Why do cats close their third eyelid? - briefly
Cats close their third eyelid, or nictitating membrane, to protect and moisten their eyes while maintaining visibility, especially during sleep or when their eyes are exposed to irritants.
Why do cats close their third eyelid? - in detail
Cats possess a unique anatomical feature known as the third eyelid, or nictitating membrane, which is a thin, translucent layer located in the inner corner of each eye. This structure serves multiple protective and functional purposes. When a cat closes its third eyelid, it acts as a barrier against potential harm, such as dust, debris, or foreign objects, ensuring the eye remains clean and safe. Additionally, the third eyelid helps maintain moisture by spreading tears evenly across the cornea, preventing dryness and irritation. In some cases, the partial or full closure of the third eyelid can indicate underlying health issues, such as infections, inflammation, or systemic illnesses. For example, conditions like conjunctivitis, eye trauma, or dehydration may cause the third eyelid to become more visible. Furthermore, the nictitating membrane contains a gland responsible for producing a portion of the tear film, contributing to overall eye health. While occasional visibility of the third eyelid is normal, especially during sleep or relaxation, persistent or abnormal closure warrants veterinary attention to address potential medical concerns. Understanding this feature highlights the evolutionary adaptation of cats to protect their vision and maintain ocular health in various environments.