No matter how large or small, how brave or fearful, how bold or gentle, your dog is at heart a hunter. Dogs descend from wolves, consummate predators of animals as large as bison, and their eyes reflect this ancestry, as well as thousands of years of breeding by humans. In fact, some eye problems in dogs are the result of breeding them for specific traits, such as a flat face.
 
How Dogs See
Eyes work much like a camera. Light enters through the pupil (see diagram). The iris, a structure that can expand and contract, controls the amount of light allowed in. Light then passes through the clear cornea and lens, which focus the light on the retina, a light-sensitive layer. This layer contains color-sensitive cones and motion- and light-sensitive rods, all of which convert light into electrical signals. The cones and rods send these signals via the optic nerve to the brain, which constructs an image from them. Dogs have only two types of cones, compared with the three types in human eyes. As a result, dogs don’t distinguish as many colors as do people.
 
Eyes also contain structures not found in a camera, such as the gel-like vitreous humor that fills the eyeball and gives it shape.
 
Canine eyes have some structures that human eyes do not. The nictitating membrane is a thin whitish-pink tissue that acts as a third eyelid and protects the eye. The tapetum lucidum is a reflective lining behind the retina; it is what makes dogs’ eyes glow eerily when light hits them.