Do cats have three eyelids?
Cats do indeed possess a third eyelid, which can sometimes be glimpsed when a pet wakes or yawns. The nictitating membrane (or palpebra tertia, as it is formally known), lies behind the other two and sweeps horizontally across the eye.
Why do cats have three eyelids?
It’s not only cats that are blessed with this ocular equipment. Nictitating membranes are the norm among birds and mammals as well as some fish and reptiles, providing additional physical protection for the delicate cornea, and helping to lubricate and clean the eyeballs. They may be translucent to allow partial vision even when closed.
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Why did we lose our third eyelid?
Humans and most other primates lack a third eyelid - functional nictitating membrane -perhaps because we tend to lead with our hands rather than our faces when exploring the world, making our eyes less vulnerable. All that remains of our own are the fleshy, triangular bumps that sit in our eyes’ inner corners.
Main image: lioness eyes © Getty images