Although this subject has been extensively debated, it is currently accepted that dogs have better night vision than humans. Their retinal cells are better at concentrating incoming light, which means they have good vision under twilight conditions. This allows them to hunt at night.
Dogs can perceive distant motion very well, but have difficulty distinguishing fixed objects at the same distance. This phenomenon is also an adaptation to the visual hunting done by dogs.
The visual angle differs among breeds, depending on the type of work the breed was developed to perform. Herding dogs, for example, need the maximum possible field of view for the best view of the livestock. Their eyes are mostly on the sides of head, giving a large visual field. In order to locate prey, hunting dogs need depth perception, with a narrower field of view, so their eyes are on the front part of the head.
The eye sits in an orbit, a cavity of the skull. It is held in place by muscles that work in different directions. The muscles move the eye and aim it.
The eye is protected by surrounding structures, the eyelids and glands. Each eye has three eyelids. The upper and lower eyelids are mucus-lined folds of skin. Their edges are protected by eyelashes, which keep dust from falling into the eyes. The third eyelid is merely a membrane in the inside corner of the eye. Normally invisible, the third eyelid covers the eye when the eye is closed or irritated, or when nerve problems occur.
The eye is exposed to the dry external environment, but the exposed part (the cornea) is protected by tears, an aqueous medium secreted by the lacrimal glands. The tears then collect in the spaces between the eyelids and the eyes and are carried away by a narrow duct that begins in the inside corner of the eye and ends in the nostrils. When tears are produced in excess, or the duct is obstructed, the tears flow out onto the eyelids, where they oxidize into red streaks on the hair that resemble blood.
The eye itself has two parts:- The anterior portion consists of the cornea, the iris and the crystalline lens. This part of the eye focuses light, in somewhat the same manner as a camera's objective lens. The cornea and the crystalline lens are transparent and act as optic lenses. The iris acts as a diaphragm to regulate the amount of light that enters through the hole it surrounds, the pupil.- The posterior portion includes the vitreous body, the choroid and the retina. It changes the optic signals received as light into nerve signals that are transmitted to the brain by the optic nerve. To continue the camera analogy, the posterior portion of the eye serves as the film and the brain develops the picture.
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