CHAPTERFIVE

GETTINGAHEAD

ItwastwonightsbeforemyanatomyfinalandIwasinthelabataroundtwointhemorning,memorizingthecranialnerves.Therearetwelvecranialnerves,eachbranchingtotakebizarretwistsandturnsthroughtheinsideoftheskull.Tostudythem,webisectedtheskullfromforeheadtochinandsawedopensomeofthebonesofthecheek.SothereIwas,holdinghalfoftheheadineachhand,tracingthetwistedpathsthatthenervestakefromourbrainstothedifferentmusclesandsenseorgansinside.Iwasenrapturedbytwoofthecranialnerves,the

trigeminalandthefacial.Theircomplicatedpatternboileddowntosomethingsosimple,sooutrageouslyeasythatIsawthehumanheadinanewway.Thatinsightcamefromunderstandingthefarsimplerstateofaffairsinsharks.Theeleganceofmyrealization—thoughnotitsnovelty;comparativeanatomistshadhaditacenturyormoreago—andthepressureoftheupcomingexamledmetoforgetwhereIwas.Atsomepoint,Ilookedaround.ItwasthemiddleofthenightandIwasaloneinthelab.Ialso

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happenedtobesurroundedbythebodiesoftwenty-fivehumanbeingsundersheets.Forthefirstandlasttime,Igotthewillies.Iworkedmyselfintosuchalatherthatthehairsonthebackofmyneckrose,myfeetdidtheirjob,andwithinananosecondIfoundmyselfatthebusstop,outofbreath.ItgoeswithoutsayingthatIfeltridiculous.Iremembertellingmyself:Shubin,you’vebecomehard-core.Thatthoughtdidnotlastlong;IsoondiscoveredIhadlockedmyhousekeysinthelab.Whatmademesohard-coreisthatheadanatomyis

deeplymesmerizing,infact,beautiful.Oneofthejoysofscienceisthat,onoccasion,weseeapatternthatrevealstheorderinwhatinitiallyseemschaotic.Ajumblebecomespartofasimpleplan,andyoufeelyouareseeingrightthroughsomethingtofinditsessence.Thischapterisaboutseeingthatessenceinsideourownheads.And,ofcourse,theheadsoffish.

THEINNERCHAOSOFTHEHEAD

Headanatomyisnotonlycomplicatedbuthardtosee,since,unlikeotherpartsofthebody,thetissuesoftheheadareencapsulatedinabonybox.Weliterallyhavetosawthroughthecheek,forehead,andcraniumtoseethevesselsandorgans.Havingthusopenedahumanhead,wefindaclumpofwhatlooksliketangledfishinglines.Vesselsandnervesmakecuriousloopsandturnsastheytravelthrough

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theskull.Thousandsofnervebranches,muscles,andbonessitwithinthissmallbox.Atfirstglance,thewholearrayisabewilderingmess.Ourskullsaremadeupofthreefundamentalparts:think

plates,blocks,androds.Theplatescoverourbrain.Patthetopofyourhead,andyouarefeelingthem.Theselargeplatesfittogetherlikejigsaw-puzzlepiecesandformmuchofourcranium.Whenwewereborn,theplateswereseparate;theopenspacesbetweenthem,thefontanelles,arevisibleininfants,occasionallythrobbingwiththebraintissueunderneath.Aswegrow,thebonesenlarge,andbythetimewereachtheageoftwotheyhavefused.Anotherpartofourskullliesunderneaththebrain,

formingaplatformthatholdsitup.Unliketheplate-likebonesatthetop,theseboneslooklikecomplicatedblocksandhavemanyarteriesandnervesrunningthroughthem.Thethirdkindofbonemakesupourjaws,somebonesinourears,andotherbonesinourthroats;thesebonesstartdevelopmentlookinglikerods,whichultimatelybreakupandchangeshapetohelpuschew,swallow,andhear.Insidetheskullareanumberofcompartmentsand

spacesthathousedifferentorgans.Obviously,thebrainoccupiesthelargestofthese.Otherspacescontainoureyes,partsofourears,andournasalstructures.Muchofthechallengeinunderstandingheadanatomycomesfromseeingthesedifferentspacesandorgansinthreedimensions.Attachedtothebonesandorgansintheheadarethe

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musclesweusetobite,totalk,andtomoveoureyesandwholehead.Twelvenervessupplythesemuscles,eachexitingthebraintotraveltoadifferentregioninsideourhead.Thesearethedreadedcranialnerves.

