Dogs actually have three eyelids: the upper, lower and a third eyelid which is called the nictitating membrane. The nictitating membrane is a thin, opaque tissue which rests in the inner corner of the eye, below the lower eyelid. The purpose of the third eyelid is to provide additional protection to the eye and cornea, and also spreads tears across the eyeball surface.
In most cases, the nictitating membrane remains retracted and is not visible. When it becomes visible, this is usually because the eyeball has sunken into the socket, and may be an indication of illness or of a painful eye. Pet owners may occasionally observe the third eyelid when their dog is relaxed and falling asleep.
In some dogs, a portion of the third eyelid is always visible; this condition is called haws. When a dog is born with the third eyelid visible, this is a normal condition and does not indicate illness, although it is not desirable in show dogs.
The third eyelid can also prolapse, leading to a condition commonly referred to as “cherry eye.” This condition is more common in certain breeds, and can be surgically corrected by your veterinarian.