"Cherry eye" is a common term for prolapse of the third eyelid gland.  dogs, have an "extra" or third eyelid located inside the lower eyelid, also referred to as the “nictitating membrane.” The third eyelid serves as an additional protective layer for the eye. The third eyelid also contains a special gland that produces a significant portion of the eye’s protective tear film. When this gland prolapses, the condition is known as "cherry eye".