So often we look at our pets, and they seem like little people in dog and cat costumes. We can be reminded that this is not true when we notice that dogs and cats actually have body parts that people are missing. Besides tails, another structure absent in people is the third eyelid.
Dogs and cats both have a third eyelid, or nictitating membrane, which is often retracted, and therefor unseen. This pink tissue appears behind the two “regular eyelids,” and sweeps up and out from the inside corner of each eye. This eyelid had several functions, including acting like a windshield washer when debris gets upon the eye. The third eyelid can also pop up very quickly and act like a shield when the eye is in danger of being injured. The third eyelid also contains additional tear glands to help lubricate the eye, as well as lymph tissue to help fight off infection.
Dogs, and especially cats, may partially raise their nictitans when they are not feeling well. Because this tissue is pink, it can make the eyes look like they are rolling back in the head, or that the eye itself is turning colors. If the third eyelid is completely raised, one cannot see the eye at all. The third eyelid will sometimes be visible when your pet is asleep, especially when they are sleeping with their eyes open. If you pet is awake and keeping their third eyelids up, they definitely need a check up, as it can be a sign of disease, or generally not feeling well.
Some disease’s, such as Horner’s syndrome, may cause the third eyelid to raise on it’s own. Dogs and cats with an eye injury may partially raise their nictitans so as to partially protect their cornea, or the outer surface of the eye. Some pets may blink with their third eyelids if their regular eyelids will not work, such as when a cocker spaniel (or another breed) has facial nerve paralysis from a severe ear infection. Cats notoriously raise their third eyelids when they are just feeling “yucky.”
“Cherry eye” is a common condition in dogs, where the tear gland on the third eyelid becomes inflamed and protrudes from the inner corner of the eye. These pink bumps can be quite large in some dogs, and very unsightly. These swollen glands will occasionally respond to medication, and otherwise can be surgically replaced and anchored into their normal position. Years ago, these prolapsed tear glands were routinely removed; unfortunately removing this gland has the potential to cause “dry eye”, where the eye cannot produce enough tears to keep itself moist and healthy.
Interestingly, it may not be unique that our pets have a third eyelid, but rather that people do not. Most other animals, from horses to lizards to birds or even manatees, have a form of this structure, while in people all we have is small bumps where this third eyelid should be. Dogs and cats may not have an extra eyelid; you and I are perhaps missing one.