The eye is one of the primary sensory organs in the human body and is responsible for sight. The eye is a highly specialised organ, allowing a good sense of depth perception, the ability to distinguish between colours, and the ability to see in varying degrees of light. The eye is a slightly asymmetrical globe, about one inch (2.5 cm) in diameter. The front part of the eye you can see when you look in the mirror consists of: the iris, the pigmented (coloured) part; the cornea, the clear layer over the iris; the pupil, the black circular opening in the iris, which lets light in; the sclera, the white of the eye; and the conjunctiva, the invisible, clear layer of tissue covering the entire front of the eye except the cornea. To understand the workings of the human eye, it’s essential to learn how these parts work to make it possible for us to see and interact with the world around us.
LET THERE BE LIGHT
In order for you to see, there must be light. Light reflects from an object and enters the eye through the thin veil of tears that coats the front of the eye onto the cornea. The cornea is a transparent layer protecting the iris and pupil with a ring called the limbus connecting it to the sclera (the white part of the eye that surrounds the iris and pupil). The cornea is a curved structure that plays a major role in focusing and refracting light. After the cornea, light passes through the iris. The iris is the circular structure that gives the eye its colour. Depending on how much light there is, the iris may contract or dilate, limiting or increasing the amount of light into the eye. After light travels through the iris it enters the pupil and the lens, which change shape to focus the light onto the smooth flat surface at the back of the eye called the retina. This process is similar to a camera focusing an image onto a movie screen. The retina at the back of the eye is made up of light-detecting rods and cones that transmit the light into nervous signals that flow via the optic nerve to the brain, the body’s control centre. The brain receives and interprets the image, and you react accordingly.
SEEING RED (AND OTHER COLOURS)
The two types of cells that detect light are rods and cones. Rods are able to function with less light and are vital in both peripheral and night vision. Cones are responsible for detecting colour and work best in abundant light. They are most heavily concentrated near the centre of the retina, around a small depression free of rods known as the fovea. The fovea is necessary for central vision activities, such as reading and writing.
Eye colour can be brown, blue, green, grey or shades of the same, and is created by the amount and type of melanin pigment in the iris. The size, shape, and spacing of the cells found in the iris also determine the intensity of blue or green shade in the eyes. Eye colour is inherited from a combination of your parents’ genes, though your eye colour can change as you grow up.
To get the clearest view of the world, the brain directs eyes to move together. The lines of sight from each eye merge into one complete image. With eye movement and peripheral vision, a human being can see roughly 270 degrees in front of them and to the sides without having to turn their head.
Your eyes are vital for seeing the world around you, so never take them for granted. They look out for you, so look after them by eating healthy, quitting smoking, limiting alcohol consumption, wearing UV sunglasses, and visiting your optometrist for regular check ups to help detect any eye problems or changes in vision.
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