Why do cats have a secondary membrane in their eyes?

[The Grimmus]

I have noticed while looking at my cat that there is a secondary membrane (first beign the mucus membrane covering his eye it is a little har do expsplain so if you have a cat pick it up and look at it's eye you will notice a white area beteen his outside eye and the side of it's nose. It it will become more apprant when the cat is at rest where it will unfold from the corner and cover most of the cat's eye. My question is why dose a cat have it i can see when my cat is sleeping his eyes are closed so what makes a membrane of this nature exist.

 

[solipsist]

I'm no expert, but I believe it is called a nictating membrane. Many animals have them. Even humans have the vestige of one.

I don't know what the third eyelid is for. I've seen some suggestions in relation to amphibians that it protects the eye whilst swimming whilst still being semi transparent.

Slightly more information is available here;
http://en.wikipedia.org/wiki/Nictating_membrane