Nictitans gland prolapse is the medical term for ‘cherry eye’. It is also called prolapse of the gland of the third eyelid. Unlike people, dogs have a ‘third eyelid’. This is located in the corner of each eye and contains a tear gland. Sometimes, for some reason, this gland prolapses along with the third eyelid. Or to put it simply, it comes out of its normal position and swells creating the condition known as cherry eye.