The human ear is an organ of hearing and balance. It is called as
statoacoustic organ. It has three parts; external ear, middle ear and inner
ear.
External ear: - The external ear consists of two
parts; pinna and auditory canal.
The pinna or helix or auricle is present on the side of head
and is broad cartilaginous structure. Its posterior end without cartilage is
called as lobule. Its hollow centre is called as concha. Its movement is
regulated by auricular muscles which is vestigial in human.It collects,
receives sound wave and sends the sound waves into the auditory canal.
The external auditory canal or external auditory meatus extends from the deepest concha andextends upto tympanic membrane or ear drum. It is about 2.5 cm long.Its wall consists of cartilage and bone. It consists of hairs and ceruminous gland which secretes ear wax or cerumen. The hairs and cerumen helps to trap dust particles.
The tympanic membrane
is also called as ear drum which is a membranous structure and transmits sound
wave from auditory canal to middle ear.
Middle ear: - It is a small air filled cavity with three
small bones called as ear ossicles. It is present in temporal bone of skull so
is also called as temporal cavity. The ear ossicles are malleus, incus and stapes.
The malleus is hammer shaped and remains articulated with
Tympanic membrane in one side and with incus in another side.
The incus is anvil shaped and remains articulated with
malleus in one side and with stapes in another side.
The stapes is stirrup shaped and remains articulated with
incus in one side and with oval window in another side.
These ear ossicles transmit the vibration of sound wave from
ear drum to the oval window. They also intensify sound wave by 20 to 25 times.
The middle ear remains connected with pharynx by a short tube
called as Eustachian tube. It helps to equalize the air pressure on both side
of ear drum.
Inner ear: - it consists of a membranous labyrinth
lying inside bony labyrinth with fluid perilymph. It is present in the petrous
part of temporal bone. The membranous labyrinth consists of vestibule,
semicircular canals and cochlea.
Vestibule is a central sac like structure and has two parts upper
utriculus and lower sacculus. They remain connected with each other by a narrow
utriculosaccular canal. It remains filled with endolymph.In both utriculus and
sacculus a sensory spot called as macula is present which consists of hair
cells or sensory cells and supporting cells. The hair cells consist otolithic membrane with otolith which is small crystals of
calcium carbonate. These cells are responsible for static balance.
Semicircular
canals:- There
are three semicircular canals which are right angle to each other. They are
anterior, posterior and lateral canals. The anterior and posterior open from
common duct called as crus commune. They arise from vestibule and end into
vestibule. The end of each canal has swelling called as ampulla.The ampulla
consists of sensory spot called as crista which is responsible for dynamic
equilibrium.
Cochlea:- it is organ of hearing. It is a
highly coiled tube of about 35mm length and is divided in to 3 canals,
vestibular canal or scala vestibuli, middle canal or scala media and tympanic
canal or scala tympani.
The Reissner's membrane separates the vestibular and middle canal while
the basilar membrane separates the middle canal and tympanic canal. The
vestibular and tympanic canals are filed with prilymph while the middle canal
is filled with endolymph. The vestibular canal is connected with the oval
window while tympanic canal is connected with the round window. On the surface
of basilar membrane sensory hair cells are present and on the top of these
cells tectonic membrane is present. All these three, sensory hair cells,
basilar membrane and tectonic membrane forms organ of corti which is
responsible for hearing.
Functions of Ear
1. Hearing:
- The sound wave is
received by pinna and passes through auditory canal and strike the ear drum.
The vibration of ear drum is transmitted to ear ossicles which magnify and
transmits the vibration up to oval window. The vibration of oval window causes
vibration of perilymph in vestibular canal and it causes to and fro movement of
round window. This vibration of perilymph causes vibration of basilar membrane,
sensory hair cells and tectonic membrane. This vibration in organ of corti is
converted in to nerve impulse and is transmitted up to brain by auditory nerve
and the sound is heard.
2. Balance:
- The balance of body
during locomotion and movement is called as dynamic equilibrium. The change in
position of head with respect to gravity is detected by cristae present in the
ampulla of semicircular canals during movement.
The balance
of body during rest is called as static equilibrium. The maculae present in
utriculus and sacculus are responsible for static equilibrium.