Many animals have a third eyelid which is called the nictitating membrane, or haw. Its function is to keep the animal’s eyes clear of debris and mucus, help produce tears and protective antibodies that fight infection. In fact, most animals including dogs, cats, birds, reptiles, fish and other mammals have a third eyelid. Humans and pigs are excluded from this in having only two eyelids per eye.
More about the nictitating membrane:
- Camels are some of the largest mammals to have a nictitating membrane which helps protect their eyes from sand in the desert.
- The third eyelid is critical for most predatory animals as it allows the eyes to remain open without blinking and obstructing vision for prolonged periods of time.
- For reptiles and amphibians, the nictitating membrane allows for clear vision below water.