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 (see Enophthalmos), 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.