One out of every fifty of your genes

你每五十個基因裡

is dedicated to it.

就有一個基因致力為嗅覺工作

It must be important, right?

嗅覺一定很重要，對吧？

Okay, take a deep breath

好，來個深呼吸

through your nose.

用你的鼻子吸氣

It's your sense of smell,

這就是你的嗅覺

and it's breathtakingly powerful.

而且它強大地令人屏息

As an adult, you can distinguish

成人可辨認出約一萬種不同的氣味

about 10,000 different smells.

成人可辨認出約一萬種不同的氣味

Here's how your nose does it.

你的鼻子是這麼作用的

Smell starts when you sniff molecules

嗅覺從你聞到從空氣中

from the air into your nostrils.

進入鼻孔的氣味分子開始

95% of your nasal cavity

在那分子到達肺部之前

is used just to filter that air

95% 的鼻腔

before it hits your lungs.

僅用來過濾空氣

But at the very back of your nose

但在你鼻子的最後方

is a region called the olfactory epithelium,

有個叫「嗅覺上皮」的區域

a little patch of skin

那片小小的皮膚

that's key to everything you smell.

是你聞到氣味的關鍵

The olfactory epithelium has a layer

嗅覺上皮有一層

of olfactory receptor cells,

嗅覺受體細胞

special neurons that sense smells,

可以感受氣味的特殊神經元

like the taste buds of your nose.

就像似你鼻子的味蕾

When odor molecules hit the back of your nose,

當氣味分子達到你鼻子的後方

they get stuck in a layer of mucus

附著在那包覆在

covering the olfactory epithelium.

嗅覺上皮的黏液層上

As they dissolve,

當氣味分子溶解時

they bind to the olfactory receptor cells,

會與嗅覺受體細胞結合

which fire and send signals

燃燒並透過嗅覺神經束

through the olfactory tract

傳送訊號

up to your brain.

至你的大腦

As a side note,

附帶一提

you can tell a lot

你可以很容易

about how good an animal's sense of smell is

依動物嗅覺上皮的大小

by the size of its olfactory epithelium.

來判斷牠們的嗅覺有多靈敏

A dog's olfactory epithelium

狗的嗅覺上皮

is 20 times bigger

比人類的大上二十倍

than your puny human one.

比人類的大上二十倍

But there's still a lot we don't know

但我們對於這一小塊的細胞群

about this little patch of cells, too.

仍有許多的未知

For example, our olfactory epithelium is pigmented,

例如，我們的嗅覺上皮是帶有色素的

and scientists don't really know why.

而科學家們並不十分清楚為什麼

But how do you actually tell the difference

但實際上，你如何分辨不同的氣味呢？

between smells?

但實際上，你如何分辨不同的氣味呢？

It turns out that your brain has

其實，你腦中有

40 million different olfactory receptor neurons,

四千萬個不同的嗅覺受體神經元

so odor A might trigger neurons 3, 427, and 988,

氣味 A 可以觸發神經元 3、427 和 988

and odor B might trigger neurons 8, 76, and 2,496,678.

而氣味 B 可以觸發神經元 8、76 和 2,496,678

All of these different combinations

所有各式不同的組合

let you detect a staggeringly broad array of smells.

讓你聞出數量驚人的各種氣味

Plus, your olfactory neurons are always fresh

加上你的神經元總是常保清新

and ready for action.

且隨時準備行動

They're the only neuron in the body

它們是我們身上唯一

that gets replaced regularly,

定時更新的神經元

every four to eight weeks.

每四到八周更新一次

Once those neurons are triggered,

一旦這些神經元被觸發

the signal travels through a bundle

氣味訊號就會經由

called the olfactory tract

一個叫做「嗅覺神經束」

to destinations all over your brain,

傳達到分佈在你整個大腦

making stops in the amygdala,

停留在扁桃腺、

the thalamus,

丘腦、

and the neocortex.

和腦皮質

This is different

這有別於

from how sight and sound are processed.

視覺與聽覺訊號的傳遞

Each of those signals goes first

每個視覺與聽覺訊號

to a relay center

會先停留在一個

in the middle of the cerebral hemisphere

位於大腦半球中央的中繼中心

and then out to other regions of the brain.

