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Published by SKNT - Digital Library, 2021-04-26 23:58:52

Human Body a Visual Encyclopedia

258 Pages · 2012

Keywords: Encyclopedia,Human,Body

Human Body
a visual encyclopedia



Human Body
a visual encyclopedia
DK PUBLISHING

LONDON, NEW YORK, Contents
MELBOURNE, MUNICH, and DELHI
BODY BASICS 6
Written by Richard Walker, John Woodward,
Shaila Brown, Ben Morgan Finding out 8
Looking inside 10
Consultant for Mind chapter Kathrin Cohen Kadosh Building blocks 12
Inside cells 14
Senior editors Shaila Brown, Ben Morgan Holding it together 16
Project editor Ruth O’Rourke-Jones Body jigsaw 18
Working together 20
Editorial assistant Damilare Olugbode All is revealed 22
Project designer Hedi Hunter Outer barrier 24
Designer Daniela Boraschi Hair and nails 26
Production editor Ben Marcus Life on us 28
Managing editor Julie Ferris
BONES AND MUSCLES 30
Managing art editor Owen Peyton Jones
Publisher Sarah Larter Supporting skeleton 32
Inside bones 34
Associate publishing director Liz Wheeler Living bones 36
Art director Phil Ormerod Moving joints 38
Hardheaded 40
Publishing director Jonathan Metcalf Bending backbone 42
Super stretchy 44
DK India Muscle machine 46
Managing editor Rohan Sinha Body movers 48
Deputy managing editor Alka Thakur Hazarika Inside muscles 50
Project editor Dharini Ganesh Making faces 52
Editors Suefa Lee, Parameshwari Sircar Reaching out 54
Deputy managing art editor Mitun Bannerjee On the move 56
Consultant art director Shefali Upadhyay In space 58
Project designer Amit Malhotra
Designers Rakesh Khundongbam, Pooja Pawwar, HEART AND BLOOD 60
Anuj Sharma, Vinita Venugopal, Shreya Anand Virmani
Production manager Pankaj Sharma Around the body 62
Senior DTP designer Jagtar Singh In your hands 64
DTP designers Arvind Kumar, Liquid life 66
Arjinder Singh, Bimlesh Tiwary Oxygen carriers 68
DTP/CTS manager Balwant Singh Super network 70
Picture researcher Sakshi Saluja A tight squeeze 72
Engine room 74
First American Edition, 2012 Working pump 76
Heartbeat 78
Published in the United States by DK Publishing
375 Hudson Street,

New York, New York 10014

12 13 14 15 16 10 9 8 7 6 5 4 3 2 1

001–181750–Jul/12

Copyright © 2012 Dorling Kindersley Limited
All rights reserved

Without limiting the rights under copyright reserved above, no part
of this publication may be reproduced, stored in or introduced into a

retrieval system, or transmitted in any form, or by any means
(electronic, mechanical, photocopying, recording, or otherwise),

without the prior written permission of both the copyright
owner and the above publisher of this book.

Published in Great Britain by Dorling Kindersley Limited.

A catalog record for this book is available from the Library of Congress.

ISBN 978-0-7566-9307-7

DK books are available at special discounts when purchased in bulk for
sales promotions, premiums, fund-raising, or educational use.
For details, contact: DK Publishing Special Markets,
375 Hudson Street, New York,
New York 10014 or [email protected].

Printed and bound in China by Hung Hing

Discover more at
www.dk.com

DISEASE DEFENSES 80 In the genes 168
The human genome 170
Germs and disease 82
Body barriers 84 IN CONTROL 172
White blood cells 86
Germ destroyer 88 Control network 174
Filtering germs 90 Sending signals 176
Immune system 92 The brain 178
Bird flu virus 94 Brain cells 180
Allergies 96 Gray matter 182
Fighting disease 98 Sleep 184
Spare parts 100 Spinal cord 186
Ancient remedies 102 Brain scans 188
On autopilot 190
LUNGS AND BREATHING 104 Hormones 192
Growth hormone 194
Airways 106
In and out 108 SUPER SENSES 196
Breathing underwater 110
Inside the lungs 112 In touch 198
Coughs and sneezes 114 Smell and taste 200
Making sounds 116 Sound detectors 202
Keeping balance 204
FUELING THE BODY 118 Balancing act 206
Eyes and seeing 208
Food for life 120 Very visual 210
Digestion 122 Visual tricks 212
What a mouthful! 124
In the stomach 126 MIND AND PERSONALITY 214
Gut reactions 128
Bumpy surface 130 Left or right? 216
End of the line 132 How memory works 218
Chemical factory 134 Are you a genius? 220
Balanced diet 136 What’s your personality? 222
Food market 138 The feeling mind 224
Vitamins 140 Thrill-seekers 226
The body clock 228
WASTE DISPOSAL 142 Teenage brain 230
Body language 232
Urinary system 144
Testing the water 146 REFERENCE SECTION 234
Waterworks 148
Filling and emptying 150 Test your personality 236
Wonderful water 152 Thinking logically 238
Thinking creatively 240
LIFE CYCLE 154 Medical discoveries 242
Glossary 246
Starting out 156 Index 250
Fertilization 158 Acknowledgments 255
Pregnancy and birth 160
Life story 162
Code of life 164
Two by two 166

BODY BASICS BODY
BASICS

Just like a jigsaw puzzle, your BODY BASICS
body is made up of thousands
of pieces. Beneath its protective
covering of skin are trillions of
tiny cells, neatly organized into
the working tissues and organs
that form you.

BODY BASICS Finding out

The process of finding out how the incredibly
complex human body works started thousands
of years ago—long before there were medical
schools and devices that enabled doctors to
look inside the body. Even today, thanks to huge
advances in technology, we are still making new
discoveries about the workings of the body.

◀ CANOPIC JARS
The ancient Egyptians
placed body organs in
stone or ceramic vessels
known as canopic jars.
There was a different
jar for each of four
main organs—the
lungs, stomach, liver,
and intestines.

ANCIENT WISDOM

Many ancient cultures contributed towards a greater understanding
of how the body works. The Egyptians, for example, realized that
the heart was at the center of a system that drove the blood and
that the pulse was related to the heartbeat. They also gained some
knowledge about the body’s internal organs during a process called
mummification. This involved removing the major organs of the dead
and preserving them in jars placed alongside the body in the tomb.

GLADIATORS’ DOCTOR

Claudius Galen was an important
Roman-Greek surgeon and philosopher.
In the early 160s ce, he was put in
charge of treating wounded gladiators.
As a result, Galen learned much about
the human body and although many
of his ideas were wrong, he discovered
that arteries carried blood and that
urine was made in the kidneys.

◀ ROMAN MOSAIC In ancient
Rome, gladiators fought to the death,
inflicting horrific wounds on each other.

