Some mammals, such as camels, polar bears, and seals, have full
nictitating membranes. That is the transparent or translucent third eyelid that can be drawn across the eye for protection and to moisten it while maintaining visibility.
In humans, the
plica semilunaris (also known as the
semilunar fold) and its associated muscles are thought to be
homologous to the nictitating membranes seen in other animals. In most
primate species, a plica semilunaris is present, but a fully developed nictitating membranes still exists in lemurs and
lorisiform primates.
Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball (shown below). In some diving animals, such as beavers and manatees, it moves across the eye to protect it while under water, and in these species it is transparent; in other diving animals, including sea lions, it is activated on land, to remove sand and other debris. This is its function in most animals, but
why did it disappear in humans?Interestingly, the
plica semilunaris has no known function in humans today. So while it is homologous to the nictitating membrane in other species, it is nothing more than a
vestigial structure. The reason for the loss of a nictitating membrane in humans remains unclear, although biologist hypothesize that changes in habitat and eye physiology may have rendered the tissue unnecessary. For example, the nictitating membrane in some animals possess an attached gland known as the
nictitans gland (also known as the
Harderian gland) that may produce up to 50% of the tear film. For us, tears are constantly produced in the lacrimal glands (tear ducts) that are in the outer corners of your eyelids. Our tears and constant blinking keep our eyes moist, hence this structure became less important for our survival over time.
