The Eyes Have It: The Secrets of Eyes and Seeing (The Gross and Goofy Body)

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Blasts of Gas: The Secrets of Breathing, Burping, and Passing Gas The Eyes Have It: The Secrets of Eyes and Seeing , It s Spit-acular!: The Secrets of Saliva Now Hear This!: The Secrets of Ears and Hearing Pump It Up!: The Secrets of the Heart and Blood Up Your Nose!: The Secrets of Schnozes and Snouts

The Secrets of Eyes and Seeing

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The Eyes Have It

The Secrets of Eyes and Seeing

The Eyes Have It

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Ste Stewart

... takes readers on a journey of the wonders of the human body from what’s up your nose to what’s going on in your brain—it’s body science at its most interesting…and absurd. The Eyes Have It takes a close look at the eyes and how it is that you can see the things you do.

By Melissa Stewart Illustrated by Janet Hamlin

The Secrets of Eyes and Seeing

This book was made possible, in part, by a grant from the Society of Children’s Book Writers and Illustrators.

Marshall Cavendish Benchmark 99 White Plains Road Tarrytown, NY 10591-5502 www.marshallcavendish.us Text copyright © 2010 by Melissa Stewart Illustrations copyright © by Marshall Cavendish Corporation All rights reserved. No part of this book may be reproduced or utilized in any form or by any means electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission from the copyright holders. All websites were available and accurate when this book was sent to press. Library of Congress Cataloging-in-Publication Data Stewart, Melissa. The eyes have it : the secrets of eyes and seeing / by Melissa Stewart. p. cm.—(The gross and goofy body) Summary: “Provides comprehensive information on the role eyes and sight play in the body science of humans and animals”—Provided by the publisher. Includes index. ISBN 978-0-7614-4594-4 1. Eyes—Juvenile literature. 2. Vision—Juvenile literature. I. Title. p475.7.S74 2009 612.8’4—dc 22 2008033566 Photo research by Tracey Engel Cover photo: © Rob Walls/Alamy The photographs in this book are used by permission and through the courtesy of: Alamy: Jack Milchanowski, 5 (bottom); Terry Prekas Photography, 6; Claudia Wiens, 7; ACE STOCK LIMITED, 9; Christina Kennedy, 24; JUPITER IMAGES/Polka Dot, 27; simon de glanville, 29; Deco Images II, 31 (top); franzfoto, 38; TOPICMedia/B. Brossette, 39 (top); David Bagley, 40. Animals Animals – Earth Scenes: RICHARD DAY, 10 (right); ZIGMUND LESZCZYNSKI, 35 (right); LON LAUBER/OSF, 39 (bottom). AP Images: Eugenio Savio, 19; CORBIS: Martin Harvey, 11 (top); Frans Lanting, 28 (top); 34. Digital Railroad: Jason Horowitz/Mirror Ball Studio, 15. Getty Images: Georgette Douwma, 11 (bottom); Paul Vozdic, 21 (bottom). Peter Arnold: Biosphoto/Heuclin Daniel, 10 (left); WILDLIFE, 35 (left). Photo Researchers: Steve Gschmeissner, 36. Shutterstock: Snowleopard1, 5 (top); Tim Mainiero, 21 (top); Christian Musat, 28 (bottom); Curtis Kautzer, 30; Karel Brož, 31 (bottom); Fotolistic, 33; Primož Cigler, 37. Editor: Joy Bean Publisher: Michelle Bisson Art Director: Anahid Hamparian Series Designer: Daniel Roode Printed in Malaysia 135642

Look at Those Peepers! rs!

4

Bushy Brows, Lush Lashes es

6

Amazing Eyelids

8

Yee Haw!

10

Don't Think, Jus Just Blink J st B Bl link

12

Ew! It's Eye Go Goo! G oo!

