thumb|Close-up of a cherry eye
Cherry eye is a disorder of the nictitating membrane (NM), also called the third eyelid, present in the eyes of dogs and cats. Cherry eye is most often seen in young dogs under the age of two. Common misnomers include adenitis, hyperplasia, adenoma of the gland of the third eyelid; however, cherry eye is not caused by hyperplasia, neoplasia, or primary inflammation. In many species, the third eyelid plays an essential role in vision by supplying oxygen and nutrients to the eye via tear production. Normally, the gland can turn inside-out without detachment. A similar condition exists in dwarf lop-eared rabbits, which occurs in the harderian gland. Similar surgical treatment is necessary. Some breeds are considered to be more susceptible to its development than others, including the Bulldog, Boston Terrier, Bull Terrier, Lhasa Apso, Cocker Spaniel, St. Bernard, Shar-pei, Shih Tzu and Poodle The NM contains many glands which merge and appear as a single gland. Typically, glands secrete tears for lubrication of the cornea. KCS is a chronic degenerative conjunctivitis that can lead to impaired vision and blindness. Surgery types are broken into two groups: anchoring procedures and pocket/envelope procedures. If exposed for extended periods of time, the gland is at risk for trauma, secondary infection, and reduced tear production.