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I am Your Dog's Third Eyelid

Today we'll be discovering some interesting stuff about the dog's third eyelid. Yes, that's right, dogs have actually three eyelids, but where on earth is the third one? Last time you checked, you only saw two, the top one and the bottom one, so where is the third one hiding? No need to run a treasure hunt. Tucked out of sight, at the inner corner of your dog's eye, the third eyelid isn't normally visible. You may have likely seen it occasionally when your dog sleeps or wakes up after catching some Zzz's, but not seeing it much is a good thing. A third eyelid showing on other occasions may be indicative of some underlying problems. So today, let's get more acquainted with our dog's third eyelid, shall we?

Introducing Your Dog's Third Eyelid


Hello, it's your dog's third eyelid talking! I am also known as the nictitating membrane or haw. You may not know me too well, as I tend to not show up much often.

I am that membrane that you may occasionally see partially drawn across a part of your dog's eye. In other animals such as certain reptiles, birds and sharks, you may see me fully drawn across the eye, in some cases covering it completely.

In humans, I am just a vestigial remnant known as the "plica semilunaris." I have basically lost my original function during evolution, so I shrunk into a small fold that's found on the inside corner of your eye. See picture for correct location of the plica semilunaris in humans.

In dogs, I am more functional, covering the eye when your dog is asleep. Like your car's wiper blades, I help wipe any mucus or debris from across the surface of your dog's eyes.

According t o the American College of Veterinary Ophthalmologists, I also play an important role in maintaining normal tear production.

 I have a gland that's responsible for about 40 to 50 percent of your dog's tear production. Last but not least, my lymphoid tissue has an immunological function producing antibodies to help fight infections.

dog third eyelid

Third-Eyelid Showing

As mentioned, you don't normally see me other than when your dog sleeps with his eyelids semi-open or you see me for a split second as I retract when your dog wakes up suddenly from a deep sleep.

You see, I cover your dog's eyes when your dog is sound asleep so to prevent his eyes from drying up too much. Your dog doesn't blink in his sleep so I'll take over the role of keeping those eyes nice and moist.

So yes, it's a good thing if you notice your dog's "red eyes" when he's sleeping. What you are seeing is basically me doing my job. You need to worry instead when I do not normally retract when your dog's eyes are open and your dog is wide awake, as this can be a sign of something wrong.

If you see me when your dog is wide awake, under the form of a pinkish triangle covering the inner corner of one eye, it could mean an eye problem. The dog's eye may be painful or injured or a nerve may have been damaged, explains veterinarian Betsy Brevitz in the book " The Complete Healthy Dog Handbook: The Definitive Guide to Keeping Your Pet Happy, Healthy & Active."

There are no muscles attached to me, so my range of motion is completely passive. If the eyeball sinks in, I will automatically cover the eye. When you see me in both eyes, unless both eyes may have been injured, something more systemic (meaning widespread) may be going on. Third eyelids showing in both of a dog's eyes could be a sign of dehydration, illness or pain. Consult with your vet for proper diagnosis and treatment.

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Prolapsed Nictitans Gland 

My gland responsible for making tears ( the nictitans gland) is located on the side that lies against the eye, so it's normally not visible. Sometimes though, a small ligament responsible for holding the gland in place, may stretch or break for unknown reasons, explains veterinary ophthalmologist Rhea V. Morgan.

When this happens, my gland will prolapse, leading to a visible reddish mass in the corner of the eye that is commonly referred to as "cherry eye."

You see, this mostly happens in certain dog breeds that are prone to it such as cocker spaniels, basset hounds, lhasa apso, shih-tzu, poodle, beagles, Newfoundlands, Chinese shar-pei, Saint Bernards and bulldogs.

To correct this, I need cherry eye surgery to get that gland replaced so that it can continue to produce tears as it was meant to and to prevent the emergence of complications such as a chronic dry eye.

As seen, I perform several duties! I hope this guide has helped you understand me better! I sometimes feel like I live in the shadow, tucked out of sight as I am. Out of sight though doesn't mean out of mind, so hopefully today you'll cherish the work I do to keep your dog's eyes healthy and bright! And if I ever show up when your dog is awake and active, please have me checked out by your vet to see what's going on. Yours dearly,

Your dog's third eyelid.

Dog Pawprint


  • Betsy Brevitz, " The Complete Healthy Dog Handbook: The Definitive Guide to Keeping Your Pet Happy, Healthy & Active." Workman Publishing Company; 1 edition (April 16, 2009)
  • Christine C. Lim, "Small Animal Ophthalmic Atlas and Guide", Wiley-Blackwell; 1 edition (February 2, 2015)

Photo credits:

  • Prolapsed gland of the third eyelid (cherry eye) in a dog, by Joel MillsCC BY-SA 3.0
  • The plica semilunaris of conjunctiva, by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body, public domain

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