human development - Boys' and girls' height curves | Britannica

During this year a boy usually grows between 7 and 12 centimetres (2.75 and 4.75 inches) and a girl between 6 and 11 centimetres (2.35 and 4.35 inches). humandevelopment TableofContents IntroductionTypesandratesofhumangrowthTypesofgrowthdataBoys’andgirls’heightcurvesDifferenttissuesandpartsofthebodyDevelopmentatpubertyAlterationsingrowthrateIncreaseinbodysizeDevelopmentofthereproductiveorgansandsecondarysexcharacteristicsNormalvariationsSexdimorphismPhysicalandbehavioralinteractionLargersizeandearliermaturationHormonesandgrowth FastFacts RelatedContent Media Videos Images More AdditionalReading MoreArticlesOnThisTopic Contributors ArticleHistory Boys’andgirls’heightcurvesThegraphsmentionedabovealsoshowtheheightcurvesfrombirthtomaturity.Uptoagetwo,thechildwasmeasuredlyingonhisback.Oneexaminerheldhisheadincontactwithafixedboard,andasecondpersonstretchedhimouttohismaximumlengthandthenbroughtamovingboardintocontactwithhisheels.Thismeasurement,calledsupinelength,averagesaboutonecentimetremorethanthemeasurementofstandingheighttakenonthesamechild,hencethebreakinthelineofthecurveatagetwo.Thisoccursevenwhen,asinthebesttechniques,thechildisurgedtostretchupwardstothefullandisaidedindoingsobyameasurer’sapplyinggentleupwardpressuretohismastoidprocesses.Thetypicalgirlisslightlyshorterthanthetypicalboyatallagesuntiladolescence.Shebecomestallershortlyafterage11becauseheradolescentspurttakesplacetwoyearsearlierthantheboy’s.Atage14sheissurpassedagaininheightbythetypicalboy,whoseadolescentspurthasnowstarted,whilehersisnearlyfinished.Inthesameway,thetypicalgirlweighsalittlelessthantheboyatbirth,equalshimatageeight,becomesheavieratagenineor10,andremainssountilaboutage141/2.Atbirththetypicalboyisgrowingslightlyfasterthanthetypicalgirl,butthevelocitiesbecomeequalataboutsevenmonths,andthenthegirlgrowsfasteruntilfouryears.Fromthenuntiladolescencenodifferencesinvelocitycanbedetected.Thesexdifferenceisbestthoughtof,perhaps,intermsofacceleration,theboydeceleratingharderthanthegirloverthefirstfouryears.DifferenttissuesandpartsofthebodyThemajorityofskeletalandmusculardimensionsfollowapproximatelythegrowthcurvedescribedforheight,andsoalsodothedimensionsoftheinternalorganssuchastheliver,thespleen,andthekidneys.Butsomeexceptionsexist,mostnotablythebrainandskull,thereproductiveorgans,thelymphoidtissueofthetonsils,adenoids,andintestines,andthesubcutaneousfat.Thesizeattainedbyvarioustissuescanbegivenasapercentageofthebirth-to-maturityincrement.Heightfollowsthe“general”curve.Thereproductiveorgans,internalandexternal,haveaslowprepubescentgrowth,followedbyalargeadolescentspurt;theyarelesssensitivethantheskeletontoonesetofhormonesandmoresensitivetoanother.Thebrain,togetherwiththeskullcoveringitandtheeyesandears,developsearlierthananyotherpartofthebodyandthushasacharacteristicpostnatalcurve.Atbirthitisalready25percentofitsadultweight,atagefiveabout90percent,andatage10about95percent.Thusifthebrainhasanyadolescentspurtatall,itisasmallone.Asmallbutdefinitespurtoccursinheadlengthandbreadth,butallormostofthisisduetothickeningoftheskullbonesandthescalp,togetherwithdevelopmentoftheairsinuses.Thedimensionsofthefacefollowapathsomewhatclosertothegeneralcurve.Thereisaconsiderableadolescentspurt,especiallyinthelowerjaw,ormandible,resultinginthejaw’sbecominglongerandmoreprojecting,theprofilestraighter,andthechinmorepointed.Asalwaysingrowth,thereareconsiderableindividualdifferences,tothepointthatafewchildrenhavenodetectablespurtatallinsomefacemeasurements.Theeyeprobablyhasaslightadolescentspurt,whichisprobablyresponsiblefortheincreaseinfrequencyofshort-sightednessinchildrenthatoccursatthetimeofpuberty.Thoughthedegreeofmyopiaincreasescontinuouslyfromatleastagesixtomaturity,aparticularlyrapidrateofchangeoccursatabout11to12ingirlsand13to14inboys,andthiswouldbeexpectediftherewasarathergreaterspurtintheaxialdimension(thedimensionfromfronttoback)oftheeyethaninitsverticaldimension.Thelymphoidtissuehasquiteadifferentgrowthcurvefromtherest.Itreachesitsmaximumamountbeforeadolescenceandthen,probablyunderthedirectinfluenceofsexhormones,declinestoitsadultvalue.Thesubcutaneousfatlayeralsohasacurveofitsown,ofaslightlycomplicatedsort.ItsthicknesscanbemeasuredeitherbyXraysor,moresimply,atcertainsitesinthebody,bypickingupafoldofskinandfatbetweenthethumbandforefingerandmeasuringitsthicknesswithaspecial,constant-pressurecaliper.Subcutaneousfatbeginstobelaiddowninthefetusatabout34weekspostmenstrualage,increasesfromthenuntilbirthandfrombirthonwarduntilaboutninemonths.(Thisisintheaveragechild;thepeakmaybereachedasearlyassixmonthsoraslateas12or15.)Afterninemonths,whenthevelocityoffatgainiszero,thefatusuallydecreases(thatis,ithasanegativevelocity)untilagesixtoeight,whenitbeginstoincreaseoncemore.Girlshavealittlemorefatthanboysatbirth,andthedifferencebecomesmoremarkedduringtheperiodofloss,sincegirlsloselessthanboys.Graphsoftheamountsofsubcutaneousfatonmalesandfemalesfrombirthto16yearsrevealedthatfromeightyearson,thecurvesforgirlsandboysdivergemoreradically,asdothecurvesforlimbandbodyfat.Atadolescencethelimbfatinboysdecreases,whilethebodyfatshowsatemporaryslowingdownofgainbutnoactualloss.Ingirlsthereisaslighthaltingofthelimb-fatgainatadolescence,butnoloss;thetrunkfatshowsonlyasteadyriseuntiladolescence.DevelopmentatpubertyAlterationsingrowthrateAtpuberty,aconsiderablealterationingrowthrateoccurs.Thereisaswiftincreaseinbodysize,achangeinshapeandcompositionofthebody,andarapiddevelopmentofthegonads,orsexglands—thereproductiveorgansandthecharacterssignallingsexualmaturity.Someofthesechangesarecommontobothsexes,butmostaresex-specific.Boyshaveagreatincreaseinmusclesizeandstrength,togetherwithaseriesofphysiologicalchangesmakingthemcapableofdoingheavierphysicalworkthangirlsandofrunningfasterandlonger.Thesechangesallspecificallyadaptthemaletohisprimitiveprimateroleofdominating,fighting,andforaging.Suchadolescentchangesoccurgenerallyinprimates(thatis,men,apes,andmonkeys)butaremoremarkedinsomespeciesthaninothers.Manliesataboutthemiddleoftheprimaterange,asregardsbothadolescentsizeincreaseanddegreeofsexualdifferentiation. 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