The third eyelid, also known as the nictitating membrane or nictitans, is a membrane that sits just inside of the lower eyelid in dogs and cats. The nictitans contains a gland that typically sits at the base of this membrane. This gland plays an important role in maintaining normal tear production and is responsible for 40-60% of the tears. Damage to this gland can lead to low-tear production. Normal tear production is important to maintain ocular surface health. The third eyelid also contains cartilage for structural support and immune tissue to provide antibodies in the tear film.