An opaque third eyelid, normally not seen, may become visible across the front of the eye, in which case the nictitating membrane is protruding. The appearance of the third eyelid indicates that the eyeball has sunken into its socket, or that the eyeball has been pulled back into its socket by spasm of the retractor muscles in response to severe eye pain.
A dog may be born with visible third eyelids, called haws. In the show ring this is often considered undesirable, because it gives the animal a somewhat haggard look. Most breed standards (if they mention it at all) require that the haws be scarcely apparent. The haws are a concern only because they give the dog an unsightly appearance. There is no medical reason to remove the third eyelid just because the haws are visible.
There is a tear gland wrapped around the cartilage of the third eyelid that is a major source of tears for the eye. In a dog with cherry eye, the fibrous attachments to the undersurface of the third eyelid are weak. This allows the gland to prolapse, or bulge out from beneath the eyelid, exposing a cherrylike growth that is really a normal-size tear gland. This growth can irritate the surface of the eye and produce recurrent conjunctivitis.
Cherry eye is a congenital defect that occurs most commonly in Cocker Spaniels, Beagles, Boston Terriers, and Bulldogs.
Treatment: Removing the third eyelid or the tear gland seriously interferes with tear production and may result in a dry eye syndrome in breeds so disposed. If the gland is removed, your dog may require artificial tears daily for life. Instead, surgery can be performed that repositions the third eyelid and the tear gland. This corrects the problem while maintaining tear production.
WebMD Veterinary Reference from "Dog Owner's Home Veterinary Handbook"