The swelling inside the cat's eye is actually the cat's third eyelid protruding. The protruding third eyelid is commonly known as cherry eye.

Cat eyes are not like human eyes. In addition to the upper and lower eyelids, cats also have a third eyelid at the inner corner of the two eyes. The third eyelid, also called the nictitating membrane, is usually hidden inside the eyelids and plays a role in protecting the eyeball and providing tears for the cornea. If you find that there are swellings in the cat's eyes that cover the eyes, it is because the third eyelid hidden in the eyelid protrudes onto the cornea.

Compared to dogs, cats have more cases of third eyelid protrusion. A small number of cats will retract their third eyelids after protruding, but most cats will not retract their protruding third eyelids. After the third eyelid protrudes and is exposed for a long time, it is prone to swelling and congestion. If the cat feels uncomfortable in the eyes, it will rub and scratch with its front paws, which can easily damage the eyelids or eyes and lead to infection. Once a cat's third eyelid is found to be protruding, if there is an Elizabethan ring at home, the cat should wear it immediately to prevent scratching, and then take the cat to a pet hospital for medical treatment.