The Human Eye
The reason people can see words on paper, the sunrise in the morning, or the bright light of the moon at night, it is because of the human eyes. A person can view some of the parts of the human eye by looking into a mirror.
The eye is situated inside a hollow area of the skull called the eye socket. The eyelid protects the front part of the eye keeping it clean and moist by blinking. This is the opening and shutting of the eye throughout the day. Blinking is both voluntary, meaning it can be controlled, and involuntary, meaning it sometimes happens automatically. When the eyelid blinks automatically, it is a reflex. This reflex occurs when the eyes need to adjust to bright light, or when they are needed to shut quickly for protection. The eyelids keep dirt and other substances from entering a person's eyes.
The white part of the eye is the sclera. The sclera is the outer covering of the eyeball, which is made of a very tough material. It also contains blood vessels that deliver blood to the sclera. The cornea is a transparent dome that is located in front of the colored part of your eye. It helps with the focusing of the eye when light enters. It is a very difficult part to see because it is made of a clear tissue. Think of the cornea as a window the person's eye sees through.
Located behind the cornea is the iris, which is the colorful part of the eye. There are muscles attached to the iris that change its shape, which controls the amount of light going through the pupil. The pupil is the black circle in the middle of the iris and lets the light enter the eye. The pupils adjust to the light by opening wider when more light is needed, and shrinks when there is plenty of light available. The anterior chamber is the space between the iris and cornea, which is filled with a transparent fluid keeping the eye healthy.
When light enters the eye it next hits the lens, which is located behind the iris and it is transparent. Like all lenses, they are usually used to focus light, and this is what the lens of the eye does. It focuses light rays on the back of the eyeball to the retina. This part is home to millions of cells that are sensitive to light. The retina takes the light and changes it to signals which are sent to the brain. The brain in turn is able to tell a person what they are seeing. The image received in the eye is actually upside down when the optic nerve, like a high speed telephone line, sends the signal to the brain, When the brain receives it, though, the image seen is flipped right-side up. This happens so fast; a person won't even realize what is happening between the eyes and brain.
There is also a muscle located in the eye called the ciliary muscle. This muscle changes the shape of the lens when things are needed to be seen up close, far away, or out of the corners of the eyes. The lens becomes thicker when a person needs to see something up close, like reading, but becomes thinner when seeing something far away, like when riding a bike. The largest eye part is the vitreous body, which takes up about 2/3 of the volume of the eye, as well as giving the eye its shape.
The retina also uses special cells called rods (about 120 million of them) and cones (7 million) located in each eye to help it process light. The rods see in black, white and gray and passes along the shape of what is seen. It also can tell the difference between colors, and helps a person's vision when there is very little light. Cones are more helpful in bright light.
In summary, there are many parts of the eyes that work together to help you see and enjoy the different sights and colors surrounding you throughout the day.