The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across … See more

Fully developed nictitating membranes are found in fish, amphibians, reptiles, birds and mammals, but are rare in primates. In humans, the plica semilunaris (also known as the semilunar fold) and its associated muscles are … See more

The nictitating membrane is normally translucent. In some diving animals, including sea lions, it is activated on land, to remove sand and other debris—its function in most animals. In crocodiles, it protects their eyes f… See more

Nictitating membranes in cats and dogs do not have many muscle fibers, so they are not usually visible; chronic visibility should be taken as a sign of poor condition or ill health. The membrane can, however, be seen clearl… See more

• Accessory abducens nucleus
• Human vestigiality See more

• Hawk, transparent eyelids (nicitating membrane slow motion video) on YouTube See more