14

Tear Talk

16

Front and Center n er nte

18

Eyes with Attitude it e itude

20

Inside Your Eyeba Eyeball yeba all

22

Eye Activitiess

24

A Perfect Pair

26

The Where and Why of Eyes

28

Other Kinds of Eyes

30

A Cast of Colors

32

How Many Eyes?

34

Seeing in the Dark

36 3 6

Tricks of the Trade

38 3 8

Extreme Eyes

40

Glossary

42 4 2

A Note on Sources

45 4 5

Find Out More

46 4 6

Index

47 4 7

CO NT

EN T

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Imagine being able to grab one of your eyeballs and pop it out of your head. How would it feel? How large would it be? Would the world look any different without it? To find out what an eyeball feels like, peel the skin off a grape. Close your eyes and gently pinch it. Then roll it around in your hand. Your eyeball feels just like that grape—soft and a little bit squishy. But your eyeball is bigger than a grape. It’s about the size of a SuperBall. Packed inside, more than two million parts work together to peek, peer, and probe your surroundings. You’ll be amazed at all the ways eyes make life better for you—and for other animals, too.

A red-eyed tree frog’s bright,

bold eyes startle predators just long enough for the little animal to jump out of sight.

to save d r a h g in k r o w le Why are peop ause they seem so c giant pandas? Be dark rings , e g r a L . ly d d u c cute and , just like ig b k o lo s e y e ir e make th baby. n a m u h a f o e s o th

A chameleon’s eyeballs can rotate in almost any direction, but they don’t always move together. One eye can look up while the other looks down. One can stare straight ahead while the other glances backward. That makes it easy for chameleons to scan their surroundings for predators and prey.

BUSHY

BROW

S,

Eyebrows and eyelashes. You probably don’t think about them very much. But maybe you should. Imagine you’re an Olympic runner. You’re in second place, and the final race is on a rainy day. What body part can help you take home the gold? Okay, besides your long, muscular legs. That’s right, your eyebrows. Those hairy tufts soak up salty sweat so it won’t sting your eyes. They also trap the raindrops trickling down your forehead. Your eyebrows’ arched shape forces rain and sweat to run down the sides of your face, so you’ll never lose sight of the finish line. Eyelashes are important, too. Every time you blink, your eyelashes flick away tiny bits of dirt and dust. Not bad for a bunch of tiny hairs!

What Eyebrows Say Eyebrows rows do do more more tthan han ct yyour our eeyes. yes. T hey protect They ell oother ther ppeople eople hhow ow can tell eeling. W hen yyou ou you’ree ffeeling. When oth eeyebrows, yebrows, iitt lowerr bboth means you’re angry. If you raise one eyebrow, people know you don’t believe what they’re saying.

Life-Saving Lashes It’s a good thing a camel has extralong lashes. They keep the humped herbivore’s eyes safe and clean during fierce desert sandstorms.

AMAZING EYELIDS

Ever seen someone turn their upper eyelids inside out? It’s gross—and cool. It gives you a chance to see what’s underneath. The oily liquid that covers your inner eyelid trickles out of tiny holes between your eyelashes. It coats your eyes so they won’t dry out. More liquid flows out of tear glands just above your eyes. Tears wash away the dust and dirt your eyelashes don’t catch. Each time you blink, your eyelids spread a fresh supply of oil and tears across the surface of your eyes. But wait, that’s not all your eyelids do. When you go outside on a bright, sunny day, you squint. Your eyelids scrunch together to protect your eyes from all that light.

When you feel tired, you close your eyelids. Your eyes take in more than 80 percent of the information you get from your surroundings. So when you shut your eyes, you’re shutting out the world. That makes it easier to fall asleep.

Your eyelids automatically close in bright sunlight.

Blink or Burn Ever noticed your eyes burning when you’re watching TV or playing a computer game? Because you don’t want to miss any of the onscreen action, your eyes blink less often than usual. And that can dry them out. A few quick blinks should solve the problem.

