Hemispheric control....

Richard Norman rsnorman at mediaone.net
Wed Jan 24 09:24:42 EST 2001


Thanks for the reference.  I certainly didn't mean to imply that you
actually held to this idea (that the retinal image reversal is related
to the contralateral control in vertebrates).  You did clearly indicate
that you had seen it proposed and that was the "best" yet.

The real problem is that there really is no explanation for the
contralateral
connections, so anything, no matter how far fetched, seems better than
nothing.

Frankly, I like the old idea based on the fact that many invertebrates
(the protostomes, like arthropods, annelids)  have ventral longitudinal
nerve cords while the chordates (like vertebrates and us) have dorsal
longitudinal nerve cords.  The idea is that the connection between the
brain and the longitudinal cord rotated 180 degrees sometime in evolution,
the chordate body developed "upside down" and that switched left and
right as well as dorsal and ventral.  The notion that the hearts of those
invertebrates are dorsal while ours is ventral might be related.  Of
course this can't be true.  There are also major developmental
differences in these two groups of organisms.  The last common
ancestor must have been something more along the lines of a flatworm
or possibly a roundworm with multiple nerve cords.   But the hypotheses
enjoyed some success early in the 1900's.

I like to use this as an example of the principle that, no matter how
weird an idea you can come up with, you can find somewhere in
biology an organism that tops it.  And, also, that evolution does
not drive organisms to "perfection".  We are only "good enough
to beat anything else".


"MS" <marshmallow5 at yahoo.com> wrote in message
news:8Xrb6.4162$mo2.473354 at news1.news.adelphia.net...
> I found it:
>
> Why are vertebrate nervous systems crossed?
> Capozzoli NJ
> Med Hypotheses 1995 Nov 45:5 471-5
>
> Abstract
> Contralateral central nervous control may be an evolutionary consequence
of
> dependence on the image-forming eye, especially in large organisms. As a
> result of the topological transformation of the visual stimulus in the
> pupillary eye, the external environmental hemispace impinges directly upon
> the contralateral internal organismal hemispace. Selective pressure leads
to
> the development of central connections capable of the most rapid and
precise
> functional association of the internal milieu with the organism's
> environment. The consequence is contralateral central sensorimotor
control.
> Previous hypotheses are discussed, including those based on bilaterality,
> binocularity the optic chiasm and avoidance behaviors.
>
>







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