The Ear: Structure, Hearing and Balance

Ear : The human ear -- the stato-acoustic organ -- performs two distinct jobs, hearing and maintaining body equilibrium, and is anatomically divided into external, middle and internal parts. The external ear consists of the pinna (an immovable, cartilage-supported structure in humans that collects sound and funnels it in), the auditory canal (which carries the sound to the eardrum, and whose skin bears fine hair and wax-secreting glands), and the tympanic membrane (eardrum) itself, a delicate structure of connective tissue covered by skin outside and mucous membrane inside that transmits the incoming vibrations to the middle ear.

The middle ear holds a chain of three ossicles -- malleus (hammer), incus (anvil) and stapes (stirrup, the smallest bone in the body) -- which amplify the eardrum's vibrations and pass them on to the cochlea; a short eustachian tube connects the middle ear to the pharynx and equalises air pressure on both sides of the eardrum. The internal ear consists of the labyrinth -- a bony labyrinth (filled with perilymph) enclosing a membranous labyrinth (filled with endolymph), the latter made of three parts: vestibule, semicircular ducts and cochlea.

The cochlea, a coiled duct, holds three fluid-filled chambers separated by Reissner's membrane and the basilar membrane -- the upper scala vestibuli and lower scala tympani (both perilymph-filled) flanking the middle, endolymph-filled scala media. Sitting on the basilar membrane (the floor of the scala media) is the pea-sized organ of Corti, whose sensory epithelium of hair cells (bearing stiff microvilli called stereocilia) contacts an overlying gelatinous tectorial membrane, with supporting cells filling the gaps between hair-cell rows; this organ transduces sound vibrations into nerve impulses.

Internal Ear and the mechanism of balance : Besides the cochlea, the inner ear's vestibular apparatus -- three semicircular ducts set at right angles to each other, plus the utriculo-saccular region with its otolith organ -- handles balance. Each semicircular canal's base has an ampulla containing a sensory crista, while the vestibule holds two sensory maculae (of the utricle and saccule); in both, hair-cell cilia project into a gelatinous otolithic membrane containing calcium-carbonate-and-protein granules (otoliths/otoconia). Cristae detect dynamic balance (movement), while the maculae of utricle and saccule detect static/linear balance (head position relative to gravity), and because the three canals lie in different planes, movement in any direction can be detected. …