15 Mind-Blowing Cat Facts You Need To Know
3. They Have Three Eyelids
Something that cats have in common with dogs, birds, fish, and some other mammals is that they have three eyelids on each of their eyes. This extra eyelid acts as a layer of protection for cats' corneas, particularly when they engage in random paw fights with other cats outside your home in the middle of the evening.
Medically referred to as a nictitating membrane, the additional eyelid can help moisten the eyes to ensure a cat's vision is left unaffected in specific situations, for example, when they're prowling through long strands of grass. Seeing as it's positioned in the corner of a feline's eye, it can often be a disheartening experience for a cat owner to see this on their beloved pet, though it usually isn't any sort of an issue. That said, a cat should be taken to the vet if the third eyelid is present to be on the safe side as this could be the cause of an infection.
It's thought that humans were also gifted with a third eyelid at some stage of the evolution process. Unlike cats though, we humans don't have to capture our prey by going in face-first, leading to the extra proteciton disappearing into the inner corner of the eyes as it wasn't being used for anything.