Whathasjustbeensaidappliestofishesaswellastootheranimals；andthusinsuchofthemasarecarnivorous，andmadeforbiting，themouthhasawidegape；whereasintherestitissmall，beingplacedattheextremityofataperingsnout。Forthisformissuitedfortheirpurposes，whiletheotherwouldbeuseless。
Inbirdsthemouthconsistsofwhatiscalledthebeak，whichinthemisasubstituteforlipsandteeth。Thisbeakpresentsvariationsinharmonywiththefunctionsandprotectivepurposeswhichitserves。
ThusinthosebirdsthatarecalledCrooked-claweditisinvariablyhooked，inasmuchasthesebirdsarecarnivorous，andeatnokindofvegetablefoodwhatsoever。Forthisformrendersitserviceabletotheminobtainingthemasteryovertheirprey，andisbettersuitedfordeedsofviolencethananyother。Moreover，astheirweaponsofoffenceconsistofthisbeakandoftheirclaws，theselatteralsoaremorecrookedinthemthaninthegeneralityofbirds。Similarlyineachotherkindofbirdthebeakissuitedtothemodeoflife。
Thus，inwoodpeckersitishardandstrong，asalsoincrowsandbirdsofcrowlikehabit，whileinthesmallerbirdsitisdelicate，soastobeofuseincollectingseedsandpickingupminuteanimals。Insuchbirds，again，aseatherbage，andsuchasliveaboutmarshes-those，forexample，thatswimandhavewebbedfeet-thebillisbroad，oradaptedinsomeotherwaytothemodeoflife。Forabroadbillenablesabirdtodigintothegroundwithease，justas，amongquadrupeds，doesthebroadsnoutofthepig，ananimalwhich，likethebirdsinquestion，livesonroots。Moreover，intheseroot-eatingbirdsandinsomeothersoflikehabitsoflife，thetipsofthebillendinhardpoints，whichgivesthemadditionalfacilityindealingwithherbaceousfood。
Theseveralpartswhicharesetontheheadhavenow，prettynearlyall，beenconsidered。Inman，however，thepartwhichliesbetweentheheadandtheneckiscalledtheface，thisname，（prosopon）being，itwouldseem，derivedfromthefunctionofthepart。Forasmanistheonlyanimalthatstandserect，heisalsotheonlyonethatlooksdirectlyinfront（proso）andtheonlyonewhosevoiceisemittedinthatdirection。
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Wehavenowtotreatofhorns；forthesealso，whenpresent，areappendagesofthehead。Theyexistinnonebutviviparousanimals；
thoughinsomeoviparacertainpartsaremetaphoricallyspokenofashorns，invirtueofacertainresemblance。Tononeofsuchparts，however，doestheproperofficeofahornbelong；fortheyareneverused，asarethehornsofvivipara，forpurposeswhichrequirestrength，whetheritbeinself-protectionorinoffensivestrife。
Soalsonopolydactylousanimalisfurnishedwithhorns。Forhornsaredefensiveweapons，andthesepolydactylousanimalspossessothermeansofsecurity。Fortosomeofthemnaturehasgivenclaws，toothersteethsuitedforcombat，andtotherestsomeotheradequatedefensiveappliance。Therearehorns，however，inmostofthecloven-hoofedanimals，andinsomeofthosethathaveasolidhoof，servingthemasanoffensiveweapon，andinsomecasesalsoasadefensiveone。
Therearehornsalsoinallanimalsthathavenotbeenprovidedbynaturewithsomeothermeansofsecurity；suchmeans，forinstance，asspeed，whichhasbeengiventohorses；orgreatsize，asincamels；
forexcessivebulk，suchashasbeengiventotheseanimals，andinastillgreatermeasuretoelephants，issufficientinitselftoprotectananimalfrombeingdestroyedbyothers。Otheranimalsagainareprotectedbythepossessionoftusks；andamongthesearetheswine，thoughtheyhaveaclovenhoof。
Allanimalsagain，whosehornsarebutuselessappendages，havebeenprovidedbynaturewithsomeadditionalmeansofsecurity。Thusdeerareendowedwithspeed；forthelargesizeandgreatbranchingoftheirhornsmakestheseasourceofdetrimentratherthanofprofittotheirpossessors。SimilarlyendowedaretheBubalusandgazelle；forthoughtheseanimalswillstandupagainstsomeenemiesanddefendthemselveswiththeirhorns，yettheyrunawayfromsuchasarefierceandpugnacious。TheBonasusagain，whoehornscurveinwardstowardseachother，isprovidedwithameansofprotectioninthedischargeofitsexcrement；andofthisitavailsitselfwhenfrightened。TherearesomeotheranimalsbesidestheBonasusthathaveasimilarmodeofdefence。Innocase，however，doesnatureevergivemorethanoneadequatemeansofprotectiontooneandthesameanimal。
Mostoftheanimalsthathavehornsarecloven-hoofed；buttheIndianass，astheycallit，isalsoreportedtobehorned，thoughitshoofissolid。
