousands of years, and in some cultures stretched and enlarged so that larger plugs can be slid into a fleshy gap. The lobe tears fairly easily, whether from the weight of heavy earrings or from an earring snagging on a sweater. Punishment by the ear runs just as deep. In Roman times an injured party who could not settle a dispute was allowed to drag a refusing witness by the ear, the origin the source gives for the French expression se faire tirer l'oreille, literally to have one's ear pulled and figuratively to take a lot of persuading.

  Protruding ears, present in about 5% of ethnic Europeans, have been considered unattractive in Western societies, especially when asymmetric. The first surgery to reduce prominent ears was published by Ernst Dieffenbach in 1845, with the first case report in 1881. Because ears are individually almost unique and keep their proportions for life, they have been used for forensic identification since the 1950s.

  Pointy ears carry their own long imaginative life, appearing on the French croquemitaine, the Brazilian curupira, and creatures from ancient Greek and medieval European art through elves, faeries, hobbits, orcs, vampires, and the Vulcan and Romulan races of Star Trek. Science gave the ear two famous moments of its own. Georg von Békésy, a Hungarian biophysicist born in Budapest, won the 1961 Nobel Prize in Physiology or Medicine for his research on the cochlea. The Vacanti mouse, with an ear-shaped cartilage structure grown on its back from seeded cow cartilage cells, drew publicity and controversy in 1997 as an alternative to ear repair and grafting.

• 8. Beyond The Human

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  Horses can aim each pinna independently to better catch a sound, and the symmetrical placement of ears on either side of a vertebrate's head aids that localisation everywhere. All mammals carry three auditory ossicles. The complex ridges on the inner surface of some mammalian ears focus the sounds of prey during echolocation, working like the acoustic equivalent of a Fresnel lens in animals including the bat, aye-aye, lesser galago, bat-eared fox, and mouse lemur.

  In the elephant, the fox, and the rabbit, the ear acts as an essential thermoregulator, its blood vessels close to the surface. Domestic rabbits show five types of ear carriage, some bred for exaggerated length at a health risk controlled in certain countries. Charles Darwin studied skull abnormalities in a half-lop rabbit in 1868. Among marine mammals, earless seals form one of three groups of Pinnipedia. Some large primates, including gorillas, orangutans, and humans, keep undeveloped ear muscles that are vestigial yet large enough to identify, evidence of homology between related species.

  Only vertebrates have ears, though many invertebrates detect sound by other means. Insects use tympanal organs, located on the head or elsewhere depending on the family. The female cricket fly Ormia ochracea carries tympanal organs on each side of her abdomen, linked by a thin bridge of exoskeleton so they work as a tiny directional pair. A timing difference as small as 50 billionths of a second lets her home in on a singing male cricket and parasitise it. Caterpillars of the Monarch butterfly respond to airborne vibration through a pair of 450 micrometre long hairs called trichoid sensilla on the upper prothorax, reacting to the flight sounds of predators and parasitoids.