Many animals have a haw, or third eyelid. It’s just below their outer lids. A haw opens and closes sideways instead of up and down. And it’s transparent, so animals can see through it.

An aardvark’s

s haws keep ant and termites . out of its eyes

When manatees, beavers,

frogs, and crocodiles dive underwater, they swim with their eyelids open. Haws protect their eyes and allow them to see.

Sometimes hungry eaglets a

ccidentally jab their parents’ eyes as they lunge for food. It’s a good thing adult eagles slam their haws shut before feed ing their young.

While a woodpecker jackhammers

a tree, it closes its haws to protect its eyes from flying bits of wood.

Other animals don’t have any eyelids at all. But that’s no problem.

Ag

ecko ue g n u o s t es its lo to k n g, f l e x i b l e eep i ts eyes moist and clean.

Tough, clear scales protect a snake’s A fish doesn’t have to worry about

keeping its eyes moist and cl ean. The water it swims in consta ntly bathes its eyes.

eyes. When the snake molts, or sheds its skin, the eye scales pop off, too. Luckily, there are new ones waiting underneath.

DON’T THINK, JUST BLINK

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When the wind hurls leaves in your face or a friend lets loose with a squirt gun, your eyelids snap shut. You don’t have to think. You just blink. Your body reacts in a split second.

And just as quickly, your eyes spring back open. It happens so fast that you don’t even notice a moment of darkness. That’s because the muscle that blinks your eyelids is the fastest muscle in your body. You can blink up to five times a second. Most of the time you flit your eyelids thirty to sixty times a minute. That adds up to more than ten thousand times a day and more than ten million times a year.

What's a Wink? Your eyes blink automatically all day long. But when you purposely close one eye and then open it again, you are winking. A wink is a way to send a silent message. It might mean you’re telling a joke or that you like someone. Many people can wink one eye but not the other. How about you? If you’re right-handed, it’s probably easier to wink your left eye. And if you’re left-handed, it’s probably easier to wink your right eye.

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EW! IT’S EYE GOO! Eye goop. Eye gunk. That caking crust is gross junk. Some mornings it’s hard and flaky. Other days it’s soft and gooey. Once in a while there’s so much goopy goo that it glues your eyelashes together. Ugh! That’s disgusting! During the day you blink hundreds of times an hour. And every time your eyelids flick down and then up, they clear away dust, dirt, and other debris. But you don’t blink while you’re asleep. All night long the debris builds up. So do small amounts of sweat, oil, and tears. All that stuff piles up in the corners of your eyes, where it mixes together to form eye gunk. 14

Eyes Under Attack

Blinking helps keep yo

ur eyes free of dust and

dirt.

Sometimes bacteria get in your eyes. At night, when your eyelids are shut, your eyes warm up. And the warmer bacteria get, the faster they grow and reproduce. Luckily, it doesn’t take long for your body to fight back. It sends an army of bacteria-destroying cells to your eyes. A fierce battle rages while you sleep, and in the morning, you see the signs. Your eyes are pasted closed with all the tiny casualties of war. Yuck! 15

Like the water slowly dripping out of a leaky faucet, your tear glands are constantly cranking out a fresh supply of tears. Most days, about 1/3 of a teaspoon (1.7 milliliters) of the watery liquid enters each eye. Some of it evaporates. It turns into a gas and rises into the air. The rest drains into tear ducts, tiny tubes that run between your eyes and your nose. But if an eyelash gets in your eye or you chop up an onion, your tear glands work overtime. They make extra tears to wash the irritating materials out of your eyes.

If you hurt yourself, you’ll probably make so many tears that your tear ducts will overflow. Liquid spills over your eyelids and runs down your face. We call that crying.

See for Yourself Stand in front of a mirror and gently pull down the inside corner of your lower eyelid. See that tiny hole? It’s your tear duct.

