Cherry eye is the term use to refer to canine nictitans gland prolapse, a common eye condition in various dog breeds where the gland of the third eyelid known as the nictitating membrane prolapses and becomes visible. Commonly affected breeds include the Bulldog, Cocker Spaniel, Beagle, Pekingese, Neapolitan Mastiff, and Basset Hound. Cherry eye may be caused by a hereditary weakness in the connective tissue surrounding the gland. It is most common in puppies.