Plates,blocks,androds:thethemeforskulls.Everyboneinourheadcanbetracedtooneofthesethings.

Thekeytounlockingthebasicsoftheheadistoseethe

cranialnervesasmorethanajumble.Indeed,mostofthemreallyaresimple.Thesimplestcranialnerveshaveonlyonefunction,andtheyattachtoonemuscleororgan.Thecranialnervethatgoestoournasalstructures,theolfactory,hasonejob:totakeinformationfromournasaltissuestoourbrain.Someofthenervesthatgotooureyesandearsarealsosimpleinthisway:theopticnerveisinvolvedwithvision;theacousticnerveworksinhearing.Aboutfourothercranialnervesonlyservemuscles—

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workingtomovetheeyesinsidetheorbits,forexample,ortomovetheheadaroundontheneck.Butfourofthecranialnerveshavegivenmedical

studentsfitsfordecades.Forgoodreason:thefourhaveverycomplexfunctionsandtaketortuouspathsthroughtheheadtodotheirjobs.Thetrigeminalnerveandthefacialnervedeservespecialmention.Bothexitthebrainandbreakupintoabewilderingnetworkofbranches.Muchlikeacablethatcancarrytelevision,Internet,andvoiceinformation,asinglebranchofthetrigeminalorfacialnervecancarryinformationaboutbothsensationandaction.Individualfibersforsensationandactionemanatefromdifferentpartsofthebrain,areconsolidatedincables(whatweendupcallingthetrigeminalandfacialnerves),thenbreakupagain,sendingbranchesalloverthehead.Thetrigeminal’sbranchesdotwomajorthings:they

controlmuscles,andtheycarrysensoryinformationfrommuchofourfacebacktoourbrain.Themusclescontrolledbythetrigeminalnerveincludethoseweusetochewaswellastinymusclesdeepinsidetheear.Thetrigeminalisalsothemajornerveforsensationintheface.Thereasonaslaptothefacehurtssomuch,beyondtheemotionalpain,isbecausethetrigeminalcarriessensoryinformationfromtheskinofourfacebacktoourbrain.Yourdentistalsoknowsthebranchesofyourtrigeminalnervewell.Differentbranchesgototherootsofourteeth;asinglejabofanestheticalongoneofthesebranchescandeadenthesensationofdifferentpartsofourtoothrow.

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Thefacialnervealsocontrolsmusclesandrelayssensoryinformation.Asitsnameimplies,itisthemainnervethatcontrolsthemusclesoffacialexpression.Weusethesetinymusclestosmile,tofrown,toraiseandloweroureyebrows,toflareournostrils,andsoon.Theyhavewonderfullyevocativenames.Oneofthemajormusclesthatweuseinfrowning—itmovesthecornersofourmouthdown—iscalledthedepressorangulioris.Anothergreatnamebelongstothemuscleweusetofurrowourbrowinconcern:thecorrugatorsupercilli.Flareyournostrilsandyouareusingyournasalis.Eachofthesemuscles,likeeveryothermuscleoffacialexpression,iscontrolledbybranchesofthefacialnerve.Thingslikeanunevensmileorasymmetricallydroopingeyelidsareasignthatsomethingmightbewrongwiththefacialnerveononesideofaperson’sface.YouareprobablybeginningtoseewhyIwasstayingup

solatetostudythesenerves.Nothingaboutthemseemstomakeanysense.Forexample,boththetrigeminalandthefacialnervessendtinybranchestomusclesinsideourears.Whydotwodifferentnerves,whichinnervateentirelydifferentpartsofthefaceandjaw,sendbranchestoearmusclesthatlieadjacenttooneanother?Evenmoreconfusing,thetrigeminalandfacialalmostcrisscrossastheysendbranchestoourfaceandjaw.Why?Withsuchoddlyredundantfunctionsandtortuouspaths,thereseemstobenorhymeorreasontotheirstructure,muchlesstohowthesenervesmatchupwiththeplates,blocks,androds