A Good, Long Cry When you feel strong emotions—sadness, fear, anger, or even joy—your tears glands produce a special kind of tears. They help get rid of chemicals your body makes when you experience strong feelings. That’s why most people feel better after they cry. 17

FRONT AND CENTER Pull up your top eyelid as far as you can. Now pull down your bottom one. What do you see? A whole lot of eyeball. The edges curve back and disappear into you head. Now feel around the outside of your eye. Under your skin you’ll feel hard bone. That’s your skull. Your eyeballs sit in circular sockets in your skull. The thick, heavy bone helps protect your eyes.

There’s a clear, dome-shaped window covering the front of your eyeball. It’s called the cornea. Right behind your cornea is your iris. It can be blue, brown, green, hazel, or even violet. Your iris contains muscles that control the amount of light entering your eye. Your iris is surrounded by white sclera. It’s made of tough fibers that cover and protect the delicate structures inside your eye. See that black spot in the middle of your iris? That’s your pupil. It’s a hole that lets light into your eye. Muscles in your iris make your pupil smaller in bright light and larger in dim light.

A Real Eye Popper Kim Goodman of Chicago, Illinois, can pop her eyeballs almost half an inch (11 millimeters) beyond her eye sockets. Her eyes were measured on the set of the TV show Guinness World Records: Primetime on June 13, 1998.

See how this man can pop his eyeballs? He’s been doing it since he was nine years old.

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EYES WITH ATTITUDE Tell your mom a lie, and she gives you the evil eye. Say something ridiculous, and your sister rolls her eyeballs. Tell your best friend a joke, and watch his eyes light up. Just like eyebrows, a person’s eyeballs can tell you how he or she feels. What’s the secret behind our expressive eyes? Muscles. Six of them work together to move eyeballs up and down, right and left. You move your eye muscles about 100,000 times a day. In fact, you’re using them right now to read this book. Humans aren’t the only animals that send messages with their eyes.

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Eye Analysis

r turtle e d li s d ie ll e b w llo When a femalet yae male, it means, “Hubba, a rolls her eyes ate.” m d o o g a e k li hubba! You look

When a male razo

rback sucker fish roll s his eyes at another ma le, he’s saying, “Back o ff, punk. This is my stre am!”

If you look into a friend’s eyes, you’ll see an image of yourself—and whatever lies behind you. With the help of computers, scientists can now analyze the eye reflections of people in photos. Sometimes they help police view the scene of a crime. And they can help historians see scenes from the past. Now that’s incredible!

Three people are reflected in the eyes of this baby. Can you see them?

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L L A B E Y E R U INSIDE YO What’s really going on inside your eye? To find out, you’d have to slice it open from top to bottom. What you’d see is all the tiny structures that work together to help you spot a friend in a crowded cafeteria, catch a baseball as it soars through the sky, and enjoy the beauty of a rainbow.

Why is your eyeball so soft and squishy? Because it’s filled with a clear jelly called vitreous humor. That’s what gives your eye its shape and feel. After light rays pass through your pupil, they hit your lens. When muscles attached to your lens stretch, your lens gets thicker and rounder. That helps you see nearby objects, like the pages of this book. When the muscles relax, your lens gets thinner and flatter. That helps you see distant objects, like an airplane in the sky. As your lens changes shape, an image comes into focus on your retina, a thin layer of tissue at the back of your eyeball. Your retina is covered with millions of light-sensing cells. They capture light rays and turn them into electrical signals. Those signals travel through the optic nerve to your brain. Your brain interprets the signals and sends out messages that tell your body how to react.

Eye ID Some airports are testing a new device that identifies people by scanning their irises. Like fingerprints, each person’s irises are unique.

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Close your eyes and gently rub your eyelids. See those flashes of light? The pressure of your fingertips tricks the light-sensing cells in your eyes. They send messages to your brain that make you see yellowish spots, stars, and circles.

Hold a finger about 6 inches (15 centimeters) in front of your face. Look at it for a few seconds, and then quickly look out the window. As your eyes refocus, you’ll feel a little tug inside your eyeballs. That’s the muscles attached to your lenses doing their job.