Againasthebody，sofarasregardsitsorgansofmotion，consistsoftwodistinctparts，therightandtheleft，soalsoandforlikereasonsthehornsofanimalsare，inthegreatmajorityofcases，twoinnumber。Stilltherearesomethathavebutasinglehorn；theOryx，forinstance，andtheso-calledIndianass；intheformerofwhichthehoofiscloven，whileinthelatteritissolid。
Insuchanimalsthehornissetinthecentreofthehead；forasthemiddlebelongsequallytobothextremes，thisarrangementistheonethatcomesnearesttoeachsidehavingitsownhorn。
Again，itwouldappearconsistentwithreasonthatthesinglehornshouldgowiththesolidratherthanwiththeclovenhoof。Forhoof，whethersolidorcloven，isofthesamenatureashorn；sothatthetwonaturallyundergodivisionsimultaneouslyandinthesameanimals。
Again，sincethedivisionoftheclovenhoofdependsondeficiencyofmaterial，itisbutrationallyconsistent，thatnature，whenshegaveananimalanexcessofmaterialforthehoofs，whichthusbecamesolid，shouldhavetakenawaysomethingfromtheupperpartsandsomadetheanimaltohavebutonehorn。Rightlytoodidsheactwhenshechosetheheadwhereontosetthehorns；andAEsop’sMomusisbesidethemark，whenhefindsfaultwiththebullfornothavingitshornsuponitsshoulders。Forfromthisposition，sayshe，theywouldhavedeliveredtheirblowwiththegreatestforce，whereasontheheadtheyoccupytheweakestpartofthewholebody。Momuswasbutdull-sightedinmakingthishostilecriticism。Forhadthehornsbeensetontheshoulders，orhadtheybeensetonanyotherpartthantheyare，theencumbranceoftheirweightwouldhavebeenincreased，notonlywithoutanycompensatinggainwhatso：：ver，butwiththedisadvantageofimpedingmanybodilyoperations。Forthepointwhencetheblowscouldbedeliveredwiththegreatestforcewasnottheonlymattertobeconsidered，butthepointalsowhencetheycouldbedeliveredwiththewidestrange。Butasthebullhasnohandsandcannotpossiblyhaveitshornsonitsfeetoronitsknees，wheretheywouldpreventflexion，thereremainsnoothersiteforthembutthehead；andthisthereforetheynecessarilyoccupy。Inthisposition，moreover，theyaremuchlessinthewayofthemovementsofthebodythantheywouldbeelsewhere。
Deeraretheonlyanimalsinwhichthehornsaresolidthroughout，andarealsotheonlyanimalsthatcastthem。Thiscastingisnotsimplyadvantageoustothedeerfromtheincreasedlightnesswhichitproduces，but，seeinghowheavythehornsare，isamatterofactualnecessity。
Inallotheranimalsthehornsarehollowforacertaindistance，andtheendaloneissolid，thisbeingthepartofuseinablow。Atthesametime，topreventeventhehollowpartfrombeingweak，thehorn，thoughitgrowsoutoftheskin，hasasolidpiecefromthebonesfittedintoitscavity。Forthisarrangementisnotonlythatwhichmakesthehornsofthegreatestserviceinfighting，butthatwhichcausesthemtobeaslittleofanimpedimentaspossibleintheotheractionsoflife。
Suchthenarethereasonsforwhichhornsexist；andsuchthereasonswhytheyarepresentinsomeanimals，absentfromothers。
Letusnowconsiderthecharacterofthematerialnaturewhosenecessaryresultshavebeenmadeavailablebyrationalnatureforafinalcause。
Inthefirstplace，then，thelargerthebulkofanimals，thegreateristheproportionofcorporealandearthymatterwhichtheycontain。Thusnoverysmallanimalisknowntohavehorns，thesmallesthornedanimalthatweareacquaintedwithbeingthegazelle。Butinallourspeculationsconcerningnature，whatwehavetoconsideristhegeneralrule；forthatisnaturalwhichapplieseitheruniversallyorgenerally。Andthuswhenwesaythatthelargestanimalshavemostearthymatter，wesaysobecausesuchisthegeneralrule。Nowthisearthymatterisusedintheanimalbodytoformbone。Butinthelargeranimalsthereisanexcessofit，andthisexcessisturnedbynaturetousefulaccount，beingconvertedintoweaponsofdefence。Partofitnecessarilyflowstotheupperportionofthebody，andthisisallottedbyherinsomecasestotheformationoftusksandteeth，inotherstotheformationofhorns。
Thusitisthatnoanimalthathashornshasalsofrontteethinbothjaws，thoseintheupperjawbeingdeficient。Fornaturebysubtractingfromtheteethaddstothehorns；thenutrimentwhichinmostanimalsgoestotheformerbeingherespentontheaugmentationofthelatter。Does，itistrue，havenohornsandyetareequallydeficientwiththemalesasregardstheteeth。Thereason，however，forthisisthatthey，asmuchasthemales，arenaturallyhorn-bearinganimals；buttheyhavebeenstrippedoftheirhorns，becausethesewouldnotonlybeuselesstothembutactuallybaneful；whereasthegreaterstrengthofthemalescausestheseorgans，thoughequallyuseless，tobelessofanimpediment。Inotheranimals，wherethismaterialisnotsecretedfromthebodyintheshapeofhorns，itisusedtoincreasethesizeoftheteeth；insomecasesofalltheteeth，inothersmerelyofthetusks，whichthusbecomesolongastoresemblehornsprojectingfromthejaws。
