Nictitating membrane

The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the membrane remains in the corner of the eye. Some mammals, such as camels, polar bears, seals and aardvarks, have full nictitating membranes. Often called a third eyelid or haw, it may be referred to in scientific terminology as the plica semilunaris, membrana nictitans, or palpebra tertia.

Function

The nictitating membrane serves three significant purposes: protection, moisture, and visibility. When an animal's eye is exposed to potential harm, the nictitating membrane will act as a barrier and will protect the eye from damage. It also moistens the eye while cleaning any debris that may be present. Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eye, starting from the side near the nose. In most primate species, a small, vestigial remnant of the membrane remains, known as the plica semilunaris.

In different animals

Birds

In birds, the nictitating membrane also functions in the same way as in other animals. It protects and moistens the eye while the bird is flying. Birds of prey, such as hawks and eagles, have a well-developed nictitating membrane.

Reptiles

Reptiles, like turtles and snakes, also have a nictitating membrane. It helps them to protect their eyes from dust and dirt while they are moving on the ground.

Sharks

Sharks have a very well-developed nictitating membrane that covers their eyes while they are hunting and eating. This helps to protect their eyes from debris and from the thrashing of their prey.

See also

• Eye
• Eyelid
• Bird anatomy
• Shark anatomy
• Reptile anatomy

References

• Bird blink
  Bird blink
• Nictating membrane of a blue shark
  Nictating membrane of a blue shark
• Haliaeetus leucocephalus
  Haliaeetus leucocephalus
• Third eyelid on a domestic dog
  Third eyelid on a domestic dog
• Common Goldeneye showing nictitating membrane
  Common Goldeneye showing nictitating membrane
• Salad waking up from a nap
  Salad waking up from a nap
• Common buzzard yawning, with nictitating membrane visible, in slow motion
  Common buzzard yawning, with nictitating membrane visible, in slow motion
• 
  Gray892