Apart from their uncanny ability to follow a trajectory (such as terriers who were bred to follow small animals with enormous precision, hence why they are usually good at flying through the air and catching Frisbees and tennis balls), a dog’s visual capacity is not normally as superior as its sense of smell.

However, apart from those amazing, sad, puppy dog eyes that most dogs can display at will when scrounging for food and attention, there is something quite interesting in the make up of the eyesight of a dog.

And that is the fact that dogs have three eyelids, the upper, the lower, and of course, the third.

Most people are unaware that dogs have a third eyelid. The third eyelid is there to provide protection to the cornea and the eye; it also contains a tear gland. The third eyelid is an opaque thin tissue in the corner of the eye which is often not seen. When it is it is usually because the dog is asleep or relaxing. Sometimes, the sight of the third eyelid when your dog is asleep can make them look quite strange.

If the third eyelid is visible when your dog is awake it may be an indication that there is a foreign object in the eye or an infection.  This is because when the third eyelid appears it generally indicates that the eyeball has sunk into its socket or in some cases, has been pulled back in response to pain.

When the third eyelid is prolapsed it is known as ‘cherry eye’ and is a congenital defect which occurs in certain breeds.  It is due to the fibrous attachments to the third eyelid being weak which in turn allows the tear gland to fall out exposing a cherry like growth hence the term ‘cherry eye’.

Of course some dogs are born with visible third eyelids and this is often not a problem unless you are showing the dog.

Sometimes when dogs get debris in their eye and the foreign matter does not dislodge itself naturally, then damage to the eye may occur in the form of scrapes, scratches and corneal ulcers.   In some cases, to enable the corneal ulcer to heal properly, the third eyelid may be sewn shut to enable the dog’s eye to heal ‘naturally’. If this is the case then the dog’s eye will be sutured closed until it heals and an Elizabethan collar is normally worn to ensure the dog does not pull out the stitches or cause further damage to the eye.

The eye is a very delicate structure and any damage can have serious implications, therefore all eye problems should at first be evaluated by a veterinarian. If you are concerned about the health of your dog you should always consult with your veterinarian first. In the case of serious eye problems you should consult a veterinary ophthalmologist.

Ref: http://pets.webmd.com/dogs/nictitating-membrane-third-eyelid-dogs