a. Posture. Posture is the specific alinement of the body parts at any given time. Humans can assume an infinite variety of postures. However, the truly erect posture is unique to humans.

b. Equilibrium. Equilibrium is the state of balance of the body. An erect standing human has a highly unstable equilibrium and therefore can easily fall. Through a variety of sensory inputs (visual, etc.) and postural reflexes, the body is maintained in its erect posture.

c. Stimulus-Gravitational Forces. A primary sensory input for equilibrium consists of gravitational forces. This input is received by the membranous labyrinth within the internal ear. The gravitational forces are of two types: static, when the body is standing still, and kinetic, when the body is moving in either linear (straight) or angular directions.

d. Membranous Labyrinth. The specific portions of the membranous labyrinth involved are the two sac-like structures--the sacculus and the utriculus. Each of these two structures has an area of special hair cells called the macula. In addition, there are three semicircular ducts located within the osseous semicircular canals of the temporal bone of the skull. Each semicircular duct has a crista, a little ridge of hair cells across the axis of the duct.

e. "Body Sense." All of the various sensory inputs related to the maintenance of equilibrium and posture are integrated within the brain as "body sense." Correct information is sent to the muscles of the body by means of specific postural reflexes in order to maintain the proper posture.

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