Where's the nictitating membrane located?

Eyes

Explanation:

Nictitating membrane is located in the eyes. It is knows as the "third eyelid". In some vertebrates, a thin membrane is found beneath the lower eyelid that has the ability to extend and cover the entire eyeball. It is transparent or translucent and thus allows vision to some extent even when it covers the eye.  It also removes dirt from the eye and maintain its moisture content.

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.