The third eyelid, also known as the nictitating membrane, is a clear membrane found in the eyes of reptiles, birds, and some mammals. It protects the eye from damage, sweeps debris, and moistens the eye. Humans and most primates lack this feature. Pets may experience ailments affecting the membrane, requiring veterinary surgery.
A third eyelid, also known as a haw or nictitating membrane, is a structure found in the eyes of reptiles, birds, and some mammals. It consists of a clear membrane that is drawn across the eye. The third eyelid is located under the outer two eyelids and performs several functions in the eye. People who have been around cats and dogs may have seen this, as these mammals possess a nictitating membrane that is sometimes visible during sleep.
Like the outer eyelids, the third eyelid is designed to protect the eye from damage. It can be used to sweep dust and other debris from the surface of the eye and to moisten it. One of the eye glands is located at the base of this eyelid, providing a constant source of fluid to keep the eye moist so it works well. Many animals that live in dusty or dirty environments may use their lids periodically to clear debris.
In some underwater animals, this structure is used to cover the eye while swimming so that the animal can see, but its eyes are still protected from debris and other substances in the water. Birds use it for protection as they feed their young, making sure the fledglings don’t accidentally peck at their parents’ eyes in their haste to eat. Creatures such as sharks may employ theirs when attacking to protect their eyes from wriggling or panicking prey.
Humans and most primates lack a third eyelid, although some vestigial structures around the eye seem to suggest that humans once possessed this anatomical feature. People curious to see this membrane in action may be able to find a compliant cat or dog. In these animals, the nictitating membrane sometimes occurs during sleep or when the animal is suddenly awakened from a nap. Chickens and other pet birds can also display theirs if they are held and gently stroked or rocked to calm them.
Animals with nictitating membranes are prone to certain ailments that can affect it, causing a variety of problems. Sometimes, the membrane rolls up or flips over so it can’t be closed, forming a small lump in the corner of the eye that can cause irritation. Pets may also sometimes have difficulty closing or opening their third eyelids, and in some cases, the gland at the base may move, resulting in a condition known as cherry eye. Veterinary surgery is usually required to correct these problems.
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