need info: semicircular canals

Jerry Larson jlarson at hsc.usc.edu
Mon Jun 10 11:31:19 EST 1996


In article <4pgi8j$c72 at news0-alterdial.uu.net>, kbonelli at capecod.net wrote:


> The 3-canals are known to be at right angles to each other; one "horizontal" 
> and the other two making a 45-degree angle with the sagittal and coronal 
> planes of the skull, in humans (Curthoys et al 1977).
> 
> The question is this:  Is this anatomical geometry the same in animals (dogs, 
> cats, horses) as it is in humans?  The question has come up because of the 
> erect posture of humans compared to the quadrupedal posture of animals.  Has 
> the vestibular system undergone a 90-degree rotation in humans, or is it
still 
> the same as in animals? 

This is a very interesting question, and of course difficult to look up; I
just tried a couple of books.  I would think there would not be such a
rotation, simply because the _head_ hasn't rotated through space, has it? 
The top of the head is still on top, the eyes are still in front, the ears
still on the sides and the mouth at the bottom front, in habitual standing
position or walking.  It's basically the _torso that has rotated to come
under the head.  There is a 45 degree bend in the human neuraxis at about
the brainstem level; I never thought about this before either, but I guess
it makes sense to think that that bend represents the spinal cord bending
(bottom end forward) to get under the head, and 45 degrees is enough; as
opposed to the immediately intuitive idea that the _telencephalon has bent
forward.  So you have 180 degrees in a quadruped and 235 in humans. 
Another question for comparative anatomy, then, is what about other
primates?

I hope when you find out the answer(s) you'll let us know.

Jerry
mailto:jlarson at usc.edu



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