Interesting facts about the Second Eyelid Found in Animals
While it is often referred to as a second eyelid, the transparent membrane commonly seen on animals is actually a third eyelid. The upper and lower eyelids, which are also found on humans, comprise the two eyelids and many other types of animals also have another or third type of eye protection.
This extra eyelid is called the nictitating membrane and is found on a wide variety of animals including reptiles, fish, birds and some mammals. It is located inside the corner of the eye and is composed of conjunctival tissue, which is part of the mucus membrane. This membrane is often triangular shaped and is pulled across the eye in a diagonal or horizontal motion. Most animals can control the movement of the membrane, which occurs about 3-4 seconds. Sometimes this membrane can be referred to as a haw.
The purpose of the nictitating membrane is to allow the animal to keep their eye moist by spreading tear solutions across its surface and help remove debris that could otherwise injured the cornea. The nictitating membrane is translucent, so it allows animals to clean and moisturize the eye without having to blink. Many animals use this feature to help protect their eyes as required by their living environments. Many animals that swim will use the membrane to cover their eyes while underwater. For example, sharks often use the membrane when biting into prey to help protect their valuable eyes. Most predators need to be extremely careful with their eyesight as it is often the main tool for finding prey.
Other types of predators that use the nictitating membrane as protection include birds of prey. Owls and falcons often close the membrane when flying at high speeds to help prevent the eye from drying out and to keep debris from injuring them. It allows the birds to continue to keep a visual on their intended prey without having to blink. It also helps owls during the day by protecting their valuable eyes from UV rays and is powerful enough to allow birds of prey like eagles to look into the sunlight without the risk of damaging their eyesight. Polar bears also protect their eyes from UV radiation with this membrane. Aardvarks have been known to close their second eyelid when raiding termite nests to protect their eyes from biting insects. Even woodpeckers close the membrane when pecking to prevent their eyes from protruding due to the force.
Common domesticated animals such as dogs, cats and horses also have a nictitating membrane. These animals have lost the ability to actively control their third eyelid. Often the membrane can be seen when the animal is sleeping as it will spread across the eye. When awake the membrane is not seen, if it is visible it usually is a signal for a medical problem such as dehydration, tetanus or an eye condition. Domestic animals, especially dogs can also develop problems with their nictitating membrane. A condition referred to as ‘cherry eye’ occurs when a tear gland attached to the nictitating membrane becomes prolapsed. This may cause other eye problems and usually requires a surgical solution. Another condition that can affect the nictitating membrane is called scrolled cartilage, where the cartilage that makes up part of the third eyelid grows abnormally. This also requires surgery to repair.
Many animals use their nictitating membrane or second eyelid as a vital part of their eyesight. Even humans once had a protective membrane over their eyes, but it has lost most of its use over time. Thought to be the vestigial remnant of the nictitating membrane, humans now have a plica semilunaris. This is the small pink bump located in the corner of the eye.
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