Research overview: the third eyelid (nictitating membrane) and ocular protection

Cats possess a third eyelid, the nictitating membrane, and this overview reviews its anatomy and function. This translucent fold sweeps across the eye from the inner corner, helping spread tears, clear debris, and protect the cornea, and it harbours a tear-producing gland. Normally barely visible, a prominent third eyelid can signal illness or pain, making it useful in veterinary assessment. Researchers describe its structure through anatomy and its movement through observation. Why it matters: the nictitating membrane is part of the protective and lubricating apparatus that keeps the cat sensitive eye functioning, and its appearance is a practical health indicator. It also reflects a widely shared vertebrate feature, well developed in cats, that complements the eyelids in guarding an organ central to feline survival.