The dog's eye is pretty much a garden-variety mammalian eye, with some notable adaptations that have evolved over the millennia. It is a globe with two fluid-filled chambers (anterior and posterior). The chambers are separated by the lens, the structure that helps focus light beams onto the rear part of the eye, the retina. The eye's outer, clear surface, the cornea, offers protection to the inner eye and helps the lens focus light onto the rear of the eyeball, the retina.
Cataracts make the lens of the eye opaque or cloudy, which gradually reduces vision to the point of blindness. In their early stages, cataracts cause blurring and distortion of vision, but they are invisible to the naked eye. By the time most owners notice them, cataracts involve more than 60 percent of the dog's eye. Cataracts often accompany other illnesses, such as diabetes and hypothyroidism (low thyroid function). Surgery performed by a veterinary ophthalmologist is the only treatment considered effective in conventional veterinary medicine and is indicated only in cases where the cataracts are not a result of a secondary disease such as diabetes.