Unveiling the third eyelid: A peek into animal eye protection
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, the third eyelid, is a structure found in most terrestrial vertebrates and some cartilaginous fish. Unlike the upper and lower eyelids, the third eyelid moves horizontally. Birds, reptiles, amphibians, and fish also possess it.
The nictitating membrane is transparent for most diurnal birds and opaque for owls and dippers. In tuataras, the nictitating 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 to 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 protect their eyes during sandstorms.
Diversity and adaptations
The third eyelid helps aquatic animals, such as the manatee, with underwater vision. Birds of prey, like falcons, use it to shield 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 called the plica semilunaris, which helps 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 admire its functionality in our pets and wild relatives.
