The ear is the organ of hearing and equilibrium (balance) in vertebrates. The ear converts sound waves in the air to nerve impulses that are relayed to the brain, where they are interpreted as sound rather than as mere vibrations. The innermost portion of the ear maintains Biological Equilibrium through the so-called vestibular apparatus, which includes the semicircular canals. Any change in the position of the head or body causes the apparatus to transmit nerve impulses to the brain, evoking muscular apparatus is basically alike in all vertebrates; hearing structures evolved later in the higher vertebrates.

Many vertebrates also have specialized sense organs, rather than ears, for hearing and equilibrium. Crickets and spiders, for example, have membranes much like sounding boards on the legs. Moths have a similar rudimentary ear on the thorax that apparently serves as a warning system for attacks by bats.

Structure of the Ear
The ear in humans and most other mammals consist of three parts; the outer, middle, and inner portions. The outer ear, or pinna, is the structure commonly called the ear. It is a skin covered flap of elastic cartilage projecting from the side of the head and funneling sound into the middle ear. The middle ear is an air filled chamber containing the eardrum, or tympanic membrane, and connected to the pharynx by the Eustachian tube, thus equalizing the pressure on the two sides of the eardrum. The inner ear contains the sensory receptors for hearing; these receptors are in a fluid filled chamber called the cochlea. The middle and outer ears serve only to receive and amplify sound waves and occurs only in amphibians and mammals, whereas the inner ear is present in all vertebrates.

In fish, the inner ear is primarily an organ of equilibrium and lacks a cochlea; nor do fish have outer or middle ears. Amphibians posses a middle ear cavity; a thin membrane separating the middle ear from the outside becomes the eardrum. The pinna occurs only in mammals. In birds and reptiles, the eardrum may be in a depression below the surface of the head.


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