Chapter 3 Anatomy of the Eye

Sclera  The white part of the eyeball is called the sclera (say: sklair- uh). The sclera is made of a tough material and has the important job of covering most of the eyeball. Think of the sclera as your eyeball's armor.

Capillaries  Look very closely at the white of the eye, and you'll see lines that look like tiny pink threads. These are blood vessels, the tiny tubes that deliver oxygen and nutrients to the sclera.

CORNEA  The part of the sclera in front of the colored part of the eye is called the cornea (say: kor-nee-uh). Unlike the rest of the sclera, which is white, the cornea is completely transparent, which lets light travel through it. The cornea helps the eye focus as light makes its way through. Like clear glass, the cornea gives your eye a clear window to view the world through

IRIS  Behind the cornea are the iris and the pupil. The iris is the colorful part of the eye. When we say a person has blue eyes, we really mean the person has blue irises!

PUPIL  The iris is a muscle. This allows the iris to control how much light goes through the pupil. The pupil is the black circle in the center of the iris, and it lets light enter the eye. The pupils will get smaller when a light shines near them and they'll open wider when the light is gone.

 Between the iris and cornea is the anterior chamber. This chamber is filled with a special transparent fluid that gives the eye water, protein, and glucose (a type of sugar in the body) to keep it healthy.

TEARS  Our tears form a protective layer at the front of the eye and also help to direct the light coming into our eye.

Lens  After light enters the pupil, it hits the lens. The lens sits behind the iris and is clear and colorless. The lens' job is to focus light rays on the back of the eyeball - a part called the retina.

Retina  Your retinais in the very back of the eye, past the vitreous body. Though it's smaller than a dime, it holds millions of cells that are sensitive to light. The retina takes the light the eye receives and changes it into chemical signals so the brain can understand what the eye is seeing.

The lens is suspended in the eye by a bunch of fibers. These fibers are attached to a muscle called the ciliary muscle. The ciliary muscle has the amazing job of changing the shape of the lens.

VITREOUS BODY  The largest part of the eye sits behind the lens and is called the vitreous body. The vitreous body forms two thirds of the eye's volume and gives the eye its shape. It's filled with a clear, gelatinous material called the vitreous humor. In a real eye, after light passes through the lens, it shines straight through the vitreous humor to the back of the eye.

Rods and Cones  The retina uses special cells called rods and cones to process light. Just how many rods and cones does your retina have? How about 120 million rods and 7 million cones - in each eye!  Rods and cones are most sensitive to bright lights. (and colors)

Rods and Cones

RODS  Rods see light, dark, and shades of gray and tell us the form or shape that something has. Rods are super- sensitive, allowing us to see when it's very very dark.

CONES  Cones sense colors and they need more light than rods to work well. Cones are most helpful in normal or bright light.

Photoreceptors  The retina has three types of cones - red, green, and blue - to help you see different wavelengths of color. Together, these cones can sense combinations of light waves that enable our eyes to see millions of colors.

I can’t see…  Sometimes someone's eyeball changes shape and the cornea, lens, and retina no longer work perfectly as a team. The person's eye may focus on what it sees in front of or behind the retina, instead of on the retina. When this happens, some of what the person sees will be out of focus.

Eye Glasses  To correct this fuzzy vision, many people, including many kids, wear glasses. glasses help the eyes focus images correctly on the retina and allow someone to see clearly.

To the Brain  Think of the optic nerve as the great messenger in the back of your eye. The rods and cones of the retina change the colors and shapes you see into millions of nerve messages. Then, the optic nerve carries those messages from the eye to the brain! The optic nerve serves as a high- speed telephone line connecting the eye to the brain.

Reflected light  Reaches the retina where it falls onto the photoreceptors  The critical part of the imaging process is the lens.  The lens gives the brain information about the size, shape, and color of an object.

 The lens is transparent with smooth surfaces.  It is bi-convex which means thicker in the center.

FOVEA  The area near the center of the retina is called the fovea.  The detectors are packed cones and details of the image are distinguished easily.

Peripheral vision  Light that enters your eye from the sides does not fall on the fovea, but on the part of the retina where there are fewer photoreceptors.  This explains why peripheral vision is limited.

 The placement and numbers of cones in your retina limit how well you see colors in your normal vision.

TThe numbers of rods and cones differ in people which accounts for the different types of vision.

Vision Problems  Myopia  An eyeball that is too long or a cornea that is too curved causes myopia (or nearsightedness). In nearsighted eyes, the image isn't focused precisely inside the eye, causing blurring in the image. The more nearsighted you are, the more blurred the distant object appears, and the thicker your glasses need to be.

Far-Sighted  Hyperopia  An eye that is too short, or a cornea that is not curved enough causes hyperopia (or Farsightedness). People with hyperopia see clearly when looking at far-away objects. Currently, there are restricted options to correct hyperopia. Most operations are still under development.

The lens  The human eye can change the shape of the lens automatically.  The range of change that the lens can accomplish varies from person to person.

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