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.
"rAm6b-` J ^t=.-a| 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.
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eU3 :mg#&MZj< 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.
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@ . 人鼻子的作用被低估了。一般認為人的感覺比動物遲鈍，(1)但這在很大程度上是因為，(2)不同于動物的是，人是直立的。這意味著我們的鼻子僅(3)限于聞到飄在空氣中的氣味，而(4)遺漏了大部分附著在物體表面的氣味。(5)而實際上，我們對氣味是極其敏感的，(6)盡管我們不常意識到。 我們鼻子能夠(7)察覺到細微的人體氣味，即使這種氣味(8)微弱到總量的百萬分之一。
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q 奇怪的是，有些人發現他們能聞出一些花香但有些花香卻聞不出，(9)而其他人都能聞出來。這可能因為他們鼻子缺少一些基因，這些基因對于產生(10)專門的嗅覺感受器是必要的。這些感受器能夠感受氣味并且把 (11)信息傳給大腦的細胞。 不過，已經發現，即使(12)起初對某些氣味不敏感的人們，如果經常(13)接觸這種氣味，也會變得對它敏感起來。
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F.> 對氣味不敏感的解釋，好像是由于大腦發現保持全部嗅覺感受器持續工作(14)效率不高，但如果必要的話可以(15)產生新的感受器。這(16)也可以解釋為什么我們通常對氣味不敏感——只是因為我們確實沒有這種需要。我們通常(17)意識不到我們自己房子熟悉的氣味，而拜訪其他人房子時候就會(18)注意到那里的氣味。大腦發現最好使嗅覺感受器(19)用于保持對不熟悉和緊急事件信號的敏感，(20)比如煙的氣味，這可能表明了火災的危險。
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