Humans do not possess a functional third eyelid, but its remnants are visible in the inner corner of the eye. This protective membrane, known as the nictitating membrane, was lost through evolutionary time. While many animals rely on this extra eyelid for protection and moisture, the human version has been reduced to a non-functional fold of tissue. This remnant is classified as a vestigial organ, meaning it has lost its original purpose. The presence of this structure offers insight into the shared ancestry between humans and other vertebrates.
Defining the Nictitating Membrane
A functional third eyelid, or nictitating membrane, is a distinct protective fold that moves horizontally across the eye’s surface. Unlike the upper and lower eyelids, which close vertically, this membrane sweeps from the inner corner toward the outer edge. In many species, the membrane is translucent or transparent, allowing the animal to maintain vision even when the eye is covered. This feature primarily shields the cornea from physical trauma and clears debris from the eye’s surface.
The membrane also plays a role in distributing the tear film across the eyeball to prevent dryness. Some species have a gland associated with this membrane that produces much of the total tear fluid. It is composed of a thin sheet of conjunctival tissue supported by a T-shaped cartilage plate, which helps it conform to the cornea. Depending on the species, its movement can be passive, triggered by eyeball retraction, or actively controlled by muscles.
The Vestigial Remnant in Human Eyes
The human eye contains two structures in the medial canthus, or inner corner, that represent the evolutionary remnants of the third eyelid. The most prominent remnant is the plica semilunaris, a small, crescent-shaped fold of conjunctiva. Adjacent to this fold is the lacrimal caruncle, a fleshy, reddish bump containing sebaceous and sweat glands. Neither structure can sweep across the cornea to protect or moisten the eye like a true third eyelid.
The plica semilunaris is classified as a vestigial organ because it no longer performs the protective function of its ancestral counterpart. However, it serves a minor mechanical role in the modern human eye. The fold helps maintain the proper positioning of the tear lake, the collection of tear fluid at the inner corner. It also permits greater rotational movement of the eyeball by preventing the bulbar conjunctiva from attaching directly to the eyelid.
The lacrimal caruncle serves a separate, limited function, primarily collecting fine debris and secreting a mixture of oils and mucus. This oily secretion, sometimes visible after sleeping, helps lubricate the surface of the eye. These human structures are considered vestigial because they have lost the main function of the nictitating membrane: a full, movable protective screen. The loss of this functional eyelid is linked to the evolutionary path of primates, which rely less on the membrane for protection than other animals.
Functional Third Eyelids in the Animal Kingdom
Many species still possess a functional nictitating membrane, often linked to their environmental needs. Birds, such as peregrine falcons, use the translucent membrane to clear debris and spread moisture across the eye while diving at high speeds. This protection allows them to maintain clear vision during intense aerodynamic activity.
Aquatic mammals and reptiles also rely on this structure for underwater protection. Seals use their third eyelid to remove sand and debris on land, and it shields their eyes while hunting in water. Beavers and manatees have a transparent membrane that acts like a built-in goggle, protecting the eye from water irritation without obscuring vision. The membrane is also important for animals in dusty environments or those that hunt aggressively, such as sharks, where it is used as a tough shield during an attack.