Victoria A. Raucci
Vision begins when light reflects off of an object and enters the eye through the cornea, the outermost layer of the eye. The cornea bends the rays and allows the light to pass through the black center of the eye, called the pupil. The pupil is expanded or contracted by the colored part of the eye, the iris, depending on how much light is available. Light rays pass through the eye into the lens, which then refracts the light and sends it through to the retina at the back of the eye. Within the retina are light-sensing nerve cells called rods and cones. Rods detect movement and allow for peripheral vision, and allow sight in dim light. The cones detect colors and finer details in bright light. The rods and cones send signals to the brain through the optic nerve, which takes in the signals and produces an image in the brain.
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The functionality of both humans and canine eyes are relatively similar. However, they have evolved over time to serve the individual needs of each species. The human is diurnal, meaning most active during the night, and relies heavily on sight as its primary sense. The dog evolved from the wolf, whose hunting habits led it to primarily chase prey during the night, using vision secondary to its sense of smell. The domestic dog is crepuscular, meaning it is most active during dawn and dusk. No eye has it all – compromises must be made in visual acuity in order to compensate for seeing in low light.
The first difference between the human eye and the dog's eye is immediately noticeable. Although eye placement varies among breeds of dogs, most dogs' eyes are placed more laterally on their heads than human eyes, which are directly in the front of the face. Humans are designed to look forward, while dogs have a greater field of vision.
Another obvious difference between the eye of the dog and that of a human is the visibility of the sclera, or the "white part" of the eye. Human eyes have much more of the sclera visible, while dogs' sclera does not show as much.
Once inside the canine eye, more differences to human eyes are visible. A dog's eye gathers more light than a human's eye. Both the lens and cornea of a dog's eye are larger than a human's eye. In addition, light entering the dog's eye hits the retina at least twice, resulting in double the light to highlight the image. This redoubling of light accounts for the "creepy dog eyes" that look reflective when we take pictures with a flash on a camera. Once the light is registered in the retina at the back of the eyeball, the photoreceptors within the retina determine the level of visual acuity and ability to detect color.
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