Section XI. THE SPECIAL SENSE OF EQUILIBRIUM (BALANCE)
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 stand-
ing 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
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
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.
11-35. SACCULUS AND UTRICULUS
a. The sacculus and the utriculus are two sac-like portions of the membranous
labyrinth. They are filled with endolymph.
b. On the wall of each sac is a collection of special hair cells known as the
macula, which serves as a receptor organ for static and linear kinetic gravitational
forces. The saccular macula and the utricular macula are oriented at more or less right
angles to each other. For the pair of maculae in the membranous labyrinth of the right
side, there is a corresponding pair in the labyrinth of the left side. Information from all of
these maculae is sent into the brain for continuous sensing of the position of the head in