The third eyelid, otherwise known as the nictitating membrane, lies in the conjunctival sac at the medial canthus. It is made up of a t-shaped piece of cartilage which is enveloped by conjunctiva. The gland of the third eyelid sits at its base and is responsible for production of up to half of the tear film in dogs. There are also many lymphoid follicles which reside beneath the inner / bulbar surface.

The third eyelid moves across the eye passively when the globe is retracted, and this occurs every time an animal blinks. There are some muscular attachments but their effect is not as great.

The main functions of the third eyelid are tear production, tear film distribution and corneal surface protection. Therefore, removal or disease, of the third eyelid can lead to drying of the cornea, chronic keratitis and conjunctivitis and decreased tear production.