It usually means your cat is sick or hurt. If it is under a bed or in some place where you can't touch him/her that is an even bigger sign. If I were you I would take them to the vet as soon as possible. Trust me.
I don't think this question us nessary.
I'm not sure but if specifically the third eyelid is showing it is a sign of illness or possibly injury and you should get the cat or dog to the vet.
Seeing your cat's third eyelid is a common sign of disease in cats. To answer your question, yes, it is very possible it means that your cat is sick.
The camel has a third eyelid.
The third eyelid in the cat (and dog) is found in the lower corner of the eye on the side near the nose. It can be visualized on occasion as it may partially elevate and partially cover the eye. In birds, the third eyelid is very functional and sweeps across the entire eye. It too is located in the same spot as the cat and dog. Even humans have a third eyelid, however, it is vestigial (a small non-functional remnant).
The third eyelid on a frog is called a nictitating membrane.
This could be due to anything from a genetic conformation of the orbit to infection in the third eyelid. If this is a new development, you should probably have your vet take a look at it.
A nictitating membrane The nictitating membrane is the THIRD eyelid, the SECOND eyelid that you asked about is just that, the eyelid, just like in humans
Yes, sometimes when they are drowsy and are so relaxed that when you stroke their head their top eyelid pulls up it is possible to see what can only be explained as a white eyelid which is the third eyelid. This is perfectly normal. However, if you are seeing this eyelid when the cat is awake then i'm not so sure this is so normal as my cats is only visible, as i've said above when drowsy. So perhaps if you are seeing this when the cat is awake, a quick trip to the vets to make certain all is ok may be advisable? But if I'm honest I'm sure it is and my initial interpretation of when you actually saw it is more accurate. Hope this answer is helpful!
Cats do have a second eyelid to show whether they are well or unwell.
Nictitating Membrane
Three. They have the upper and lower lids, similar to humans. They also have a third membrane that acts as an eyelid, so to speak. The third "eyelid" moves from the outside edge to the inside edge.
The eyelids act to protect the anterior surface of the globe from local injury. Additionally, they aid in regulation of light reaching the eye; in tear film maintenance, by distributing the protective and optically important tear film over the cornea during blinking; and in tear flow, by their pumping action on the conjunctival sac and lacrimal sac. The third eyelid which is seem in some animals and birds is called the nictitating membrane or haw as it is called, closes across the eye (unlike the upper and lower eyelids) and are there as extra protection for that all important eye. It is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining vision.