The sense of equilibrium is provided by structures in the inner ear, collectively known as the vestibular apparatus. Movements of the head cause fluid within these structures to bend extensions of sensory hair cells, and this bending results in the production of action potentials.
The sense of equilibrium, which provides orientation with respect to gravity, is due to the function of an organ called the vestibular apparatus. The vestibular apparatus and a snail-like structure called the cochlea, which is involved in hearing, form the inner ear within the temporal bones of the skull. The vestibular apparatus consists of two parts: (1) the otolith organs, which include the utricle and saccule, and (2) the semicircular canals (fig. 10.11).
The sensory structures of the vestibular apparatus and cochlea are located within the membranous labyrinth (fig. 10.12), a tubular structure that is filled with a fluid similar in composition to intracellular fluid. This fluid is called endolymph. The membranous labyrinth is located within a bony cavity in the skull, the bony labyrinth. Within this cavity, between the membranous labyrinth and the bone, is a fluid called perilymph. Perilymph is similar in composition to cerebrospinal fluid.
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