structure of human ear
Answers
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NCERT based Answer for Class 11 Biology Students
The ears perform two sensory functions: hearing and maintenance of body balance (equilibrium).
External ear:
Pinna consists of elastic cartilage. It collects sound waves from environment.
External auditory canal (meatus) directs sound waves to eardrum.
Note: External auditory canal lies in temporal bone of skull.
There are fine hairs and wax secreting sebaceous glands in the skin of pinna and the auditory meatus.
Combination of hairs and ear wax helps prevent dust and pathogens from entering the ear.
Eardrum or tympanic membrane lies between the external auditory canal and middle ear.
Sound waves cause eardrum to vibrate, which, in turn, causes malleus to vibrate.
Middle ear:
Eustachian tube connects middle ear with pharynx. It equalizes pressure between middle ear and oral cavity.
Three ear ossicles are malleus (hammer shaped), incus (anvil shaped) and stapes (stirrup shaped). They are connected with synovial joints.
Malleus is attached to inner surface of eardrum.
Stapes lies against a membrane–covered opening between middle and inner ear called the oval window (fenestra ovalis).
Just below the oval window is another opening called the round window (fenestra rotandus).
Ear ossicles transmit and amplify vibrations from tympanic membrane to oval window.
Internal ear:
Internal ear is also called labyrinth because of its complicated series of canals.
It consists two main divisions: an outer bony labyrinth that encloses an inner membranous labyrinth.
Bony labyrinth is a series of cavities in the temporal bone of skull.
Space between bony and membranous labyrinth is filled with watery fluid called perilymph.
Membranous labyrinth consists of vestibular apparatus (organ of equilibrium) and cochlea (organ of hearing).
Vestibular apparatus further consists of three semicircular canals and an otolith organ.
Three semicircular canals project from utricle of the vestibule. They lie at right angle to each other.
One end of each canal is a swollen enlargement called ampulla (plural: ampullae).
Receptor cells in ampullae are called cristae (singular: crista). These cells detect rotational movements.
When head moves, endolymph in semicircular ducts flows over hair cells (receptors) and bends them. The movement of hair cells generates nerve impulses which pass along auditory cranial nerve.
Utricle and saccule contain receptors called maculae. These receptors are meant for static equilibrium and linear movement.
Utricle and saccule together are called otolith organ as they have crystals of CaCO3 called otoliths; in their fluid.
Cochlea:
It is a spiral body and is divided into three channels: upper scala vestibuli, middle scala media and lower scala tympani.
Reissner’s membrane lies between scala vestibule and scala media whereas basilar membrane separates scala media from scala tympani.
Cavities of scala vestibule and scala tympani are filled with perilymph.
Cavity of scala media is filled with endolymph.
Note: Scala vestibuli and scala tympani communicate with each other through a narrow canal called helicotrema; at the end of spiral of cochlea.
Scala media communicates with saccule of vestibular apparatus.
Oval window communicates with scala vestibuli whereas round window communicates with scala tympani.
Organ of Corti (the organ of hearing) rests on the basilar membrane in scala media.
It contains hair cells which are the receptors for auditory sensations.
These hair cells synapse with auditory cranial nerve.
Above hair cells is a thin elastic membrane called tectorial membrane.
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