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Transverse section of the dorsal or ventral eyelid of a pigeon (HE)
Birds have three eyelids, the upper or dorsal eyelid (palpebra dorsalis), the lower or ventral eyelid (palpebra ventralis) and the third eyelid (palpebra tertia) or nictitating membrane (membrana nictitans). Closure of the eye is mainly due to movement of the lower eyelid. The nictitating membrane serves to protect, moisten and clean the eye.
The dorsal and ventral eyelids may be feathered or unfeathered. In most pigeon breeds, the eyelids are unfeathered. There are no glands in the dorsal and ventral eyelid. The cornea is kept moist by the gland of the nictitating membrane (glandula membranae nictitans) and the lacrimal gland (glandula lacrimalis).
The eyelid can have multiple inner folds to allow for stretching when closing the eye.