Your dog has three eyelids. The third eyelid is called the nictating membrane which provides your dog’s eye with extra protection by closing upward from the bottom of the eye. Cherry eye occurs when the nictating membrane and the tear gland — the gland that is responsible for producing about 30% of your dog's tears  — located on nictating membrane's underside become everted. This eversion blocks your dog’s abiity to retract the lid back into the eye. Cherry eye occurs more frequently in certain breeds such as Basset Hounds and Beagles.

Your dog will have a red, fleshy-looking bulb in the inner corner of the eye (the side closest to his nose). Your dog may also blink or squint more than normal. Cherry eye itself is not painful, but the growth can irritate the eye and cause pain, and, over time, the exposed tear gland can become irritated, inflamed and painful. Also, if your dog rubs his eye, the gland can become infected and painful. Finally, again over time, the tear gland's ability to produce tears can be hindered which may also cause your dog pain.