Get a white piece of paper, and draw a dot on the left side. Then draw a plus sign about 7 inches (18 cm) to the right. Hold the paper about 20 inches (51 cm) from your face, close your right eye, and look at the plus sign. Slowly bring the paper closer, and the dot disappears. What’s going on? Most of your retina is packed with lightsensing cells. But not the area where your optic nerve exits your eyeball. When the dot is in front of this blind spot, you can’t see it at all.

Ever tried playing catch with one eye closed? It’s not so easy, is it? When you look at the world with just one eye, everything looks sort of flat. You can’t tell how far away objects are. And that makes it hard to catch a ball whizzing through the air.

But when your eyes work together, you get a 3-D (threedimensional) view of the world. That comes in handy when you want to catch a ball or eat a bowl of cereal. And it can help you escape from your angry little sister after you put a spider in her bed. As she chases after you, each of your eyes sees her from a slightly different angle. Your brain combines the two images, so you can tell exactly how far away she is — and exactly how fast you need to run.

Got Glasses? If your cornea, lens, or entire eyeball isn’t quite the right shape, glass lenses can solve the problem. Know how many U.S. presidents have worn eyeglasses? All of them. So if you have glasses, wear them proudly. You might be headed for the White House.

THE W HERE A ND e-dimensional Your eyes give you a thre ’re on the ey th e us ca be ld or w e th view of e other animals front of your head. Som D vision helps 3. do u yo ay w e m sa e see th to tree. It helps monkeys jump from tree and coyotes owls and eagles, cougars pinpoint their prey. eyes in But some animals have different places.

Hippopotamuses and crocodiles have eyes high up on their heads. They can see what’s going on while most of their bodies are hidden underwater.

Chipmunks and chickadees, giraffes and goldfish have many enemies. Because their eyes are on the sides of their heads, they can see far to the right and left. That means they can constantly scan their surroundings—without moving their heads.

Holy Cow! Looking for a tasty treat? In some countries people snack on cows’ eyes. After removing the vitreous humor, lens, cornea, and iris, they boil what’s left of the eye. Then they stuff the eyes with coleslaw, beef, or cream cheese. Sounds yummy!

Like to spy on your famil

y and friends? Then you’ll wish you had eyes like some crabs an d snails. They’re on antenna-like stalks that can move up and down and swive l in any direction. What a great way to view the world!

OTHER KI NDS OF E YES Imagine having eyes that can sense changes in light but can’t see objects. You wouldn’t be able to see a friend’s face or catch a Frisbee or watch TV. That’s exactly how many spiders, caterpillars, snails, sea jellies, and sea stars see the world. Their simple eyes have light-sensing cells, but they can’t form images. What good are simple eyes? They tell land animals whether it’s day or night, and they tell ocean creatures whether they’re moving up or down. Animals with simple eyes can’t see a predator’s body, but they can detect its movements.

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Now imagine having huge eyes with thousands of little lenses. Each lens would get a slightly different view of the world. That’s how the compound eyes of most insects, lobsters, and crabs work. Because their brains process all the different images at the same time, these animals can get a complete view of their surroundings.

the e e s

A praying mantis has supersharp compound eyes.

The hungry hunter uses them to spoott the slightest movements up to 60 feet (18 meters) away.

A dragonfly’s 29,000 lenses

wrap almost all the way around its head. They help the ferocious flier see what’s going on behind it.

How many colors can you name? Ten? Maybe twenty? That’s nothing. Your eyes can see about two million different shades of color. Light-sensing cells on your retina do the job. During the day six million cells detect the light rays that enter your eye. They look like tiny, upside-down ice cream cones—but they aren’t nearly as delicious! Some cone cells are tuned in to red light. Others sense blue or green light. When your brain combines the signals from all three kinds of cone cells, you can see yellows and browns, pinks and purples, and all the other colors in the world.

