托福阅读理解冲刺题

hormonesinthebody

uptothebeginningofthetwentiethcentury,thenervoussystemwasthoughttocontrolallmunicationwithinthebodyandtheresultingintegrationofbehavior.scientistshaddeterminedthatnervesran,essentially,onelectricalimpulses.theseimpulseswerethoughttobetheengineforthought,emotion,movement,andinternalprocessessuchasdigestion.however,experimentsbywilliambaylissanderneststarlingonthechemicalsecretin,whichisproducedinthesmallintestinewhenfoodentersthestomach,eventuallychallengedthatview.fromthesmallintestine,secretintravelsthroughthebloodstreamtothepancreas.there,itstimulatesthereleaseofdigestivechemicals.inthisfashion,theintestinalcellsthatproducesecretinultimatelyregulatetheproductionofdifferentchemicalsinadifferentorgan,thepancreas.

suchacoordinationofprocesseshadbeenthoughttorequirecontrolbythenervoussystem;baylissandstarlingshowedthatitcouldoccurthroughchemicalsalone.thisdiscoveryspurredstarlingtocointhetermhormonetorefertosecretin,takingitfromthegreekwordhormon,meaning“toexcite”or“tosetinmotion.”ahormoneisachemicalproducedbyonetissuetomakethingshappenelsewhere.

asmorehormoneswerediscovered,theywerecategorized,primarilyaccordingtotheprocessbywhichtheyoperatedonthebody.someglands(whichmakeuptheendocrinesystem)secretehormonesdirectlyintothebloodstream.suchglandsincludethethyroidandthepituitary.theexocrinesystemconsistsoforgansandglandsthatproducesubstancesthatareusedoutsidethebloodstream,primarilyfordigestion.thepancreasisonesuchorgan,althoughitsecretessomechemicalsintothebloodandthusisalsopartoftheendocrinesystem.

muchhasbeenlearnedabouthormonessincetheirdiscovery.someplaysuchkeyrolesinregulatingbodilyprocessesorbehaviorthattheirabsencewouldcauseimmediatedeath.themostabundanthormoneshaveeffectsthatarelessobviouslyurgentbutcanbemorefar-reachinganddifficulttotrack:theymodifymoodsandaffecthumanbehavior,evensomebehaviorwenormallythinkofasvoluntary.hormonalsystemsareveryintricate.ev

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