Via the cochlear aqueduct.

Organ of Corti

The organ of Corti is the sense organ of hearing and is situated on the basilar

membrane of the cochlea. It consists of a complex arrangement of supporting and

hair cells. The basilar membrane and the tectorial membrane, which is in contact

with the hair cells of the organ of Corti, are an integral part of the

structure. In its ascent from the basal coil of the scala media to the apical

coil structural changes are found in the organ, consisting of an increase in the

width of the basilar membrane and in its fibrous tissue content; the tectorial

membrane becomes larger; the tunnel of Corti increases in height and width; and

the nerve supply to the hair cells decreases.

Blood Vessels

The main supply comes from the labyrinthine artery which arises from the basilar

or anterior inferior cerebellar artery. The veins unite to form the labyrinthine

vein which opens into the inferior petrosal sinus or the sigmoid sinus. Small

veins pass via the aqueducts of the vestibule and cochlea to the superior and

inferior petrosal sinuses respectively.

Nerve Supply

The vestibulocochlear (acoustic) nerve is formed by cochlear and vestibular

parts in the internal acoustic meatus from which it emerges on the lateral side

of the sensory root of the facial nerve and enters the brain stem between the

pons and the medulla. The cochlear part is composed of fibres which are the

central processes of bipolar cells in the spiral ganglion in the modiolus of the

cochlea. The peripheral processes of the ganglion cells pierce the bony spiral

lamina to reach the inner and outer hair cells of the organ of Corti. Other

fibres follow a spiral course on the internal part of the basilar membrane. The

vestibular part consists of the processes of the bipolar cells of the vestibular

ganglion in the internal acoustic meatus. From the superior part of the ganglion

fibres pass via the superior vestibular nerve, the utricle, the ampullae of the

frontal and horizontal semicircular ducts and the anterior part of the macula of

the saccule. The inferior part of the ganglion sends fibres via the inferior

vestibular nerve to the macula of the saccule and the ampulla of the sagital

semicircular duct.

PHYSIOLOGY OF HEARING

External and Middle Ears

Airborne sound consists of vibrations of the atmosphere, that is, of alternate

phases of condensation and rarefaction. The purpose of the auditory apparatus is

to convert these vibrations in air to vibrations in the inner-ear fluids, and

then to nerve impulses to be transmitted along the auditory nerve to the higher

centres of hearing.

The auricle collects the sound waves to some extent, and they pass along the

external acoustic meatus to the tympanic membrane which is set in motion. The

vibrations of the tympanic membrane are transmitted to the malleus, incus and

stapes. The malleus and incus rotate around a common fulcrum and transmit

vibrations to the stapes in the oval window, causing vibrations to be set up in

the endolymphatic and perilymphatic compartments of the inner ear. The

conversion of sound from air into fluid is accomplished by the middle-ear

structures. To some extent the lever system of the malleus and incus helps, but

the main effect comes from the tympanic membrane. This system increases the

sound pressure at the footplate to a degree which causes the fluids of the inner

ear to vibrate. The stapes moves in a rocking rather than a piston motion and,

as fluids cannot be compressed, these vibrations are transmitted to the round

window membrane. This reciprocal action of the oval and round windows is

essential. In the normal ear the presence of the tympanic membrane and an

air-containing middle ear prevents the sound-pressure waves from reaching the

round window and opposing the outward movement of the round window membrane.

This protection of the round window is lost where there is a large perforation

of the tympanic membrane, and this is one of the factors which may produce

deafness.

The tympanic membrane is at its most efficient when the air pressure in the

external acoustic canal and the middle ear is equal. This is achieved by the

eustachian tube which normally opens during each act of swallowing. In this way

the air pressure on both sides of the tympanic membrane can be kept equal. The

stapedius and tensor tympani muscle seem to have a protective function; loud

sound causes a reflex contraction of the muscles and this serves to stiffen up

the conducting mechanism and possibly to protect the inner ear from damage.