A predisposing factor in the production of blindness is that of old age, many aged dogs having what is known as senile cataracts. Injuries of any kind to the eye are liable to end in either partial, or complete, blindness.

In cataract, it is the lens, its covering, or both, that are the seats of disease.

In the so-called glass eye (Amaurosis), the appearance of the eye is that of a normal one, the optic nerve being in a paralytic state.

Opacity of the cornea does, in accordance with the degree and situation of such deposit, produce corresponding deficiency of sight.