What Are The White Lids On Cats Eyes?

Cats have a third eyelid, also known as a nictitating membrane, which is a protective fold of skin in the corner of their eyes. This translucent or semi-transparent eyelid functions like windshield wipers, helping keep the eye moist and clean. While the third eyelid is normal anatomy in cats, sometimes excessive visibility of this membrane can indicate an issue with the eye or surrounding structures.

In this article, we will examine the anatomy of the feline third eyelid, understand its key functions, learn about common third eyelid conditions, explore treatment options, and provide tips for everyday care and myths surrounding this unique structure.[“Third eyelid – Definition, Meaning & Synonyms”, Vocabulary.com] [“Nictitating membrane”, Wikipedia] [“THIRD EYELID Definition & Usage”, Dictionary.com]


The third eyelid, also known as the nictitating membrane, is located in the inner corner of a cat’s eye, under the lower lid. It is a thin layer of cartilage and conjunctive tissue covered by mucous membrane. The nictitating membrane can move horizontally across the eye to cover and protect the cornea.

The movement of the third eyelid is controlled by the retractor bulbi muscle and other eye muscles (https://www.sciencedirect.com/topics/medicine-and-dentistry/nictitating-membrane). When these muscles contract, the third eyelid extends across the eye. It also moves passively when the eye itself retracts into the eye socket.

In addition to protection, the nictitating membrane also helps spread tears and lubricate the eye when it moves across the surface of the eye.


The third eyelid, also known as the nictitating membrane, serves several important functions in cats and other animals that have it. The main functions are:

  • Protection – The third eyelid can sweep across the eye to protect it from dust, debris, and mechanical damage. It provides an extra layer of protection in addition to the upper and lower eyelids.
  • Tear distribution – The third eyelid helps to spread tears across the surface of the eyeball, keeping it moist and lubricated. The third eyelid contains the third eyelid gland which produces about 1/3 of the aqueous portion of tears in cats.
  • Appearance – When partially drawn across the eyeball, the third eyelid gives cats that typical “sleepy” look. It is often visible when cats are sleepy,relaxed, or showing contentment.

The nictitating membrane is semi-transparent so it can protect the eye without completely obscuring vision. It allows cats to keep their eyes open and maintain awareness of their surroundings while still keeping the eyes moist and protected (https://en.wikipedia.org/wiki/Nictitating_membrane). The third eyelid plays an essential role in ocular health and function in cats.

Common Conditions

Two conditions commonly affect the third eyelid in dogs: cherry eye and third eyelid prolapse.

Cherry eye occurs when the gland of the third eyelid prolapses and protrudes as a red, cherry-like mass in the corner of the eye. It is most often seen in young dogs under 2 years old, especially breeds like Bulldogs, Beagles, Boston Terriers, and Lhasa Apsos. While the protruding gland may look bothersome, it usually does not cause any pain or vision problems. However, the exposed gland is prone to irritation and infection. Cherry eye is treated by surgically replacing or removing the gland.[1]

Third eyelid prolapse happens when the third eyelid itself protrudes across part of the eye. It can occur from trauma, infection, inflammation, or inadequate gland production. The protruding membrane may appear swollen, red, or brownish in color. Prolapse can irritate the eye and cause tearing, squinting, or impaired vision. Mild prolapse often resolves on its own, but severe cases may need surgery to tack the gland back into place.[2]


There are several treatment options for problems related to the third eyelid in cats:

Medications: Antibiotic and/or anti-inflammatory eye drops or ointment may be prescribed for 7-10 days to treat infection or inflammation of the third eyelid. Oral antibiotics and anti-inflammatory medications may also be prescribed. According to the Vetspecialists Veterinary Fact Sheet, “Antibiotic and/or anti-inflammatory eye drops or ointment will be prescribed for 7-10 days. Oral medication will also usually be prescribed for 5-10 days.” https://vetspecialists.co.uk/fact-sheets-post/conditions-of-the-third-eyelid/

Surgery: If the third eyelid is persistently protruding and irritating the eye, surgery may be recommended to partially remove it. This is known as a partial nictitans resection. According to WikiHow, “If medical management does not return the third eyelid back to its normal position, your vet may recommend partial removal, or resection, of the protruding portion of the third eyelid.” https://www.wikihow.com/Treat-Third-Eyelid-Protrusion-in-Cats

Everyday Care

It’s important to keep your cat’s third eyelids clean as part of their regular care. Use a soft damp cloth to gently wipe away any discharge or debris that accumulates on the third eyelid. Be very gentle, as the tissue is delicate. According to Aristopet, you can use sterile saline eye wash to help flush out the eye and keep it clean.