8

MEDIEVAL THINKING WOW! BODY BASICS

In early medieval times, knowledge Body snatching—digging
about the workings of the human body up and stealing dead bodies
was still largely based on many of
Galen’s theories. It was not until the from graves for medical
1400s, when laws on dissecting corpses students to dissect—was
were relaxed in some countries, that
anatomists could study the body widespread in some
and try to find out how the bones, countries from about
muscles, and body systems worked.
1750 to 1850.
◀ TEACHING ANATOMY During the
1400s, wooden anatomical models, such as the
ones show here, were used as medical teaching aids.

BODY ART

In the late 1400s, when Italian
artist Leonardo da Vinci was
given permission to dissect
corpses, he used his skill to
draw many anatomically precise
pictures. But in an age when
the printing press was still a
new invention, Leonardo’s
drawings remained unseen.
The breakthrough came in
1543, when Flemish doctor
Andreas Vesalius published
his book On the Structure of
the Human Body, providing a
valuable medical reference tool.

◀ THE GREAT LADY
This drawing by Leonardo,
known as The Great Lady,
shows the internal body
organs in great detail.

WAXWORKS OF ART

Detailed anatomical wax models became popular

in the 1700s for teaching medical students about

the human body. They were brightly colored

and clearly showed the muscles, nerves,

blood vessels, and internal organs in 3-D.

The most famous and earliest collection

of anatomical wax models is at ◀ LIFELIKE LIMB
the oldest scientific museum This wax model of an arm

in Europe, La Specola in was made in Italy in the 1800s.
Florence, Italy. Such models provided medical students
with a fantastic tool for understanding

the internal workings of the body.

9

Looking inside

BODY BASICS From the outside, the body rarely gives any clues
about what is going on inside. One way of discovering
how the body works is to use some of the astonishing
medical technology that allows doctors to look inside
a person without causing any harm. This also helps
them spot any hidden injury or signs of disease.

X-RAY

When X-ray imaging was discovered in the late 1800s,
doctors were able, for the first time, to see inside the living
body without having to cut it open. This body imaging
technique fires X-rays—a form of radiation—through a
particular part of the body onto a photographic plate.
Hard, dense parts of the body, such as bones, show up
clearly since they absorb the rays. Softer body tissues are
not visible since the X-rays pass right through them.

CT SCAN Hard tissues
such as bone show
In a CT (computed tomography) scan, beams up as white areas.
of X-rays are sent through the body and are
then analyzed by computer. This turns scans Softer internal
into images that show “slices” through the organs show up as
body. These slices can be built up into 3-D gray or black areas
pictures to give an all-around view of an
organ. CT scans are more revealing than in an X-ray.
ordinary X-rays because they show the
soft tissues in greater detail.

▲ CROSSSECTION This CT scan shows FAST FACTS
a slice through the abdomen, revealing the liver
(orange), stomach (green), and spleen (pink). ■ X-rays were once used in shoe stores
to examine how well the new shoes
10 fitted each customer’s feet.
■ Patients undergoing an MRI scan have
to remove any metal objects, such as
jewelry, from their body because the
scanner contains very strong magnets.
■ Ultrasound is used in medicine to
clean teeth, break up kidney stones,
and remove cataracts from the eyes.

ULTRASOUND

With ultrasound, high-pitched
sound waves that we cannot hear are
beamed into the body. These bounce
off organs, producing echoes that are
picked up and turned into pictures by
a computer. Ultrasound is very safe,
so it can be used to make sure that an
unborn baby is growing normally.

Taken in 1895, an imag ▶ BABY REVEALED BODY BASICS
This ultrasound scan shows the face
e of hand bones was the first ever X-ray.
and fingers of a seven-month-old
baby inside its mother’s womb.

MRI SCAN

A person having an MRI
(magnetic resonance imaging)
scan is put inside a tunnel-like
scanner. When water
molecules in the body are
exposed to strong magnets
they can be made to “flip”
and give out tiny bursts
of radio waves. These are
picked up by a scanner
and turned into an image.

◀ BRAIN SCAN This MRI scan
shows a slice through the head. Brain
tissues are shown in different colors.

ENDOSCOPE

An endoscope is a flexible tube with a camera at
one end. It is pushed inside the body through an
opening, such as the mouth. It has its own light
source to light up body cavities, and a transmitter
sends back images to be viewed on a screen.

◀ BODY FRAMEWORK
Bones show up best in an X-ray, as can
be seen in this image of the incredibly
flexible skeleton of a gymnast.

▶ INSIDE VIEW
An endoscope takes
an inside look at the
small intestine. This
is an ideal way to
check for blockages

and growths.

11

Building blocks Unlike other body cells, a stem
cell does not have a distinctive
Your body is made of trillions of cells. These are living shape because it does not have
building blocks that are really tiny. The cells shown a specialized role.
here have all been photographed with the help of a
BODY BASICS powerful microscope to make them visible. Cells are
constantly dividing to produce new cells that
enable your body to grow and to repair itself.

THE CELL FACTORY

All cells inside your body are produced by stem cells.
These stem cells are unique because they can change
into the many different types of cells that make and
shape your body, including skin, muscle, and blood
cells. Cell production is a job that starts in
the earliest days of life.

FAST FACTS

■ The body’s biggest cells—egg
cells or ova—are 0.004 in (0.1 mm)
across and just visible without
a microscope.
■ The smallest body cells are just
0.00016 in (0.004 mm) long and are
found in the cerebellum (part of
the brain).
■ Placed in a row, 40 average-sized
cells would stretch across a period.

TYPES OF CELLS

There are more than 200 types of cell ▲ EPITHELIAL CELLS ▲ NERVE CELL ▲ FAT CELL
in the body, six of which are shown here. Protective epithelial cells— Wirelike nerve cells (neurons) Also called adipocytes, these
Each has its own specific shape and size, these are from an air passage— carry electrical signals and rounded cells store fat, one
related to the job it does. Nerve cells, line flat surfaces and organs. form the nervous system. of the body’s energy sources.
for example, are long and thin, ideal
for carrying electrical signals. Fat cells,
crammed with fat droplets, provide fuel
for the body and help insulate and
keep you warm.

12

A cell membrane surrounds and MAKING NEW CELLS
protects the stem cell, holding
the contents together. Cells divide repeatedly to replace worn-out cells—a process known
as mitosis. Before a cell divides, chromosomes, which carry the
instructions needed to build and run a cell, copy themselves within the
nucleus (control center) of the cell. Then the cell splits into two identical
cells, each with a full set of instructions.

1. Single cell Duplicated 2. In line
chromosome
Nucleus Chromosomes line
Cell up across the cell

3. Separation Fibers pull BODY BASICS
chromosome
strands apart

4. New cells

Cell starts Cells separate,
to divide each with identical
chromosomes

▲ STEM CELL We start WOW!
life as a tiny embryo growing
inside our mother. An embryo The human body is
contains stem cells like this one built from 100 trillion
that can develop into any one of cells and produces about
the cells found inside the body. 5 million cells every

second to replace
those that die.