Did You Know? Not all animals have three kinds of cone cells. That means they can’t see all the colors you do. Some monkeys can’t see red, and many insects see only yellow and blue. Dogs probably can’t see any colors at all.

Blind to Color About 8 percent of all boys and about 1 percent of all girls are color-blind. They have trouble seeing some colors. Some color-blind people are born without red or green cones. Others have all their cones, but some don’t work.

HOW MANY EYES? yes on your You have two e and frogs, head. So do fish iles. But cats and crocod ave some animals h many more.

Giant clams c

an have tho of eyes, bu t they’re ve usands ry s They can s ense light a imple. nd motion, but they ca n’t form im ages.

e hundred small, on an th e or m ve ha ps lo al sc Some d a retina, so an ns le a s ha e on ch Ea . es blue ey h images. ug ro e se to le ab be ht ig m scallops

When a tuatara hatches, it has a third eye on the top of its head. But as the reptile grows, its extra eye becomes covered with scales. So far scientists haven’t figured out how the tuatara uses its third eye.

Tarantulas have eight clo

sely clustered eyes—two large on es and six small ones. With so many eyes, you might think tarantulas ha ve excellent eyesigh t. Think again! They r ely mainly on their sense of touch to u nderstand the world .

Heads or Tails? The false-eyed frog has two large eyespots on its butt. But they can’t see a thing. When the frog feels scared, it buries its head in the sand and sticks its rump in the air. Its fierce false face scares most enemies away.

What’s the best way to see something at night? Look slightly to the right or left of the object—not directly at it. Most of your cone cells are clustered in a circle called your fovea—the area of the retina right behind your pupil. It sees objects most clearly in bright daylight. But when evening comes, a different set of light-sensing cells takes over. More than 125 million rod cells line the edges of your retina. They work best in dim light. They’re great at detecting shapes and movements— but not colors. That’s why at night, you see things in shades of gray.

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Some animal eyes can detect light rays that your rods and cones can’t.

Bees and butterflies can see ultraviolet (UV) light. Many flowers have UV lines and splotches that guide insects to their nectar.

Some snakes have p

its in their mout that detect infra hs red (IR) rays. T hey form images from th e body heat give n off by mice, birds, and other prey.

Hooray for A! To see better at night, eat lots of carrots, eggs, broccoli, spinach, sweet potatoes, and cantaloupe. These foods are full of vitamin A. Your rods need this vitamin to do their job.

TRICKS OF THE TRA DE You can see pretty well at night, but many animals can see even better.

An owl’s huge eyes let in lots of light.

That helps these nighttime hu nters spot their prey. But such enor mous eyes can’t swivel in their sock ets. Luckily, owls can turn their he ads almost all the way around.

lots of e v a h s d r a z s and li e k a n s e m ee well o s S y e h T . ll a t cones a rods but no day. e h t g in r u d t no at night, but

ls

At night many animals open theiBur tpudupiring

wide and let the light flow in. So lions . ht lig h uc m o to s e’ er th y da the crocodiles, d an s or at lig al , ds ar op le d an r pupils ei th ow rr na ks ar sh d an es snak to thin slits.

Ever seen a cat’s eyes glow at night? Like dogs and deer, rabbits and raccoons, cats have a mirrorlike membrane behind the retina of each eye. This tapetum lucidum reflects light back through the cat’s retina, so it has a second chance to strike the rods. The cat’s eyes seem to glow because whatever light isn’t absorbed by its rods passes out of its eyes through its pupils.

st as the large h l a im n a h whic Do you know sharpest e h t t u o b a orld? How eyes in the w e eyes that v a h n e v e ls nima eyes? Two a ow that's N . e c n o t a s thing can see two incredible!

tarsier e th d lle ca re tu ea cr e lik A tiny, monkey ed to its body ar p m co es ey t es rg la e th has to the g on el b l al of es ey st ge ig size. But the b zed eyes are a si lal yb lle vo s It . d ui sq t gian yours! hundred times larger than

The eyes of hawks and eagles see eight times more clearly than the sharpest human eyes. They have two foveae and five times more cones. A golden eagle can see a rabbit from 1 mile (1.6 kilometers) away. Just imagine having eyesight like that!

