A cat's third eyelid, or nictitating membrane, is typically not visible unless there is an issue. Common causes for the appearance of the third eyelid include sleepiness, sedation, or medical ...

A swollen eyelid is usually a symptom, not a condition. It's very common and is usually due to allergy, inflammation, infection, or injury. The skin of your eyelid is less than 1 millimeter thick. But ...