▲ BONE CELL ▲ MUSCLE CELLS ▲ RED BLOOD CELLS SEEING SMALL
This bone cell (osteocyte)
maintains the bone tissue All movement is produced These small, dimpled blood The invention of the microscope in the
(brown) that surrounds it. 1600s opened up a hidden world for
by muscle cells. These cardiac cells carry oxygen from the English scientist Robert Hooke. He devised
a microscope (shown above) to study
muscle cells make the heart beat. lungs to body tissues. tiny objects such as plants and insects.
In 1665, Hooke recorded his findings in his
groundbreaking book, Micrographia (meaning
“small drawings”). He also invented the
term cell to describe the basic unit of life.

13

Inside cells Ribosomes are tiny structures that
make important substances called
Right now, the trillions of cells inside your proteins, using instructions
body are buzzing with activity. Inside each received from the nucleus.
tiny cell there are many parts that make
BODY BASICS substances, release energy, and recycle
worn-out materials. Although body cells
come in many shapes and sizes, they all
share the same basic structure.

IN CHARGE

Except for red blood cells, every cell in your body
has a control center called the nucleus. It contains
the essential instructions to build, maintain, and
run the cell. Outside the controlling nucleus is the
cytoplasm, a clear, jellylike liquid that contains
cell parts called organelles (“tiny organs”).
These organelles have their own specific task.

The nucleus is the
largest organelle
within the cell.

Lysosomes break
down and recycle
worn-out organelles.

DISCOVERING CELLS ▲ CELL MEMBRANE Guarding the ▲ CLOSEUP VIEW
contents of the cell is a protective layer called This model of a typical cell
In the 1670s, Dutch textile merchant the cell membrane. It consists of various shows the features that can
Antonie van Leeuwenhoek made some parts that help control the movement of
amazing discoveries using his simple substances into and out of the cell. be seen using a powerful
but powerful microscope. His interest electron microscope.
in studying living things led to the
discovery of bacteria, red blood cells,
and sperm (shown here in his drawing).

14

The cytoplasm, which consists FAST FACTS
mainly of water, helps move
materials around the cell. ■ Humans start life as a single cell,
which divides and gives rise to the
▲ POWERHOUSES trillions of cells that make up a body. BODY BASICS
Mitochondria are the ■ All living things—including
powerhouses of the cell, animals and plants—are made of cells.
releasing energy to keep Some, such as bacteria, consist of
the cell alive. a single cell.
■ The cells with the shortest lifespan are
those lining the small intestine. They live
for about 36 hours. Brain cells, however,
may last a lifetime.

◀ PROTEIN FACTORIES
This organelle, called the
endoplasmic reticulum,
is the busiest area of the cell.
It helps to move and store
proteins made by tiny
ribosomes on its surface.

LOOK CLOSER: SEEING BIG

Invented in the 1930s, electron microscopes can magnify the
insides of cells up to 200,000 times. Ordinary light microscopes
have a maximum magnification of just 2,000 times, so they
reveal much less detail.

Microtubules are fine
rods that support and
shape the cell, and also
move organelles through
the cytoplasm.

◀ PROCESS AND PACKAGE ▲ ELECTRON MICROSCOPE A scientist looks at a 3-D
The Golgi complex is a stack of image created by a scanning electron microscope. The same
flattened bags that processes and image can be viewed on the monitor’s screen.
packages proteins for use inside
and outside the cell. 15

BODY BASICS

HOLDING IT TOGETHER

These highly magnified cells (blue),
each with a nucleus (purple), are called
fibroblasts. They play an important role
in the body by producing connective
tissues. These provide the scaffolding
for many organs, including the skin,
and also form the tendons that link
bones to muscles.

BODY BASICS

BODY BASICS Body jigsaw

Your body’s trillions of cells fit into an incredibly complicated
jigsaw to give you shape and make your body work. If they didn’t,
you would be a squishy mass of random cells. Cells that have
the same function are organized into working groups called tissues.
When groups of tissues join together to do a particular job they
are called organs—your heart, lungs, and brain are all body organs.

TISSUE TYPES Your lungs are organs
that take in oxygen
There are four basic types of tissues in your body. Epithelial from the air and pass
tissues are sheets of cells that cover the inner surfaces of your it to your blood.
organs and form your outer skin. Connective tissues are
fibrous and hold organs and other body structures together.
Muscle tissues make you move by contracting (pulling)
and relaxing. A network of nerve tissues is used to
send messages all around the body.

▶ MUSCLE TISSUE
Muscle cells form long

and striped skeletal
muscle tissue, which
pulls on your bones

to make you move.

WOW! Your bones are organs
that are made mainly from
Your body’s biggest and hard connective tissues.
heaviest organ is the one
that covers all the other

parts of your body—
your skin!

▶ EPITHELIAL TISSUE
Packed tightly together,

these cells (pink) form the
lining of the small intestine,
part of your digestive system.

18

Made up of more than 100 billion GETTING ORGANIZED
nerve cells, the brain is your body’s
most complex organ, controlling For your body to work like clockwork, it has to be
nearly all your activities. organized—from the tiniest cells, to tissues, organs,
and finally into systems that have specific roles.
◀ NERVE TISSUE
These neurons (green) BODY BASICS
are cells that make up
nerve tissue. They carry
information in the
form of electrical signals
between your brain and
the rest of your body.

The thyroid gland contains ▲ CELL A living human body cell cannot
tissue that releases chemical exist on its own. The more complex the cell is,
messengers called hormones. the more it depends on other cells to survive.
Simple cells are better at surviving on their
▼ CONNECTIVE TISSUE own, but cannot perform complicated tasks.
This is cartilage, a type of tough, flexible
tissue that covers the ends of your bones
(where they form joints) to keep them

from wearing out. It also gives
your nose and ears their shape.

▲ TISSUE These cardiac (heart) muscle cells
form cardiac muscle tissue, which contracts
(pulls) to make the heart beat.

Your blood vessels
are made up of
connective, muscle,
and epithelial tissues.

The stomach is ▲ ORGAN The heart is an organ made
the organ that from not only muscle tissue but also epithelial,
churns food and connective, and nerve tissues. It is the main
breaks it down organ in your circulatory system, pumping the
into smaller pieces. blood—liquid tissue—around your body.

Nerves carry
electrical signals
to all your
body’s organs.

19

BODY BASICS Working together

The human body is an incredible machine. It performs countless tasks
effortlessly every second, every day. It supports and moves us, delivers
food and oxygen to our cells, protects us from disease, gets rid of waste,
enables us to reproduce, and allows us to think and feel. All these tasks
are performed by the 12 body systems you can see below.

