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

 

[ravenprp]

The secondary membrane, also known as the nictitating membrane, is a thin, translucent layer of tissue that covers the inner corner of a cat's eye. It is a natural part of a cat's anatomy and is present in many other animals, such as birds and reptiles.

The purpose of this membrane is to protect the eye from potential harm, such as dust or debris, while still allowing the cat to see. It also helps to keep the eye moist and clean by spreading tears over the surface of the eye.

In cats, the secondary membrane is controlled by a small muscle that allows it to move quickly over the eye when needed. This is why you may see it unfold from the inner corner of the eye when your cat is at rest or sleeping. It is a reflexive action that helps to protect the eye while the cat is in a vulnerable state.

Overall, the secondary membrane is a natural and important part of a cat's eye anatomy. It serves as a protective barrier and helps to maintain the health and function of the eye. So, the next time you see your cat's secondary membrane in action, know that it is just another amazing adaptation that helps keep your feline friend safe and healthy.