Why do cats have two eye lids and humans only have one?
Asked by Claire Ennis via Facebook
Presumably you mean three eyelids, Claire?
THE SHORT ANSWER:
While we no longer have a third eyelid to the same extent as some animals, we do have a very small remnant of it. This small remnant has not retained much of its original functionality. The reason for this is probably because over the course of evolution our species’ forerunners no longer needed it to the same extent as the animals which have a third eyelid to this day.
THE FULL ANSWER:
As I’m sure you’re well aware of, human beings have two palpebrae (eyelids) over each eye; one at the top and one at the bottom.
Many other animals, from cats, to certain birds and fish, have what’s known as a third eyelid. This third eyelid, technically referred to as the nictitating membrane, is located towards the corner of the eye closest to the nose.
Often times it is made out of cartilage that is covered by a thin skin-like layer which helps to protect and lubricate the eye. We call this layer the conjunctiva.
More importantly, a gland, appropriately called the gland of the third eyelid, is attached to the nictitating membrane of some animals. This gland helps to produce up to 50% of the lubrication (tears) necessary to keep the eye moist and clean. Some species of animal have another gland, called Harder’s gland, which does this as well.
Another important structure in some third eyelids is an area of immune cells we call lymphoid tissue. The cells in this tissue help to kill off invaders, such as bacteria, which may enter your eye.
If you’ve ever seen the third eyelid move across your cat’s eyes, you’ll have noticed it moves in a horizontal fashion across the eye. This is in contrast to the other two eyelids, which move in a vertical plane. However, regardless of which eyelid we’re talking about, you should have already gathered that they serve the same main function: protection of the eye.
In the case of the nictitating membrane, it is able to clear away debris from the eye, keep it moist, or actually keep away excess moisture or sunlight; depending on the circumstances and animal we are talking about.
Some animals are able to actively move the third eyelid using a subset of eye muscles (this includes the cat); while other animals employ passive means of accomplishing this movement.
However, having this third eyelid isn’t without its own set of problems. Among many other issues, the gland of the third eyelid can pop out and cause a condition known as “cherry eye” and in other cases you can get some nasty cancers of this eyelid.
You have a third eyelid too!
Ok, not exactly. You do have what we call a vestigial remnant of the third eyelids our evolutionary ancestors may have had. A vestigial remnant is something which has lost much or all of its original functionality over the course of evolution (like your appendix). In humans, this structure is called the plica semilunaris, and it has significantly less functionality when compared to animals which have a full blown nictitating membrane.
Answered by Artem Cheprasov