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thatmakeupourskull.Inthinkingaboutthesenerves,Iamremindedofmyfirst

dayshereinChicagoin2001.Ihadbeengivenspaceforaresearchlaboratoryinahundred-year-oldbuildingandthelabneedednewutilitycables,plumbing,andairhandling.Irememberthedaywhenthecontractorsfirstopenedthewallstogetaccesstotheinnardsofthebuilding.TheirreactiontotheplumbingandwiringinsidemywallwasalmostexactlylikeminewhenIopenedthehumanheadandsawthetrigeminalandfacialnervesforthefirsttime.Thewires,cables,andpipesinsidethewallswereajumble.Nobodyinhisrightmindwouldhavedesignedabuildingfromscratchthisway,withcablesandpipestakingbizarreloopsandturnsthroughoutthebuilding.Andthat’sexactlythepoint.Mybuildingwasconstructed

in1896,andtheutilitiesreflectanolddesignthathasbeenjerry-riggedfurtherwitheachrenovation.Ifyouwanttounderstandthewiringandplumbinginmybuilding,youhavetounderstanditshistory,howitwasrenovatedforeachnewgenerationofscientists.Myheadhasalonghistoryalso,andthathistoryexplainscomplicatednerveslikethetrigeminalandthefacial.Forus,thathistorybeginswithafertilizedegg.

THEESSENCEINEMBRYOS

Nobodystartslifewithahead:spermandeggcome

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togethertomakeasinglecell.Betweenthemomentofconceptionandthethirdweekthereafter,wegofromthatsinglecelltoaballofcells,thentoaFrisbee-shapedcollectionofcells,thentosomethingthatlooksvaguelylikeatubeandincludesdifferentkindsoftissues.Betweenthetwenty-thirdandtwenty-eighthdaysafterconception,thefrontendofthetubethickensandfoldsoverthebody,sotheembryolooksasifit’salreadycurledupinthefetalposition.Theheadatthisstagelookslikeabigglob.Thebaseofthisglobholdsthekeytomuchofthebasicorganizationofourheads.Fourlittleswellingsdeveloparoundtheareathatwill

becomethethroat.Ataboutthreeweeksweseethefirsttwo;theothertwoemergeaboutfourdayslater.Eachswellinglooksquitehumbleontheoutside:asimpleblob,separatedfromthenextbyalittlecrease.Whenyoufollowwhathappenstotheblobsandcreases,youbegintoseetheorderandbeautyofthehead,includingthetrigeminalandfacialnerves.Ofthecellsinsideeachblob,knownasarches,somewill

formbonetissueandothersmuscleandbloodvessels.Thereisacomplexmixofcellsinsideeacharch;somecellsdividedrighttherewhileothersmigratedalongwaytoenterthearchitself.Whenweidentifythecellsineacharchaccordingtowheretheyendupintheadult,thingsstarttomakealotofsense.Ultimately,thefirstarchtissuesformtheupperand

lowerjaws,twotinyearbones(themalleusandincus),and

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allthevesselsandmusclesthatsupplythem.Thesecondarchformsthethirdsmallearbone(thestapes),atinythroatbone,andmostofthemusclesthatcontrolfacialexpression.Thethirdarchformsbones,muscles,andnervesdeeperinthethroat;weusethesetoswallow.Finally,thefourtharchformsthedeepestpartsofourthroat,includingpartsofourlarynxandthemusclesandvesselsthatsurrounditandhelpitfunction.Ifyouweretoshrinkyourselftothesizeofapinheadand

travelinsidethemouthofthedevelopingembryo,youwouldseeindentationsthatcorrespondtoeachswelling.Therearefouroftheseindentations.And,likethearchesontheoutside,cellsontheindentationsformimportantstructures.ThefirstelongatestoformourEustachiantubeandsomestructuresintheear.Thesecondformsthecavitythatholdsourtonsils.Thethirdandfourthformimportantglands,includingtheparathyroid,thymus,andthyroid.WhatI’vejustgivenyouisoneofthebigtricksfor

understandingthemostcomplicatedcranialnervesandlargeportionsofthehead.Whenyouthinktrigeminalnerve,thinkfirstarch.Facialnerve,secondarch.Thereasonthetrigeminalnervegoestoboththejawsandtheearisthatallthestructuresitsuppliesoriginallydevelopedinthefirstarch.Thesamethingistrueforthefacialnerveandthesecondarch.Whatdothemusclesoffacialexpressionhaveincommonwiththemusclesintheearthatthefacialnervesupplies?Theyareallsecondarchderivatives.Asforthenervesofthethirdandfourtharches,theircomplexpaths

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allrelatetothefactthattheyinnervatestructuresthatarosefromtheirrespectivearches.Thosethirdandfourtharchnerves,amongthemtheglossopharyngealandvagus,followthesamepatternastheonesinfront,eachgoingtostructuresthatdevelopedfromthearchtheyareassociatedwith.

