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inner ear[¦in·ər ′ir]
a membranous labyrinth; main part of the organs of hearing and equilibrium in vertebrates and man. The inner ear is filled with a fluid—endolymph—and embedded in the cartilaginous or bony skeletal labyrinth. The slitlike cavity between the inner ear and skeletal labyrinth is filled with perilymph; in terrestrial vertebrates this cavity is connected with the lymphatic cavities of the head through the perilymphatic duct. Two openings, or windows, are formed in the skeletal labyrinth of terrestrial vertebrates. The base of an auditory ossicle (stapes) enters the oval window from the middle ear. Below it is the round window, which is covered with an elastic membrane to permit the fluid in the inner ear to shift when the stapes moves.
The inner ear originates as a depression in the ectoderm in the posterior part of the head. As the embryo develops, the rudiment of the inner ear assumes the form of a vesicle connected with the external environment by a thin endolymphatic duct and later completely separated from the ectoderm. The rudiment of the inner ear is subsequently differentiated into upper and lower portions that are joined together. Three semicircular canals appear in the upper portion in all vertebrates (in Cyclostomata, one or two canals). A swelling— ampulla—is formed at one end of each of the canals. The remaining part of the upper portion of the inner ear, which connects the semicircular canals to each other, is called the oval saccule (utricle). A round saccule (sacculus) is formed in the lower portion of the inner ear; it has a peculiar swelling called the lagena, or cochlea.
The sensory (receptor) epithelium of the inner ear is distributed unevenly. In the oval and round saccules it forms so-called acoustic spots (maculae)—sensory cells with short hairs and acoustic (ampullar) crests that protrude in the form of plates into the inner cavity of the ampullae of the semicircular canals; the sensory cells of the crests have long hairs. In most vertebrates, the cochlea has a receptor apparatus in the form of a primary acoustic papilla that is formed when the round saccule separates from the acoustic spot. In fish, amphibians, and some other vertebrates, there is one small acoustic spot near the junction of the oval and round saccules. In amphibians, the main acoustic papilla separates from the primary acoustic papilla, and the corresponding part of its wall forms the so-called main (basal) membrane. In reptiles, the prominence of the saccule is more strongly developed. In crocodiles, it becomes a long, somewhat curved cochlear canal; the development of the main membrane with sensory hair cells on it causes the cochlear canal to separate into upper (scala vestibuli) and lower (scala tympani) portions. A cover plate develops over the main membrane and hair cells as the receptor acoustic apparatus becomes more complex. Birds and monotrematous mammals have a curved cochlear canal separated from the round saccule by a narrow canal. The organ of hearing is most highly developed in viviparous mammals and man. The cochlear canal becomes even more elongated and is twisted in a spiral with IVi to five turns. The primary acoustic papilla disappears, and the main acoustic papilla becomes the organ of Corti.
The bases of the receptor cells in all the structures of the inner ear come into contact with the short processes (dendrites) of the nerve cells whose bodies are grouped together in the so-called cochlear ganglion, while the long processes (axons) of the nerve cells form the acoustic nerve, which transmits excitation to the vestibular and acoustic centers of the brain. The endolymph of the inner ear contains calcareous deposits characteristic of the organs of equilibrium— otoliths (statoliths) of different sizes that are often replaced by a mass of tiny granules, or otoconia. In Cyclostomata, the calcareous deposits of the inner ear appear in the protoplasmatic reticulum in the form of otoconia, which may coalesce into an otolith. In most fish and all terrestrial vertebrates, the large otoliths are contained in sacs, while the small calcareous inclusions are frequently found in other parts of the inner ear as well (for example, in the endolymphatic duct). The calcareous inclusions and cupulae in the ampullae of the semicircular canals and the accumulations of ciliated cells and endolymph on which they act make up the structural and functional foundation of the vestibular apparatus.
REFERENCESShimkevich, V. Kurs sravnitel’ noi anatomii pozvonochnykh zhivotnykh, 3rd. ed. Moscow-Petrograd, 1922.
Shmal’gauzen, I. I. Osnovy sravnitel’ noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Prosser, L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. (Translated from English.)
Kisliakov, V. A., and I. V. Orlov. “Fiziologiia vestibuliarnoi sistemy (sovremenoe sostoianie problemy).” In the collection Voprosy fiziologii sensornykh sistem [issue 1]. Moscow-Leningrad, 1966.
G. N. SIMKIN