Cats possess a visual system that includes more than the two eyelids seen in humans. Cats have three distinct eyelids for each eye. This unique triple-eyelid structure provides enhanced protection and specialized functions that help maintain the health and clarity of the feline eye. This extra anatomical feature, while usually unseen, is a defining characteristic of many animals, including birds and reptiles.
The Outer Eyelids
The upper and lower eyelids in cats operate in a manner similar to human eyelids, providing primary mechanical closure and protection for the eye. These outer lids are composed of haired skin on the exterior, supported by a fibrous plate, and lined internally with conjunctiva. Their movement is governed by muscles, such as the orbicularis oculi, which facilitate the blinking reflex.
The main function of these two visible lids is to protect the ocular surface from foreign debris and injury. Blinking distributes the tear film evenly across the cornea, keeping the eye moist and healthy. Unlike dogs, the cat’s upper eyelid is the more mobile of the two and handles most of the blinking action.
The Third Eyelid Structure
The third eyelid is a unique structure medically termed the nictitating membrane. This membrane is a fold of tissue located in the inner corner of the eye, closest to the nose. In a healthy, alert cat, the third eyelid is retracted and largely invisible, appearing as a small, pinkish-white crescent.
The structure is supported by a T-shaped piece of cartilage and covered on both sides by conjunctiva. Movement of this accessory lid is largely passive, meaning the cat cannot voluntarily control it. The membrane slides horizontally across the eye when the eyeball retracts slightly into the eye socket, which occurs reflexively during sleep, sedation, or in response to pain.
Function and Health Significance
The nictitating membrane plays a specialized role in maintaining the health and functionality of the eye. It acts as a windshield wiper, moving across the cornea to sweep away dust and debris. This action is integral for distributing the tear film effectively and keeping the surface of the eye lubricated.
A significant portion of the eye’s moisture comes from a gland located at the base of the third eyelid, which can contribute up to 30–50% of the total aqueous tear volume. Because the membrane is typically hidden, its persistent visibility in an awake cat is often a non-specific indicator of a health issue. This sustained protrusion can signal underlying problems such as general illness, dehydration, sedation, or specific eye conditions like a corneal ulcer or “cherry eye,” where the gland prolapses.