Ifwefollowthegillarchesfromanembryotoanadult,wecantracetheoriginsofjaws,ears,larynx,andthroat.Bones,muscles,nerves,andarteriesalldevelopinsidethesegillarches.

Thisfundamentalblueprintofheadshelpsusmakesense

ofoneoftheapocryphaltalesinanatomy.In1820,sothestorygoes,JohannesGoethewaswalkingthroughtheJewishcemeteryinViennawhenhespottedthedecomposingskeletonofaram.Thevertebraewereexposedandabovethemlayadamagedskull.Goethe,inamomentofepiphany,sawthatthebreaksintheskullmade

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itlooklikeagnarledmessofvertebrae.ToGoethe,thisrevealedtheessentialpatternwithin:theheadismadeupofvertebraethatfusedandgrewavaulttoholdourbrainsandsenseorgans.Thiswasarevolutionaryideabecauseitlinkedheadsandbodiesastwoversionsofthesamefundamentalplan.Thenotionmusthavebeeninthedrinkingwaterintheearly1800sbecauseotherpeople,amongthemLorenzOken,allegedlycameupwithvirtuallythesameideainasimilarsetting.GoetheandOkenwerebothpickingupsomethingvery

profound,althoughtheycouldnothaveknownitatthetime.Ourbodyissegmented,andthispatternismostclearlyseeninourvertebrae.Eachvertebraisablockthatrepresentsasegmentofourbody.Theorganizationofournervesisalsosegmental,correlatingcloselywiththepatternofthevertebrae.Nervesexitthespinalcordtosupplythebody.Thesegmentalconfigurationisobviouswhenyoulookatthelevelsofthespinalcordthatareassociatedwitheachpartofourbody.Forexample,themusclesinourlegsaresuppliedbynervesthatexitfromlowerpartsofthespinalcordthanthosethatsupplyourarms.Headsmaynotlookit,buttheyalsocontainaverydeepsegmentalpattern.Ourarchesdefinesegmentsofbones,muscles,arteries,andnerves.Lookintheadult,andyouwon’tseethispattern.Weseeitonlyintheembryo.Ourskullsloseallovertevidenceoftheirsegmental

originsaswegofromembryotoadult.Theplate-likebonesofourskullsformoverourgillarches,andthemuscles,

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nerves,andarteries,whichallhadaverysimplesegmentalpatternearlyon,arerewiredtomakeouradultheads.Knowingsomethingaboutdevelopmentcanhelpus

predictwheretolookforwhatismissinginchildrenwhohavecertainbirthdefects.Forexample,childrenbornwithfirstarchsyndromehaveatinyjawandnonfunctioningearswithnomalleusorincusbone.Missingarestructuresthatnormallywouldhaveformedfromthefirstarch.Thearchesaretheroadmapformajorchunksofthe

skull,fromthemostcomplicatedcranialnervestothemuscles,arteries,bones,andglandsinside.Thearchesarealsoaguidetosomethingelse:ourverydeepconnectionwithsharks.

OURINNERSHARK

Thetake-homemessageofmanyalawyerjokeisthatlawyersareanespeciallyvoraciouskindofshark.Teachingembryologyduringoneoftherecurringvoguesforthesejokes,Irememberthinkingthatthejokeisonallofus.We’reallmodifiedsharks—or,worse,thereisalawyerinsideeachofus.Aswe’veseen,muchofthesecretofheadsliesinthe

arches,theswellingsthatgaveustheroadmapforthecomplicatedcranialnervesandkeystructuresinsidethehead.Thoseinsignificant-lookingswellingsandindentationshavecapturedtheimaginationofanatomists

