What is the function of a rabbit's third eyelid?

The function of a rabbit's third eyelid, also known as the nictitating membrane, is primarily to protect the cornea while still allowing the rabbit to maintain its vision. This specialized structure acts as a shield against debris, dust, and other irritants that could occur in the rabbit's environment, such as during dust storms. The third eyelid moves horizontally across the eye, providing a protective barrier without significantly obstructing the rabbit's ability to see. This adaptation is particularly important for a prey animal like the rabbit, which needs to remain vigilant for potential threats even while its eyes are partially covered.

Other options relate to visual enhancement or moisture retention, but they do not capture the main role of the third eyelid as effectively as the protective function it serves during adverse conditions.