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.

Frequently Asked Questions

What is the third eyelid in animals?

The third eyelid, also known as the nictitating membrane, is an additional protective eye covering found in some animals. It is a translucent or opaque flap of tissue that can move horizontally across the eyeball, providing moisture, protection from debris, and can help with vision underwater.

Which animals have a third eyelid?

Many animals have a third eyelid, including birds, reptiles, fish, and mammals. Notably, domestic animals such as cats and dogs possess this feature. It's particularly common in animals that need extra eye protection, such as those that burrow, swim, or hunt.

How does the third eyelid benefit animals?

The third eyelid serves several functions: it helps to moisten and clean the eye surface without obstructing vision, provides protection from environmental hazards, and in some species, it is essential for maintaining eye health during hunting or swimming by shielding the eyes from debris and water.

Can humans see an animal's third eyelid?

Yes, humans can often see an animal's third eyelid, especially when it is partially or fully extended across the eye. In cats and dogs, for example, the third eyelid can sometimes be seen as a white or pale pink membrane at the inner corner of the eye.

Is the third eyelid a sign of illness in pets?

While the third eyelid is a normal anatomical feature, its prominence can sometimes indicate illness. If a pet's third eyelid is persistently visible or there are changes in its appearance, it could be a sign of conditions like dehydration, eye infections, or other health issues, and veterinary attention should be sought.

Do any primates have a third eyelid?

Most primates, including humans, do not have a functional third eyelid. However, the plica semilunaris of the conjunctiva in humans is thought to be a vestigial remnant of a nictitating membrane, indicating a common evolutionary ancestry with species that do have a fully functional third eyelid.

More Info: animal.discovery.com

Discussion Comments

ysmina

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.

candyquilt

@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.

bluedolphin

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

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