Do Animals Have a Third Eyelid?

Many animals have a third eyelid which is called the nictitating membrane, or haw. Its function is to keep the animal’s eyes clear of debris and mucus, help produce tears and protective antibodies that fight infection. In fact, most animals including dogs, cats, birds, reptiles, fish and other mammals have a third eyelid. Humans and pigs are excluded from this in having only two eyelids per eye.

More about the nictitating membrane:

  • Camels are some of the largest mammals to have a nictitating membrane which helps protect their eyes from sand in the desert.
  • The third eyelid is critical for most predatory animals as it allows the eyes to remain open without blinking and obstructing vision for prolonged periods of time.
  • For reptiles and amphibians, the nictitating membrane allows for clear vision below water.
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Discussion Comments


I guess a dog's third eyelid is different a cat's third eyelid. My dog's third eyelid became visible because of an eye infection. The vet said that not all breeds have a nictitating membraine (third eyelid) and if the third eyelid becomes visible, it's a sign of eye pain.

My dog's third eyelid disappeared again as soon as the infection was treated. So if any of you can see your dog's third eyelid, see a vet. It's not always normal like it is with cats.


@ankara-- You can see it if you pull your cat's lower eyelid down while he's sleeping. You may be able to see it without doing anything when your cat wakes up or when he goes into bushes where his eyes need extra protection.

I see my cat's third eyelid all the time. It's a whitish tissue that appears from the sides of her eyes. Her third eyelid became clearly visible when we had to give her a sedative while traveling. It's like she couldn't control her third eyelid and it was half out for several hours until the medication wore off.


Cats have a third eyelid?! Is it possible to see it?

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