DEFINITION
The ear is the organ responsible for hearing and balance in
humans and many animals.
The anatomical structure of the ear is divided into three main
regions: the outer ear, middle ear, and inner ear.
1. Outer Ear (External Ear)
The outer ear gathers and channels sound waves toward the middle ear.
Structures:
Pinna (Auricle):
The visible, cartilaginous part of the ear that gathers sound.
It is shaped to help collect and direct sound waves into the
external auditory canal.
2. External Auditory Canal (Ear Canal)
A tube-like structure that extends from the concha to the tympanic membrane
(eardrum).
About 2.5 cm long and slightly S-shaped.
Lining:
The outer third contains skin, hair, and ceruminous glands that produce earwax
(cerumen), which protects the ear canal.
Function:
Channels sound waves to the tympanic membrane.
Protects the middle ear by trapping debris and maintaining a stable environment.
3. Tympanic Membrane (Eardrum)
A thin, semi-transparent membrane at the end of the external auditory canal.
Separates the outer ear from the middle ear.
Structure:
o Composed of three layers: outer (skin), middle (fibrous), and inner (mucosal).
Function- Vibrates in response to sound waves and transmits these vibrations to the ossicles
in the middle ear.
2. Middle Ear
The middle ear amplifies sound and transmits it to the inner ear.
Structures:
Ossicles (Three Small Bones):
o Malleus (Hammer): Attached to the tympanic membrane and transmits
vibrations to the incus.
o Incus (Anvil): Connects the malleus to the stapes..
o Stapes (Stirrup): The smallest bone in the body; its footplate fits into the oval
window of the cochlea.
Tympanic Cavity:
o An air-filled space containing the ossicles.
o Lined with mucous membrane.
Eustachian Tube:
o A canal connecting the middle ear to the nasopharynx.
o Equalizes air pressure on both sides of the tympanic membrane.
Oval Window:
A membrane-covered opening that connects the stapes to the cochlea.
Transmits vibrations from the stapes to the fluid in the cochlea.
Round Window:
A secondary membrane-covered opening in the cochlea.
Allows displacement of fluid in the cochlea to relieve pressure from incoming sound
waves.
3. Inner Ear
The inner ear consists of fluid-filled chambers and canals, collectively referred to as
the labyrinth, and is divided into two main parts: the bony labyrinth and
the membranous labyrinth.
1. Bony Labyrinth
The bony labyrinth is a rigid, hollow structure surrounds and
protects the membranous labyrinth and contains perilymph, a
fluid that cushions and supports the membranous labyrinth.
Divisions of the Bony Labyrinth:
Cochlea: For hearing.
Vestibule: Expanded part that forms the base for cochlea and semicircular canals.The
oval and round window opens into the vestibule.
Semicircular Canals: There are three semicircular canals that are present in three
planes.
. 2. Membranous Labyrinth
The membranous labyrinth lies within the bony labyrinth and is filled with endolymph, a
fluid that facilitates sensory signal transmission.
Divisions of the Membranous Labyrinth:
Cochlear Duct (Scala Media): For hearing.
Utricle and Saccule: Three semicircular canals open into utricle.
Semicircular Ducts: For rotational acceleration.
3. Cochlea
The cochlea is a spiral-shaped organ involved in hearing. It resembles a snail shell
o and makes about 2.5 turns.
Chambers of the Cochlea:
Vestibular Duct
o Located above the cochlear duct.
o Filled with perilymph.
o Begins at the oval window.
Tympanic Ducts
Located below the cochlear duct.
Filled with perilymph.
Ends at the round window.
Cochlear Duct
o Located between the vestibular duct and tympanic duct.
o Filled with endolymph.
Fluids of the Inner Ear
Perilymph:
o Found between the bony and membranous labyrinths rich in sodium.
Endolymph:
o Found within the membranous labyrinth rich in potassium.
Sensory Organs
Organ of Corti (Hearing):
o Located on the basilar membrane of the cochlear duct.
o Contains inner hair cells (primary sensory receptors) and outer hair
cells(amplify sound vibrations).
Maculae (Balance):
Found in the utricle and saccule.
Detects linear acceleration and gravitational pull.
Nerve Supply
Vestibulocochlear Nerve (Cranial Nerve VIII):
o Divided into two branches:
Cochlear Nerve: Transmits auditory signals from the cochlea to the
brain.
Vestibular Nerve: Transmits balance information from the utricle,
saccule, and semicircular canals.
