Do cats have two eyelids?

Cats like humans have eyelids. This thin fold of skin can be involuntarily or voluntarily opened to protect this very important organ from foreign bodies by covering its surface. Cats are predators. In the wild, these animals need to hunt to survive. Cats also need to fight their predators. Nature has gifted cats not only with a pair of eyelids that will provide the eyes protection. Cats do not only have two eyelids (upper and lower). Nature has given cats double eyelids and these structures are called third eyelids. This inner eyelid tucked inside the corner of the cat’s eye has the very important function of ensuring that the surface of the eyes is kept free of debris that can damage this most important organ.

Cats’ acute vision has made these animals highly efficient hunters. Cats are famed for their enhanced night vision. The structure and the location of the eyes afforded these feline friends depth perception and greater field of view. Needless to say, the eyes have to be protected as it is a vital tracking and hunting tool. The eyes must be protected from being scratched by blades of grass when it runs after a prey. Domestic cats do not need to hunt to survive but these animals are often seen fighting with other cats in the neighbourhood. Cats can get their eyes injured from these fights. Cats are famous for their stares. An unblinking eye is probably an advantage when the cat is hunting. However, the surface of the eyes has to be kept lubricated by tears and other eye secretions. This is where the importance of the eyelids comes in. The closing of the upper and lower eyelid will keep the surface of the eyes moist. A blink is a reflex action of the eyelid to protect the eyes from foreign bodies. The third eyelid provides the cat’s eyes with extra level of protection.

The third or inner eyelid is commonly called haw but it is called palpebral tertia in scientific terminology. Just like the wisdom teeth and the appendix of humans, the third eyelid or the nictitating membrane is considered as a biological oddity. The third eyelid was once considered functionless and an irrelevant structure. Scientists have even considered its removal so that the cat’s eyes can be fully studied. However, because this nictitating membrane is present in most mammals and in birds, scientists have deemed it necessary to study this structure so as to know its functions. The nictitating membrane is a fold of very thin tissue that is covered with conjunctiva or a specialized mucus membrane. This thin membrane is large enough to completely cover the surface of the eyes. Unlike the outer eyelids that is moved up and down to cover the eyes, the nictitating membrane moves horizontally so that the eyeball is partially or totally covered by the transparent or translucent fold of tissue. One side of this special membrane faces the cat’s inner eyelid and the other side face the cornea. As the nictitating membrane can cover the entire surface of the eyes, its main function is to remove debris that gets into the eyes and to distribute tears to keep the cornea moist. The eyelid also protects the cornea from injuries that can be sustained by the cat while hunting or while fighting its predators. Normally, only a small portion of the inner eyelid is visible as this thin membrane is hidden within the eye socket. It would only be visible when the cat blinks and causes the skeletal muscles of the eyeball to retract so that the thin tissue would move across the surface of the eye.