然後分送到大腦的其他區域

But smell, because it evolved

但嗅覺呢，因為它的演化

before most of your other senses,

早於你大部分的其他感官

takes a direct route

嗅覺是直接進入

to these different regions of the brain,

腦中不同的區域

where it can trigger your fight-or-flight response,

這些區域是可以觸發戰鬥或逃跑反應、

help you recall memories,

幫助你喚起記憶、

or make your mouth water.

或讓你分泌唾液

But even though we've all got

但即便我們有一樣的生理構造──

the same physiological set-up,

但即便我們有一樣的生理構造──

two nostrils and millions of olfactory neurons,

兩個鼻孔和數百萬的嗅覺神經元

not everybody smells the same things.

卻不是每個人聞到的氣味都一樣

One of the most famous examples of this

其中最著名的一個例子是

is the ability to smell so-called "asparagus pee."

嗅出所謂的「蘆筍尿」的能力

For about a quarter of the population,

大約 1/4 的人們

urinating after eating asparagus

吃了蘆筍後排尿時

means smelling a distinct odor.

會聞到尿液裡的特殊異味

The other 75% of us don't notice.

但其他 3/4 的人卻聞不出來

And this isn't the only case

而這還不是

of smells differing from nose to nose.

每個鼻子聞到的氣味不同的唯一例子

For some people,

對某些人而言

the chemical androstenone smells like vanilla;

化學物質「雄甾酮」聞起來像香草

to others, it smells like sweaty urine,

別人聞起來卻像汗尿味

which is unfortunate

這很可惜

because androstenone is commonly found

因為雄甾酮通常存在像豬肉這樣好吃的東西裡

in tasty things like pork.

因為雄甾酮通常存在像豬肉這樣好吃的東西裡

So with the sweaty urine smellers in mind,

因顧慮到這些會聞到尿汗味的人

pork producers will castrate male pigs

豬肉供應商將公豬去勢

to stop them from making androstenone.

來防止公豬製造雄甾酮

The inability to smell a scent

無法聞到某種氣味

is called anosmia,

稱為嗅覺喪失症

and there are about 100 known examples.

而且已知的例子大概有一百種

People with allicin anosmia can't smell garlic.

有大蒜素嗅覺喪失症的人聞不出大蒜味

Those with eugenol anosmia can't smell cloves.

有丁香酸嗅覺喪失症的人聞不出丁香味

And some people can't smell anything

而有些人則完全聞不出任何氣味

at all.

而有些人則完全聞不出任何氣味

This kind of full anosmia

這種完全嗅覺喪失症

could have several causes.

可能有幾個肇因

Some people are born without a sense of smell.

有些人天生就沒有嗅覺

Others lose it after an accident

有些人是意外傷害後

or during an illness.

或是患病時喪失嗅覺

If the olfactory epithelium gets swollen or infected,

如果嗅覺上皮腫脹或感染

it can hamper your sense of smell,

會對你的嗅覺造成阻礙

something you might have experienced

當你生病時你可能有這樣的經驗

when you were sick.

當你生病時你可能有這樣的經驗

And not being able to smell anything

然而聞不到任何氣味

can mess with your other senses, too.

也會影響你其他的感官

Many people who can't smell at all

許多完全聞不到氣味的人

also can't really taste the same way

嚐到的味道也和我們不盡相同

the rest of us do.

嚐到的味道也和我們不盡相同

It turns out that how something tastes

事實證明，東西嚐起來的味道

is closely related to how it smells.

和聞起來的氣味是息息相關的

As you chew your food,

當你咀嚼食物時

air is pushed up your nasal passage,

夾帶食物氣味的空氣

carrying with it the smell of your food.

被推上你的鼻腔

Those scents hit your olfactory epithelium

那氣味到達你的嗅覺上皮

and tell your brain a lot

告訴你的大腦許多

about what you're eating.

關於你正在進食的東西的訊息

Without the ability to smell,

沒了嗅覺的能力

you lose the ability to taste

你喪失了品嚐任何

anything more complicated

比五味更複雜的味道

than the five tastes

比五味更複雜的味道

your taste buds can detect:

你的味蕾可以分辨

sweet,

甜、

salty,

鹹、

bitter,

苦、

sour,

酸、

and savory.

香

So, the next time you smell exhaust fumes,

所以，下次你聞到排出的煙、

salty sea air,

海的鹹味、

or roast chicken,

或烤雞的香味

you'll know exactly how you've done it

你就知道你到底怎麼辦到的

and, perhaps, be a little more thankful that you can.

也或許，為你可以聞到那些氣味有多一點點的感激