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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]
The parietal eye is found in the tuatara, most lizards, frogs, salamanders, certain bony fish, sharks, and lampreys. [7] [8] [9] It is absent in mammals but was present in their closest extinct relatives, the therapsids, suggesting that it was lost during the course of the mammalian evolution due to it being useless in endothermic animals. [10]
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
Common misnomers include adenitis, hyperplasia, adenoma of the gland of the third eyelid; however, cherry eye is not caused by hyperplasia, neoplasia, or primary inflammation. [3] In many species, the third eyelid plays an essential role in vision by supplying oxygen and nutrients to the eye via tear production. [4]
Taoism teaches that the third eye, also called the mind's eye, is situated between the two physical eyes, and expands up to the middle of the forehead when opened. Taoism asserts that the third eye is one of the main energy centers of the body located at the sixth Chakra , forming a part of the main meridian, the line separating left and right ...
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").
Diagram of a human eye; note that not all eyes have the same anatomy as a human eye. The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment. [10] The human eye is not a plain sphere but is like two spheres combined, a smaller, more sharply curved one and a larger lesser curved sphere.