The dog's eye is very similar in structure and function to the human eye, although dogs tend to be less reliant on
their sense of sight than humans. The dog's eye is thought not to be capable of
true colour vision and their ability to see at night is not as acute as our own.
The cornea is the transparent "window" at the front of the eye which is normally clear. The next structure is the iris, a circular opening that regulates how much light enters the eye by being dilated or constricted according to the light conditions. Behind the iris is the lens, a transparent structure that focuses incoming light on to the light-sensitive area at the back of the eye (the retina). Muscles attached to the lens can change its shape and thickness to enable clear vision at different distances.
The interior of the eye is filled with a clear, jelly-like material, the vitreous humor, which maintains the position and shape of the eye and its component structures. The optic nerve at the back of the eyeball collects the sensory information from the retina and transmits it to the brain where the information is processed and interpreted as sight.
In the young dog, the cornea and lens are completely transparent and the pupil appears black. As your dog becomes middle-aged or older, you may notice that the cornea develops a slightly opaque bluish appearance and is most noticeable in bright light. This progresses and becomes more obvious in old dogs, but it occurs gradually and does not generally affect the dog's vision. However, many other eye diseases may cause inflammation of the cornea giving it a cloudy, grey or white appearance.
A cataract is a partial or complete opacity of the lens, due to a change in the structure of the lens, and interferes with the passage of light through to the retina at the back of the eye and can cause total blindness. Cataracts may be present at birth (congenital) or acquired later in life. Diabetes mellitus is a risk factor for the development of cataracts. Blindness due to cataracts may be addressed by surgically removing the affected lens and enables light to once again reach the retina, although the eye will lose its ability to focus. If you notice any sudden changes in the appearance of your dog's eyes, or he is showing signs of discomfort, discharge or impaired vision, see your vet without delay since most eye diseases are best treated early.