Dogs actually have three eyelids; the upper, lower and a third
eyelid which is called 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 offer additional protection to the eye and cornea and
also spreads tears across the eyeball surface. Most animals,
except humans and pigs, have a nictitating membrane.
In most cases, the nictitating membrane remains retracted and is
not visible. When it becomes visible, this is usually because
the eyeball has sunken, and may be an indication of illness or of a
painful eye. You can sometimes see the third eyelid when your
dog is relaxed and falling asleep.
In some dogs, a portion of the third eyelid is always visible
and this condition is called haws. For some dogs this is
If the membrane suddenly pops forward, this condition is called
"cherry eye" because it looks like a small red cherry on the inside
corner of the eye. The correct term is prolapsed bland of the
third eyelid and when this happens, the supporting cartilage of the
membrane fold over and the third eyelid loses its
If you note any sudden change in the appearance of your dog's
eye such as redness, cloudiness, ocular discharge, squinting,
rubbing the eye, change in vision, protruded third eyelid, you
should set up a visit with your veterinarian.