Childrens Illustrated Encyclopedia

262

Cell

every second, millions of cells die and millions

more replace them. An average cell measures

about one-thousandth of an inch (0.025 mm)

across, but there are many different kinds of

cells in the body, each adapted for a certain

job. Nerve cells are long and thin. like wires,

nerve cells conduct (carry) electrical nerve

signals. Red blood cells are doughnut-

shaped and contain chemicals that carry

oxygen around the body. epithelial cells

on body surfaces, such as

the lining of the mouth,

are broad and flat 

and fit together like

paving stones.

The body’s AbiliTies

The human body is capable

of amazing feats of balance and

coordination. Many animals can

run faster or jump higher, but

our bodies are very adaptable.

An extremely complex brain

controls the body and gives us the

intelligence to use our physical

abilities to the best advantage.

skiN

The body is covered by skin.

skin is flexible and helps

protect the body. it keeps

water and harmful bacteria

out, and keeps body fluids

in. skin is also wear-resistant

because it continually

renews itself. The base

of the upper layer,

or epidermis, divides

constantly to make new

cells. The new cells

move upwards as if on a

conveyor belt, to

replace cells that are

worn out.

FRoM The MoMeNT we are born to the moment we die, our bodies do not

stop working for a second. The human body is a complex collection of more

than 100,000 million living units called cells. There are more than 200 different

types of cells, including nerve cells or neurons, muscle cells, fat cells, epidermal

cells, blood cells, and gland cells that release secretions, such as hormones and

enzymes. each type of cell in the body does a particular job. Cells that do similar

jobs are grouped together to form tissues, such as muscle tissue and nerve tissue.

Tissues, in turn, are grouped together to form organs, which are separate

structures within the body. The lungs, heart, liver, and kidneys are some of the

main organs. linked organs work together as systems, and each system carries

out one or more major functions. For example, the heart, blood vessels, and

blood form the circulatory system, which carries oxygen and nutrients around

the body and carries away waste products. All the different systems work together,

under the control of the brain, to produce a living, walking, talking human body.

NeRvous sysTeM

The brain and the nerves make up the nervous system.

Nerves extend from the brain and spinal cord to all body parts, carrying

signals in the form of tiny electrical impulses. The signals bring information

from the sense organs to the brain and take instructions from the brain to the

muscles. The brain controls many processes automatically, such as breathing,

heartbeat, and digestion, without us having to think about them.

Hair root

Dermis

Hair

Epidermis

Hair

follicle

Human body 262-

Skin cell

Red

blood

cell

Nerve cell

Bone cell

Muscle cell

Fat cell

Sweat

gland

Skull

Neck vertebra

Rib cage

Humerus

(upper-arm

bone)

skeleTAl sysTeM

Two hundred and

six bones form the

body’s strong internal

framework. some are

connected at flexible

joints; joints in the leg,

for example, allow us to

move. others are fixed

firmly together, as in the

skull. The vertebral

column, or backbone,

supports the head at

the top and the limbs

on either side. The

backbone also encases

and protects the delicate

spinal cord.

Nerves

Femur

(thigh

Patella bone)

(knee-

cap)

Tibia

(shin-

bone)

Elbow

joint

Radius

Metacarpal (bone

joining wrist with

fingers)

Talus (topmost bone

of the ankle joint)

Cerebrum (main

part of brain)

Spinal

cord

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