Tyrannosaurus rex may have had the best eyesight ever. Some scientists say the fearsome dinosaur may have been able to see thirteen times more clearly than a person.

Whirligig beetles and four-eyed fish are the only animals that can see clearly above and below water at the same time. What’s their secret? Their eyeballs are divided in half. The top half of each eye watches for predators and prey above the water’s surface while the bottom half keeps an eye on the watery world below. What a great trick!

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bacterium (pl. bacteria)—A tiny, one-celled living thing that reproduces by dividing. blind spot—The area of the retina that is attached to the optic nerve. It cannot detect light or color because it has no rod or cone cells. color–blind—Unable to see certain colors due to missing or damaged cones. cone cell—A light-sensitive cell on the retina of the eye. In bright light it allows animals to see color. cornea—The thin, clear layer of tissue that covers the front of the eyeball. debris—Bits and pieces of various materials. evaporate—To change from a liquid to a gas. focus—To bring into clear view. In the eye the lens bends light rays to focus images on the retina. fovea—The area of the retina that sees objects most clearly. It is packed with cone cells. haw—A third eyelid located below the outer eyelid. It is transparent and opens and closes sideways, instead of up and down. herbivore —An animal that eats only plants.

infrared (IR)—Light rays that are just beyond the red portion of visible light. People can’t see IR rays, but some animals can. iris—A ring-shaped flap of tissue that controls the amount of light entering the eye. It also gives the eye its color. lens—A clear layer of tissue that changes shape to focus images on the retina. molt—To shed an outer covering, such as skin in a snake or an exoskeleton in insects and spiders. nectar— A sugary liquid that many flowers produce. It attracts insects that spread the plant’s pollen. optic nerve—A large nerve that carries messages from the retina to the brain. predator—An animal that hunts and kills other animals for food. prey—An animal that is hunted by a predator. pupil—The circular opening in the center of the iris. retina—A thin layer of tissue that lines the back of the eyeball. rod cell—A light-sensitive cell on the retina of the eye. It detects shapes and movement in low light. sclera—The white outer layer of the eyeball. tapetum lucidum—Present in some animals, the thin layer of tissue behind the retina that reflects light to help creatures see better at night.

tear duct—A tube that drains tears out of the eye. It connects to the nose. tear gland—A tiny sac that produces and releases tears. 3-D vision (three-dimensional vision)—The ability to determine the precise location of objects by combining of information from the two eyes. ultraviolet (UV)—Light rays that are just beyond the violet portion of visible light. People can’t see UV rays, but some animals can. vitreous humor—The soft, transparent jelly that fills most of the eyeball.

Dear Readers, I’ve written about eyes and vision before, so I knew quite a bit about the topic before I began to work on this book. But I also knew that The Eyes Have It! would look at eyes in a whole new way. I wanted to discuss gross stuff like eye gunk and clearly explain the importance of eyelids and all the ways our eyes express feelings. Plus I wanted to include the latest information about how animals see and how our knowledge about eyes is being used by police to solve crimes. I started my research by reading books. But when it came to understanding how eyes function, I realized that some of the books I was reading were out of date. At that point I began relying heavily on recent articles in magazines and scientific journals. I also searched for information on the Internet. Along the way I learned that the T. rex may have had the best vision ever and that iris scans can be used to identify people. I knew these kinds of facts would really fascinate young readers. My final step was to speak to doctors and scientists doing eye research. These interviews ensure that the book includes the most up-to-date information about animals with a third eyelid, night vision, and the reasons we cry. — M elissa Stewart

