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.
The visual streak is a horizontal band in the retina right above the optic nerve; this area has the highest concentration of rods and cones and vision is sharpest here. The visual streak varies greatly among breeds, and studies suggest that different breeds see the world differently.* In wolves and in dogs with long heads like wolves, the streak is wide, with the nerves evenly distributed. The shorter a breed’s head, the narrower (more circular) the streak tends to be. Pugs, for example, have a small spot of sharp vision—an “area centralis”—as humans do. Even within breeds, the visual streak can vary from type to type.
These features and others equip the dog to be a good hunter under various light conditions. The tapetum lucidum improves vision in poor light, as does the high proportion of rods to cones. A rod-dense retina also makes dogs excellent at detecting motion and shapes. Because most dogs’ eyes angle slightly to the side, they have a wider field of view than humans. When a wide field of vision combines with a wide visual streak, as in a German Shepherd, the dog can see the whole horizon at once (instead of having to scan the eyes back and forth as humans do).
As hunters of large prey with keen senses of smell and hearing, dogs don’t need to see well close up, and near vision is blurry in long-nosed dogs. (Short-nosed dogs, with their humanlike area centralis, do appear to see well close up. Though the area centralis may lessen their ability as hunters, it may make them better lapdogs, more able to “read” their owners’ faces.) Overall vision is also less sharp.
Common Eye Disorders
Some eye disorders occur more often than others. “As a general practitioner, I was often presented with problems such as conjunctivitis, dry eye and corneal ulcer,” says Christine Lim, DVM, Resident III in Comparative Ophthalmology at the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California–Davis. “Now that I specialize in ophthalmology, I am more often presented with cataracts, glaucoma and retinal disorders.”
Following are a few of the more common canine eye problems. (Note: Some breeds are more prone to eye problems than others, and a mixed-breed with one of those types in the mix could also be affected.)
Conjunctivitis. A condition in which the lining of the eyelids and the front of the sclera (the white of the eye) become inflamed. It can be caused by infection, an object in the eye, an allergic reaction, dry eye, a scratch, or even smoke or dust, and can also be a symptom of other diseases. Treatment depends on the cause.