Blinking is a universal biological mechanism that protects and lubricates the eyes, maintaining the clarity required for vision. While most animals blink vertically using upper and lower eyelids, many species employ a specialized third eyelid that moves horizontally across the eye. This alternative method of cleaning and protecting the eye is an adaptation tailored to specific environmental demands.
The Anatomy of Sideways Blinking
The structure responsible for this horizontal movement is known scientifically as the nictitating membrane, often referred to as the third eyelid. This membrane is a fold of conjunctiva, typically located in the inner corner of the eye. Unlike standard eyelids, the nictitating membrane is frequently translucent, allowing the animal to maintain some vision even while the eye is protected.
The membrane contains cartilage for structural support and is associated with a gland that contributes significantly to tear production and lubrication. Its movement sweeps across the eyeball in a horizontal or diagonal direction. In some animals, the action is controlled by smooth muscle, while in others, it is passive, moving into place when the eyeball is retracted slightly into the socket.
The Diverse Animals That Use This Trait
A functional nictitating membrane is widespread across various groups of vertebrates. Nearly all species of birds possess this trait, using it actively during flight and foraging. Raptors, such as the peregrine falcon, rely on the membrane to protect their eyes during high-speed dives.
Reptiles and amphibians commonly feature this specialized eyelid, including crocodiles, lizards, and most tailless amphibians like frogs. In aquatic environments, many fish, notably various species of sharks, utilize a highly developed third eyelid.
While less common among mammals, the membrane is fully functional in select species that face harsh environmental conditions. Specific examples include polar bears, which need protection from snow glare, and beavers, which use it for underwater vision. Domestic animals like cats and dogs also possess the nictitating membrane, though it is often less visible unless it prolapses, a condition known as “cherry eye.”
Primary Functions of the Nictitating Membrane
The nictitating membrane is primarily protective and lubricating, tailored to the unique lifestyle of each species. For birds, it acts as a high-speed shield, allowing them to clear debris and distribute tear film without momentary vision loss during rapid flight. A woodpecker deploys the membrane milliseconds before striking wood to prevent retinal damage from intense vibration.
The third eyelid protects the eye from environmental extremes. In animals like camels, it shields against fine sand in desert wind, and in polar bears, it guards against intense ultraviolet radiation that causes snow blindness.
For aquatic animals like crocodiles and sea lions, the membrane acts like a built-in goggle, protecting the corneal surface from water and debris while preserving visual capacity underwater. Predatory animals like sharks also use the membrane to protect their eyes from flailing prey just before a strike, preventing injury during the hunt.