The third eyelid

The plica semilunaris is a small fold of bulbar conjunctiva on the medial canthus of the eye. It is loose, so that eye movements are not restricted. It is the vestigial remnant of the nictitating membrane (the “third eyelid”) which is present in other animals such as birds, reptiles, and fish. It is rare in mammals, mainly found in monotremes and marsupials. Its associated muscles are also vestigial. The plica semilunaris of Africans and Indigenous Australians are slightly larger than in other peoples. Only one species of primate, the Calabar Angwantibo, is known to have a functioning nictitating membrane.

via Plica semilunaris of conjunctiva

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One Response to The third eyelid

  1. alfy says:

    i was surprised to hear that the nictitating membrane is rare in mammals. Long before I became a student of biology I recall that our domestic cats, when disturbed asleep would open their eyes and the NM could seen sliding across the eyeball to its place of rest. A particularly pugnacious cat had a NM with a jagged edge through close contact fighting.

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