SKELETAL SYSTEM CARDIOVASCULAR SYSTEM RESPIRATORY SYSTEM
pages 3045 pages 6079 pages 104117

SKIN, HAIR, AND NAILS MUSCULAR SYSTEM LYMPHATIC AND IMMUNE
pages 2429 pages 4659 SYSTEM pages 80103

20

EXTRAORDINARY SYSTEMS FAST FACTS BODY BASICS

Each body system consists of a group of ■ Your outer layer of skin is completely replaced
vital organs that carry out specific tasks. every month.
The systems do not operate alone, but ■ We use our respiratory system to take about 30,000
work together to maintain a healthy and breaths a day, breathing out enough air to blow up
efficient body. For example, all systems 3,750 party balloons.
require a constant supply of oxygen, which ■ In an average lifetime, the urinary system makes
is taken in by the respiratory system and and releases about 10,000 gallons (40,000 liters) of
transported by the cardiovascular system. urine, enough to fill a small swimming pool.

URINARY SYSTEM NERVOUS SYSTEM
pages 142153 pages 172191

female

male

DIGESTIVE SYSTEM REPRODUCTIVE SYSTEM ENDOCRINE SYSTEM
pages 118141 pages 154171 pages 192195

21

BODY BASICS

ALL IS REVEALED

This is an amazing scan of a living body.
It reveals the parts of your body that lie
hidden beneath your skin. Holding you
up is your spine (pink/purple) and the
rest of your skeleton (orange). The brain
and lungs can also be seen, as can some
of the main leg and arm muscles (blue).

BODY BASICS

BODY BASICS Outer barrier

Wrapped around your body like a protective overcoat, your skin
forms a barrier between the inside of your body and the outside
world. Your skin is waterproof, keeps out germs, and repairs itself.
It also filters out harmful rays in sunlight, enables you to feel
your surroundings, and helps control your body temperature.

UNDER THE SKIN The hair shaft sprouts
above the skin’s surface
The largest organ in your body, skin is just from a follicle.
millimeters thick. It has two layers—the
epidermis and dermis. Cells in the base of The epidermis is the
the epidermis divide constantly, producing upper part of the skin.
cells that move to the surface, flatten, and It has several layers.
fill with a tough, waterproof protein called
keratin. The thicker dermis contains sweat The dermis is the
glands, blood vessels, and other structures lower part of the skin.
that perform the skin’s many tasks. It contains blood vessels.

LOOK CLOSER: TEMPERATURE CONTROL Sebaceous glands
release an oily
Skin performs a vital role in keeping your body temperature at a steady 98.6 °F substance, sebum,
(37 °C). In hot conditions, dermis blood vessels widen to lose extra heat, that softens hair
and sweat evaporates from the skin to cool the body. When it’s cold, dermis and skin.
blood vessels narrow to keep heat from escaping and tiny muscles pull body
hairs upright in an effort to trap warm air, lifting the skin into goosebumps. Arteries supply
food and oxygen
to skin cells.

Nerve carry WOW!
signals from
touch receptors We lose about
toward the brain. 50,000 skin flakes
every minute, and 44 lbs
▲ SWEAT This magnified view ▲ GOOSEBUMPS The skin’s (20 kg) of flakes
shows tiny sweat droplets on the surface is covered in little bumps
surface of a fingertip. if we get too cold. in a lifetime.

24

◀ SKIN CELLS SUN PROTECTION
Continually worn away,
these dead, flat skin cells Sunlight contains harmful ultraviolet rays
are replaced by cells that that can damage skin cells. As a defensive
move to the surface barrier, skin contains special cells called
from the lowest layer melanocytes. These cells produce melanin,
of the epidermis. a natural brown pigment (coloring substance)
that passes into neighboring epidermis cells.
Here, melanocytes form a screen that blocks BODY BASICS
ultraviolet rays. In sunny weather melanin
production increases, producing a suntan.

Receptors detect
touch, cold, heat,
or pain.

Flat, dead cells
on surface of
epidermis

Melanin granules
spread out in
epidermis cells

This tiny muscle Epidermis cell Melanocyte
pulls the hair
upright in cold
conditions.

FINGERPRINTS

Packed with receptors, your fingers are incredibly
sensitive. They also have tiny ridges that help you
grip objects. On hard surfaces, such as glass, ridges
leave behind patterns called fingerprints. There are
three main types of patterns—loop, whorl, and
arch—but each person’s fingerprint is unique.
That’s why the police use them to identify criminals.

A hair follicle is a Arch
narrow pocket in the
skin from which
a hair grows.

Loop

The fat layer under the Sweat glands release watery
skin stores energy and sweat onto the skin’s surface
helps keep you warm. to cool you down.

Whorl

25

BODY BASICS Hair and nails

Made from dead cells packed with a tough substance
called keratin, both hair and nails grow from the skin.
Millions of hairs grow on your body. These include
thicker hairs on your head and finer vellus hairs
elsewhere. Nails cover the ends of your fingers
and toes, and also help you grip small
objects when you pick them up.

BODY PROTECTION ▶ EYELASHES
These help protect
Hairs are flexible strands that have the eyes by shading
different protective roles according to
where they grow. Scalp hairs, for example, them from sunlight and
protect the head from sunlight. Nails are trapping dust particles.
hard plates that protect and support the Anything that touches the
sensitive tips of your fingers and toes. eyelashes triggers blinking.
Both of these body protectors are made
from dead cells, so it doesn’t hurt when ▼ VELLUS HAIRS These fine hairs
you get your hair cut or trim your nails. make us more sensitive to touch. For
example, if an insect lands on vellus
hairs you will feel it, warning you
that you may be bitten.

STRAIGHT, WAVY, OR CURLY?

What type of hair you have depends on the shape
of your scalp hairs. The shafts of straight hair are
round, those of wavy hair are oval, while curly hair
shafts are flat. A hair’s shape depends on whether
it grows from a round, oval, or flat follicle.

Straight hair

Wavy hair
Curly hair
26

HOW HAIR GROWS Old hair is pushed
out of the follicle
Each hair grows from a narrow hole called a follicle. by new hair
At the hair’s base, living cells divide, pushing upward
to make the shaft longer. As they do so, cells fill with
keratin and die. After a phase of growth, a hair is
pushed out by its replacement.

