The clear window at the front of the eye is the cornea. Around the cornea it is a narrow rim of white connective tissue called the sclera, which is less visible in dogs than people. The sclera surrounds and supports the entire eyeball. In some breeds the sclera can be spotted or colored.
The round opening at the center of the eye is the pupil. Around the pupil is the iris. Like a camera shutter, the iris opens and closes to control the amount of light getting into the eye. The iris contains the eye color. While most dogs have eyes that are brown, blue eyes are normal for some breeds. In some Northern breeds and dogs with the merle pattern, odd eyes (one brown and one blue) are not uncommon.
A pinkish membrane called the conjunctiva, which contains blood vessels and nerve endings, covers the white of the eye and doubles back to cover the inner surface of the eyelid. When irritated, the conjunctiva appears red and swollen.
The eyelids are tight folds of skin that support the front of the eyeball. The upper lid has eyelashes, but the lower lid does not. There are small hairs on the edge of the lower lids.
The dog has an important third eyelid, the nictitating membrane, found at the inner corner of the eye. It is normally inconspicuous, but when it extends across the surface of the eye it looks like the eyeball has rolled back into its socket.
Tears come from the lacrimal glands and each eye has two: one under the bony ridge at the top of the eye socket and the other is part of the third eyelid. Tears get to the surface of the eye using small ducts that empty behind the lids. They prevent the cornea from drying out. They also fight infections and help flush out any debris that gets into the eye. The inner eye has three fluid-filled chambers. The anterior chamber is between the cornea and the iris. The posterior chamber is between the iris and front of the large vitreous chamber which is filled with a clear jelly.
The lens is held in place by strands called suspensory ligaments. They attach to the ciliary body which is made up of muscle, connective tissue, and blood vessels. The ciliary body secretes the jelly that fills the anterior and posterior chambers. Contraction of the ciliary muscles changes the shape of the lens so objects at different distances are in focus.
Light enters the eye through the cornea and anterior chamber and then through the pupil and lens. It then travels through the vitreous and is received by the retina. The retina is a layer of photoreceptor cells that convert light into electrical impulses. These impulses are carried to the brain by way of the optic nerves.