The nictitans, or third eyelid, is composed of a fold of conjunctival tissue at the medial-inferior aspect of the eye. Giving support to the fold is a T-shaped cartilage. At the ventral base of the cartilage lies the gland of the nictitans, which produces mucus and aqueous tears. Along with the eyelids, the nictitans functions to protect the cornea, to spread tears over the cornea and to clean the corneal surface.

Prolapse of the nictitans gland is commonly seen in cocker spaniels, bulldogs, Boston terriers, Shih tzus and many other breeds. In this condition, the base of the gland flips up and is seen above and behind the border of the nictitans. Prolapse frequently involves both eyes and usually is encountered in young dogs. Treatment is surgical replacement of the gland and cartilage. Even though the condition can sometimes recur, the gland should not be removed. Because the gland produces a significant portion of the tears, removal can contribute to the development of KCS.