homework encyclopedia

This strong yet flexible material is a living tissue, made up of bone

cells embedded in a matrix of fibres. Bone is not solid – blood

vessels and nerves run through tunnels within it, and some areas

are a honeycomb of small spaces. In the centre of many bones is

a cavity packed with a jelly-like substance called bone marrow.

WHAT DOES BONE MARROW DO?

Bone marrow makes millions of blood cells every

second to replace old, worn-out blood cells, which the

body destroys. There are two types of marrow: red

and yellow. Red marrow makes blood cells. Yellow

marrow is mainly a fat store, but it can turn into

red marrow if the body needs extra blood cells. At

birth, nearly all bone marrow is red. During the teens,

much of it turns into yellow bone marrow.

HOW DO DIFFERENT JOINTS WORK?

Most joints are free-moving. These are called synovial

joints, and they allow varying degrees of

movement. Hinge joints, like those in the

fingers, knees, and elbows, can only bend

and straighten. Others, such as the ball-

and-socket joints in the shoulders and

hips, allow movement in all directions.

WHY DON’T BONES RUB AGAINST EACH OTHER?

In synovial joints, the bone endings are covered

with a smooth, glossy material called hyaline cartilage,

which is slippery yet hard-wearing. This cartilage

allows the bone ends to slide smoothly past each

other. Also, a capsule of fluid surrounds the joint.

The fluid lubricates the joint, reducing friction just

as oil helps the movement of a bicycle chain.

WHAT IS BONE MADE OF?

The hard matrix of bone is made of crystals of

calcium phosphate and other minerals, and fibres of

protein called collagen. The minerals make bone hard,

while the collagen fibres are arranged lengthways to

make bone flexible. Both are produced by cells called

osteocytes, found throughout the matrix.

Bones are joined at joints. Different types

of joint allow different movements. Joints

are often held together by straps of tough

fibrous tissue, called ligaments, and the

muscles that cross the joint.

FIND OUT MORE. Circulation 134 • Growth 149 • Muscle 132

BONE

SPONGY BONE 1

A honeycomb of struts and spaces makes spongy bone

lightweight yet resilient. Bone marrow fills the spaces.

4 ARM MOVEMENT

The joints in the shoulder, elbow,

and wrist work together to give the

arm an amazing range of movement.

KNEE JOINT 3

Synovial fluid

and cartilage ease

movement where the

femur (thigh bone) and

tibia (shin bone) meet.

4 COMPACT BONE

Densely packed, concentric rings

of minerals and collagen give

compact bone immense strength.

Blood vessels run through tunnels

in the centre of each set of rings.

1 SECTION THROUGH A BONE

This femur has been cut to show

its different layers. Because this is

not a living bone the hollow

centre has no bone marrow in it.

JOINTS

The shoulder

is a ball-and-socket joint,

which allows the greatest

range of movement

A column

of 24 vertebrae

holds the trunk

upright, but twists

and bends to

allow movement

The elbow
is a hinge joint,
allowing the arm
to bend and
straighten

Cartilage

(pink) lines the

surfaces where

the bones meet

Synovial fluid

(blue) allows the

joint to move

more freely

BALL-AND-

SOCKET

JOINT

HINGE JOINT

Spongy bone

Compact bone