The third eye (lid)

Most creatures have two eye lids. They serve the purposes of a. opening so as to allow vision, b. spreading tears across the cornea so that it remains moist, and c. closing quickly so as to protect the cornea from damage from debris.

Many creatures also have a third eye lid called a nictitating membrane, from the Latin word ‘nictare’ meaning to ‘blink’, that wipes horizontally across the cornea. It serves as a protective layer from dust and debris for creatures such as Blue-tongued Lizards (Tiliqua scincoides) pictured, that live close to the ground. For water creatures such as rakali, the Australian water rat, the membrane is draw across the eye when it is swimming underwater.

The nictitating membrane can be translucent when its main purpose is being a ‘windscreen wiper’ or transparent when its purpose is to protect the cornea from the outside environment such as under water or for animals e.g. Peregrine Falcon that travel at high speed. Birds, fish, reptiles and some mammals have nictitating membranes. In humans the structure in the corner of the eye known as the semilunar fold is a vestige of a nictitating membrane.

I could use some when I’m on my bike!