填空题 The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, ---|||________|||--- this is largely because, ---|||________|||--- animals, we stand upright. This means that our noses are ---|||________|||--- to perceiving those smells which float through the air, ---|||________|||--- the majority of smells which stick to surfaces. In fact, ---|||________|||--- , we are extremely sensitive to smells, ---|||________|||--- we do not generally realize it. Our noses are capable of ---|||________|||--- human smells even these are ---|||________|||---to far below one part in one million. 　　Strangely, some people find that they can smell one type of flower but not another, ---|||________|||--- others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate ---|||________|||--- smell receptors in the nose. These receptors are the cells which sense smells and send ---|||________|||--- to the brain. However, it has been found that even people insensitive to a certain smell ---|||________|||--- can suddenly become sensitive to it ---|||________|||--- to it often enough. The explanation for insensitivity to smell seems to be that the brain finds it ---|||________|||--- to keep all smell receptors working all the time but can ---|||________|||--- new receptors if necessary. This may ---|||________|||--- explain why we are not usually sensitive to our own smells―we simply do not need to be. We are not ---|||________|||--- of the usual smell of our own house, but we ---|||________|||--- new smells we visit someone else'' s. The brain finds it best to keep smell receptors ---|||________|||--- for unfamiliar and emergency signals ---|||________|||--- the smell of smoke, which might indicate the danger of fire.