homework encyclopedia

Human Body^143

The main entrance to the body’s airways is the nose.

It also contains our smell sensors. The

outer part of the nose is made mostly

of a rubbery tissue called cartilage.

HOW DOES THE NOSE DETECT SMELLS?

In the roof of each nostril is a small patch of nerve

endings, called the olfactory epithelium, covered

by mucus. Odour molecules from smells dissolve

in the mucus and stimulate the nerve endings,

which send signals to the olfactory bulb and the

brain. We can recognize more than 10,000 smells

and detect just a few molecules of some odours.

WHY DO WE BREATHE THROUGH THE NOSE?

Breathing through the nose helps to protect the

tissues of the lungs from germs, dirt, and very dry or

cold air. The moist lining of the nasal passages warms

and moistens air. Sticky mucus traps particles of dust,

soot, and bacteria. Tiny beating hairs, called cilia,

drive the mucus towards the throat to be swallowed.

WHY DO TEETH DECAY?

If teeth are not properly cleaned, a mixture of bacteria

and food, called plaque, builds up on them. As the

bacteria feed on the sugary food debris they produce

acid, which dissolves the calcium minerals in enamel

and dentine, forming a cavity. If the sensitive dentine

layer is exposed, hot and cold foods can cause pain.

Anchored firmly in the jaw bones are the teeth. Our

teeth are used to tear, chop, and grind food to make

it soft enough to swallow and easier to digest.

WHAT ARE TEETH MADE OF?

The white, outer shell of a tooth is made of enamel,

the hardest substance in the body. Under the enamel is

a softer substance called dentine, and inside this is the

soft, living heart of the tooth, called the pulp cavity.

4 ORAL X-RAY

This X-ray shows all the teeth

in an adult’s mouth in one

picture. It reveals their long roots,

usually hidden beneath the gums.

The white areas in the crowns are fillings.

There are four main types of tooth, each

suited to a particular task. Incisors (1) bite

and chew, canines (2) pierce and tear,

premolars (3) and molars (4) crush and grind.

FIND OUT MORE Brain 139 • Digestion 144–145 • Respiration 136

PLAQUE 3

A sticky deposit called plaque

builds up on teeth that are

not cleaned regularly.

Plaque consists of mucus,

food debris, and bacteria

that cause tooth decay.

FIND OUT MORE. Digestion 144–145 • Micro-organisms 85

INSIDE A TOOTH 3

Beneath the hard outer surface,

dentine forms the body of a

tooth. Nerves (green) and blood

vessels (red and blue) run through

the tissue of the pulp cavity and

out through root canals at the

base of the tooth.

Teeth

Nose

INSIDE THE NOSE 3

Inside the nose is the hollow nasal cavity

where air is warmed, moistened, and

cleaned before it continues on its way

down the pharynx (throat) to the lungs.

1 SMELL RECEPTORS

This magnified view shows a

tangle of cilia (microscopic hairs)

emerging from a smell receptor

cell in the nose.

The olfactory

epithelium is a small

patch of tissue

containing smell-

sensitive cells

Plaque

(yellow) has built up

on the enamel (grey)

of this tooth

The olfactory bulb

carries signals from

smell receptor cells

to the brain

Pulp cavity

containing blood

vessels and nerves

Roots

secure teeth in

the jaw bones

The crown of a

tooth is the part

above the gum

Hairs in the

nostrils filter

out large

particles

Nasal cavity

Cartilage

Enamel

Dentine

123

4

nose

teeth