As humans, we possess two eyelids that cover our eyes when we blink or close them. At first glance, it may appear that your canine possesses these same features, but take a closer look, and you’ll discover that they have a third eyelid.
Find out why dogs have a third eyelid and why this is integral to maintaining good eye health and vision. Plus, learn the reasons why you do not want this membrane to be visible and the steps to take if it is.
Dogs Have a Third Protective Eyelid
We know that most mammals – including humans – possess an upper eyelid and a lower eyelid. We use these to protect our eyes from dust, dirt, and light. But did you know that dogs have a third eyelid, or that it serves a particular purpose?
No matter your dog’s breed, they will possess a third eyelid called the nictitating membrane. This extra lid sits beneath the outer two and draws horizontally from the inner to the outer points of the eye.
There is no definitive color to the nictitating membrane; instead, it depends on the breed and health of your dog. Some canines could possess a cloudy, yellow eyelid, while others could have one that is red, pink, or clear. The eyelid should not be bright red in nature; if it is, it could signal irritation or inflammation.
The nictitating membrane possesses some clear differences between your dog’s two main eyelids. The biggest difference is that it is made of a thin, moist membrane rather than the skin and fur that comprises their other eyelids.
A Dog’s Third Eyelid Offers Protection from Debris
Humans seem to function okay with two eyelids, providing ample cover for our entire eyeball, so what’s the purpose of the third eyelid in dogs? It works in conjunction with the other two eyelids to perform many of the same functions, such as keeping the eye clear and healthy through a kind of windshield wiper mechanism.
The nictitating membrane also produces around a third of a dog’s tears, thanks to the tear glands at its base. All these elements are crucial structures to help your dog fight infections.
The glands in the third eyelid can produce and provide antibodies to the eye, which help to wash away irritants and foreign bodies, which could otherwise cause harm to your canine. This mechanism also helps to protect the cornea – a vital component in your dog’s eyesight.
By itself, the cornea does not produce any blood vessels that can fight against infections, so the nictitating membrane does an excellent job of protecting this vital feature.
Dogs have a third eyelid because they need the extra cover, moisture, and protection – and they aren’t the only ones to employ this mechanism. Other animals such as cats, camels, birds, reptiles, and even fish possess three eyelids to increase the protection of their sensory organs.
While some species – such as sharks and reptiles – can close the third eyelid over their entire eye, a dog’s nictitating membrane only covers around 50% of their eye surface. This means the third eyelid is a vestigial membrane – a kind of leftover remnant of evolution that isn’t fully formed, much like a human coccyx.
Routinely Inspect Your Dog’s Eyes
A healthy membrane means healthy eyes for your canine, and the nictitating membrane does a great job of keeping your dog’s eyes clear and healthy. Still, it can’t fight every infection, so you need to look for changes in eye health.
Discharge, swelling, itchiness, repetitive blinking, and discoloration are all signs of eye infection in dogs. In this case, book an appointment with your vet as soon as you can; they will be able to address any problems before the infection spreads or causes issues with your dog’s eyesight.
If you want to check the general health of your dog’s eyes, try the following:
- Check both eyes for signs of dirt, debris, or foreign objects.
- Ensure that neither eye is emitting any discharge of excess tears.
- Check that the whites around both eyes are clear and free from redness.
- Ensure your dog’s eyes aren’t crusty or cloudy.
- Check the upper and lower eyelids for any signs of redness and irritation.
Before touching your dog’s eyes, wash your hands thoroughly to avoid introducing any new bacteria or debris.
Why Is My Dog’s Third Eyelid Showing?
In general, you shouldn’t be able to see your dog’s third eyelid. Exceptions to this rule include times when they’re suddenly woken up or are slowly opening their eyes following deep sleep. Visible membranes outside of these circumstances – or a membrane that doesn’t recede fully – could be a sign of a medical issue.
The most common causes of a visible third eyelid are cherry eye, neurological disorder, or poor physical health.
“Cherry eye” refers to the appearance of a bright red, cherry-like patch in the corner of the eye caused by the prolapse of one of the glands within the third eyelid. This congenital disorder is prevalent in breeds (such as some flat-faced breeds and Cocker Spaniels) and means that the nictitating membrane can no longer employ its “windshield wiper” mechanism.
Several neurological conditions can also cause the third eyelid to show, such as Horner’s syndrome, characterized by droopy or sunken eyes and eyelids. Causes are currently unknown though bites or trauma to the head can trigger this condition.
In addition, a visible third eyelid could point to general health concerns such as dehydration, malnourishment, or eye problems. If you notice that your canine’s third eyelid suddenly becomes visible, check in with your vet for the best advice.
