Cherry eye is the term used to refer to canine nictitans gland prolapse

Prolapse

Prolapse literally means "to fall out of place", from the Latin prolabi meaning "to fall out". In medicine, prolapse is a condition where organs, such as the uterus, fall down or slip out of place. It is used for organs protruding through the vagina or the rectum or for the misalignment of the...

, a common congenital eye defect in various dog breeds where the gland of the third eyelid known as the nictitating membrane

Nictitating membrane

The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...

 prolapses and becomes visible. Commonly affected breeds include the Bulldog

Bulldog

Bulldog is the name for a breed of dog commonly referred to as the English Bulldog. Other Bulldog breeds include the American Bulldog, Olde English Bulldogge and the French Bulldog. The Bulldog is a muscular heavy dog with a wrinkled face and a distinctive pushed-in nose...

, Chihuahua

Chihuahua (dog)

The ' is the smallest breed of dog and is so named for the state of Chihuahua in Mexico. Chihuahuas come in a wide variety of sizes, head shapes, colors and coat lengths.-History:...

, Cocker Spaniel, Beagle

Beagle

The Beagle is a breed of small to medium-sized dog. A member of the Hound Group, it is similar in appearance to the Foxhound, but smaller, with shorter legs and longer, softer ears. Beagles are scent hounds, developed primarily for tracking hare, rabbit, and other game...

, Pekingese

Pekingese

The Pekingese, or "Peke" is an ancient breed of toy dog, originating in China. The breed was favored by the Chinese Imperial court, and its name refers to the city of Beijing where the Forbidden City resides...

, Neapolitan Mastiff

Neapolitan Mastiff

The Neapolitan Mastiff, Italian Mastiff, is a large, ancient dog breed. This massive breed is often used as a guard and defender of family and property due to their protective instincts and their fearsome appearance....

, and Basset Hound

Basset Hound

The Basset Hound is a short-legged breed of dog of the hound family. They are scent hounds, bred to hunt rabbits and hare by scent. Their sense of smell for tracking is second only to that of the Bloodhound....

. Cherry eye may be caused by a hereditary weakness in the connective tissue

Connective tissue

"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 surrounding the gland.