What’s That Creepy Third Eyelid in Dogs and Cats?
What Is the Third Eyelid?
That third eyelid—in vetspeak, the nictitating membrane—is also known as the haw, a term used more commonly in dog show rings.
It’s usually considered unsightly and probably costs some points if it’s exposed during judging.
Appearing from opaque to clear, many species of animals have them. Unless our dogs and cats spend a lot of time in tall grasses and brush, they’ve sort of lost the need for that third eyelid mainly because they no longer have to hunt prey to earn a living.
But, wild species sure need it. Sliding across the eye horizontally or diagonally, the nictitating membrane has a variety of functions.
Some species of animals can deploy the membrane at will, while in others it’s a matter of reflex.
What Is the Purpose of the Nictitating Membrane?
Predators usually deploy the nictitating membrane while hunting and feeding in order to minimize damage caused by struggling prey.
While most prey are not in a position to fight back effectively, their teeth, claws, fur and feathers can come into close proximity to the predator’s eyes, potentially causing injury.
We’re all familiar with the shark’s nictitating membrane because we always get a good look at it (especially during shark week) as the shark bites down on something.
It doesn’t matter that the prey is torn to shreds. The turbulence in the water surrounding the activity keeps the eyes vulnerable.
While chasing prey, or navigating the landscape in brush and grass, the third eyelid in land animals protects the eyes from abrasions and plant debris.
For birds, the nictitating membrane protects the eye from the considerable airborne dust and debris they encounter during flight, and it also protects their eyes while feeding their young.
Since they’re clear to opaque, animals can deploy the membrane and still see while chasing and subduing prey, flying, swimming, and engaging in other pursuits that expose the eyes to wind, dust, debris and injury.
But wait, there’s more! That third eyelid protects polar bears from snow blindness, and woodpeckers deploy it a millisecond before pecking a surface to prevent injury to the retina.
When the peregrine falcon goes into its trademark 200 mph dive, it flutters the membrane to wipe debris from the eye’s surface, keeping vision crystal clear.
The fluttering also spreads moisture across the eye, keeping it lubricated, which brings us to another important function of the nictitating membrane: tear production.
The Nictitating Membrane and Lubricating Fluid
Depending upon the animal, there are a couple of different glands in the membrane that produce up to 50 percent of the lubricating fluid of the eye.
The membrane and those glands are subject to medical problems, too. Depending upon the injury or condition, treatment runs from simply instilling drops or ointments, to using antibiotics, to surgery.
One of the more common conditions is known as “cherry eye” and is actually the membrane in prolapse.
Veterinarians can often manually correct the condition, but chronic cases usually result in surgery.
Oh, yeah, that reddish tissue in the corner or our eyes is the vestigial remnant of our own nictitating membrane.
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© 2017 Bob Bamberg