The ears are the sensory organs involved the hearing as well as in maintainance of the body eqilibrium. Automically, the ear is composed of the three divisions namely the external ear, middle ear and internal ear. The external ear consists of the pinna and external auditory meatus (canal). The pinna collects the sound waves coming from the environment. The external auditory canal is the circular tube which leads inside upto the tympanic membrane (ear drum). The skin of the pinna and the meatus contains fine hairs and wax secreting ceruminous glands. The tympanic membrane or ear drum is formed of connective tissues having outer skin cover and inner mucus membrane. Three ear ossicles called malleus, incus and stapes are present in the middle ear. The malleus is attached to the tympanic membrane and the stapes is connected to the oval window of the internal ear. The transmission of sound waves from external auditory canal to internal ear occur due to ear ossicles. An Eustachian tube is present in the middle ear, which connect the middle ear cavity with the pharynx. It helps in equalizing the air pressure on either side of the tympanic membrane.
The internal ear is fluid filled structure called labyrinth. It consists of the bony and the membranous labyrinths. The outer bony labyrinth is formed by the series of channels in which the membranous labyrinth. The outer bony labyrinth is formed by the series of channels in which the membranous labyrinth is present. The membranous labyrinth is filled with a fluid called endolymph. The membranous labyrinth consists o coiled cochlea, the reisser's and basilar membranes. These membranes divide the surrounding perilymph filled bony labyrinth into an upper scala vestibuli and a lower scala tympani. The space within cochlea is known as scala media is filled with endolymph. The scala vestibuli ends at the oval window at the base of cochlea. The scala tympani terminates at the round window which opens to the middle ear.
The organ of Corti is located on the basilar membrane. It contains the hair cells which act as auditory receptors. The hair cells are columnar ciliated cells present in rows. The basal ends of the hair cells are in close contact with the afferent nerve fibres while their apical end contains numerous cilia. A thin elastic membrane projects above the rows of the hair cells called tectorial membrane.
Above the cochlea, the internal ear also contains vestibular apparatus. It consists of three semicircular canals and the otolith organ formed of the sacculus and utriculus. The semicircular canals lie in different plane at right angles to each other and are suspended in the perilymph. The bases of canals are swollen called ampullae, which contain a projecting ridge known as crista ampullaris which contain hair cells. The sacculus and utriculus also have projecting ridge called macula. The crista and macula are the specific receptors or vestibular apparatus. They are responsible for maintenance of body posture and the balance.
Mechanism Of Hearing :
The sound waves are received by the external ear and sent to the tympanic membrane. The ear drum vibrates and the vibrations are transmitted by the chain of three ossicles to the oval window. The vibrations are further passed to the fluid of cochlea. The waves in the perilymph and endolymph induce movement in the basilar membrane. The hair cells of organ of Corti bend and pressed against the tectorial membrane. The nerve impulses are generated and sent to the afferent nerve fibres. The impulses are carried by the auditory nerves to the auditory centre of the brain, where the impulse are analyzed and the sound is perceived.