The original function of the ear in vertebrate evolution was not hearing, it was equilibrium—coordination and balance. Only later did vertebrates evolve the cochlea, middle-ear structures, and auditory function of the ear. In humans, the receptors for equilibrium constitute the vestibular apparatus, which consists of three semicircular ducts and two chambers—an anterior saccule (SAC-yule) and a posterior utricle33 (YOU-trih-cul) (see fig. 16.11).
The sense of equilibrium is divided into static equilibrium, the perception of the orientation of the head when the body is stationary, and dynamic equilibrium, the perception of motion or acceleration. Acceleration is divided into linear acceleration, a change in velocity in a straight line, as when riding in a car or elevator, and angular acceleration, a change in the rate of rotation. The sac-cule and utricle are responsible for static equilibrium and the sense of linear acceleration; the semicircular ducts detect only angular acceleration.
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