The human ear is the anatomical structure responsible for hearing and balance. The ear consists of three parts, the outer, middle, and inner ears.

The outer ear collects sounds from the environment and funnels them through the auditory system. The outer ear is composed of three parts, the pinna (or auricle), the external auditory canal (or external auditory meatus), and the tympanic membrane (or eardrum).

The two flap-like structures on either side of the head commonly called earsare actually the pinnas of the outer ear. Pinnas are skin-covered cartilage,not bone, and are therefore flexible. The lowest portion of the pinna is called the lobe or lobule and is the most likely site for earrings. The pinnas ofmost humans cannot move, but these structures are very mobile in other mammals, such as cats and dogs.

The external auditory canal is a passageway in the temporal lobe of the skullthat begins at the ear and extends inward and slightly upwards. In the adulthuman it is lined with skin and hairs and is approximately one inch (2.5 cm)long.

The outer 1/3 portion of the canal is lined with a membrane containing ceruminous (ear wax producing) cells and hair cells. The purpose of the cerumen andhairs is to protect the eardrum (which lies at the end of the canal) by trapping dirt and foreign bodies and keeping the canal moist. In most individuals, cleaning of the external auditory canal (with Q-tips for example) is not needed. The inner portion of the external auditory canal contains no glands orhair cells.

The human tympanic membrane or eardrum is a thin, concave membrane stretchedacross the inner end of the external auditory canal much like the skin covering the top of a drum. The eardrum marks the border between the outer ear andmiddle ear. The eardrum serves as a transmitter of sound by vibrating in response to sounds traveling down the external auditory canal; it begins sound conduction in the middle ear.

In the adult human, the tympanic membrane has a total area of approximately 63 square mm. It consists of three layers that contribute to the membrane's ability to vibrate while maintaining a protective thickness. The middle point of the tympanic membrane (the umbo) is attached to the stirrup, the first of three bones contained within the middle ear.

The middle ear transmits sound from the outer ear to the inner ear. The middle ear consists of an oval, air-filled space approximately 2 cubic cm in volume. The middle ear can be thought of as a room, the outer wall of which contains the tympanic membrane. The back wall, separating the middle ear from the inner ear, has two windows, the oval window and the round window. There is a long hallway leading away from the side wall of the room, known as the eustachian tube. The brain lies above the room and the jugular vein lies below. Themiddle ear is lined entirely with mucous membrane (similar to the nose) and is surrounded by the bones of the skull.

The eustachian tube connects the middle ear to the nasopharynx. This tube isnormally closed, opening only as a result of muscle movement during yawning,sneezing, or swallowing. The eustachian tube allows for air pressure equalization, permitting the air pressure in the middle ear to match the air pressurein the outer ear. The most noticeable example of eustachian tube function occurs when there is a quick change in altitude, such as when a plane takes off. Prior to takeoff, the pressure in the outer ear is equal to the pressure inthe middle ear. When the plane gains altitude, the air pressure in the outerear decreases, while the pressure in the middle ear remains the same, causing the ear to feel "plugged." In response to this the ear may "pop." The popping sensation is actually the quick opening and closing of the eustachian tube, and the equalization of pressure between the outer and middle ear.

Three tiny bones (the ossicles) in the middle ear form a chain that conductssound waves from the tympanic membrane (outer ear) to the oval window (innerear). The three bones are the hammer (malleus), the anvil (incus), and the stirrup (stapes). These bones are connected and move as a link chain might, causing pressure at the oval window and the transmission of energy from the middle ear to the inner ear. Sound waves cause the tympanic membrane to vibrate,which sets up vibrations in the ossicles, which amplify the sounds and transmit them to the inner ear via the oval window. In addition to bones, the middle ear houses the two muscles, the stapedius and the tensor tympani, which respond reflexively (that is, without conscious control) to sounds.

The inner ear is responsible for interpreting and transmitting sound (auditory) sensations and balance (vestibular) sensations to the brain. The inner earis small (about the size of a pea) and complex in shape; its series of winding interconnected chambers has been compared to (and called) a labyrinth. Themain components of the inner ear are the vestibule, semicircular canals, andthe cochlea.

The vestibule, a round open space that accesses various passageways, is the central structure within the inner ear. The outer wall of the vestibule contains the oval and round windows (which are the connection sites between the middle and inner ear). Internally, the vestibule contains two membranous sacs, the utricle and the saccule, which are lined with tiny hair cells and attachedto nerve fibers, and which serve as the vestibular (balance/equilibrium) sense organs.

Attached to the utricle within the vestibular portion of the inner ear are three loop-shaped, fluid filled tubes called the semicircular canals. The semicircular canals are named according to their location ("lateral," "superior,"and "posterior") and are arranged perpendicularly to each other, like the floor and two corner walls of a box. The semicircular canals are a key part of the vestibular system and allow for maintenance of balance when the head or body rotates.

The cochlea is the site of the sense organs for hearing. The cochlea consistsof a bony, snail-like shell that contains three separate fluid-filled ductsor canals. The upper canal, the scala vestibuli, begins at the oval window; the lower canal, the scala tympani, begins at the round window. Between the two canals lies the third canal, the scala media. The scala media is separatedfrom the scala vestibuli by Reissner's membrane and from the scala tympani bythe basilar membrane. The scala media contains the organ of Corti, (named after the 19th-century anatomist who first described it). The organ of Corti lies along the entire length of the basilar membrane. The organ contains hair cells and is the site of the conversion of sound waves into nerve impulses, which are sent to the brain for auditory interpretation along Cranial Nerve VIII, also known as the Auditory Nerve.

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