Why do dogs’ and cats’ eyes shine?

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

Send questions to Cecil via cecil@straightdope.com.

STAFF REPORTS ARE WRITTEN BY THE STRAIGHT DOPE SCIENCE ADVISORY BOARD, CECIL’S ONLINE AUXILIARY. THOUGH THE SDSAB DOES ITS BEST, THESE COLUMNS ARE EDITED BY ED ZOTTI, NOT CECIL, SO ACCURACYWISE YOU’D BETTER KEEP YOUR FINGERS CROSSED.