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Outer Ear: Comprising the pinna, ear canal, and outer layer of the eardrum. Sound enters the ear canal. At the eardrum, sound energy (air pressure changes) are transformed into mechanical energy of eardrum movement.	Pinna: The pinna is composed of cartilage and has a relatively poor blood supply. It presence on both sides of the head allows us to localize the source of sound from the front vs. the back. Our ability to localize from side to side depends on the relative intensity and relative phase of sound reaching each ear and the analysis of the phase/intensity differences within the brainstem. And, of course, the lobe of the pinna is handy for single or multiple hoops, loops, diamonds, etc.

	Cartilaginous Portion of External Ear Canal: The ceruminous and sebaceous glands in the cartilaginous portion of the ear canal combine to produce cerumen. See the link, Earwax for further discussion of this substance. The total length of the ear canal in adults is approximately one inch, which gives it a resonance frequency of approximately 3400 Hz, an important frequency region for understanding speech.

	Bony Portion of External Ear Canal: The bony portion of the ear canal is surrounded by the mastoid bone, occupies the inner third, and is very tender. Occasionally, completely in the canal (CHIC) hearing aids will reach as far as this portion of the canal. Outgrowths of bone, called exostoses (see the link on Surfer's Ear) occasionally will grow in the bony portion of the ear canal in response to cold-water exposure.

	Tympanic Membrane: The tympanic membrane actually has three layers, with the outer layer continuous with the skin of the outer ear canal. The upper portion of the TM is called the pars flaccida, while the lower portion is called the pars tensa. The central portion of the pars tensa provides the active vibrating area in response to sound. The TM is a continually growing structure, which allows it to close if it has a hole in it and to extrude a ventilation tube.

	Mastoid Air Cells: A portion of the temporal bone (surrounding the ear). Under normal circumstances this honeycombed area cells are filled with air. They can fill with fluid or pus when chronically infected.

Middle Ear: The middle ear serves as an impedance-matching transformer, matching the impedance of air in the ear canal to the impedance of the perilymph of the inner ear.	Malleus: The malleus is the most lateral (toward the side of the head) of the three ear bones (ossicles) in the middle ear. The long process of the malleus is attached to the inner layer of tympanic membrane. When the TM vibrates in response to sound, the malleus vibrates in concert.

	Incus: The incus is attached to the malleus, and so vibrates as the malleus vibrates. The long process of the incus is also attached to the head of the stapes. Because the long process of the incus is slightly shorter than the long process (manubrium) of the malleus, incoming sound is given a slight (2.5 dB) boost in energy. This is referred to as the lever advantage.

	Stapes: The stapes has a footplate and a superstructure. Its footplate is seated in the oval window, which separates the middle ear from perilymph of the inner ear. As the long process of the incus vibrates, so does the footplate of the stapes. Because the vibrating area of the tympanic membrane is larger than the area of the stapes, incoming sound is given a significant boost in energy of over 20 dB. This is referred to as the hydraulic advantage.

	Round Window Niche: The round window is located within the round window niche. The round window is the most basal end of the scala tympani, and allows release of hydraulic pressure of perilymph that is caused by vibration of the stapes within the oval window.

	Eustachian Tube: The eustachian tube connects the middle ear with the nasopharynx of the throat. This tube "opens" with swallowing or coughing to equalize pressure between the middle ear and ambient pressure that is found in the throat. Because the eustachian tube is shorter and has a more horizontal orientation in young children, it is less likely to open. Therefore, fluid collect more often within the middle ear space of young children. See our link on Serous Otitis Media for more detail on this.

*Inner Ear:* The last energy transformation occurs here. Mechanical energy is transformed into the traveling wave pattern of the basilar membrane.	Cochlea: A snail shaped structure that is the sensory organ of hearing. The vibrational patterns that are initiated by vibration of the stapes footplate set up a traveling wave pattern within the cochlea. This wavelike pattern causes a shearing of the cilia of the outer and inner hair cells. This shearing causes hair cell depolarization resulting in all or none neural impulses that the brain interprets as sound.

	Vestibular Labyrinth: The vestibular labyrinth is comprised of the saccule and utricle, sense organs of balance which inform our brain about our linear position in space. The horizontal, anterior, and posterior semi-circular canals are also part of our vestibular labyrinth, and inform our brains about rotational movement in space.

