The Eyes And How They Work

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If you’re like all of us here at the London Veterinary Clinic you probably look at your pet's eyes more often and more intensely than any other part of its body. We’re magnetically drawn to their eyes, and for good reason. Our dogs and cats communicate with us with their eyes. We get to know their moods and emotions.

Their eyes are constructed a little different to ours. The eyelids support and protect the eyeball. Eyelashes, restricted to the upper lid only in dogs and cats help prevent debris from contacting the eyeball. More dramatically, a third eyelid, or nictitating membrane, acts as a windscreen wiper, brushing aside debris. It and the rest of the pink membrane, the conjunctiva, lining the eyelids is constantly bathed in a protective film of tears that ensures clear vision, prevents drying and fights infection. Excess tears drain through a duct, the nasolacrimal duct, into the nasal cavity.

The eyeball itself sits deep in a bony socket, cushioned on a layer of fat. The tough, transparent surface of the eye, the cornea, protects the eye but allows light to pass through the round opening in the centre of the eye, the pupil, onto the lens which focuses light onto the retina at the back. The amount of light entering is controlled by contracting or dilating muscles in the coloured iris.

What Your Pet Sees

With large pupils and more laterally placed eyes, compared to us, dogs and cats have better peripheral vision and follow moving objects more accurately. Both have lots of weak-light-detecting cells, called rods, in their retinas so they see better in relative darkness. Although they have ample numbers of cells called cones in their retinas, necessary for seeing colour, the ability to see colour depends on a combination of retinal cone cells and brain function. While our brains (and those of all primates) are wired to see three primary colours (trichromatic) a pet's brain is wired for seeing two primary colours (dichromatic). Pets do see in colour but what they see is more like what we see when one of the colours on a television fails.

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