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for150years,becausetheylooklikethegillslitsinthethroatregionsoffishandsharks.Fishembryoshavethesebulgesandindentations,too.In

fish,theindentationsultimatelyopenuptoformthespacesbetweenthegillswherewaterflows.Inus,theindentationsnormallysealover.Inabnormalcases,gillslitsfailtocloseandremainopenaspouchesorcysts.Abranchialcyst,forexample,isoftenabenignfluid-filledcystthatformsinanopenpouchinsidetheneck;thepouchiscreatedbythefailureofthethirdorfourtharchtoclose.Rarely,childrenarebornwithanactualvestigeofanancientgillarchcartilage,alittlerodthatrepresentsagillbarfromthethirdarch.Intheseinstances,mysurgicalcolleaguesareoperatingonaninnerfishthatunfortunatelyhascomebacktobiteus.Everyheadoneveryanimalfromasharktoahuman

sharesthosefourarchesindevelopment.Therichnessofthestoryliesinwhathappensinsideeacharch.Here,wecanmakeapoint-by-pointcomparisonbetweenourheadsandthoseofsharks.

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Thegillregionofadevelopinghumanandadevelopingsharklookthesameearlyon.

Lookatthefirstarchinahumanandashark,andyou

findaverysimilarstateofaffairs:jaws.Themajordifferenceisthathumans’firstarchalsoformssomeearbones,whichwedonotseeinsharks.Unsurprisingly,thecranialnervethatsuppliesthejawsofhumansandsharksisthefirstarchnerve,thetrigeminalnerve.Thecellsinsidethesecondgillarchdivide,change,and

giverisetoabarofcartilageandmuscle.Inus,thebarofcartilagebreaksuptoformoneofthethreebonesofourmiddleear(thestapes)andsomeothersmallstructuresatthebaseoftheheadandthroat.Oneofthesebones,calledthehyoid,assistsusinswallowing.Takeagulp,listentomusic,andthankthestructuresthatformfromyoursecondarch.Inashark,thesecondarchrodbreaksuptoformtwo

bonesthatsupportthejaws:aloweronethatcompares

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withourhyoidandanupperonethatsupportstheupperjaw.Ifyouhaveeverwatchedagreatwhitesharktrytochompsomething—adiverinacage,forexample—youhaveprobablynoticedthattheupperjawcanextendandretractasthesharkbites.Theupperboneofthissecondarchispartoftheleversystemofbonesthatrotatetomakethatpossible.Thatupperboneisremarkableinanotherway,too.Itcompareswithoneofthebonesinourmiddleear:thestapes.Bonesthatsupporttheupperandlowerjawsinsharksareusedinustoswallowandhear.Asforthethirdandfourtharches,wefindthatmanyof

thestructuresweusetotalkandswalloware,insharks,partsoftissuesthatsupportthegills.Themusclesandcranialnervesweusetoswallowandtalkmovethegillsinsharksandfish.

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Atfirstglance,ourcranialnerves(bottomright)appeardifferentfromthoseofashark(bottomleft).Butlookcloselyandyouwillfindprofoundsimilarities.Virtuallyallofournervesarepresentinsharks.Theparallelsgodeeperstill:equivalentnervesinsharksandhumanssupplysimilarstructures,andtheyevenexitthebraininthesameorder(topleftandright).

Ourheadmaylookincrediblycomplicated,butitisbuilt

fromasimpleandelegantblueprint.Thereisapatterncommontoeveryskullonearth,whetheritbelongstoashark,abonyfish,asalamander,orahuman.Thediscovery

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ofthispatternwasamajoraccomplishmentofnineteenth-centuryanatomy,atimewhenanatomistswereputtingembryosofallkindsofspeciesunderthemicroscope.In1872,theOxfordanatomistFrancisMaitlandBalfourfirstsawthebasicplanofheadswhenhelookedatsharksandsawthebulges,thegillarches,andthestructuresinside.Unfortunately,hediedsoonafterinamountaineeringaccidentintheSwissAlps.Hewasonlyinhisthirties.

GILLARCHGENES

Duringthefirstthreeweeksafterconception,wholebatteriesofgenesareturnedonandoffinourgillarchesandthroughoutthetissuesthatwillbecomeourfuturebrain.Thesegenesinstructcellstomakethedifferentportionsofourhead.Thinkofeachregionofourheadasgainingageneticaddressthatmakesitdistinctive.Modifythisgeneticaddressandwecanmodifythekindsofstructuresthatdevelopthere.Forexample,ageneknownasOtxisactiveinthefront

region,wherethefirstgillarchforms.Behindit,towardthebackofthehead,anumberofso-calledHoxgenesareactive.EachgillarchhasadifferentcomplementofHoxgenesactiveinit.Withthisinformation,wecanmakeamapofourgillarchesandtheconstellationofgenesactiveinmakingeach.Nowwecandoexperiments:changethegeneticaddress