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BOOKS

Amazing Animals of the World. New York: Scholastic Library, 2006. Miller, Sara Swan. All Kinds of Eyes. Tarrytown, NY: Benchmark Books, 2007. Stewart, Melissa. Eye See You. North Adams, MA: Storey Books, 2006. WEBSITES

Guinness World Records This site contains up-to-date information on some of the strangest world’s records you can imagine. http://www.guinnessworldrecords.com/default.aspx Kids Health This site answers just about any question you might have about your body and keeping it healthy can be found on this site. http://kidshealth.org/kid/ Night Creatures of the Kalahari With this site, you can learn about the amazing adaptations that help a variety of creatures see at night. http://www.pbs.org/wgbh/nova/kalahari/nightvision.html That Explains It! This site contains all kinds of interesting information about the human body, animals, food, inventions and machines, and more. http://www.coolquiz.com/trivia/explain/

Page numbers in bold are illustrations. animals, 5, 10–11, 21, 28–31, 33, 34–35, 37–41 bees, 37, 37 blinking, 6, 8, 9, 12–15, 15 brain, messages to, 23, 24, 27, 32 butterflies, 31, 31, 37 camels, 7, 7 cats, 34, 39, 39 cells, light-sensing, 23, 24, 25, 30, 32, 36–39, 40 chameleons, 5, 5 colors, seeing, 32, 33 cone cells, 32, 36, 37, 38, 40 corneas, 18, 19, 27 cows, 29 crabs, 29, 29, 31 crocodiles, 10, 28, 34, 39 dinosaurs, 41 dogs, 33, 33 dragonflies, 31, 31 eagles/eaglets, 10, 28, 40, 40 eyeballs, 4–5, 18–19, 18, 20, 22–23, 22, 25, 27, 41

eyebrows, 6, 6–7, 20 eyelashes, 6, 6–7, 14, 16 eyelids, 8–15, 9, 10, 15, 18, 24, 24 eyes, 4–5, 6, 15, 30–31, 34, 39 eyesight, 25-28, 40–41 fish, 11, 11, 21, 28, 30–31, 30, 34, 41 foveae, 36, 40 frogs, 5, 5, 10, 34, 35, 35 geckos, 11, 11 giraffes, 28, 28 glasses, 37, 37 haws, 10, 10 hippopotamuses, 28, 28 insects, 31, 33, 37, 41 irises, 18, 19, 22, 23 lenses, 22, 23, 25, 27, 31, 34 monkeys, 28, 33, 40 muscles, eye, 20–21, 23, 25 night vision, 36–38

optic nerves, 22, 23, 25 owls, 28, 38, 38 pupils, 18, 19, 22, 36, 39 reflections, in eyes, 21, 21 retinas, 22, 23, 25, 32, 34, 36, 39 rod cells, 36, 36, 37, 38, 39 sclera, 18, 19, 22 snakes, 11, 37, 38, 39 tarantulas, 35, 35 tear ducts/glands, 8, 16–17 tuataras (reptiles), 34, 34 turtles, 21, 21 vitamin A, 37 vitreous humor, 22, 23 winking, 13 woodpeckers, 10, 10

ABOUT THE AUTHOR

Melissa Stewart has written everything from board books for preschoolers to magazine articles for adults. She is the award-winning author of more than one hundred books for young readers. She serves on the board of advisors for the Society of Children’s Book Writers and Illustrators and is a judge for the American Institute of Physics Children’s Science Writing Award. Stewart earned a B.S. in biology from Union College and an M.A. in science journalism from New York University. She lives in Acton, Massachusetts, with her husband, Gerard. To learn more about Stewart, please visit her website: www.melissa-stewart.com. ABOUT THE ILLUSTRATOR

Janet Hamlin has illustrated many children’s books, games, newspapers, and even Harry Potter stuff. She is also a court artist. Gross and Goofy is one of her all-time favorite series, and she now considers herself the factoid queen of bodily functions. She lives and draws in New York and loves it.