lets learn - structure of ears

8/12/2019 Lets learn - Structure of Ears

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The ear is the organ that detects sound . It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system .

Often the entire organ is considered the ear, though it may also be considered just the visible portion. In most mammals, the visible ear is a flap of tissue that is also called the pinna (or

auricle in humans) and is the first of many steps in hearing . Vertebrates have a pair of ears placed somewhat symmetrically on opposite sides of the head. This arrangement aids in theability to localize sound sources.

The shape of the outer ear of mammals varies widely across species. However the innerworkings of mammalian ears (including humans') are very similar.

Outer ear

Main article: Outer ear

The outer ear is the most external portion of the ear. The outer ear includes the fleshy visibleouter ear, called the pinna or auricle, the ear canal, and the outer layer of the tympanicmembrane, also known as the ear drum .[1] :855 The outer ear is the only visible portion of the earin humans and almost all vertebrates, and consequently the word "ear" may be used to refer tothe pinna alone.

Human ear (from Descent of Man )

The auricle consists of the curving outer rim (the helix), the inner curved rim (the antihelix), andopens into the ear canal , properly called the external acoustic meatus . The tragus protrudes and

partially obscures the ear canal. The ear canal stretches for a distance of about 1 inch, andconsists of an inner portion surrounded by bone , and an outer portion surrounded by cartilage . The skin surrounding the external acoustic meatus contains glands that produce ear wax(cerumen) . The ear canal ends at the external surface of the ear drum (tympanic membrane) [1] :856
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Two sets of muscles are associated with the outer ear; the intrinsic and extrinsic muscles. Insome mammals these muscles can adjust the direction of the pinna .[1] :855 In humans these musclehave very little action if any at all. [citation needed ] These muscles are supplied by the facial nerve , which also supplies sensation to the skin of the ear itself, as well as the ear cavity. The vagusnerve , auriculotemporal nerve of the mandibular nerve , lesser occipital branch of C2 , and the

greater occipital nerve branch of C3 all supply sensation to portions of the outer ear andsurrounding skin .[1] :855

The auricle consists of a single piece of fibrocartilage with a complicated relief on the anterior,concave side and a fairly smooth configuration on the posterior, convex side. The Darwiniantubercle , which is present in some people, lies in the descending part of the helix andcorresponds to the true ear tip of the long-eared mammals. The lobule merely containssubcutaneous tissue .[2] In some animals with mobile pinnae (like the horse), each pinna can beaimed independently to better receive the sound. For these animals, the pinnae help localize thedirection of the sound source. Human beings localize sound within the central nervous system,

by comparing arrival-time differences and loudness from each ear, in brain circuits that are

connected to both ears. This process is commonly referred to as EPS, or Echo PositioningSystem.

Middle ear

Middle Ear

Malleus

Tensor Tympani

Incus

Stapedius

Labyrinth

Stapes

Auditory Canal Tympanic Membrane

(Ear Drum)

Eustachian Tube
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Tympanic cavity

Components of the middle ear

Main article: Middle ear

The middle ear is an air-filled cavity behind the tympanic membrane, includes three bones(ossicles) : the malleus (or hammer), incus (or anvil), and stapes (or stirrup). The middle ear alsoconnects to the upper throat via the Eustachian tube .[1] :858

The three ossicles transmit sound from the tympanic membrane to the ventricles of the ear. Themalleus is connected to the tympanic membrane, and transmits vibrations of the membrane

produced by sound waves . The malleus has a long process (the manubrium, or handle) that isattached to the mobile portion of the eardrum. The incus is the bridge between the malleus andstapes. The stapes connects to the oval window, and is the smallest named bone in the human

body. The three bones are arranged so that movement of the tympanic membrane causesmovement of the malleus, which causes movement of the incus, which causes movement of the

stapes. When the stapes footplate pushes on the oval window, it causes movement of fluid withinthe cochlea (a portion of the inner ear) .[1] :858 The ossicles help in amplification of sound waves by nearly thirty times.

In humans and other land animals the middle ear (like the ear canal) is normally filled with air.Unlike the open ear canal, however, the air of the middle ear is not in direct contact with theatmosphere outside the body. The Eustachian tube connects from the chamber of the middle earto the back of the nasopharynx. The middle ear is very much like a specialized paranasal sinus , called the tympanic cavity; it, like the paranasal sinuses, is a hollow mucosa-lined cavity in theskull that is ventilated through the nose. The mastoid portion of the human temporal bone, whichcan be felt as a bump in the skull behind the pinna, also contains air, which is ventilated through

the middle ear. Normally, the Eustachian tube is collapsed, but it gapes open both with swallowing and with positive pressure. When taking off in an airplane, the surrounding air pressure goes from higher(on the ground) to lower (in the sky). The air in the middle ear expands as the plane gainsaltitude, and pushes its way into the back of the nose and mouth. On the way down, the volumeof air in the middle ear shrinks, and a slight vacuum is produced. Active opening of theEustachian tube is required to equalize the pressure between the middle ear and the surroundingatmosphere as the plane descends. The diver also experiences this change in pressure, but withgreater rates of pressure change; active opening of the Eustachian tube is required morefrequently as the diver goes deeper into higher pressure.

Abnormalities such as impacted ear wax (occlusion of the external ear canal), fixed or missingossicles, or holes in the tympanic membrane generally produce conductive hearing loss . Conductive hearing loss may also result from middle ear inflammation causing fluid build-up inthe normally air-filled space. Tympanoplasty is the general name of the operation to repair themiddle ear's tympanic membrane and ossicles. Grafts from muscle fascia are ordinarily used torebuild an intact ear drum. Sometimes artificial ear bones are placed to substitute for damagedones, or a disrupted ossicular chain is rebuilt in order to conduct sound effectively.
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lets learn - structure of ears

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