The Enigmatic Cat Third Eyelid: Unveiling Its Purpose and Importance

The eyes of cats have a unique feature in the anatomy of their eyes which is cat third eyelid also known as the nictitating membrane.

Unlike humans and many other animals, cats possess an additional protective eyelid located in the inner corner of each eye, hidden from view under normal circumstances.

Now let’s start understanding the structure and function of cat third eyelid’s unique membrane. 

Cat third eyelid performs many important functions which help to maintain the cat’s eye functions including providing protection to the eye and also provide lubrication to the eye which saves the cat’s eye from injuries and irritants.

Our purpose is to explain extensively cat third eyelid including its structure function and its importance in the life of cats.

Anatomy of the Cat Third Eyelid

The cat third eyelid is also pronounced as a nictitating membrane which is not only present in cats but in some other animals In cats, the nictitating membrane is a thin, semi-transparent, and triangular-shaped fold of tissue located at the inner corner of each eye, next to the nose. When at rest, it is mostly hidden under the lower eyelid and not readily visible.

The nictitating membrane is mobile and can be voluntarily extended across the surface of the eye to provide additional protection and moisture. It moves diagonally across the eye from the inner corner to cover the cornea, leaving only a narrow slit for vision. During hunting grooming, fighting, or any other activity cat’s third eyelid protects there eyes from any type of damage.

The presence and prominence of the third eyelid can vary among different species. The Cat third eyelid is not only present in cat third eyelid is also present in some other animals including dogs rabbits and birds. In these species, the third eyelid serves significant protective functions.

However, in some other animals, the nictitating membrane may be less developed or even absent. For instance, humans and most primates do not possess a third eyelid, and it is not as prominent or functional in these species. 

In some other animals, another type of membrane is present which is structurally different but performs the same function of protection such as horizontally moving membrane in reptiles and sharks

The cat third eyelid is a complex structure that is composed of many tissues and other structures which work together.

1. Conjunctiva: The nictitating membrane is covered by a specialized mucous membrane called the conjunctiva. The conjunctiva is the same type of tissue that lines the inner surface of the eyelids and the front surface of the eye (except the cornea). It provides lubrication and protection to the eye

2. Cartilage: The third eyelid contains a supporting framework of cartilage, which gives it structure and stability. The cartilage allows the membrane to retain its shape and move smoothly across the eye when extended.

3. Gland of the Third Eyelid: Also known as the Harderian gland, it is a significant gland located within the third eyelid. 

Water-based fluid is released on the eye by the gland which helps in lubrication and protection of the eye especially when cat third eyelid is moving across the cornea

4. Smooth muscles: The nictitating membrane is controlled by a set of smooth muscles that enable it to be drawn across the eye and retracted back under the lower eyelid. These muscles allow the cat to control the movement of the third eyelid and protect the eye as needed.

Evolutionary History

The evolutionary history of Cat third eyelid shows a relationship with early vertebrates this third eyelid is not only present in cats it is also found in some other animals.

The nictitating membrane is believed to have evolved as a protective adaptation to safeguard the eyes from various environmental factors and potential injuries.

In fish, amphibians, reptiles, birds, and some mammals, including cats, the nictitating membrane serves to provide additional protection to the eyes beyond the regular upper and lower eyelids. This membrane likely evolved as an extension of the lower eyelid to cover the cornea, the transparent outer covering of the eye, and help keep it moist and clean.

In the case of cats, the nictitating membrane serves several adaptive advantages in their natural environment:

1. Protection: The nictitating membrane helps protect the cat’s eyes from debris, dust, and potential damage during hunting and exploration. It acts as a transparent shield that can quickly cover the eye in situations where there is a risk of injury.

2. Moisture and Lubrication: The membrane contributes to keeping the eyes moist and lubricated. This is particularly important in arid environments or during prolonged periods of hunting, when blinking might not be sufficient to maintain the eye’s moisture.

cat under water

3. Camouflage: The nictitating membrane can partially cover the eyes while allowing the cat to continue observing its surroundings. This feature might be advantageous during hunting, as it helps the cat remain inconspicuous and avoid alerting potential prey.

4. Underwater Vision: Cats are known for their aversion to water, but they can still swim if necessary. The cat third eyelid helps cats in vision in a watery environment 

Over generations of living alongside humans, the role of the nictitating membrane in domestic cats may have undergone some alterations:

1. Reduced Need for Camouflage: Domestic cats do not rely on camouflage as heavily as their wild counterparts. They are often kept in controlled environments, and hunting is less of a survival necessity. Consequently, the need for the nictitating membrane to aid in camouflaging during hunting might have diminished.

cat hunting

2. Altered Environmental Factors: Domestic cats experience different environmental challenges compared to wild cats. Their living spaces are generally safer and cleaner, reducing the likelihood of encountering debris or harmful substances that would require frequent activation of the nictitating membrane.

3. Changes in Hunting Behavior: Domestic cats might display different hunting behaviors due to regular access to food provided by their owners. This shift in hunting patterns could impact the frequency and significance of nictitating membrane usage.

Function and Purpose

Cat third eyelid is a  specialized membrane that is not only present in cats but the third eyelid is also present in some other animals including birds reptiles and some mammals. Its primary functions include:

Protection and defense mechanisms

Shielding the eyes from debris, foreign objects, and injury: The third eyelid can quickly move across the eye’s surface, acting as a protective barrier against potential hazards like dust, dirt, or flying debris. This helps the animal to save itself from environmental damage, especially the animals living in windy or dusty areas.

