A reflective layer of cells behind the retina of the eye, present in many mammals. Familiar animals with a tapidum lucidum include cats and dogs.

Photons (light particles) sometimes aren't absorbed by the retina; instead they go right through, as it is a very thin layer of cells. The tapidum lucidum reflects the light back through the retina so it has a chance to be detected again. This greatly increases the light sensitivity of the eyes.

The tapidum lucidum is also responsible for the well-known myth that cats' eyes glow in the dark. The truth is you are simply seeing ambient light reflecting off this reflective layer.

Unfortunately, humans didn't evolve a tapidum lucidum, so while our cats are effortly bounding about in the dark, we are tripping over things and walking into walls. The fact that we are diurnal mammals probably contributed to this.

When you really get down to it, Homo sapiens really sucks in the great cage match that is natural selection, and the only lucky break we've got is the three or so pounds of axon and myelin encased in our laughably fragile cranial vaults. I mean, we can't fly, and even if we could, we don't have the turbocharged cardiopulmonary system that is the jewel in the crown of the order Aves. We can't swim very well, no matter what Elaine Morgan and Michael Phelps may think about between bong hits. We can run really slowly for really long periods of time in the midday heat, but any decent ambush predator needs only a second or two to catch up with you, noonday sun be damned.

We're the gangly, sniffling Jerry Lewises of the animal kingdom; we're weak, slow, have no natural weapons and pretty fragile. Any one of our ancestors (and many of our extant relatives) would have given us a wedgie, stolen our lunch money and stuffed us in our locker without even breaking a sweat.

Well, to all of these disadvantages, add poor night vision.

Ever wonder why your cat's eyes glow when you shine a light into them? It has nothing to do with diabolical cahootery, and has everything to do with their retinas. Cats, and in fact, many other vertebrate animals, have a layer of cells lining the inside of their eyes, which reflect light back onto the retina, greatly improving their low light vision. It basically recycles photons that would otherwise smack themselves against the eye's cones and rods and have that energy wasted after just a single triggering of a retinal cell. This layer of cells is what the nerdiest of the nerdy call the 'tapetum lucidum', which is Latin for 'bright tapestry'. Personally, I'd have called it the 'dator gulielmuum', or 'giver of the willies', but that's just me. Think on this the next time a pack of ravening hyenas tear through your village at twilight.

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