The third eyelid (membrana nictitans) lies between the eyelids and the eyeball at the inside corner of each eye. Its purpose is to protect the eye and to help contain and spread tears over the cornea. The third eyelid also contains tear-producing glands and therefore contributes to the overall lubrication of the eye.
The gland of the third eyelid lies within the substance of the third eyelid and occasionally protrudes over the free edge of the third eyelid. As a result, the exposed, sensitive tissue becomes very irritated and inflamed, causing considerable discomfort. The reddened, swollen tissue resembles a cherry; hence the common name of this condition is “cherry eye.”
Use of medication alone often fails to alleviate the condition. Surgery is usually required for correction.