Have you noticed the third eyelid (TE) in your dog? You can see it when looking closely on the inside corner of the eyelids near to his/her nose. Its color varies depending on his/her breed, from clear and almost transparent to a cloudy pinky-white pigmentation. We call it the nictitating membrane or haw.
The outer eyelid moves up and down, but this inner one moves across the eyes of your dog. It moves from the inner part of his/her eye towards his/her ears. All dogs have a third eyelid and so you don’t be alarmed when you see it! He/she was born with it just like all his/her furry friends.
When the nictitating membrane closes across your dog’s eye, it looks as though his/her eye is rolling back in his/her head. What actually happens is that the upper and lower eyelids open making it appear he/she has white eyeballs because the third eyelid is also closed, it can happen when he/she is falling asleep for example.
We have both an upper lid and a lower lid, your dog also has these, plus the ‘third eyelid’, all effectively protecting his/her eyes, maintaining them moist and clear from debris. This is useful as it avoids your dog from having to blink when looking at a prey for example for hunting dogs, and of course for focusing on a game. Cats, birds, fish, reptiles and camels also have three eyelids.
If you feel your dog needs to be checked if something looks abnormal, make sure you visit your veterinarian for a check-up.