Nictitating membrane
Overview
The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eyelid
present in some animals that can be drawn across the eye
for protection and to moisten it while maintaining visibility. Some reptiles, birds, and shark
s have a full nictitating membrane; in many mammals, there is a small vestigial remnant of the membrane in the corner of the eye.
Eyelid
An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...
present in some animals that can be drawn across the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
for protection and to moisten it while maintaining visibility. Some reptiles, birds, and shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s have a full nictitating membrane; in many mammals, there is a small vestigial remnant of the membrane in the corner of the eye.
