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The term comes from the Latin word nictare, meaning "to blink". It is often called a third eyelid or haw, and may be referred to in scientific terminology as the plica semilunaris, membrana nictitans, or palpebra tertia. Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball.
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]
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]
Image of a human eye showing the blood vessels of the bulbar conjunctiva Hyperaemia of the superficial bulbar conjunctiva blood vessels. In the anatomy of the eye, the conjunctiva (pl.: conjunctivae) is a thin mucous membrane that lines the inside of the eyelids and covers the sclera (the white of the eye). [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.
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 ...
All geckos except those in the subfamily Eublepharinae (eyelid geckos) possess brilles. Some groups of bony fish have a transparent eyelid known as the adipose eyelid . [ 3 ] Some reptiles, mammals and birds have a translucent third eyelid that moves horizontally across the eye called the nictitating membrane .
Entoptic phenomena (from Ancient Greek ἐντός (entós) 'within' and ὀπτικός (optikós) 'visual') are visual effects whose source is within the human eye itself. (Occasionally, these are called entopic phenomena , which is probably a typographical mistake.)