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The feline eyelid is very different from human eyelids. First of all cats do not have cilia, which we know as eyelashes, though they do have a thin layer of lashes that are somewhat like eyelashes, simply because they are different from any other kind of hair. The cat's eyelids are also more tightly fit against the cornea than humans.
The conjunctiva is a thin membrane that covers the eyeball. The conjunctiva and eyelid touch. Felines have less of a conjunctival sac than humans. Because the eyelid is so close to the conjunctiva it can be subject to several kinds of infections such as conjunctivitis (reddening of the eye). Persian and Himalayan cats suffer from various glandular cysts in the eyelids.
Humans have a third eyelid called the Nictitating membrane which is the pink lump found in the corners of the eye. The nictitating membrane in humans serves no useful function today. However, the nictitating membrane in cats and other animals still have a function.
The pupil of the cat is larger than in the human eye and the rods, which are the receptors for light and are more numerous. There are as much as three times more rods in the cat's pupil than found in human pupils. Therefore cats need an extra protection against light to protect their sensitive eyesight.
Feline's have an additional protective layer called the tapetum ludium. Finally they too have the nictitating membrane or third eyelid, which can serve as an extra protection against light as well.
Like humans the nictitating membrane is found in the corner of the eyes, but most of the time it is unnoticeable except when the cat is sick, then you can see the membrane protrude from the corner of the eye and stretch towards the middle of the eye.
Cherry eye is an uncommon condition found in cats whereby the gland in the nictitating membrane prolapses (fall down or fall out of place).
Causes
The cause of Cherry Eye is believed to be due to a weakness in the ligaments, which keep the gland in place.
It appears to be a congenital disease in Burmese cats, though the exact pattern (gene pattern) is unknown at present.
The gland may fall due to inflammation
Other causes of an unknown nature (Idiopathic nature) which could include
dermatitis, cancers, immune system anomalies, bacteria, fungus, parasites, damage from the sun, or some other internal or external trauma.
Symptoms
A pink or oval shaped protrusion out of the corner of the eye closest to the nose
Conjunctivitis (redness of the eyelid)
Discharge - watery or thick substance leaking from the eye or eyes
Diagnosis
Visual inspection of the eye
Treatment
Surgical removal of the third eyelid
Partial removal of the gland
Surgical reparation surgery to put the gland back in place
Prognosis
Cats with cherry eye should be seen by the veterinarian immediately because the disease is tricky. It may go away in a few weeks on its own, or it may also lead to some serious eye condition. Much more research will have to be conducted for a better understanding of the condition
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