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The Nictitating Membrane: The Third Eyelid

Monday 29 May 2017

From these photographs you could easily imagine that the animal kingdom had suddenly been enveloped in its own zombie apocalypse.  Yet these pictures do not feature the Squawking Dead. Thanks to high speed photography, these pictures capture the nictitating membrane in action. It is also known as the third eyelid, haw and the inner eyelid. It is drawn across the eye to protect and moisturize it while retaining visibility.

The term, unsurprisingly, comes from Latin.  Nictare means to blink and although many mammals have vestigial nictitating membranes in the corner of the eye, a number have the full version, including seals, polar bears and camels. Some birds, reptiles and sharks have full nictitating membranes. However, the full nictitating membrane is rare among primates, with only lemurs and the lorisoid primates possessing one.

Image Credit Flickr User Lip Kee
However, the nictitating membrane is so vital that among mammals and birds the norm is for a species to have this third eyelid and those which do not possess one – such as ourselves – are the odd ones out.  It may not come as a shock but in this particular case we are the oddity of nature.

What makes the nictitating membrane different from the upper and lower eyelids is the fact that it moves horizontally, across the eyeball.  Many diving animals use it for protecting their eyes while they are swimming underwater and in these species it is transparent so that they retain 100 percent of their visual acuity while submerged.

Some species are not so fortunate.  The crocodile uses its nictitating membrane while underwater but it really hinders their ability to focus.

The membrane looks strange enough on mammals and birds.  However, it is perhaps with amphibians that it really looks extraordinary.

Other animals, such as the sea lion, activate it while on land and use it to get anything caught in their eyes out and this is its primary function in most animals.  Birds of prey use it to protect themselves from their chicks while they feed them.  No one wants their eyes damaged by over eager offspring after all. Another raptor, the peregrine falcon, activates its membrane when it is performing one of its dives which enable the bird to reach up to 200 miles per hour.  It is used as much to moisturize the eye as to remove any dust and debris which might hit them while the dive is underway.

You may have wondered how the woodpecker doesn’t end up with damaged eyes as it drills in to a tree.  A millisecond before its beak hits the trunk the woodpecker unconsciously tautens its nictitating membrane (birds can also actively control it).  This helps to prevent the bird from developing eye injuries which would otherwise be induced by its continual hammering.

The nictitating membrane in dogs and cats is not usually visible and if it is then it may be a sign that the animal is ill or even that it has fleas.  Yet if one of your household is a cat or dog then when they are asleep, gently open one of its eyes and you should be able to see it.  Until the turn of the last century it was considered an extraneous structure and was often removed by veterinarians so that they could better examine the eyes of their unfortunate patients. Happily, this practice has long ceased.

Although the exact function of the nictitating membrane remains unknown in cats it is thought that it is used to protect the cat’s very large cornea when it is stalking through long grass. Anyone who has ever cut themselves on grass will know why it might come in useful during this activity.  While birds of prey use it to protect their eyes from their young, it is believed that cats use their nictitating membrane to protect their eyes when attacking their prey – as it might just try to fight back. One thing is known for sure – its role in protecting the surface of the eye is an important one.

Perhaps these pictures are all the more interesting for us because we do not possess a nictitating membrane.  If we did then possibly they might not be quite so fascinating. Yet this amazing structure is quite something – whether we have one or not.

First Image Credit Flickr User alde

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