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Sight

© Diffomédia/Royal Canin

Although the topic is strongly debated, current thinking is that dogs have better night vision than humans. Their retinal cells concentrate more light information, which means that they have good twilight vision.

Dogs are very good at perceiving movement far away, but they have more trouble distinguishing stationary objects at the same distance. This is a practical advantage on a hunt.

The viewing angle differs between breeds, depending on the work they have been bred to do. Herders need the widest possible viewing angle to enable them to monitor livestock optimally. To achieve this, their eyes are typically on the side of the head. Hunters need narrow binocular vision to be able to perceive prey, so they have their eyes on the front of the head.

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How well do dogs see?

It is generally accepted that most dogs are myopic (nearsighted), therefore cannot clearly image far away objects. Visual acuity in the dog is thought to be more sensitive to motion than actual imagery of the object reviewed.

Do dogs see colour?

Recent studies at the University of California indicate that dogs have limited colour vision. The spectrum is divided into two hues: (human, violet range) seen as blue in dogs; (human, yellow to red) seen as yellow by dogs. The spectrum between the two is probably viewed as white to gray.

quinn

Art J.Quinn, BS, DVM
Diplomate, American College of Ophthalmology
110 Cedar Lane
Sand Springs, OK (USA

Eye and associated structures

The eye fits into the eye socket, a cavity of the skull, where it is held in place by muscles with different orientations, which enables it to move in a specific direction.

Eyelids and glands protect the eye. Each eye has three eyelids. The upper and lower eyelids are folds of skin lined by a mucous membrane on the inside. They are edged with lashes, which protect the eye from falling dust and other foreign particles. The third eyelid is a simple membrane in the inside corner of the eye which is usually invisible. It covers the eye when it is closed or irritated, or in the event of a nerve problem.

Given that the eye is exposed to dry air, it needs the protection of the lacrimal glands, which produce an aqueous medium (tears) which bathes the cornea. The tears are collected in the space between the eyelids and the eye and evacuated via a narrow duct that begins at the inside corner of the eye socket and ends in the nostrils. If too many tears are produced or the duct is obstructed, they will run out onto the dog’s face forming red streaks in the coat due to oxidisation, which can cause them to be confused with blood.

The eye is made up of two distinct parts:

• The anterior segment comprises the cornea, iris and lens. Its role is to concentrate the light, rather like a camera lens. The cornea and lens are transparent surfaces which act as optical lenses, whereas the iris, which has an aperture called the pupil, acts as a diaphragm, regulating the amount of light which gets through.

• The posterior segment comprises the vitreous chamber, choroid and retina. Its role is to convert the optical signals of the light into nerve signals sent to the brain by the optical nerve. Returning to the example of the camera, the posterior segment serves as the film, which is developed by the brain.

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