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It's the first sense you use when you're born.
它是你出生时第一个使用的感觉。
One out of every fifty of your genes is dedicated to it. It must be important, right?
你每五十个基因之中就有一个用于它。它肯定很重要，对不对？
Okay, take a deep breath through your nose.
好吧，做个深呼吸，从你的鼻子吸气。
It's your sense of smell, and it's breathtakingly powerful.
它就是你的嗅觉，它的强大简直要让你倒吸一口凉气。
As an adult, you can distinguish about 10,000 different smells. Here's how your nose does it.
一个成人可以区分出大约一万种不同的气味。你的鼻子是这样做到的：
Smell starts when you sniff molecules from the air into your nostrils.
开始时你把空气中的分子从鼻孔吸入。
95% of your nasal cavity is used just to filter that air before it hits your lungs.
你的鼻腔中95%的部分用来过滤刚吸入的空气，然后这些空气才进入你的肺里。
But at the very back of your nose is a region called the olfactory epithelium,
但是在你的鼻子的最后面有一个区域叫做嗅上皮，
a little patch of skin that's key to everything you smell.
是一小块皮肤，它是你能闻到东西的关键。
The olfactory epithelium has a layer of olfactory receptor cells,
嗅上皮是一层嗅觉受体细胞组成的，
special neurons that sense smells, like the taste buds of your nose.
它们是一群能感觉到气味的特殊神经元，就好像你鼻子里的味蕾。
When odor molecules hit the back of your nose,
当气味分子撞到你的鼻子后部，
they get stuck in a layer of mucus covering the olfactory epithelium.
他们就会被困在一层粘液里，这层粘液就盖在嗅上皮表面。
As they dissolve, they bind to the olfactory receptor cells,
气味分子被（粘液）溶解之后，就结合到嗅觉受体细胞上，
which fire and send signals through the olfactory tract up to your brain.
这些细胞再产生和发送出信号，通过嗅束传到你的大脑。
As a side note, you can tell a lot about how good an animal's sense of smell is by the size of its olfactory epithelium.
一个题外话，你可以根据一种动物嗅上皮的大小来判断它的嗅觉有多好。
A dog's olfactory epithelium is 20 times bigger than your puny human one.
狗的嗅上皮比人类的小小嗅上皮要大20倍。
But there's still a lot we don't know about this little patch of cells, too.
不过关于这一小群细胞，我们还有很多不知道的呢。
For example, our olfactory epithelium is pigmented, and scientists don't really know why.
比如说，我们的嗅上皮是带色素的，科学家们还不知道这是为什么。
But how do you actually tell the difference between smells?
但你究竟是如何区分不同的气味的呢？
It turns out that your brain has 40 million different olfactory receptor neurons,
事实证明你的大脑里有4千万个不同的嗅觉受体神经元，
so odor A might trigger neurons 3, 427, and 988, and odor B might trigger neurons 8, 76, and 2,496,678.
所以气味A可能会触发神经元3、427、和988，而气味B可能触发神经元8、76和2496678。
All of these different combinations let you detect a staggeringly broad array of smells.
所有这些不同的组合可以让你闻出多到惊人的各种气味。
Olfactory neurons are always fresh and ready for action.
而且，你的嗅觉神经元总是精神抖擞并随时准备行动。
They're the only neuron in the body that gets replaced regularly, every four to eight weeks.
他们是身体里唯一被定期更新的神经元，每四到八个星期更新一次。
Once they are triggered, the signal travels through a bundle called the olfactory tract to destinations all over your brain,
一旦这些神经元被触发，信号通过一束称为嗅束的线路传播，直到遍布你的大脑里的它们目的地。
making stops in the amygdala, the thalamus, and the neocortex.
信号会在扁桃体、丘脑和大脑皮层都有停留。
This is different from how sight and sound are processed.
这种传播方式和视觉及声音信号的处理方式不同。
Each of those signals goes first to a relay center
这些信号都先要到达一个中继中心，
in the middle of the cerebral hemisphere and then out to other regions of the brain.
