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Smell

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 dangers.

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.

SUBCONSCIOUS SMELLS
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 fragrance.

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.

     
     

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Smell

 

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