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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.
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]
Eyelid edema is a condition in which the eyelids are swollen and tissues contain excess fluid. It may affect eye function when it increases the intraocular pressure. Eyelid edema is caused by allergy, trichiasis or infections. [4] The main symptoms are swollen red eyelids, pain, and itching. Chronic eyelid edema can lead to blepharochalasis.
Thelazia is a genus of nematode worms which parasitize the eyes and associated tissues of various bird and mammal hosts, including humans. [2] [3] They are often called "eyeworms", and infestation with Thelazia species is referred to as "thelaziasis" (occasionally spelled "thelaziosis").
Cherry eye is a disorder of the nictitating membrane (NM), also called the third eyelid, present in the eyes of dogs and cats. [1] Cherry eye is most often seen in young dogs under the age of two. [2]
Humans have two eyes, situated on the left and the right of the face. The eyes sit in bony cavities called the orbits, in the skull. There are six extraocular muscles that control eye movements. The front visible part of the eye is made up of the whitish sclera, a coloured iris, and the pupil. A thin layer called the conjunctiva sits
A parietal eye (third eye, pineal eye) is a part of the epithalamus in some vertebrates. The eye is at the top of the head; is photoreceptive; and is associated with the pineal gland , which regulates circadian rhythmicity and hormone production for thermoregulation . [ 1 ]
Some mammals, such as cats, camels, polar bears, seals and aardvarks, have a full translucent third eyelid called a nictitating membrane, while others have a vestigial nictitating membrane. [13] The membrane works to protect and moisten the eyelid while maintaining visibility.