Reading Comprehension/human Body (Page 2 of 4) in pdf

Reading Comprehension/Human Body

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The Human Eye, continued

The back of the eye is like a movie screen for the images you see. The retina is made

up of very specialized cells called rods and cones. There are about 120 million rods and

about 7 million cones in each eye! Rods cannot sense things in color, but they can see

black and white and different shades of gray. After the sun has set, when you can no

longer see anything in color, the rods are working very hard. Rods also allow you to see

the shape of different objects. There are three different types of cones that work in

bright light: red, blue, and green. Together, these cones process the light waves that

come into the eye and let you see the many different colors you do. The rods and cones

send all the information they gather through the optic nerve at the back of the eye. The

brain then uses the nerve signals to put together a picture of the outside world. The

place in the retina where the optic nerve exits is called the blind spot. This is because

there are no rods or cones in this area, and if an image is projected onto this part of the

retina, you cannot see it.

The eye also has a couple of ways to protect itself. The biggest protection for the eye is

the eyelid. The eyelid helps to keep the eye clean and moist. Opening and closing the

eyelid is called blinking, and you can do it when you want to and without even thinking

about it. This means that blinking is both a voluntary (it sounds like the word volunteer)

and involuntary action. Your eyelids will shut automatically to stop things from getting

into your eye. Don’t forget your eyelashes! They form a team with your eyelids to keep

dirt and dust out of your eye. But what happens when a piece of dust does get in your

eye? That’s when tears come in to play. Tears come from glands called lacrimal glands

which are located just above the outer corner of your eyes. Every time you blink a small

amount of tears is released into your eye to keep it moist. Tears also help to wash away

bacteria and dust from the surface of your eye. Tears drain out of your eye by entering

the lacrimal duct or tear duct.

Some people wear glasses; do you know why? Sometimes people have trouble seeing

and glasses help them. When you have your eyes tested, the doctor asks you to read

letters from different lines of an eye chart. If you have normal vision, you have 20/20

vision. This means that you can read line twenty at twenty feet away. If your vision is

20/200, this means that you can read the same letter at 20 feet that a normal eye can

read at 200 feet! A person with 20/200 eyesight is legally blind in the United States.

There are many other degrees of vision between 20/20 and 20/200, and some people

even have better than 20/20 vision. People wear glasses for different reasons. Some

people cannot see things very far away, and they are called nearsighted (myopic). In

these people, the image is focused in front of the retina. Some people have trouble

seeing things that are close, and these people are called far-sighted

(hyperopia). In these people the image is focused behind the retina.

Glasses help to bend the light enough so that the image is always

focused on the retina.

Your eyes do a very important job. They help you see the colors and

shapes of things around you. The eyes you have now will be with you

forever, so make sure you take good care of them!

Reading Comprehension/human Body Page 2

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