The human nose is an underrated tool. Humans are often thought to be insensitive smellier compared with animals, 1 this is largely because, 2 animals, we stand upright. This means that our noses are 3 to perceiving those smells which float through the air, 4 the majority of smells which stick to surfaces. In fact 5 , we are extremely sensitive to smells, 6 we do not generally realize it. Our noses are capable of 7 human smells even when these are 8 to far below one part in one million. |QxT "`rT
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Strangely, some people find that they can smell one type of flower but not another, 9 others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate 10 smell receptors in the nose. These receptors are the cells which sense smells and send 11 to the brain. However, it has been found that even people insensitive to a certain smell 12 can suddenly become sensitive to it when 13 to it often enough. 6*% E4#4
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The explanation for insensitivity to smell seems to be that brain finds it 14 to keep all smell receptors working all the time but can 15 new receptors if necessary. This may 16 explain why we are not usually sensitive to our own smells we simply do not need to be. We are not 17 of the usual smell of our own house but we 18 new smells when we visit someone else’s. The brain finds it best to keep smell receptors 19 for unfamiliar and emergency signals 20 the smell of smoke, which might indicate the danger of fire. uH=^ILN.
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人鼻子的作用被低估了。一般认为人的感觉比动物迟钝，(1)但这在很大程度上是因为，(2)不同于动物的是，人是直立的。这意味着我们的鼻子仅(3)限于闻到飘在空气中的气味，而(4)遗漏了大部分附着在物体表面的气味。(5)而实际上，我们对气味是极其敏感的，(6)尽管我们不常意识到。 我们鼻子能够(7)察觉到细微的人体气味，即使这种气味(8)微弱到总量的百万分之一。 *|^}=ioj*
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奇怪的是，有些人发现他们能闻出一些花香但有些花香却闻不出，(9)而其他人都能闻出来。这可能因为他们鼻子缺少一些基因，这些基因对于产生(10)专门的嗅觉感受器是必要的。这些感受器能够感受气味并且把 (11)信息传给大脑的细胞。 不过，已经发现，即使(12)起初对某些气味不敏感的人们，如果经常(13)接触这种气味，也会变得对它敏感起来。 dBM> ;S;v
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对气味不敏感的解释，好像是由于大脑发现保持全部嗅觉感受器持续工作(14)效率不高，但如果必要的话可以(15)产生新的感受器。这(16)也可以解释为什么我们通常对气味不敏感——只是因为我们确实没有这种需要。我们通常(17)意识不到我们自己房子熟悉的气味，而拜访其他人房子时候就会(18)注意到那里的气味。大脑发现最好使嗅觉感受器(19)用于保持对不熟悉和紧急事件信号的敏感，(20)比如烟的气味，这可能表明了火灾的危险。 y-k]Tr
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