The nictitating membrane, often called the “third eyelid,” is found in a wide array of animal species. This structure allows animals to protect and clean their eyes without fully losing their vision, which is necessary for survival in challenging environments.
Defining the Nictitating Membrane
The nictitating membrane is a translucent sheet of tissue positioned beneath the upper and lower eyelids. Unlike the primary eyelids, which close vertically, this membrane sweeps horizontally or diagonally across the eye, typically originating from the medial canthus (the inner corner near the nose). It is primarily a fold of the conjunctiva, the moist membrane that covers the eye’s surface and the inner lining of the eyelids.
The membrane includes connective tissue and, in many species, a supporting T-shaped cartilaginous plate that helps it conform to the cornea. Many animals also have a specialized gland, often called the gland of the third eyelid, embedded within this structure that contributes significantly to tear film production. This design facilitates its dual functions of protection and lubrication without completely obscuring sight.
Primary Functions Across Species
The most prominent role of the nictitating membrane is to act as a shield, providing rapid protection for the delicate cornea and eyeball. This is particularly useful against physical hazards such as flying debris, sudden impacts, or when an animal is engaged in combat or capturing prey. For instance, certain shark species deploy this membrane just before striking a target, safeguarding their eyes from injury during the struggle.
The membrane operates as an efficient cleaning and moisturizing apparatus, functioning like a biological windshield wiper. As it sweeps across the eye, it distributes the tear film evenly over the ocular surface, preventing dryness and clearing away foreign particles like dust, sand, and other irritants. In some mammals, the associated gland can produce up to 50% of the total tear fluid, underscoring its importance in lubrication.
The third primary function relates to maintaining vision clarity even when the membrane is deployed. Because the membrane is often translucent, it allows the animal to retain some degree of vision while the eye is protected. This capability is especially important for predators or animals in high-speed flight, where a momentary loss of sight from a full blink could be hazardous.
Distribution in the Animal Kingdom
A functional nictitating membrane is widely present across vertebrate classes, signifying its utility in diverse ecological niches. Birds, especially raptors like falcons and eagles, use it extensively during high-speed dives and flight to protect their eyes from wind and debris. For these species, the membrane is actively controlled, allowing for rapid deployment during aerial maneuvers.
Reptiles and amphibians commonly possess this structure, using it to protect their eyes in aquatic or terrestrial environments. Sharks, as aquatic predators, utilize a tough, mobile membrane to shield their eyes from struggling prey and the abrasive nature of the water. In contrast, some aquatic mammals, such as beavers and manatees, have a transparent membrane that acts like built-in goggles for clear underwater vision.
Terrestrial mammals in harsh environments rely on a full membrane for specialized protection. Camels, for example, have a translucent membrane that shields their eyes during sandstorms while still allowing them to navigate. Polar bears use a nictitating membrane to protect against snow blindness and aid visibility underwater.
The Human Vestige
While the nictitating membrane is common across much of the animal kingdom, most primates, including humans, lack a fully functional version. Humans possess a small, reddish-pink fold of tissue in the inner corner of the eye, known as the plica semilunaris. This small fold represents the evolutionary remnant of the once-functional third eyelid.
The plica semilunaris is largely non-functional in its ancestral role of protection and sweeping, though it helps with tear drainage and permits greater rotation of the eyeball. Its presence serves as evidence of a shared evolutionary history with other vertebrates, suggesting a full nictitating membrane was present in ancestral primates. The reduction of this feature is linked to changes in habitat and lifestyle, which lessened the environmental need for such a specialized protective layer.