[00:06.89]The human nose is an underrated tool.
[00:09.60]Humans are often thought to be insensitive smellers
[00:12.53]compared with animals,
[00:14.04](1)but this is largely because, (2)unlike animals,
[00:16.97]we stand upright.
[00:18.69]This means that our noses are (3)limited to
[00:20.81]perceiving those smells which float through the air,
[00:24.03](4)missing the majority of smells
[00:25.54]which stick to surfaces.
[00:27.66]In fact, (5)though, we are extremely sensitive to smells,
[00:31.59](6)even if we do not generally realize it.
[00:34.42]Our noses are capable of (7)detecting human smells
[00:37.44]even when these are (8)diluted to far below
[00:40.06]one part in one million.
[00:42.68]Strangely, some people find that they can smell
[00:45.71]one type of flower but not another,
[00:47.72](9)whereas others are sensitive
[00:49.14]to the smells of both flowers.
[00:51.35]This may be because some people do not have
[00:53.46]the genes necessary to generate (10)particular smell receptors
[00:57.12]in the nose.
[00:58.00]These receptors are the cells which sense smells
[01:01.19]and send (11)messages to the brain.
[01:03.61]However, it has been found
[01:05.22]that even people insensitive to
[01:06.84]a certain smell (12)at first
[01:08.66]can suddenly become sensitive to it
[01:10.47]when (13)exposed to it often enough.
[01:13.30]The explanation for insensitivity to smell
[01:16.01]seems to be that the brain finds it
[01:17.93](14)inefficient to keep all smell receptors working all the time
[01:21.77]but can (15)create new receptors if necessary.
[01:24.50]This may (16)also explain
[01:25.91]why we are not usually sensitive to our own smells
[01:29.14]--we simply do not need to be.
[01:30.96]We are not (17)aware of the usual smell
[01:33.11]of our own house,
[01:34.52]but we (18)notice new smells
[01:35.93]when we visit someone else's.
[01:37.84]The brain finds it best to
[01:39.37]keep smell receptors (19)available for unfamiliar
[01:42.09]and emergency signals
[01:43.71](20)such as the smell of smoke,
[01:45.11]which might indicate the danger of fire.内容来自 听力课堂网：http://www.tingclass.net/show-8692-250725-1.html