At Animal Vision Center of Virginia, we often assume pet parents know about their pet’s ‘third eyelid.” However, some owners may not be familiar with this anomaly. If you are among the curious, here’s a quick tutorial! Dogs, cats, birds, and some reptiles and mammals have three eyelids that help protect and lubricate their eyes: Upper eyelid: Like humans, this eyelid helps close the eye. Lower eyelid: This eyelid protects the eye and keeps it moist, like the upper one. Nictitating membrane: Often called the “third eyelid,” this thin, translucent membrane sits inside the lower eyelid in dogs and cats. It plays a crucial role in tear production, responsible for 40-60% of the tears.