Smell is the most evocative of our senses; the slightest scent can bring
on a heady rush of long-forgotten memories. Complex yet primitive, it
retains its intimate associations with pleasure and warns of possible
Smell - the ability to detect and analyse chemical traces in the air - is
perhaps the least understood of the five senses. The sensory receptors for
smell are tucked away in the roof of the nasal cavity, just beneath the
frontal lobes of the brain. This olfactory area is tightly packed with
millions of small cells, each with about a dozen fine hairs, or cilia,
projecting into a layer of mucus. The mucus keeps the cilia moist and traps
odours, while the cilia effectively enlarge the area of each olfactory cell
and so increase its sensitivity.
Although the actual process is not fully understood, it is thought that
smelly substances dissolve in the mucous fluids, stick on to the cilia and
cause the cells to fire off electrical signals. Olfactory nerves channel
these signals to the two olfactory bulbs in the brain, where the information
is processed and passed on through to the olfactory area of the cerebral
cortex. This is at the base of the brain, the same area which is the sorting
house for our emotional responses, which may account for the way that smells
evoke deep emotional responses and unlock buried memories.
It is also the area mainly associated with the subconscious mind, and many
of our responses to smell go on beneath the conscious level, not reaching
the cerebral cortex at all.
To be smelly, a substance must give off chemical particles. Such
particles must be fairly complex - simple substances such as table salt
rarely have any detectable smell - and remain in suspension in the air in
order to be swept into the nostrils and dissolve in the mucus. Volatile
substances such as petrol are very smelly as high concentrations of the
chemicals are able to reach the cells; perfumes are deliberately structured
in a similar way.
A SENSE OF PURPOSE
Smell is the main way many animals sense the world, but humans have
gradually become less dependent on it. It now has four specific roles:
stimulating the salivary glands prior to eating; sexual attraction; a basic
warning system; and a back-up information gathering system.
Taste and smell are strongly linked. Our taste buds are fairly crude
mechanisms, capable only of distinguishing salt, sweet, sour and bitter. All
the subtle information about the taste of food comes from smell; this is why
we enjoy our food much less when we have a cold.
At the subconscious level, we are all susceptible to pheromones. sexual
hormones that are present in sweat. The popularity of perfumes illustrates
the role that noses play in sexual attraction. Men and women
are susceptible to different smells; parfumiers play on this when creating a
Our sense of smell is often the first warning we get of any unpleasant
gases in the air - if something is burning, for instance, - and is a good
way of telling if meat is good or bad or whether milk has turned. However,
it is a fairly crude information gatherer, as we find it hard to detect
differences in the intensity of a smell. It might have to change by upwards
of 30 per cent before we can detect a difference. By contrast, our eyes can
detect changes of one per cent in the intensity of light.
SNIFFING OUT TROUBLE
Problems with the ability to smell are most commonly due to short-term
problems in the nose, such as the common cold or sinusitis, that affect our
ability to get air to pass over the olfactory area. Heavy smokers also
suffer a loss of smell because the delicate tissues dry up.
Anosmia - the loss of the sense of smell - can be caused by head injury.
It does not have to be a serious injury; the delicate olfactory nerves,
which pierce the bottom of the skull, can be bruised or even sheared off by
a knock to the head. Anosmia through injury can be permanent, and may affect
one or both nostrils.
More rarely, a permanent or temporary loss of the ability to smell could
be caused by a tumour or aneurysm pressing on the olfactory nerves, by
meningitis or by internal haemorrhaging.