Many people take for granted that they can see without any conscious effort, but in reality, vision is very complex. Many different parts of the eye must work together to help a person see. Each part has a unique and vital purpose, and there are a surprising number of moving parts when it comes to the human eye.
The vision process begins with the cornea, the front surface of the eyeball. It is located in front of the iris and pupil and allows light to come into the eye. The cornea has a vertical diameter of around 11 millimeters and a horizontal diameter of 12 millimeters. It is around 550 microns thick, which is just over 0.5 millimeters. It is made up of five layers: the corneal epithelium, Bowman's layer, the corneal stroma, Descemet's membrane, and the corneal endothelium.
The sclera is the white part of our eyes. It slightly overlaps the top and bottom of the back end of the cornea. Its exterior is smooth and white, but the interior is a brown, grooved surface. It is surprisingly strong and flexible, and this flexibility gives the sclera even more strength. It is made up of collagen and elastic fibers, and its job is to protect the eye, as well as maintain its shape.
The anterior and posterior chambers are the first of three liquid-filled chambers in the eye. The former is between the cornea and the iris, the latter between the iris and the lens. The liquid here helps refract the light bought in through the cornea. This "aqueous humor" has a viscosity almost identical to water (aqua). Aqueous humor is generated from blood plasma and is renewed every hour. Its presence in the anterior and posterior chambers creates pressure that helps the eye maintain its shape. Too much pressure in these chambers is the cause of glaucoma.
The iris is arguably the most noticeable part of the eye. It is the ring of color around the dark pupil in the center. The iris doesn't just give us our eye color, however; it is a circular muscle that expands and contracts to change the size of the pupil based on how much light is present. The color depends on the ratio of eumelanin (melanin that is brown or black) and pheomelanin (melanin that is red or yellow), which genetics primarily determines. More eumelanin leads to brown eyes and more pheomelanin leads to blue or green eyes.
The pupil is the black circle visible inside the ring-like iris. It is the hole through which light passes to get to the other parts inside the eye. Pupil size ranges from three to seven millimeters, and changes depending on the amount of light the eye needs to take in. If there is less light, the muscles in the iris make the pupil larger so it cantake in as much light as possible. If there is a lot of light, the pupil contracts because it doesn't need to capture any extra light to see.
The lens is a transparent part of the eye made from proteins and water. It has several layers, including the firm nucleus and the soft cortex. When a person is young, the shape and curvature of their lens can change by the use of their ciliary muscle. This change in shape allows the eye to focus on objects closer and farther away. By the age of 50, the lens has become static and can no longer change shape. It also becomes yellower with age, which is the cause of cataracts.
Though similar in name to the aqueous humor, the vitreous humor is a different type of liquid that helps maintain the eye's shape. This liquid is more gelatinous and fills the space between the lens and the retina. The jelly-like substance is 98-99 percent water mixed with various sugars, salts, and collagen. Vitreous humor is responsible for the "floaters" we sometimes see in our vision: these are shadows of debris floating around in the vitreous.
The retina is one of the most complex parts of the eye. All of the parts mentioned previously are layers meant to protect the retina, which is vital to vision. Located at the back of the eye, it contains photosensitive cells that are sensitive to light, and trigger impulses that pass through the optic nerve to the brain. When the signal gets to the brain, a visual image forms. In short, without the retina, we could not process the light that comes into the eye and could not see.
The fovea is part of the retina, a small depression where the vision is sharpest. The retinal cones are most dense here, so the region offers the best color vision. Here, some of the retinal layers are pulled aside, making a clear path for incoming light to reach the receptors at the back of the eye. The fovea, like the retina itself, can only be seen during eye exams, using special medical equipment.
The macula is a pigment that covers the fovea. According to scientific theory, it serves as a filter of sorts. It stretches over the fovea and absorbs blue and ultraviolet radiation. The exact pigment here varies from person to person and is responsible for the fact that different people perceive colors differently.
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