Re: Sclerotic rings

["Matthew Bonnan" <mbonnan@hotmail.com>]

Dann Pidgon writes:

"I had always assumed that sclerotic rings were merely there to support
the eye in animals where the orbit was much larger than the eye itself.
In mammals the eye fits the orbit snuggly, hence we have no need for
additional support. Is this not the case?"

> From Hildebrand, 1995:

"[In reptiles and birds] the eye is large, particularly in birds; eyes of 
some hawks are larger than those of humans.  The sclera [white part of the 
eye] is stiffened by a cartilaginous cup behind and by a ring of about 15 
small, overlapping bones on the forward wall, where the eyeball might 
otherwise be distorted by the ciliary muscles [the muscles which control the 
shape of the lens].  These [ciliary] muscles are striated ... and, hence, 
faster in action than those of other vertebrates.  The small, soft, somewhat 
flattened lens has a peripheral annular pad that makes firm contact with the 
ciliary apparatus.  Accomodation is active and instant; muscular contraction 
causes the lens to bulge in front ..."

So, basically, the function of the sclerotic ring is to prevent distortion 
of the eyeball by ciliary muscles when a reptile or bird focuses on 
something.  Because they have powerful ciliary muscles, the sclerotic ring 
braces their large eyes.

Hope this helps,

Matt Bonnan
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