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The nictitating membrane of a masked lapwing as it closes over the left eye, originating from the medial canthus. The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision.
In addition to the two eyelids usually found in vertebrates, bird's eyes are protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds. Some bird groups have specific modifications to their visual system linked to their way of life.
The plica semilunaris is a small fold of bulbar conjunctiva on the medial canthus of the eye.It functions during movement of the eye, to help maintain tear drainage via the lacrimal lake, and to permit greater rotation of the globe, for without the plica the conjunctiva would attach directly to the eyeball, restricting movement. [1]
The Harderian gland is a gland found within the eye's orbit that occurs in tetrapods (reptiles, amphibians, birds and mammals) that possess a nictitating membrane. [ 1 ] The gland can be compound tubular or compound tubuloalveolar , and the fluid it secretes ( mucous , serous or lipid ) varies between different groups of animals.
In squamate reptiles both the nictitating membrane and the eyelids have been suggested as the evolutionary origin of the brille, but embryonic studies supports the fusion of the dorsal and ventral eyelids. [1] Brille means "spectacles" or "glasses" in German, Norwegian, and Danish.
Cherry eye is typically diagnosed by examination of the conjunctiva and nictitating membrane. [2] The most obvious symptom of cherry eye is a round fleshy mass in the medial canthus of the eye, similar in appearance to the fruit it is named for. [ 8 ]
An eyelid (/ ˈ aɪ. l ɪ d / EYE-lid) is a thin fold of skin that covers and protects an eye.The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision.
The nictitating membrane as it covers the eye of a masked lapwing. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet reflective patches on their feathers.