ANATOMY OF THE HUMAN EAR
Sound enters the Ear Canal (Meatus) and vibrates the Eardrum (Tympanic Membrane). The eardrum moves the ossicles by exerting a twisting force on the Umbo of the Malleus (handle of the hammer) and this in turn causes the Incus (anvil) to move the Stapes (stirrup). Movement of the Stapes vibrates the fluid in the Cochea (inner ear). The cochlear fluid causes tiny stereocilia (stiff hairs) to bend and initiate a reaction in the so-called “hair cells” which send a message via the Auditory Nerve to the hearing centres in the brain,
The Semi-Circular Canals are the mechanism which detects head movement and helps us to keep our balance. The three canals are in different planes and can be thought of as a sort of three dimensional gyroscope. There is a curious connection between the balance mechanism and the eyes - as the head is turned from side to side the eyes remain pointed at the same position in space. When the eyes wobble with head movement it is a sign that the balance mechanism is not working properly and this can be used as a diagnostic test.
The complicated structure of the Ossicles (malleus, incus and stapes) in the Middle Ear is not an accident of nature. The inertia of the mass concentrated at the junction of the Malleus and Incus creates a swivel point and is positioned so that it cancels vertical vibration of the head caused by the larynx. If it did not you would be deafened by your own voice. At high sound intensities the additional force applied to the ossicles causes their twisting motion to change so that the twisting movement at the Stapes changes from horizontal to vertical. But when the narrow Stapes twists vertically it transmits very little movement to the cochlea fluid so it acts as a simple mechanical volume control to protect the cochlea from loud sound. The brain also sends a signal to a tiny muscle (Stapedius Tensor) that reduces movement of the Stapes at high sound levels. The normally closed Eustachian Tube opens when we swallow or yawn. It ensures that the air pressure is equal on both sides of the eardrum.
The snail-like structure of the Cochlea has a long thin membrane called the Basilar Membrane going from the base near the Stapes to the apex at the narrow end. The Basilar membrane is narrow near the base and wide near the apex and the width differential causes it to resonate at different points along its length depending on the frequency of the sound. The narrow end near the stapes responds to the higher frequencies and it is the nerves on this part of the Basilar Membrane that are most vulnerable to excessive noise.
The Semi-
The complicated structure of the Ossicles (malleus, incus and stapes) in the Middle Ear is not an accident of nature. The inertia of the mass concentrated at the junction of the Malleus and Incus creates a swivel point and is positioned so that it cancels vertical vibration of the head caused by the larynx. If it did not you would be deafened by your own voice. At high sound intensities the additional force applied to the ossicles causes their twisting motion to change so that the twisting movement at the Stapes changes from horizontal to vertical. But when the narrow Stapes twists vertically it transmits very little movement to the cochlea fluid so it acts as a simple mechanical volume control to protect the cochlea from loud sound. The brain also sends a signal to a tiny muscle (Stapedius Tensor) that reduces movement of the Stapes at high sound levels. The normally closed Eustachian Tube opens when we swallow or yawn. It ensures that the air pressure is equal on both sides of the eardrum.
The snail-