Bald eagles, like other birds, have incredible eyesight. To help their vision stay sharp, they have the help of an inner eyelid called the nictitating membrane or plica semilunaris. This membrane is clearly visible, mid-blink, in the picture above and can be captured with a fast shutter speed camera.
Eagles and other birds have three eyelids: top, bottom and horizontal. As the bird blinks, the nictitating membrane slides horizontally across the eye beginning in the inside corner, closest to the beak, then moves across to the outside corner. In addition to protection while attacking prey, this membrane keeps the eye moist and clear of dust. And, because the membrane is transparent, eagles never lose sight of their prey when the membrane draws across the eye.
In most mammals, there is only a small vestigial remnant of the nictitating membrane in the corner of the eye. However, polar bears, aardvarks, camels, seals, marsupials and monotremes do have fully functional membranes. A good video of a crocodile’s nictitating membrane in slow motion can be viewed here:
The first anatomical description of the nictitating membrane in the eye was made by an English biologist named Richard Owen in 1866. In the context of human evolution, we, too, have a remnant of this membrane in the corners of our eyes. If you look in a mirror at the inner corner of your eye, you will see a small fold of pink tissue which is the vestigial remnant of the nictitating membrane. Vestigial organs are things in our anatomy like our appendix and wisdom teeth that are leftover remnants from a time in early human evolution when we needed them for survival for grinding leaves and digesting our food.
The reason that primates lost the real function of this membrane is that evolution, by natural selection, takes away functions and organs that are no longer necessary for survival. There have been rare cases of persistent nictitating membranes found in our species which you can read about and see here: A rare case report
And the same report with pictures in the 2017 Indian Journal of Ophthalmology can be viewed here: Persistent unilateral nictitating membrane in a 9‑year‑old girl: A rare case report
To read about other interesting human vestigial organs and functions, please see here. 15 Human Vestigial Organs and Functions