At 12:30 PM 11/20/04, Chris Barker wrote:
>But the inner ear's balance mechanism has not measure of gravity per
>se, by my understanding. It is possible I have forgotten something
>over the years, but my aeromedical training (grand name for survival
>skils in the air) mentioned the semi-circular canals, the fluid that
>they contain and the tiny hairs which bend with the fluid's movement as
>the head moves measured in 3 axes. In this way you can feel movement
>from a position; once the fluid stops moving you have reached a new
>starting point for movement. But there is nothing to tell you which
>way is up except for other cues: visual, external feel and of course
>the way your bloo's pressure feels in your head (if you are inverted
>the blood rushes to your head). Tell me if I'm wrong, because I need
>to know these things! ;-)
>
>A diagram is at this url: http://oto.wustl.edu/cochlea/intro1.htm
You are quite correct that it is a form of tri-axial accelerometer and
therefore cannot sense anything unless there is an acceleration. Gravity
is a constant force; i.e. it induces constant accleration in the absence of
another equal, counteractive force. In the _changing_ motion that most
people experience simply moving about self-propelled on the Earth's
surface, which requires acceleration to produce it, the inner ear
accelerometer (as with all accelerometers) senses the vector sum of all the
forces when they do not add to zero. Moving about while making the
photograph gives a sense of "up" and "down" . . . and therefore
"horizontal" as orthogonal to that. You are correct that it (the cochlea)
is not, and cannot be used alone; its data must be compared to and
integrated with that from other senses.
There is also data from musculo-skeletal system you mention . . . forces
exerted by muscles and felt by bone joints and internal organs . . . and
some portion of the muscle forces must counteract gravity lest we fall
over. The human brain, in all its magnificence, integrates the data from
all sources to get a sense of "up" and "down" relative to local
gravity. If the cochlea "accelerometer" fails, physical balance (a
counteraction to gravity to keep from falling over) gets all screwed up . .
. the brain has extreme difficulty without it, left only with what the
other senses provide. That's why I cited the inner ear as "notable" but
not exclusive (my Bad for not stating this outright). When properly
functioning it seems to enable considerable precision under most
conditions. I'm certain your aviation experience has been amply able to
create conditions that cause complete sensory confusion about "up" and
"down" as defined by local gravity, and is why certain cockpit
instrumentation is crucial to remaining aloft in defiance of local gravity
. . . to prevent having a Bad Day.
BTW, gravitational mass is equivalent to inertial mass, and therefore
gravitational and inertial forces and accelerations are equivalent. This
is the principle postulate of General Relativity (as an extension of
Special Relativity).
-- John Lind
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