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1、Passage7 Noseshaveitprettyhard.Boxersfattenthem.Doctorsrearrangethem.Peoplemakejokesabouttheirunflatteringcharacteristics.Worstofall,whenitcomestosmell,noonereallyunderstandsthem. Despitethenose'sconspicuouspresence,itsworkingsaresubtle.Smell,orolfactionisachem
2、osense,relyingonspecializedinteractionsbetweenchemicalsandnerveendings.Whenarose,forexample,issniffed,odormoleculesarecarriedbytherisingairstreamtothetopofthenasalcavity,justbehindthebridgeofthenose,wherethetipsofthetensofmillionsofolfactorynervecellsareclustered
3、inthemucouslining.Themoleculessomehowtriggerthenerveendings,whichcarrythemessagetotheolfactorylobesofthebrain.Becausesmellinformationthentravelstootherregionsofthebrain,thescentofarosecanelicitnotonlyapleasurablesensationbutemotionsandmemoriesaswell. Thoughjusth
4、owodorsstimulatethenervesisunknown,scientistsdoknowthatoursenseofsmellissurprisinglykeen,capableofdistinguishinguptotensofthousandsofchemicalodors.Thelaboratorytaskofisolatingthecomponentsofanodorisfarfromsimple.Tobaccosmoke,forexample,ismadeupofseveralthousanddi
5、fferentchemicals.Moreover,smellresearchersmustgrapplewiththeproblemofwhattocallthedifferentodorsthatthenosedetects.Peoplegenerallyrefertosmellsbytheirsourcesofassociations.Descriptionssuchas"likeawetdog"or"likemyelementaryschool"mayconveyperceptionsbutarevastlyin
6、adequateforlabelingthechemistryinvolved. Tofurthercomplicateresearch,olfactionisconnectedtoothersensations.Besidesolfactorynerves,thenasalcavitycontainspain-sensitivenervesthatperceivesensationssuchasthekickinammoniaoftheburninginchilipeppers.Smellalsointerwines
7、withtastetocreateflavor.Acoffeedrinkerholdinghisnosewhilesippingwouldtasteonlythebitterinhisbrew,fortastereceptorsgenerallyappearlimitedtobitter,salty,sourandsweet.Thesenseofsmellistenthousandtimesmoresensitivethantasteandmakessubtledistinctionsamonglemon,chocola
8、te,andmanymoreflavors. Sohowdoesthenosemanagethissophisticateddiscrimination?Lakeofevidencehasn'tkeptscientistsfromspeculating.Oneideaisthateveryodormoleculev