Hair shaft is the part New hair
above the skin’s surface

Epidermis
BODY BASICS
▲ SCALP HAIRS Growing Dermis
from its follicle (pink), thisscalp hairs are lost and replace
is one of the 100,000 hairsEach day Hair follicle
on the scalp. Scales on the
outside of the hair overlap, Cells divide here
just like roof tiles. to make hair grow

, about 120 Plug blocks
opening to follicle
Red, inflamed
tissue GETTING PIMPLES

White blood cells Sebum—the skin’s natural oil—
and bacteria is released into hair follicles to
form pus soften hairs. But too much sebum
can block hair follicles, causing
Oily sebum pimples. Bacteria infecting the
trapped sebum are attacked by
Sebaceous gland white blood cells. This makes the
releases sebum follicle red, painful, and filled
with creamy pus.
Hair follicle

NAIL STRUCTURE

As this cutaway view shows, nails have a root, a body, and
a free edge. Living cells in the matrix behind the root divide

d. constantly. The new cells push the nail body forward, filling
with keratin and dying as they do so. This makes the edge of
the nail grow by about 0.11 in (3 mm) a month.

◀ NAILS Seen under Fat underneath skin
a microscope, these are Nail matrix is where
the dead, flattened new nail is formed
cells that nails are
made of. Packed with Nail root
keratin, the cells form
plates that interlock to Body of nail
make nails really hard.
Free edge
of nail

Finger
bone

27

Life on us

BODY BASICS Warm and moist, your skin makes a perfect home for millions
of germs and bugs that are often too small to see with the
naked eye. Unknowingly, we carry around these passengers
day and night. Many feed on skin cells and oils; others suck
blood. Luckily for us, these passengers are often harmless,
and some even protect us from dangerous germs.

Head lice have gray-brown bodies, The needle-like sucking
but after feeding on blood they can mouthparts withdraw
often look reddish. Hungry when a louse is
head lice can feed up to not feeding.
five times a day.

◀ HEAD LOUSE
In microscopic view,
this head louse can be
seen gripping on to a
strand of hair.

HAIR GRIPPERS BURROWING MITES BODY BASICS

With their flattened bodies and sharp, grasping Eyelash mites are wormlike creatures that live head
claws, tiny head lice crawl through hair, piercing downward inside the eyelash follicles. Here they feast
the scalp to suck blood. Between feeds they on oily sebum and dead skin cells. You cannot see or
grip hairs, making it difficult to wash or comb feel them, and they are often harmless. They emerge
them away. The wingless insects live for about from their human homes at night and wander over
the skin, as can be seen in this magnified view.
30 days. They mainly infect children and
are passed on when heads touch.

A head louse relies
on its two sensitive
feelers (antennae)
to detect movement
and to smell.

The legs end in sharp, FRIENDLY GERMS
curved claws that are
perfect for grasping hairs, Millions of germs, such as these
preventing the louse from ball-shaped skin bacteria called
being dislodged. Staphylococcus (meaning “bunches

of grapes”) live on your body,
favoring damper, warmer places
such as armpits. Many deter other
harmful bacteria from settling on

your body’s surface. But if the
skin is cut, bacteria can get inside

the body, multiply rapidly,
and cause infection.

A head louse egg, called BED CRAWLERS
a nit, is glued firmly
to a scalp hair. A female Wingless bedbugs find their victims by detecting body
head louse can lay up warmth. Most active at night, they emerge from their
to 10 eggs a day. hiding places, pierce the skin of sleeping victims, and
feed on blood, leaving itchy rashes and bite marks.
WOW! After feeding for about five minutes, the
blood-swollen bugs scurry back to their
If thousands of skin hiding places to digest their meal.
bacteria can fit onto the
period at the end of this Bedbugs have flattened,
sentence, imagine how oval bodies with three
pairs of curved legs.
many are living
on your skin.

BONES AND MUSCLES BONES AND
MUSCLES

Find out how your bones, BONES AND MUSCLES
muscles, and joints work
together to support and
move your body—from
throwing and kicking a
ball, to shaping your
facial expressions.

Supporting skeleton

Without the skeleton, your body would collapse into a shapeless heap. Built
from hard, strong bones, the skeleton not only holds up your body but also
allows it to move. It surrounds organs such as the brain and heart, and protects
them from damage caused by everyday knocks and jolts. Your bones also make
blood cells and store calcium, a mineral that is essential for healthy teeth.
BONES AND MUSCLES
The kneecap (patella)
ton makes up around 20 percent of your bo
TWO SKELETONS protects the front of

The skeleton can be divided the knee joint.
into two main parts. The axial
skeleton (cream) consists The shoulder The thighbone (femur)
of the backbone, (pectoral girdle)
skull, ribs, and is formed by the dy’s weight. is the skeleton’s biggest
breastbone. It runs clavicle and bone, and supports the
down the center scapula bones. weight of the upper body.
of the body and
supports and protects The hip (pelvic
important organs. girdle) is formed
The appendicular from two linked
skeleton (blue) consists hipbones.
of the arms and legs and
the girdles that attach
them to the axial
skeleton.

▲ BONE NUMBERS The shinbone
Eighty bones make (tibia) links knee
up the axial skeleton. to ankle and
The appendicular carries most of
skeleton has 126. the body’s weight.

The fibula is the smaller
lower leg bone that
forms part of the ankle.

The metatarsal is Your skele FLEXIBLE FRAMEWORK
one of the five sole
bones in the foot. An adult skeleton is made up of
206 bones, many of which you can
see in this model. The framework
that these bones create is not rigid.
Movable joints, such as the knee
joint between the thighbone and
shinbone, make the skeleton flexible.
They allow the body to move when
bones are pulled by muscles.

32

FAST FACTS The skull shapes ▶ SMALLEST BONE The size of
the head and a rice grain, the stirrup (stapes) is
■ Bone tissue is as strong as steel but face, surrounds the body’s smallest bone. It is one
as light as aluminum. and protects the of three bones, called ossicles,
■ More than half your bones are located brain, and houses
in your hands and feet. the eyes and ears. found inside each ear.
■ Your first bones, when you were born,
were made of flexible cartilage. This BONES AND MUSCLES
turned into bones as you grew older.

The ulna is the inner The radius is the outer The collarbone (clavicle)
bone of the forearm. forearm bone that forms helps hold the shoulder
the elbow joint with the and arm out to the
The humerus is the ulna and humerus. side of the body.
upper arm bone that
links the shoulder to The shoulder WILHELM ROENTGEN
the elbow. blade (scapula) forms
the shoulder joint In 1895, German physicist Wilhelm Roentgen
The breastbone with the humerus. discovered X-rays. He found that when
(sternum) provides X-rays were projected through the body
an attachment point The backbone is a onto a photographic plate, they produced
for the ribs and flexible column of bones clear images of bones.
protects the heart. (vertebrae) that holds
the head and upper
The ribs are 12 pairs body upright.
of curved bones that
support the chest SEEING THE SKELETON
and play a role
in breathing. Doctors use X-rays to see the skeleton inside living
bodies. X-rays help doctors to diagnose damaged or
The hip girdle broken bones. X-rays also reveal how some bones,
supports organs in the such as the ribs, perform an important role by
lower abdomen and surrounding and protecting soft organs.
forms a hip joint
with each thighbone. ◀ CHEST XRAY This X-ray shows a man’s
healthy lungs (dark areas) and heart (pale blue)
enclosed by the ribs (pale bands).

