Domestic cats have three eyelids per eye. I will explain where they are. There is one eyelid above the eye which comes down over the eye when the cat blinks. The second eyelid is below the eye. This mirrors human anatomy. The third eyelid is interesting in that it is vestigial in domestic cats and dogs. By that I mean it is not fully functioning as is the case in animals such as the bald eagle and the masked lapwing. In these animals it is a transparent eyelid which can be drawn across the eye to moisten and protect it while maintaining vision.
Upper and Lower Eyelids
The cat’s upper and lower eyelids are folds of skin which support the front of the eye. Interestingly, they don’t make direct contact with the surface of the eye because there is a thin layer of tears between the inside of the eyelid and the front of the eye. The edges of the lower eyelid and the upper should meet when the eyes are closed. If it doesn’t happen the cornea dries out which can cause irritation to the cat. Cats don’t normally have eyelashes but they can be present and they can be turned inwards irritating the surface of the eye.
In the domestic cat the third eyelid is not usually visible. When it is visible it can be indicative of feline ill-health. It can be seen if the upper eyelid of the cat is gently drawn back when they are asleep.
Sometimes you can see the domestic cat’s third eyelid when they asleep or are resting and relaxed with their upper eyelid half open. The third eyelid is drawn halfway across the eyeball and clearly visible sometimes even in healthy cats. I have see my cat in deep slumber with his eyes open but the third eyelid drawn partly across his eye.
When the cat wakes up the third eyelid retracts towards the corner of the eye. It sometimes protrudes slightly from the inside corner of the eye when the cat is awake and healthy. However this visibility is often associated with conditions such as bulging eye and retracted or sunken eye. In the case of bulging eye its visibility is caused by an infection in the tissue behind the eyeball, bleeding behind the eye or a tumour. When associated with sunken eye its visibility is caused by a painful eye illness resulting in spasms of the muscles around the eye caused by e.g. tetanus, dehydration or chronic weight loss.
Other illnesses causing prolapse (coming over the eye) of the third eyelid include, Key-Gaskell Syndrome, Horner’s Syndrome and Haw Syndrome. These are examples and your veterinarian will obviously discuss the matter further if you think that your cat is ill because of the visibility of their third eyelid.