The Nictitating Membrane: An Evolved Protective Feature in Animals and Humans

Sci-Five | The Hindu Science Quiz: Exploring the Third Eyelid

The nictitating membrane, also known as the third eyelid, is a protective feature present in some animals. In birds, this membrane serves to protect their eyes from debris and injury during high-speed dives. Polar bears use their nictitating membrane to shield their eyes from UV radiation to prevent snow blindness.

In humans, the plica semilunaris is a fold of conjunctiva at the inner corner of the eye believed to be a vestigial organ of the nictitating membrane found in other animals. This fold is similar in appearance to the third eyelid and led to the idea that it may be a remnant of such a structure. While the function of this fold in humans remains unclear, it is typically transparent or translucent in color and can be seen when looking at one’s own eye or that of another person.

The nictitating membrane is typically found in various animals for protective purposes, such as protecting against debris and injury during high-speed dives or shielding from UV radiation. This feature is an example of evolutionary adaptation that has allowed certain species to survive in harsh environments by providing them with additional protection against external threats.

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