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From Cecil's Mailbag by the Straight Dope Science Advisory Board
Dear Straight Dope:
Why do dog and cat's eyes shine, and why doesn't the same thing
happen to humans?
--Davidbw1
SDSTAFF Dex replies:
Ed Zotti, Cecil's long-time editor, has written his own book,
called KNOW IT ALL, which is aimed at answering Cecil-type
questions for young folks. Since this isn't available online (but
is available at your local or on-line bookstore, and Ed needs the
money), we thought we'd answer.
Cats' and deer's (and other nocturnal animals') eyes shine because
of a special irridescent layer called the tapetum lucidum, behind
the retina and around the optic nerve, that acts like a mirror.
Light passing into the cat's eyeball bounces offf the tapetum
lucidum, giving the animal the ability to reflect (within its own
eye) what little light is incoming, thus allowing it see in near
darkness.
The eyes shine at night because that's when the pupils are dilated
wide enough for the tapetum lucidum to be visible.
And humans' eyes don't shine because they don't have a tapetum
lucidum. We're not night-hunters, despire the evidence of lots of
guys hanging around bars way after dark.
--SDSTAFF Dex
Straight Dope Science Advisory Board
Cecil's Mailbag is researched and written by members of the Straight Dope Science Advisory Board, Cecil's online auxiliary. Although the SDSAB does its best, these articles are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.
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