位于大脑半球的中央，然后再输出到大脑的其他区域。
But smell, because it evolved before most of your other senses,
但是嗅觉，它比其他感官更早进化出来，
takes a direct route to these different regions of the brain,
它是通过一条直接的途径进入大脑的不同区域，
where it can trigger your fight-or-flight response, help you recall memories, or make your mouth water.
它在这些区域可以触发你的战逃反应，帮助你回忆或者让你流口水。
But even though we've all got the same physiological set-up,
但即使我们都有相同的生理结构，
two nostrils and millions of olfactory neurons, not everybody smells the same things.
两个鼻孔和数以百万计的嗅觉神经元，也不是每个人都能闻到同样的气味。
One of the most famous examples of this is the ability to smell so-called 'asparagus pee.'
这方面最著名的例子之一就是能够闻到所谓的芦笋尿。
For about a quarter of the population, urinating after eating asparagus means smelling a distinct odor.
人群中大约四分之一的人，在吃了芦笋后排尿时，会嗅到一种特定的气味。
The other 75% of us don't notice.
而另外75%的人就闻不到。
And this isn't the only case of smells differing from nose to nose.
这还不是唯一一种不同的鼻子闻到不同的气味的例子。
For some people, the chemical androstenone smells like vanilla;
对某些人来说，化合物雄甾酮闻起来像香草；
to others, it smells like sweaty urine, which is unfortunate because androstenone is commonly found in tasty things like pork.
而对其他人来说，闻起来就像汗尿，这些人太不幸了，因为雄甾酮常见于猪肉这一类好吃的东西里。
So with the sweaty urine smellers in mind, pork producers will castrate male pigs to stop them from making androstenone.
所以考虑到这些闻到汗尿味的人，猪肉生产商们会将公猪阉割，使它们不能产生雄甾酮。
The inability to smell a scent is called anosmia, and there are about 100 known examples.
不能闻到某种气味被称为嗅觉障碍，大约已有100个已知的例子。
People with allicin anosmia can't smell garlic.
大蒜素嗅觉障碍的人闻不到大蒜味。
Those with eugenol anosmia can't smell cloves.
丁香酚嗅觉障碍的人闻不到丁香味。
And some people can't smell anything at all.
有些人什么味也闻不到。
This kind of full anosmia could have several causes.
这种完全性的嗅觉障碍可能有几个成因。
Some people are born without a sense of smell.
有些人天生没有嗅觉。
Others lose it after an accident or during an illness.
有些人在发生意外之后或患病期间失去嗅觉。
If the olfactory epithelium gets swollen or infected, it can hamper your sense of smell,
如果嗅上皮肿涨或受了感染，会影响你的嗅觉，
something you might have experienced when you were sick.
你可能已经在生病时有过这种经历。
Not being able to smell anything can mess with your other senses, too.
而且闻不到气味的时候，你的其他感官也会受到影响。
Many people who can't smell at all also can't really taste the same way the rest of us do.
很多闻不到任何味道的人，尝到的味道也和我们不一样。
It turns out that how something tastes is closely related to how it smells.
其实东西吃起来的味道和它闻起来的味道是密切相关的。
As you chew your food, air is pushed up your nasal passage, carrying with it the smell of your food.
你咀嚼食物的时候，空气被推入你的鼻腔，夹带着你的食物的味道。
Those scents hit your olfactory epithelium and tell your brain a lot about what you're eating.
这些气味接触到你的嗅上皮，告诉你的大脑很多关于你在吃的东西的信息。
Without the ability to smell,
如果不能闻，
you lose the ability to taste anything more complicated than the five tastes your taste buds can detect:
你也就不能品尝到任何比简单的五味复杂一点的味道了：
sweet, salty, bitter, sour, and savory.
甜、咸、苦、酸和薄荷味。
So, the next time you smell exhaust fumes, salty sea air, or roast chicken,
所以下次你闻到尾气油烟、咸咸的海洋空气或者烤鸡，
you'll know exactly how you've done it and, perhaps, be a little more thankful that you can.
你就知道你到底是怎么闻到的了，而且或许，对于你可以闻到味多一点点感激的心。
