Childrens Illustrated Encyclopedia

465

Skeletons 465-

Humans and other mammals, fish, birds, and reptiles

all have an endoskeleton, or inner skeleton, made up of

many separate

bones. the central

part of the skeleton

is the spine (vertebral

column or backbone).

the spinal joints can

move only a little, but

the spine as a whole is

very flexible. some creatures,

such as worms, have no bones.

Instead, the pressure of fluid

inside their bodies helps them

keep their shape. they are said

to have a hydrostatic skeleton.

InsIDe tHe HUMan BoDY hundreds of bones link

together like scaffolding to form the skeleton. Without a

skeleton, the body would collapse. the skeleton holds the

body rigid and gives shape to all the softer parts. It also

protects the organs—the skull surrounds the brain, and

the ribs act as a protective cage around the lungs and

heart. the skeleton is also an anchor for the muscles,

which move the different parts of the body.

Bone is made of living cells surrounded by a

framework of minerals, particularly calcium

and phosphate, and tough fibers of a

substance called collagen. In a newborn baby

many of the bones are made of a soft, rubbery

substance called cartilage. as a baby grows, the

cartilage is gradually replaced by hard bone. our

wrists and ankles are among the last to become

bone. In later life, bones gradually become more

fragile and brittle and break more easily.

Fibula

(calf bone)

Thin, tough outer layer

called the periosteum

Hard, compact

bone outside

Medullary cavity

Bone

living bone is tough and slightly flexible—

only dead bone is white and brittle. Blood

vessels pass through small holes in the

bone’s surface and carry a steady supply

of blood to the bone. some bones contain

a jellylike substance called red bone

marrow, which makes blood cells.

HUMan skeleton

there are 206 bones in the

human skeleton, including 28

in the skull, 26 in the spine,

30 in each arm, 30 in each

leg, and 25 in the chest. the

largest bone is in the thigh,

and the smallest ones are the

ossicles, which are three tiny

bones inside each ear.

SKELETONS

JoInts

Bones are linked together at joints. there are several types of

joints, including fixed, hinge, and ball-and-socket joints. Fixed

joints, such as those between the separate bones in the skull,

cannot move. Hinge joints, such as those

in the elbow, allow movement in one

direction only. Ball-and-socket joints,

such as the hip, allow the bones to

swing in two directions and also to twist.

Hip joint

Carpals

(wrist bones)

Metacarpals

(palm bones)

Patella

(kneecap)

Radius

(forearm bone)

Lumbar (lower back)

vertebrae

Cervical (neck)

vertebrae

Maxilla

(upper jaw)

Mandible

(lower jaw)

Skull

The elbow is a

hinge joint, the

simplest of

joints, and

moves mainly

back and forth.

A pivot joint

allows the head

to turn from

side to side.

The wrist is formed

by an ellipsoidal

joint, which can be

flexed or extended

and moved from

side to side.

Soft, spongy

bone inside

The shoulder and

the hip are both

ball-and-socket

joints and allow

the greatest range

of movement.

Tibia

(shin bone)

Humerus

(upper arm bone)

12 pairs of ribs

Sternum

(breastbone)

Scapula

(shoulder blade)

Clavicle

(collarbone)

Pelvic

(hip) girdle Phalanges

(finger bones)

Femur

(thighbone)

Tarsals

(ankle bones)

Metatarsals

(foot bones)

Phalanges

(toe bones)

Ulna

(forearm bone)

Lizard has an internal

skeleton, like other

vertebrates.

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