Eyes are sensitive and easily damaged by injury, infection or disease. If your dog has a painful eye, one eye starts to look different from the other one, is tearing or squinting, avoiding light, or any other change, get your dog checked out by your vet.
To help understand eye health and disease, it is useful to know what the structure of dog eye looks like.
Dogs have three eyelids
Dogs have a third eyelid, known as the nictitating membrane. When the eye is open the membrane is located in the inner corner of the eye under the lower lid and is usually not obvious. The closing of this membrane, horizontally across the eye, while the other eye lids are open gives a dog the appearance of rolling their eyes back into their head. Some dogs when they are sleeping may briefly open the upper and lower lids, and look like they have completely white eyeballs due to the closed third eye lid.
The third eyelid has a major tear duct associated with it. One of the functions of the third eyelid is to spread tears across the surface of the eye.
In some dogs the edge of the third eyelid is visible, this is called haws. Haws are present from birth and of no concern. The may be a consideration for show dogs.
If the third eyelid suddenly become visible, or more visible if the case of haws, then it is likely that the third eyelid has shifted out of position. Cherry eyeoccurs when there is a problem with the third eyelid and its associated tear duct, usually prolapse (sticking out) due to weakened attachment. Occasionally the third eyelid can fold over on itself (eversion). This can usually be corrected with surgery.
Since dogs rely more on their hearing and sense of smell, they have relatively poor vision. Dogs are nearsighted, so they can see clearly at close distances, but less well at distance.
However, dogs have large pupils which give them a wide field of vision and they are good at detecting moving objects. Dogs do not recognize colors well and their color vision is thought to be similar to a red/green color blind person.
An interesting fact:- Dogs with long muzzles tend to have extra eyes cells for a wide field of vision and detecting motion. Dogs with short muzzles tend to be able to see close detail better and are more likely to watch television.