EAR

vertebrate organ of hearing and equilibrium that detects and analyzes noises by transduction (or the conversion of sound waves into electrochemical impulses) and maintains sense of balance (equilibrium). The hearing apparatus of vertebrates is surprisingly uniform, considering the range of body types and habitats of vertebrates. In the evolution from jawless fishes to mammals and birds, the ear has become progressively complex and increasingly able to discriminate sound frequencies. The basic sound-sensitive structures of the fish ear are three interconnecting, fluid-filled cavities derived from the lateral-line system. In amphibians the ear is basically like that of fishes, except that frogs have an additional cavity, the air-filled middle ear, with a membrane (the eardrum, or tympanic membrane) at the skin surface and connected to the inner ear by a small bone, the columella. In reptiles one structure is somewhat elongated (the start of a true cochlea, or inner ear), and the eardrum is withdrawn into the head. Snakes have lost the middle ear and the external opening but have retained the columella; they hear airborne sound poorly but are especially sensitive to vibrations of the ground. The ear of birds is basically reptilian, but the cochlea is longer and more sensitive. Pitch discrimination is far more acute in birds, many of which perceive frequencies well above the range of human hearing. The mammalian ear consists of three main parts: the outer ear, comprising a largely cartilaginous external projection called the pinna (lacking in cetaceans and phocid seals and reduced or lacking in many burrowing mammals) and the auditory canal; the middle ear, comprising the tympanic cavity, which is separated from the external ear by the eardrum (tympanic membrane) and contains the auditory ossicles, a chain of three tiny bones (the malleus, or hammer; the incus, or anvil; and the stapes, or stirrup); and the inner ear, or labyrinth, comprising a complex cavity in the petrous portion of the temporal bonethe bony labyrinthand an inner membranous labyrinth, a delicate system of fluid-filled ducts and sacs (the semicircular canals, the vestibule, and the cochlea) that occupies only a small part of the available space. In hearing, sound waves in the air are directed by the pinna into the auditory canal. Vibrations of the eardrum, positioned across the end of the canal, are conducted and intensified by the auditory ossicles through the tympanic cavity to the oval window, a membrane in the outer wall of the vestibule. Through the oval window, the intensified vibrations are transmitted to the fluid of the inner ear. It is in the coiled cochlea that the vibrations of this fluid are converted by the hair cells of the organ of Corti into nerve impulses that are transmitted to the brain. The inner ear also functions, independently of hearing, as the organ of equilibrium. Acceleration in space and rotation of the body cause an inertial lag of fluid in the semicircular canals, which stimulates sensory hairs. Continued stimulation of the hairs after body rotation has stopped is experienced as dizziness. Static pressures, such as gravitational forces, are detected by hair cells in two sacs within the vestibule, permitting perception of the orientation of the head. Each of the ear's three sections can be afflicted by a particular set of diseases. The most common cause of progressive hearing loss in human beings is otosclerosis, a disease of the bone enclosing the inner ear that causes fixation of the base of the stirrup in the oval window; this fixation prevents the conveyance of sound vibrations to the inner ear. The cause of otosclerosis is unknown, but it can often be corrected surgically. A common inner ear disease is congenital nerve deafness, which is caused by defective hearing nerves in the cochlea that usually become evident at or soon after birth. Resulting hearing impairment is most often severe, and there is no known medical or surgical treatment for the disease. The gradual decay of the hearing nerve in the inner ear due to old age is called presbycusis. While the ear is, strictly speaking, a vertebrate organ, invertebrates possess certain analogous structures for hearing. Many arthropods have tactile hairs that are stimulated by sound energy, in addition to other forms of energy. Certain roaches and crickets possess cercal organs (tufts of hairs supplied with nerves) that are located at the tip of the abdomen and are sensitive to a wide range of sound frequencies. Tympanal organs, found on the abdomen, thorax, or first legs of certain members of the insect orders Orthoptera (crickets), Homoptera (cicadas), and Lepidoptera (moths), consist of a membrane at the body surface or in a trachea (respiratory tubule). Changes in the tension of the tympanal membrane induced by impinging sound waves are sensed by chordotonal, or scolophore, organs, highly specialized nervous structures that record tension changes in a variety of situations in the insect body.