The third eyelid is a membrane at the inner corner of a dog's eye. You may sometimes glimpse it during rest, but a new or persistent change while your dog is alert warrants veterinary advice. Do not assume that a visible membrane, a pink lump and an eye that looks sunken are all the same condition.
For more on eye care and changes to look out for, see Dog Eye Stye. For more detail on eye care and changes to look out for, start with How to Get Something Out of My Dog's Eye, Dog Eye Twitching and The Origins of Veterinary Clinical Nutrition.
Seek urgent care for pain, squinting, an eye held shut, cloudiness, injury or a vision change. Obtain veterinary advice promptly if your dog is also unwell, weak or vomiting; new balance or neurological changes need immediate veterinary attention. Do not wait for the membrane to cover a larger part of the eye.
For more on eye care, see Eye Supplements for Dogs.
Describe what you can see without moving it
Note when the change began, whether one or both sides are affected and whether it disappears when your dog becomes alert. A photo taken without delaying care can help document an intermittent change. Do not lift the membrane, press on the eyeball or push a lump back with a finger.
Describe any swelling around the eye separately. Squinting can indicate discomfort, and cloudiness needs examination of the eye structures. These signs matter even when the membrane is the feature you noticed first.
Read Cataracts in Dogs UK for more detail on eye care and changes to look out for.
A visible membrane has more than one explanation
Eye pain, changes in the position of the eye, disorders of the membrane or gland, and some nerve or general health problems can affect what is visible. The veterinarian assesses the whole dog as well as the eye. One affected side does not identify a local cause, and two affected sides do not identify a particular systemic disease.
For related questions about eye care and changes to look out for, read Entropion in Dogs, Pannus in Dogs UK and Ectropion in Dogs. For more on eye care and changes to look out for, see Bacterial Eye Infection in a Dog.
A painful corneal ulcer is one possible ocular problem to consider, but a raised third eyelid is not an ulcer test. Conjunctival inflammation and other surface problems can also produce changes around the eye. Do not choose treatment by comparing a photograph with an online example.
Our guide Uveitis in Dogs UK covers a related part of eye care and changes to look out for.
Report recurring watering, green or yellow discharge or a change from ordinary eye-corner debris. These observations help establish the history. They do not identify an organism, tell you that an eye is adequately lubricated or show that the membrane itself needs cleaning. To work the relevant checks into your usual routine, see Best Dog Eye Cleaner for Tear Stains.
For more on eye care and changes to look out for, see Angel Eyes for Dogs UK.
A pink corner lump is not a home diagnosis
A prolapsed third-eyelid gland, often called cherry eye, is one possible explanation for a pink or red mass. Cartilage abnormalities and other lesions can look similar. A veterinary examination establishes what tissue is involved and what treatment is suitable. To work the relevant checks into your usual routine, see Dog Eye Cleaner.
For more detail on eye care and changes to look out for, start with Dog Blinking: Eyelids, Tears and Everyday Eye Comfort and Subconjunctival Haemorrhage in Dogs.
The guide Cherry Eye in Dogs goes into more detail on this part of eye care.
The gland contributes to tear production. When prolapse is confirmed, preserving and repositioning it is generally preferred to removing functioning tear tissue. That does not mean an owner should attempt manual replacement. Ask the clinician about the procedure, individual risks and review plan rather than applying a fixed recovery promise.
A history of dry eye is particularly useful to report because tear protection is part of the assessment. It does not show whether a newly visible membrane has the same cause as a previous eye problem.
If you want to look at the options in more detail, start with Blocked Tear Duct in Dogs.
What to tell the clinic
Bring the names of recent medicines, any sedation or anaesthesia, possible trauma, appetite changes and relevant ear or neurological history. The veterinarian may compare pupils and eye position, examine the membrane and perform tests chosen for the findings. Avoid expecting every examination to be painless or follow one fixed sequence.
Redness and the warning signs commonly associated with infection should be reported without labelling the cause yourself. Concurrent itching elsewhere can be relevant, but allergy and other ocular conditions cannot be separated simply by counting affected eyes.
Read Glaucoma in Dogs in the UK for more detail on eye care and changes to look out for.
Protect the eye while getting advice
Prevent rubbing as safely as you can and ask whether a suitable protective collar is needed. Do not probe beneath the membrane or start human redness-relief, leftover steroid or anaesthetic drops. The non-prescription eye-drop guide explains why a familiar bottle may be unsuitable for a new problem.
For more detail on eye care and changes to look out for, start with Dog Ear and Eye Cleaners and Pekingese Dog Care.
For a chemical splash, start gentle irrigation immediately with copious clean lukewarm water or appropriate saline while arranging urgent veterinary help. Do not delay for a particular product or neutralise the chemical. Keep the packaging and follow the clinic's instructions about continued flushing.
For related questions about eye care and changes to look out for, read Optixcare Eye Lube for Dogs and Best Tear Stain Remover for Dogs.
Care after assessment
If external grooming is appropriate, the eye-cleaning guide describes softening accessible material on surrounding fur without rubbing the eyeball. It does not teach how to reposition a gland or treat a nerve disorder. Stop if handling causes discomfort.
To compare cleaning and daily care, see Dog Eye Wipes and Saline Solution for Dogs' Eyes.
Use the written review plan and contact the clinic sooner if comfort, eye appearance, vision or general health worsens. The dog eye-health guide can help organise observations without turning one visible membrane into a diagnosis.
Read Can Dogs See Color? What Blue, Yellow, Red and Green Look Like for more detail on eye care and changes to look out for.
Clinical references
Cornell: cherry eye; Merck Veterinary Manual: the cornea; Cornell: keratoconjunctivitis sicca; Merck Veterinary Manual: the conjunctiva; Merck Veterinary Manual: corrosive exposure; ACVO: cleaning around the eyes.
This educational article does not replace individual veterinary assessment or prescribed treatment.