Potential issues to watch for include redness, swelling, or any changes to the appearance of the third eyelid. Discharge or swelling may indicate infection or inflammation. If the third eyelid appears to be protruding more than normal or covering a large portion of the eye, it could signal an underlying condition that requires veterinary attention. Monitor your cat’s eyes closely and contact your vet if you notice anything abnormal about the third eyelids.


The third eyelid, also known as the nictitating membrane, has been known about for centuries. The term “nictitating membrane” comes from the Latin words “nictare” meaning “to blink” and “membrana” meaning “membrane”.

According to the Wikipedia article on nictitating membranes https://en.wikipedia.org/wiki/Nictitating_membrane, the third eyelid was first described by the French physician Guichard Joseph Duverney in 1676. However, its function was not fully understood until the 1700s when comparative anatomists like Claude Perrault began studying it in animals.

By the 1800s, the nictitating membrane was well documented in veterinary medicine texts about the anatomy of domestic animals. But its vestigial presence in humans was still being debated by scientists and doctors during this time.

Today the third eyelid is universally recognized as a normal anatomical structure in many animals that has largely lost its original function in humans over evolutionary time. Though it goes by many colloquial names like “haw” and “third eyelid”, the medical term “nictitating membrane” has endured since its early origins.

Other Animals

Dogs, reptiles and birds also have a third eyelid called a nictitating membrane. The nictitating membrane in dogs is sometimes referred to as the “haw.” It functions to keep the eyes moist and protected.

In dogs, the nictitating membrane sweeps horizontally across the eye to remove debris and distribute tears. It can become inflamed or infected, requiring veterinary treatment. Sometimes it becomes visible if a dog is unwell. In severe cases, it may need to be surgically removed.

Reptiles and birds evolved nictitating membranes to help keep their eyes clean and moist while hunting and flying. A reptile’s nictitating membrane is transparent and sweeps diagonally across the eye. It helps protect their eyes while burrowing. Birds can actively control their nictitating membrane to remove debris during flight.

Overall, the nictitating membrane serves an important protective purpose in many animals. While humans lost this structure through evolution, it remains an essential adaption for other creatures. Understanding the third eyelid in animals provides insight into ocular anatomy and evolution.


There are some common misconceptions and myths about the white lids on cat’s eyes:

Some people believe that cats have a third eyelid. However, the white lid that sometimes shows in the corner of a cat’s eye is not actually a third eyelid. Cats only have two eyelids – an upper and lower lid like humans do. The white lid that sometimes appears is called the nictitating membrane or third eyelid flap, but it is not an actual eyelid.1

Another myth is that this white lid appeared in early humans or mammalian ancestors. While some mammals like dogs have a third eyelid, there is no evidence that humans ever had a third eyelid. The nictitating membrane evolved in animals like reptiles, birds, and some mammals, but not in primates like humans.2

Some people think that if the white lid is showing, it means the cat has an eye problem or condition. However, it is normal for this third eyelid flap to show periodically, especially when the cat is sleepy or sick. Only if the membrane is consistently covering the eye or looks swollen or inflamed is it a cause for concern.3


The third eyelid, or nictitating membrane, is an important anatomical structure in many animals including cats, dogs, and birds. It serves critical functions such as lubricating the eye, removing debris, and protecting the eye from scratches or injury 1. While humans do not have a nictitating membrane, this specialized eyelid allows animals to see clearly while also blinking. Overall, the third eyelid provides key health benefits for animals and enables comfortable vision. It is an example of the amazing adaptations in nature.

Proper third eyelid health and function is critical for animals like cats and dogs. If the nictitating membrane is damaged, infected, or develops abnormalities, it can greatly impact eye health and comfort. Pet owners should monitor their animal’s third eyelid regularly and see a veterinarian if any issues develop. With proper everyday care and attention, the third eyelid can support lifelong ocular wellbeing.

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