33

BONES AND MUSCLES Inside bones ◀ COMPACT BONE
This outer part of the bone
Just like your heart or eyes, your bones are living is made from cylinders of
organs. Bones are made from cells as well as hard bone tissue that run
a mix of materials that make them hard and along the length of the
tough, but also slightly springy. Their structure bone, making it strong.
makes them strong enough to support your At the center are
body, but light enough not to weigh you down. blood vessels (red).

ALL ABOUT BONES ▶ INSIDE A BONE
Part of a long bone, this thighbone
Although all bones differ in shape and size, (femur) has been cut away so
they have the same basic structure. A hard, you can see its structure.
tough outer layer of compact bone surrounds
a lighter mass of spongy bone, while a network Vein
of blood vessels delivers the essential supplies
that keep your bones alive. Some bones also Yellow bone
have a central space filled with a jellylike marrow stores fat
material called bone marrow. rich in energy.

Artery

◀ SPONGY BONE Spongy bone is thickest ▲ RED BONE MARROW
Despite its name, spongy bone near the ends of the This soft tissue fills the spaces in
is not soft but strong. Lighter bone because it is here spongy bone. It produces all the
than compact bone, it helps that your bone has to different types of blood cells.
reduce the bone’s total weight. bear the most stress. This close-up image shows both red
blood cells (red) and white blood
cells (blue) in the bone marrow.

34

Long bones like this These blood vessels supply LOOK CLOSER: BUILD OR BREAK
one are also found oxygen and nutrients
in your arms. to the bone and Your bones are constantly being broken down and
remove waste. built up to make sure they are as strong as possible.
Two types of cells are responsible. These cells are
A tough membrane known as osteoblasts and osteoclasts.
protects the bone’s
surface and provides BONES AND MUSCLES
an anchor for
your muscles.

The shaft links WOW! ▲ OSTEOBLASTS Found on the surface of the bone,
the bone’s two osteoblasts are bone-building cells. They are surrounded
rounded heads. Your red bone here by the bone tissue (orange) they have just made.
marrow makes red
Bone tissue contains calcium, blood cells at a rate
a substance obtained from of about 2 million
food such as milk and cheese. cells every second.

BUILT FOR STRENGTH The head of the ▲ OSTEOCLASTS These cells (pink) break down
thighbone carries worn-out bone. Here they are busy eating away at
Your bones are far stronger and lighter than the body’s weight. the surrounding tissue (cream).
concrete or steel. If your skeleton was made
from steel it would weigh five times as much,
making it almost impossible for you to
move around. The complex honeycomblike
design of tiny struts and hollow spaces
inside bones has been copied in many
man-made structures.

These criss-cross struts
in spongy bone are
arranged in a way
that provides
maximum support.

Compact bone is
tough and dense
so that it doesn’t
buckle under stress.

◀ THIGHBONE The arched shape of the ▲ EIFFEL TOWER, FRANCE The arch
thighbone, and the arrangement of struts at the base of this tower supports its weight,
inside it, enable it to help support the body. helped by the criss-cross pattern of metal struts.

35

Living bones ▼ IN THE WOMB By 17 weeks,
the bones in this skeleton have
Your bones are not lifeless. Packed with blood vessels, grown enough for the unborn
nerves, and cells, they can bleed when cut and can grow baby to make movements.
and repair themselves. Bones appear a few weeks
after a baby starts to develop in its mother’s Bone
womb, and continue growing after
birth, through childhood, and Cartilage
into early adulthood.
BONES AND MUSCLES
EARLY SKELETON

Babies are born with more than
300 bones, but these are not fully
formed. As the baby grows, some
of these bones fuse together to
form bigger bones. A baby’s
skull is flexible to help the
mother give birth.

The strong, flat bones
of the skull protect
the brain.

HOW BONES GROW The spaces between a
baby’s skull bones allow
When bones first form, they are made of flexible the brain to grow.
cartilage, like the tissue that supports your ear.
As the weeks and years pass, bone cells turn this Lower jaw
soft cartilage framework into much harder bone.
This process eventually finishes at the age of 20.

▶ CARTILAGE TO BONE Seven-week-old The long bone is made The upper arm bone
This sequence shows the embryo of flexible cartilage. (humerus) is one of the
development of a long bone. skeleton’s long bones.
Hard, compact bone
Cartilage tissue has formed. Shoulder blade

Compact bone Twelve-week-old These blood vessels The curved ribs surround
fetus (unborn baby) supply the bone cells. delicate organs to protect
them from being crushed.
Strong but light spongy
bone has formed.

The cartilage protects
the ends of the bone.

During childhood The long bone has growth areas
36 made from cartilage, which
allows the bone to lengthen.

HEALING BONES

If bones break or fracture, the healing A clot helps The new blood vessels The new bone BONES AND MUSCLES
process begins immediately. Within to stop the start to grow through tissue connects
hours, as you can see in this section bleeding. the cartilage. the bone ends.
of a broken long bone, a blood clot
forms to stop bleeding from vessels ▲ THREE HOURS ▲ THREE WEEKS ▲ THREE MONTHS
inside the bone. Then rebuilding begins. Within hours a blood clot The cartilage replaces the clot The bone replaces the cartilage
Cartilage is used to make a temporary forms between the bone ends. to bridge the gap in the bone. and the break is repaired.
repair and is gradually replaced by
stronger bone cells. The whole process
takes about 12 weeks, but is longer for
leg bones that carry the body’s weight.

Your body rebuilds bones throughout your life. In the womb, cartilage
still makes up much
of a baby’s skeleton.

The thighbone (femur) This cartilage will be replaced
will become the longest by the small, knobby bones
bone in the body. known as short bones,
as the baby grows.

This part of the WOW! LOOK CLOSER: FIXING FRACTURES
hip bone forms a
flexible joint with When an arm or leg bone is
the thighbone. broken, a doctor first takes
an X-ray to see where the
The chain of bones that makes break is. Once realigned,
up the backbone is made the bones are wrapped in
mostly of light spongy bone. a rigid cast to make sure
that they hold together and
Peoinpltehemirofistncgoemrs,mwornisltys,barneadkabrmonse. s heal properly. If a fracture
is really bad, doctors use
metal pins and plates to fix
the bones in place.

▶ PINNED BONES
This X-ray shows a small
number of tiny pins and

a plate (yellow) being
used to hold the broken

leg bones together.

37

Moving joints

BONES AND MUSCLES Wherever two or more bones meet in your ▶ PIVOT JOINT
skeleton, they form a joint. Some of these joints In this joint, the
are fixed and rigid. But most are free-moving rounded end of one
joints that give your skeleton incredible flexibility, bone fits within another,
allowing you to run, write, and perform many like a peg pushed through
other movements. Joints also provide stability a hole. Found at the top of
by holding your bones together. your neck, the joint allows
you to turn your head
from side to side.