	VIII Nerve: The VIII nerve, also known as the auditory nerve, transmits information from the cochlear and vestibular labyrinth to our brains. Essentially, it is the transmission line from the sense organs to the central processor of the brain.

	Facial Nerve: The facial nerve is the VII cranial nerve, and travels in parallel with the VIII cranial nerve through the internal acoustic canal. The facial nerve innervates the face and provides both sensory and motor function to the face.

*Cochlear partition: consisting of the scala vestibuli, scala media, and scala tympani*
	Scala Vestibuli: One of the three partitions within the cochlea that is filled with perilymph, which attaches via the cochlear aqueduct (not shown) to the subarachnoid space. Perilymph, therefore, is the same as cerebro spinal fluid (CSF). Perilymph has a low potassium - K+ - concentration and a high sodium - Na+ - concentration.

	Scala Media: Also one of the three partitions within the cochlea, the scala media is filled with endolymph. In contrast to perilymph, endolymph has a high K+ concentration and a low Na+ concentration. The endolymph that is found within the scala media is continuous with the endolymph of the saccule, communicating through the ductus reuniens.

	Scala Tympani: The third partition, the scala tympani also contains perilymph. The scala vestibuli and the scala tympani are continuous with each other at the most apical end of the cochlear partition called the helicotrema.

	Organ Of Corti: Within the scala media, the organ of Corti is the sense organ of hearing. The outer and inner hair cells of the organ of Corti change vibrational energy into neural energy, that is transmitted via the VIII nerve to the brain.

	Reissner's Membrane: Separates the endolymph of the scala media from the perilymph of the scala vestibuli.

	Tectorial Membrane: A delicate, flexible, gelatinous membrane overlying the sensory receptive inner and outer hair cells. The cilia (hair like tufts that extend from both outer and inner hair cells) of the outer hair cells are embedded in the tectorial membrane. For inner hair cells, the cilia may or may not be embedded in the tectorial membrane. When the cochlear partition changes position in response to the traveling wave, the shearing of the cilia is thought to be the stimulus that causes depolarization of the hair cells to produce an action potential.

	Stria Vascularis: A highly vascularized layer of cells that is thought to secrete endolymph.

Organ Of Corti: consisting of the tectorial membrane, outer hair cells, inner hair cells, basilar membrane, and supporting cells
	Outer Hair Cells: There are three rows of approximately 12000 outer hair cells. Although they are much greater in number than the inner hair cells, they receive only about 5% of the innervations of the nerve fibers from the acoustic portion of the VIII nerve. These cells contain muscle-like filaments that contract upon stimulation and fine tune the response of the basilar membrane to the movement of the traveling wave. Because of their tuned response, healthy outer hair cells will ring following stimulation. This "ringing" provides the sound source for Otoacoustic Emissions.

	Inner Hair Cell: There is one row of approximately 3500 inner hair cells . These cells receive about 95% of the innervations from the nerve fibers from the acoustic portion of the VIII nerve. These cells have primary responsibility for producing our sensation of hearing. When lost or damaged, a severe to profound hearing loss usually occurs.

	Tunnel Of Corti: A space filled with endolymph that is bordered by the pillars of Corti and the basilar membrane.

	Basilar Membrane: A ribbon like structure upon which rests the organ of Corti. The basilar membrane is less wide but stiffer at its basal end toward the oval window, but is wider and more compliant toward its apical end away from the oval window. Because of these two gradients of size and stiffness, high frequencies are coded at the basal end with low frequencies progressively coded toward the apical end.

	Osseous Spiral Lamina: A delicate bony plate that extends from the mid portion of the coiled cochlea, helping to separate scala vestibuli from scala tympani.

	Pillars Of Corti: Supporting cells that bound the tunnel of Corti. The tunnel o Corti runs the entire length of the cochlear partition.

	Spiral Ganglia: The cell bodies of nerve fibers that innervate the inner and outer hair cells.

	Inner Sulcus: Inert supporting cells to the inner hair cells.

	Deiter's Cells: Inert supporting cells of the outer hair cells.

	Hensen's Cells: Inert supporting cells.

	Claudius' Cells: Inert supporting cells.

	Tectorial Membrane: See above