Keeping the cornea moist and lubricated: The third eyelid is equipped with glands that produce a mucous-like substance, aiding in keeping the cornea moist and lubricated. The cornea provides clarity to the eye and cat third eyelid help to stay clean cornea which is very important for good vision.

Regulation of tear production

The third eyelid’s role in tear distribution and drainage: The third eyelid plays a part in spreading tears over the surface of the eye, ensuring even distribution and contributing to adequate hydration. It also assists in draining excess tears away from the eye, maintaining a balanced tear film.

How it contributes to eye health and clarity: By regulating tear production, the third eyelid helps to keep the eyes clean, lubricated, and free from irritants. This contributes to overall eye health and ensures better clarity of vision.

The enhanced vision and predatory advantages

Investigation of the connection between the third eyelid and night vision: Some studies have shown that the third eyelid might aid in protecting it and also help the cats in their night vision. Possible improvements in hunting and survival: For animals that are nocturnal or have a predatory lifestyle, the third eyelid’s ability to protect the eyes and maintain clear vision can be advantageous for successful hunting and survival.

Common Issues and Health Considerations

Cherry Eye

Causes: Cherry eye occurs when the nictitating membrane (also known as the third eyelid) prolapses and becomes visible, resembling a red or pink mass in the corner of the eye. It happens when the connective tissue of the eyes becomes weak and could not keep the gland in its place.

Symptoms: The most obvious symptom is the protrusion of the third eyelid, which can be accompanied by eye irritation, redness, swelling, and discharge.

Treatment: Cherry eye is typically treated surgically. The veterinarian will reposition the prolapsed gland and suture it back into place to prevent further prolapse. In some cases, the gland may be removed if necessary.

Long-term Implications and Potential Complications:

Recurrence: In some cases, the cherry eye may recur even after surgical correction.

Dry Eye: Surgical intervention can sometimes damage tear gland tissue, leading to a condition called dry eye (keratoconjunctivitis sicca). This condition requires ongoing management and eye lubrication.

Conjunctivitis and Infections

Conjunctivitis: Inflammation of the conjunctiva (the membrane lining the eyelids and covering the eye) can cause redness, discharge, and discomfort in the eye. Bacterial viral or allergic factors are major causes of this condition and in this condition proper treatment is necessary.

Tumors and Growths

Tumors: Tumors can affect the third eyelid and surrounding tissues. Some may be benign, while others can be malignant and require surgical removal or other treatments.

Regular Eye Examinations

Regular veterinary check-ups are essential for the early detection of eye issues, including third eyelid problems. Early intervention can prevent complications and improve treatment outcomes

Practices for Maintaining Healthy Third Eyelids

 Environmental Safety: Ensure your pet’s environment is free from potential eye irritants or hazards that could cause eye injuries or infections.

Eye Hygiene: If your pet’s eyes have discharge, gently clean them with a moist, soft cloth or eye wipes recommended by your veterinarian.

Avoidance of Overexertion: Some dog breeds may be more prone to cherry eye due to genetic factors. Be cautious with activities that may cause excessive eye rubbing or stress.

 Proper Nutrition: Provide a balanced and appropriate diet to support overall eye health.

Veterinary Attention: If you notice any abnormalities or changes in your pet’s eyes, seek veterinary care promptly.

The Role of the Third Eyelid in Cat Behavior

Blinking and the third eyelid’s significance in communication

Cats have a third eyelid, known as the nictitating membrane, located in the inner corner of their eyes. The third eyelid is usually not visible unless the cat is sick, scared, or undergoing certain medical conditions. In these cases, it may cover part of the eye, giving the cat a “haw” appearance.

The third eyelid’s movements can be subtle and brief, serving various functions, including moistening and protecting the cornea, especially during grooming or hunting activities. Cats use blinking as a form of non-verbal communication, especially with humans. Slow blinking is often considered a sign of trust and affection. When a cat blinks slowly at you, it may be a way of expressing comfort and contentment, and they might appreciate a similar response from you

Expressions of emotions through the nictitating membrane

cat emotions

While there isn’t much scientific evidence regarding emotions expressed solely through the nictitating membrane, a cat’s overall eye expression and body language play a significant role in communicating their emotions. Dilated pupils may indicate excitement, fear, or aggression, while constricted pupils might suggest contentment or focus.

Cats can tell about their emotions with the help of their eyes and body language it is very important to save cat third eyelids to understand the overall behavior of the cat. For example, if a cat’s nictitating membrane is frequently visible or covering its eye, it may indicate discomfort, illness, or pain, and it should prompt a visit to the veterinarian.

Understanding cat behavior through their eye movements

Cat behavior is a complex subject, and while eye movements, including the cat third eyelid’s role, can provide some insights, they are just one aspect of the overall picture.


The extra protection layer present on the eye of some animals is known as the third eyelid and it cats it is known as cat third eyelid .which helps to provide protection and lubrication to the eyes and maintains the proper function of the eyes.

As responsible cat owners, we must prioritize our feline companions’ eye health. Regularly monitoring their eyes for any signs of discomfort, redness, discharge, or changes in the appearance of the cat third eyelid is crucial. If any abnormalities are observed, seeking prompt veterinary attention is essential to ensure early detection and proper treatment of potential eye issues.

The third eyelid in cats is a fascinating and relatively less explored area of research. Future studies could focus on gaining a deeper understanding of its exact function and how it relates to the overall eye health of cats. Advancements in imaging technology and diagnostic tools may also provide more insights into any underlying conditions or diseases that could affect the third eyelid.

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