FLEXIBLE JOINTS ▲ SADDLE JOINT
Your thumb has the only saddle joint
You have six main types of free-moving (synovial) in your body. It allows movement in
joints in your body. Each joint has its own range two directions—from side to side and
of movement depending on how the bones fit back and forth—making your thumb
together in the joint. The ball-and-socket joint in more flexible than your fingers.
your hip, for example, allows you to move your leg
in most directions, while the hinge joint in your
elbow allows for movement in one direction only.

HOW JOINTS WORK

This is an inside view of a synovial joint. It is WOW!
held together by a tough fibrous capsule. The
bone ends are covered by a flexible, rubbery There are about
tissue called cartilage and separated by synovial 400 joints in the body,
fluid, which helps oil the joint. Together they and more than 250 of
allow the joint to move smoothly. these are free-moving,

Ligaments are strong, synovial joints.
fibrous straps that hold
the bones together. ▲ BALLANDSOCKET JOINT
This flexible joint is formed where
Bone the ball-like head of one bone fits
into the shallow cup-shaped socket of
Synovial fluid fills another. It allows you to swing your
the space between arms and legs in most directions.
the bones.

This smooth cartilage
keeps the bones from
rubbing against
each other.

The capsule, together
with the ligaments,
surounds the joint.

38

The joints in your skull Lower leg bone STRONG HOLD
are fused together to
protect your brain. Many joints, such as
these between the foot
A hinged joint in Heel bone and leg bones, are held BONES AND MUSCLES
your jaw allows together by tough
for movement. Sole bone straps of fibrous tissue
called ligaments. These
▲ ELLIPSOIDAL JOINT allow the joints to move
Found in your knuckles and in but prevent the bones
your wrists, a dome-shaped bone from being pulled apart.
fits into the cavity of another
bone, allowing up-and-down More than a dozen ligaments
and side-to-side movements. are attached to your ankle, heel,
and sole bones, providing both
support and stability.

▲ HINGE JOINT SUPER FLEXIBLE
Just like a door hinge that
moves in one direction, this People whose bodies are very
joint works in the same way, flexible are sometimes called
allowing you to bend or double-jointed. This doesn’t
straighten your elbows,
knees, fingers, and toes. mean that they have more
joints than normal; rather,

their ligaments are more
stretchy, making their joints
more flexible. Gymnasts in

particular have unusually
flexible joints, but they also

need to maintain this
flexibility by exercising.

LOOK CLOSER: PULLED APART

Sometimes a sudden knock or blow can force bones out
of their normal position. The joint between those bones
is said to be dislocated. Doctors treat a dislocation by
forcefully moving bones back into their correct position.

▲ PLANE JOINT This type of ▲ DISLOCATED FINGER This X-ray of the hand
joint allows you to make small gliding shows clearly how two finger bones have been pulled
movements only. It forms where two apart at a knuckle joint.
flat bone ends fit tightly together,
such as in your ankle. 39

Hardheaded

BONES AND MUSCLES Your skull is the most complex part of your skeleton. It is WOW!
made up of 22 bones, but only your lower jaw bone can move.
The others are fused together to form a superstrong case that The U-shaped hyoid bone
protects your brain and the main sense organs. It also forms in your lower jaw is the only
a foundation for your face, shaping how you look. bone in the body that is not

Eight thin but incredibly attached to another bone.
strong bones form the It provides a moving base
domed part of
your skull. for the tongue.

TIGHT FIT Each eye socket is a
ball-shaped hollow area
The skull bones of a newborn baby are loosely with a hole at the back for
joined by soft tissue. This allows the skull to the optic nerve. This nerve
expand as the baby’s brain grows. By about the age carries information from
of 18 months, the soft tissue is replaced by bony your eye to your brain.
joints called sutures. These lock together like the
pieces of a jigsaw puzzle to form a rigid case.

A triangular hole in the
front of the skull leads
to an air passage behind
the nose called the
nasal cavity.

An adult has a ▲ SIDE VIEW
full set of 32 teeth, The sutures, where most of
firmly rooted in the skull bones are joined
the bones of the together, are clearly visible
upper and in this side view of a skull.
lower jaws.

40

SKULL KIT DOUBLE PROTECTION

If you were to separate all the bones in a human skull, The brain is your body’s control center. It has double
you would find that most of the biggest bones fit together protection against damage. First, the skull’s rounded
to form the cranium—the strong bony case that surrounds shape makes it extra strong, like a crash helmet,
and protects your brain. The others are the facial bones that as can be seen in this image. Then, within the skull,
contain your teeth and anchor the face muscles, which give there is a layer of shock-absorbing padding.
you a wide range of facial expressions.

Cranial bones Your spinal cord passes The skull surrounds Brain
Facial bones through this big hole where and guards the
the skull joins the spine. delicate brain. BONES AND MUSCLES

Your upper jaw Small blood vessels NOSE SUPPORTER
is made up of two that serve the brain
bones that also pass through tiny Most of your nose is not
form the base of holes in the bones. supported by bone, but by a
each eye socket. tough, flexible material called
◀ SEPARATE BONES cartilage. The same fibrous
Built for strength, your This view of the skull shows material supports your ears,
mobile lower jaw is a 20 of the 22 bones (the two tiny which is why they are so
single bone, studded bones that sit on each corner of bendable. Cartilage also
with 16 teeth. the eye are not shown). Most forms springy pads in your
of the bones of the skull are joints to keep the ends of the
paired to give your face shape. bones from grinding together.

The skull sutures WEIGHT SAVERS Your sinuses are
form wavy lines where found on either
the bones interlock. Some of the skull bones side of your nose.
around your nose contain
Each cheekbone hollow, air-filled spaces called The hollow
arches out below sinuses. These spaces help areas are like
the eye socket, reduce the weight of your echo chambers.
rejoining the skull skull, and also act as sound
behind the jaw. 41
boxes that add tone
Your movable to your voice. They are
lower jaw enables connected to your nasal
you to breathe, cavity through small openings,
speak, and eat. which may become blocked

if you catch a bad cold.

