One of the first symptoms of COVID appears
to be that a person loses their sense of smell. I wondered about this and how
important smell is to us and came across this wonderfully written treatise on
smell by Brain Pickings. Enjoy the read.
Our sense of smell can be
extraordinarily precise, yet it's almost impossible to describe how something
smells to someone who hasn't smelled it. That's precisely what science
historian Diane Ackerman explores in A Natural History of the Senses.
We see only where there is light enough,
taste only when we put things into our mouths, touch only when we make contact
with someone or something, hear only sounds that are loud enough to hear. But
we smell always and with every breath. Cover your eyes and you will stop
seeing, cover your ears and you will stop hearing, but if you cover your nose
and stop smelling, you will die.
In fact, every breath we take to live is
saturated with an extraordinary amount of olfactory information – a fact largely
a matter of scale:
Each day, we breathe about 23,040 times
and move around 438 cubic feet of air. It takes us about five seconds to
breathe – two seconds to inhale and three seconds to exhale – and, in that
time, molecules of odour flood through our systems. Inhaling and exhaling, we
smell odors. Smells coat us, swirl around us, enter our bodies, emanate from
us. We live in a constant wash of them. Still, when we try to describe a smell,
words fail us like the fabrications they are… The charm of language is that,
though it is human made, it can on rare occasions capture emotions and
sensations that aren't. But the physiological links between the smell and
language centers of the brain are pitifully weak. Not so the links between the
smell and the memory centers, a route that carries us nimbly across time and
distance.
Indeed, that route is a greater shortcut
to our cognition and psychoemotional circuitry than any of our other senses can
offer. Ackerman outlines the singular qualities of our smell-sensation that set
it apart from all other bodily functions:
Smell is the most direct of all our
senses. When I hold a violet to my nose and inhale, odor molecules float back
into the nasal cavity behind the bridge of the nose, where they are absorbed by
the mucosa containing receptor cells bearing microscopic hairs called cilia.
Five million of these cells fire impulses to the brain’s olfactory bulb or
smell center. Such cells are unique to the nose. If you destroy a neuron in the
brain, it’s finished forever; it won’t regrow. If you damage neurons in your
eyes or ears, both organs will be irreparably damaged. But the neurons in the
nose are replaced about every thirty days and, unlike any other neurons in the
body, they stick right out and wave in the air current like anemones on a coral
reef.
That's also what makes perfumes so
powerful – if you've ever walked into a crowded room and instantly experienced
a pang of emotion as you thought you smelled your ex, or your mother, or your
third-grade teacher, you've had a first-hand testimony to the potency of smell
as a trigger of emotional memory. Ackerman explains:
A smell can be overwhelmingly nostalgic
because it triggers powerful images and emotions before we have time to edit
them… When we give perfume to someone, we give them liquid memory. Kipling was
right: “Smells are surer than sights and sounds to make your heart-strings
crack."
What's perhaps most extraordinary is that
scent lodges itself largely in the long-term memory system of the brain. And
yet, we remain inept at mapping those links and associative chains when it
comes to describing smells and their emotional echoes. To shed light on how
perfumery plays into this paradox, Ackerman offers a taxonomy of the basic
types of natural smells and how they became synthetically replicated,
unleashing an intimate dance of art, science, and commerce:
All smells fall into a few basic
categories, almost like primary colors: minty (peppermint), floral (roses),
ethereal (pears), musky (musk), resinous (camphor), foul (rotten eggs), and
acrid (vinegar). Therefore perfume manufacturers have had such success in
concocting floral bouquets or just the right threshold of muskiness or fruitiness.
Natural substances are no longer required; perfumes can be made on the
molecular level in laboratories. One of the first perfumes based on a
completely synthetic smell (an aldehyde) was Chanel No. 5, which was created in
1922 and has remained a classic of sensual femininity. It has led to classic
comments, too. When Marilyn Monroe was asked by a reporter what she wore to
bed, she answered coyly, "Chanel No. 5." Its top note – the one you
smell first – is the aldehyde, then your nose detects the middle note of
jasmine, rose, lily of the valley, orris, and ylang-ylang, and finally the base
note, which carries the perfume and makes it linger: vetiver, sandalwood,
cedar, vanilla, amber, civet, and musk. Base notes are almost always of animal
origin, ancient emissaries of smell that transport us across woodlands and
savannas.
And so, we get to the actual science of
smell – what actually makes us have an olfactory experience, and why we often
confuse those with taste:
We need only eight molecules of a
substance to trigger an impulse in a nerve ending, but forty nerve endings must
be aroused before we smell something. Not everything has a smell: only
substances volatile enough to spray microscopic particles into the air. Many
things we encounter each day – including stone, glass, steel, and ivory – don't
evaporate when they stand at room temperature, so we don't smell them. If you
heat cabbage, it becomes more volatile (some of its particles evaporate into
the air) and it suddenly smells stronger. Weightlessness makes astronauts lose
taste and smell in space. In the absence of gravity, molecules cannot be
volatile, so few of them get into our noses deeply enough to register as odors.
This is a problem for nutritionists designing space food. Much of the taste of
food depends on its smell; some chemists have gone so far as to claim that wine
is simply a tasteless liquid that is deeply fragrant. Drink wine with a head
cold, and you'll taste water, they say. Before something can be tasted, it has
to be dissolved in liquid (for example hard candy has to melt in saliva); and
before something can be smelled, it has to be airborne. We taste only four
flavors: sweet, sour, salt, and bitter. That means that everything else we call
"flavor" is really "odor." And many of the foods we think
we can smell we can only taste. Sugar isn't volatile, so we don't smell it,
even though we taste it intensely. If we have a mouthful of something
delicious, which we want to savor and contemplate, we exhale; this drives the
air in our mouths across our olfactory receptors, so we can smell it better.
