How to examine a cat's nictitating membrane
Your cat, yes your innocent cat, has (gasp) six* eyelids! That's right, little Furbles has nictitating membranes. One of the myriad fascinating little tweaks made in the basic design by a capricious God.
*Thanks to rootbeer277 - I had thought 'four' but in fact the proper answer is 'six.'
I can hear the cry of alarm from cute-addicted cat-lovers everywhere. Ewww! Six eyelids? Gross! Fear not. Mrs. Norris's little ocular secret isn't disgusting; rather, it's a slightly cute method of protecting the eyes from all manner of potential harm. One use of said membrane in land-based hunters is to allow them to 'blink' without losing sight of prey, since the nictitating membrane is translucent (and almost transparent to infrared, in which band cats can see); also, to blink without risking contamination if the fur or whiskers around the eye are wet and/or dirty, and to provide extra protection for the eye in case of trauma to the eyelid (claws, twigs, beaks, etc).
You're lying. How could Garfield have an additional eyelid (much less two - one extra on each side!) without my noticing? Easy - he just keeps it out of sight. The nictitating membrane usually only closes when the eyelid is already lowered. Then how can I see it? Aha - now we're getting somewhere.
The MOST IMPORTANT STEP in the following is #1. At least, if you like your epidermis intact.
- Make sure your cat is in a good mood, or at least receptive to a scratch between the ears!
- with the cat facing you, begin by scritching the top of the head with four fingers on one hand. Pay special attention to the base of the ears. Your cat will close his or her eyes in sheer transported bliss if he or she is relaxed.
- Now, with the thumb of the same hand, stroke the cat's nose from the middle of the nose up to the forehead. Give them a good five or six strokes, with some pressure behind them. Most cats will push up against your hand at this point.
- Now, quickly, remove your hand and induce the cat to open its eyes. This won't be hard, they'll probably open 'em right away to find out why you stopped, you silly human. For a couple of seconds, until it is retracted below the pupil, you should be able to see the nictitating membrane slide down off the eye where it was raised to protect the pupils from potential damage from your thumb.
Silly, I know, but interesting biology. Plus, as a side bonus, most cats enjoy having their heads stroked, and may reward you with a nuzzle or a purr. | 