Bending backbone BONES AND MUSCLES

Running from your skull down to your hips is a long chain ▼ ATLAS AND AXIS
of bones, forming your backbone. This strong column of The atlas, the topmost bone,
bones holds your head and body upright but at the same allows your head to nod. It
time allows your neck and back to twist and bend. Your also rotates on the axis bone
backbone also protects your delicate spinal cord—a large Atlas beneath it so your head can
bundle of nerve fibers that sends messages to and turn from side to side.
from your brain to the rest of your body.
Axis
Neck (cervical)
bones FLEXIBLE SPINE

STACK OF BONES The joints between any two
vertebrae allow only limited
Twenty-six ring-shaped bones called vertebrae form movements. But added together,
your S-shaped backbone. There are seven neck bones these small movements make
supporting your head and 12 chest bones that are linked to your backbone very flexible.
your ribs. Your lower back has five lumbar bones that bear It can bend forward, backward,
most of the weight of your upper body, and a group of from side to side, and twist and
small bones in the sacrum and coccyx are fused together turn. The backbone’s soft curve
to form the tail-end of your backbone. gives it the springiness to absorb
jolts as you move.
WOW! Chest
(thoracic) A hole through
You have the same bones each bone forms
number of neck bones as a tunnel-like area
a giraffe—the difference for the spinal cord. Your backbone curves
gently, protecting the
being that a giraffe’s spinal cord from
neck bones are snapping.
much longer.
Nerve roots exit
through narrow spaces
along the backbone.

▶ LUMBAR VERTEBRAE A disk made of fibrous
These bones are sturdy and cartilage with a jellylike
have a large circular area, center separates each bone.
helping to carry the weight
that pushes down from Muscles and ligaments
your upper body. are attached along the
back of the bones.
The breastbone
(sternum) is at the
front of the ribcage.

SHOCK ABSORBERS

Lumbar bones Sandwiched between each bone is a tough,
springy disk of cartilage. The disks squash
The ribs curve slightly under pressure to absorb shocks when
forward from you run or jump, allow small bending and
the backbone. twisting movements, and prevent the bones
from rubbing against each other.

Your spinal cord Each bone can
runs through twist slightly in
this space. both directions.

A flexible cartilage
connects each rib
to the sternum.

Backbone The floating ribs are
not attached to your
breastbone.

Sacrum

ORGAN PROTECTOR The nerves branch The disk squashes
out from the to absorb knocks.
Your backbone provides a strong anchor for your ribcage. spinal cord. A springy ligament
This cagelike structure consists of 12 pairs of curved ribs, between each bone
each attached to your backbone. The ribcage helps protect helps control movement.
your delicate organs, such as the heart, lungs, and liver.

Coccyx

43

BONES AND MUSCLES

BONES AND MUSCLES SUPER STRETCHY

Contortionists are people who have
unusually flexible joints and can twist
their bodies into incredible shapes.
They are often described as being
double-jointed, but actually have the
same number of joints as everyone
else. Contortionists train hard to
maintain and increase their flexibility.

BONES AND MUSCLES

BONES AND MUSCLES Muscle machine

You can move because you have muscles—fleshy machines that turn fuel
into motion by moving the bones of your skeleton. Some muscles are under
your concious control, such as when you decide to move your hand and turn
the page of this book. However, there are many other muscles working away
without you knowing to keep your body alive, healthy, and upright.

MUSCLE TYPES The bulkiest muscle in
your body pulls your
Your body has three types of muscle, seen here leg straight at the hip.
under a microscope. You use skeletal muscle
to move your arms and legs, heart muscle to AMAZING MUSCLES
power your heart, and smooth muscle to move
food along your digestive system. The big muscles that cover your skeleton make up
almost half your body weight. They form layers
▲ SKELETAL MUSCLE Made up of bundles that work together to move all the parts of your
of threadlike fibers, skeletal muscle shrinks and body. About 640 of these skeletal muscles are
shortens to pull tight and move bones. under your conscious control. Some are very
strong, like the ones at the tops of your legs, but
others are built for precision rather than power.

The Achilles The calf muscle pulls your
tendon connects heel up so you can walk,
your calf muscle run, or stand on tiptoe.
to the heel bone.

▲ HEART MUSCLE This is a special type of
muscle that never gets tired, because it has to
keep your heart beating every day of your life.

▲ SMOOTH MUSCLE Made of long cells, Bone Muscle
often linked together in muscular sheets, smooth
muscles form part of organs such as your stomach. Tendon

46 ◀ BONE
CONNECTORS
Muscles are connected
to your skeleton by tough,
slightly stretchy straps of
tendons. They allow muscles
to move bones into position.

This powerful neck Muscles anchored to
muscle bends your your skull work your
head forward. jaw when you chew.

Your triceps WOW!
straightens your There are
arm at the elbow. more than 14 muscles
in your tongue,
Your chest muscles which is why it’s BONES AND MUSCLES
pull your arms in so flexible!
toward your body.

The biceps bends your
elbow by pulling on
the forearm.

There are no bones in your
abdomen, so tough straps
of muscle help strengthen
this part of your body.

HOT WORK FAST FACTS

Working muscles release ■ You have three times as
heat—and the harder they work, many muscles as bones
the hotter they get. This image of a in your body.
man exercising shows the hottest ■ You use at least 12 muscles
parts of his body in white, red, and in your face just to smile
yellow. His arms are hot because at a friend.
these muscles are working hard. ■ The most powerful muscle
But a lot of the heat is being carried is the masseter muscle in your
away to the man’s face, where it is jaw. You use it to chew food.

more easily lost to the air. This 47
keeps his body from overheating.

STAYING UPRIGHT

Some of your skeletal muscles
are kept busy keeping you
upright—especially your back
and neck muscles. When you
fall asleep, like this boy sleeping
on a train, your muscles get
the chance to relax and go
floppy. Sleeping on a train is
uncomfortable, but at night in
your bed, the body is properly
supported and your muscles
get the rest they need.

Body movers

BONES AND MUSCLES Every movement you make depends on your muscles— Skeletal muscle
from blinking and eating, to walking and riding a bike.
Your movements are controlled by nerve signals from your The blood
brain, which instruct the muscles to work together to vessels supply
produce precise movements. To make these movements, fuel and oxygen
your muscles need energy. This energy comes from food, to your muscles.
which is delivered to your muscles by your blood.

Muscle fiber

HOW MUSCLES WORK This is one of
the myofibrils
Skeletal muscles, such as those found in your inside a fiber.
arms, are formed by bundles of muscle fibers.
Each muscle fiber is made of smaller rod-shaped There are thousands
fibers called myofibrils, and within these are of filaments within
superthin fibers known as filaments. When the a myofibril.
filaments slide together, all the muscle fibers
shorten, so the whole muscle contracts, pulls
on your bones, and moves your arm.

TEAMWORK

When a muscle is stimulated by a nerve signal,
it contracts and gets shorter. It cannot make itself
longer. This means that muscles can pull, but
not push. Many muscles, such as the those in
your upper arm, are arranged in pairs that work
against each other, pulling in different directions.
For instance, the biceps pulls your forearm up,
and the triceps pulls it down again.

Bending When your biceps Located on the opposite
your arm contracts and gets side of the upper arm
shorter and thicker, bone, your triceps relaxes
A tendon links your arm bends. and gets longer.
a muscle to the
bone it pulls.

48


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