Important Information…the advice I am providing is solely based on the information you are giving me. To accurately diagnose and treat a patient, a veterinary doctor-patient relationship is required. Before I can tell you what is wrong, I must see you pet and perform an examination. Over the internet, your description may be misleading and you may not see what a trained veterinarian would see. Therefore, the advice I give you is not to be taken as 100% accurate. I can tell you how I have treated cases in the past with similar symptoms and you must make a decision whether you want to follow my advice or not. This service is not a substitute for visiting your vet. With that in mind, let us start…
Here is a link that may help explain in more detail. I must caution you that I have not reviewed this link and unless it is written by a veterinarian, the information can't be held to be medically accurate. But just to give you information that might help here it is:
The following link covers Horner's Syndrome which I don't think you are describing. However, look at the illustration and see the cats left eye is partially closed with the 3rd eyelid protruding.
Here is another link that gives a little more description of the anatomy of the eye and they include information about the third eyelid. Go down to where it says, "Accessory structures" and you'll see the "nictitating membrane" or third eyelid in the diagram.
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