The third eyelid, formally known as the nictitating membrane, is a mobile, fold-like structure that functions as an extra protective layer for the eye. Distinct from the upper and lower eyelids, it is largely hidden in the inner corner of the eye when not in use. Many species across the animal kingdom possess a fully developed and functional third eyelid.
Anatomy and Protective Role
The third eyelid is a specialized structure composed of conjunctival tissue, a T-shaped cartilage core for rigidity, and a significant tear-producing gland. This gland contributes substantially to the eye’s total tear film, sometimes producing up to 50% of the aqueous layer in dogs. The membrane also contains numerous lymphoid follicles, which are part of the ocular immune defense system.
Its primary function is a rapid, horizontal sweeping motion across the cornea, acting like a biological “windshield wiper.” This action quickly clears debris and foreign bodies while simultaneously distributing the tear film. This lubricates the cornea and protects the eye without requiring the animal to fully close its primary eyelids. In many domestic species, this movement is passive, triggered by the retraction of the eyeball into the socket.
Animals That Possess a Third Eyelid
The third eyelid is widespread, appearing in various classes of vertebrates, including fish, amphibians, reptiles, birds, and many mammals. The nictitating membrane is particularly prominent in birds, where its translucent quality allows them to maintain clear vision even at high speeds or during dives.
Aquatic mammals, such as seals and beavers, utilize the membrane to protect their eyes from water and debris while swimming. Land-dwelling mammals like dogs, cats, and camels use it for rapid protection from environmental hazards such as sand or brush. Some animals, such as polar bears, even use it as a form of natural “sunglasses” to prevent snow blindness from intense glare.
The Human Vestige
Humans and many primates do not possess a functional third eyelid, but an evolutionary remnant of the structure can still be observed. This vestigial structure is called the plica semilunaris. It is located in the inner corner of the human eye, appearing as a small, pinkish fold of conjunctiva next to the tear duct opening.
The reduction of the nictitating membrane in the human lineage is linked to changes in habitat and the evolution of the eye’s physiology. While it no longer sweeps across the eye for protection, the plica semilunaris helps maintain tear drainage and permits greater rotation of the eyeball. The muscles associated with the original membrane are now non-functional in humans.
Common Issues and Pathology
The most common health issue in animals with a functional third eyelid is “Cherry Eye,” which is the prolapse of the tear gland. This condition occurs when the gland detaches from its normal anchor point and protrudes, appearing as a red, swollen mass in the inner corner of the eye. Breeds like Bulldogs, Beagles, and Cocker Spaniels are frequently predisposed to this genetic weakness.
Veterinary treatment typically involves surgically repositioning the gland back into its proper pocket. Removal of the gland is avoided because it produces a substantial portion of the necessary tear film. Removing it can lead to keratoconjunctivitis sicca, commonly known as “dry eye.” Other issues include inflammation, such as Nictitans Plasmacytic Conjunctivitis, or physical trauma from foreign bodies lodging beneath the membrane.