Re: Sclerotics and nose bending (fwd)

["Mickey P. Rowe" <mrowe@indiana.edu>]

First as an administrative point I fear I need to state that messages
from John V. Jackson should only come to the list through me.  If
anyone wants details on that feel free to contact me or him; for the
moment that's all I'll say here.

Second, to address the subject about which John (via Betty) wrote.
Accommodation is *not* defined as a change in shape or position of the
lens.  Accommodation is defined as any mechanism that allows an eye to
bring closer objects into focus on the retina.  Animals generally
accommodate by changing the shape or position of the lens within the
eye, but some animals (e.g., snakes) accommodate by squeezing the eye
and thus causing it to change shape as well as move the lens within
it.  Conversely, some birds (e.g., cormorants) drastically change the
shape of their lenses when they go underwater so that they can
compensate for the fact that their corneas no longer provide much
focussing power (it's because we can't do this that we can't see well
underwater without trapping a layer of air around our eyes like in a
diving mask).  This change in shape is not accommodation in the
traditional sense (so far as I know anyways) because the purpose is
different.

Anyhoo, sclerotic rings haven't received much focus (ouch) from what I
can tell, so G. L. Walls 1942 bibl...  book, _The Vertebrate Eye and
Its Adaptive Radiation_ is probably still about the best source you're
going to find on the subject.  What John wrote about eye shape is
partly true in that the ossicles give the eye a kink at the interface
between the cornea and the sclera.  It was also partly false in that
the shape of the rest of the eye varies quite a bit amongst animals,
particularly among birds.  Bird eyes range from almost flat though
almost spherical to long and tube-shaped.  You can't really tell this
just by looking at the exterior of the animal.  And I have no idea
what John was on about with respect to sclerotic rings "allow[ing]
accommodation [...] to produce a sharp image all over the retina at
once" or even why he thinks it would be a good idea to move the lens
into the center of the eye.  John, if you have any references
describing a) how the former statement is true in animals with
sclerotic rings and/or b) how it's a problem for animals without them
I'd appreciate it if you'd contact me about it.  Back to the subject
at hand, Walls states that the function of the scleral ossicles is to
prevent the kink at the scleral corneal-surface from bulging outward
as intra-ocular pressure increases during accommodation in reptiles
(excepting snakes).  In non-reptiles intra-ocular pressure does not
drastically increase with accommodation because the musculature
involved is different.

Hope this information is of some use.

--
Mickey Rowe     (mrowe@indiana.edu)