Third eyelid: A hidden shield in animals' eyes
Our pets, like dogs and cats, have a third eyelid known as the nictitating membrane. This thin, moist membrane protects the eye, especially during sleep.
The nictitating membrane, or third eyelid, is found in most terrestrial vertebrates and some cartilaginous fish. Unlike the upper and lower eyelids, it moves horizontally. Birds, reptiles, amphibians, and certain fish also possess it.
For most diurnal birds, the nictitating membrane is transparent. However, for owls and dippers, it is opaque. In tuataras, the membrane can move horizontally from the inner to the outer part of the eye while the upper and lower eyelids remain open.
Functions of the third eyelid
According to IFLScience, the third eyelid protects the eye from debris and helps spread tears, which is particularly important for flat-faced dogs like pugs. Wild animals need protection from dust, sand, and other contaminants. For example, camels use the third eyelid to shield their eyes during sandstorms.
Diversity and adaptations
The third eyelid helps aquatic animals, such as manatees, with underwater vision. Birds of prey, like falcons, use it to guard their eyes during rapid flight. In polar regions, where the sun reflects off the snow, the third eyelid can protect against snow blindness.
Why don't humans have it?
Humans and most primates do not have a fully developed third eyelid. The upper and lower eyelids sufficiently protect our eyes. However, we do have a remnant of this feature called the plica semilunaris, which assists with tear drainage.
The third eyelid is a fascinating anatomical feature in many animals. It aids them in adapting to various environments. Although humans do not have it, we can appreciate its functionality in our pets and wild relatives.