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ofonegillarchintothatofanother.Takeafrogembryo,turnoffsomegenes,makethegeneticsignalssimilarinthefirstandsecondarches,andyouendupwithafrogthathastwojaws:amandibledevelopswhereahyoidbonewouldnormallybe.Thisshowsthepowerofthegeneticaddressesinmakingourgillarches.Changetheaddress,andyouchangethestructuresinthearch.Thepowerofthisapproachisthatwecannowexperimentwiththebasicdesignofheads:wecanmanipulatetheidentityofthegillarchesalmostatwill,bychangingtheactivityofthegenesinside.

TRACINGHEADS:FROMHEADLESSWONDERSTOOURHEADEDANCESTORS

Whystopatfrogsandsharks?Whynotextendourcomparisontoothercreatures,likeinsectsorworms?Butwhywouldwedothiswhennoneofthesecreatureshasaskull,muchlesscranialnerves?Noneofthemevenhasbones.Whenweleavefishforworms,wegettoaverysoftandheadlessworld.Bitsofourselvesarethere,though,ifyoulookclosely.Thoseofuswhoteachcomparativeanatomyto

undergraduatesusuallybeginthecoursewithaslideofAmphioxus.EverySeptember,hundredsofAmphioxusslidesappearonscreensincollegelecturehallsfromMainetoCalifornia.Why?Rememberthesimpledichotomybetweeninvertebratesandvertebrates?Amphioxusisaworm,an

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invertebrate,thatsharesmanyfeatureswithbackbonedanimalssuchasfish,amphibians,andmammals.Amphioxuslacksabackbone,butlikeallcreatureswithbackbones,ithasanervecordthatrunsalongitsback.Inaddition,arodrunsthelengthofitsbody,paralleltothenervecord.Thisrod,knownasthenotochord,isfilledwithajelly-likesubstanceandprovidessupportforthebody.Asembryos,wehaveanotochord,too,butunlikeAmphioxus’s,oursbreaksupandultimatelybecomespartofthedisksthatliebetweenourvertebrae.Ruptureadiskandthejelly-likesubstanceofwhatwasonceanotochordcanwreakhavocwhenitpinchesnervesorinterfereswiththeabilityofonevertebratomovealongthenext.Whenweinjureadisk,averyancientpartofourbodyplanisrupturing.Thanksalot,Amphioxus.

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Theclosestrelativestoanimalswithheadsarewormswithgillslits.ShownareAmphioxusandareconstructionofafossilworm(Haikouella)over530millionyearsold.Bothwormshaveanotochord,anervecord,andgillslits.ThefossilwormisknownfromoverthreehundredindividualspecimensfromsouthernChina.

Amphioxusisnotuniqueamongworms.Someofthebest

examplesarenotintheoceansoftodaybutinancientrocksofChinaandCanada.Buriedinsedimentsover500millionyearsoldaresmallwormsthatlackheads,complexbrains,orcranialnerves.Theymaynotlooklikemuch,beingsmallsmudgesintherock,butthepreservationofthesefossilsis

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incredible.Whenyoulookunderamicroscope,youfindbeautifullypreservedimpressionsthatdisplaytheirsoftanatomyinfinedetail,occasionallyevenwithimpressionsofskin.Theyshowsomethingelsewonderful,too.Theyaretheearliestcreatureswithnotochordsandnervecords.Thesewormsaretellingussomethingabouttheoriginofpartsofourbodies.Butthereissomethingelsewesharewiththeselittle

worms:gillarches.Amphioxus,forexample,hastheminabundance,andassociatedwitheacharchisalittlebarofcartilage.Likethecartilagesthatformourjaws,ourearbones,andpartsofourvoicebox,theserodssupportthegillslit.Theessenceofourheadgoesbacktoworms,organismsthatdonotevenhaveahead.WhatdoesAmphioxusdowiththegillarches?Itpumpswaterthroughthemtofilteroutlittleparticlesoffood.Fromsohumbleabeginningcomesthebasicstructuresofourownhead.Justasteeth,genes,andlimbshavebeenmodifiedandtheirfunctionsrepurposedovertheages,so,too,hasthebasicstructureofourhead.

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