Somuch，then，ofthepartswhichappertaintothehead。
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Belowtheheadliestheneck，insuchanimalsashaveone。Thisisthecasewiththoseonlythathavethepartstowhichaneckissubservient。Thesepartsarethelarynxandwhatiscalledtheoesophagus。Ofthesetheformer，orlarynx，existsforthesakeofrespiration，beingtheinstrumentbywhichsuchanimalsasbreatheinhaleanddischargetheair。Thereforeitisthat，whenthereisnolung，thereisalsononeck。OfthisconditiontheFishesareanexample。Theotherpart，oroesophagus，isthechannelthroughwhichfoodisconveyedtothestomach；sothatallanimalsthatarewithoutaneckarealsowithoutadistinctoesophagus；Suchapartisinfactnotrequiredofnecessityfornutritivepurposes；forithasnoactionwhatsoeveronthefood。Indeedthereisnothingtopreventthestomachfrombeingplaceddirectlyafterthemouth。
This，however，isquiteimpossibleinthecaseofthelung。Fortheremustbesomesortoftubecommontothetwodivisionsofthelung，bywhich——itbeingbipartite——thebreathmaybeapportionedtotheirrespectivebronchi，andthencepassintotheair-pipes；andsuchanarrangementwillbethebestforgivingperfectiontoinspirationandexpiration。Theorganthenconcernedinrespirationmustofnecessitybeofsomelength；andthis，again，necessitatestherebeinganoesophagustounitemouthandstomach。Thisoesophagusisofaflesh-likecharacter，andyetadmitsofextensionlikeasinew。Thislatterpropertyisgiventoit，thatitmaystretchwhenfoodisintroduced；whiletheflesh-likecharacterisintendedtomakeitsoftandyielding，andtopreventitfrombeingraspedbyparticlesastheypassdownwards，andsosufferingdamage。Ontheotherhand，thewindpipeandtheso-calledlarynxareconstructedoutofacartilaginoussubstance。Fortheyhavetoservenotonlyforrespiration，butalsoforvocalpurposes；andaninstrumentthatistoproducesoundsmustnecessarilybenotonlysmoothbutfirm。Thewindpipeliesinfrontoftheoesophagus，althoughthispositioncausesittobesomehindrancetothelatterintheactofdeglutition。Forifamorseloffood，fluidorsolid，slipsintoitbyaccident，chokingandmuchdistressandviolentfitsofcoughingensue。Thismustbeamatterofastonishmenttoanyofthosewhoassertthatitisbythewindpipethatananimalimbibesfluid。Fortheconsequencesjustmentionedoccurinvariably，wheneveraparticleoffoodslipsin，andarequiteobvious。Indeedonmanygroundsitisridiculoustosaythatthisisthechannelthroughwhichanimalsimbibefluid。Forthereisnopassageleadingfromthelungtothestomach，suchastheoesophaguswhichweseeleadingthitherfromthemouth。Moreover，whenanycauseproducessicknessandvomiting，itisplainenoughwhenthefluidisdischarged。Itismanifestalsothatfluid，whenswallowed，doesnotpassdirectlyintothebladderandcollectthere，butgoesfirstintothestomach。
For，whenredwineistaken，thedejectionsofthestomachareseentobecolouredbyitsdregs；andsuchdiscolorationhasbeenevenseenonmanyoccasionsinsidethestomachitself，incaseswheretherehavebeenwoundsopeningintothatorgan。However，itisperhapssillytobeminutelyparticularindealingwithsillystatementssuchasthis。
Thewindpipethen，owingtoitspositioninfrontoftheoesophagus，isexposed，aswehavesaid，toannoyancefromthefood。Toobviatethis，however，naturehascontrivedtheepiglottis。Thispartisnotfoundinallsanguineousanimals，butonlyinsuchofthemashavealung；norinallofthese，butonlyinsuchasatthesametimehavetheirskincoveredwithhairs，andnoteitherwithscalyplatesorwithfeathers。Insuchscalyandfeatheredanimalsthereisnoepiglottis，butitsofficeissuppliedbythelarynx，whichclosesandopens，justasintheothercasetheepiglottisfallsdownandrisesup；risingupduringtheingressoregressofbreath，andfallingdownduringtheingestionoffood，soastopreventanyparticlefromslippingintothewindpipe。Shouldtherebetheslightestwantofaccuracyinthismovement，orshouldaninspirationbemadeduringtheingestionoffood，chokingandcoughingensue，asalreadyhasbeennoticed。Soadmirablycontrived，however，isthemovementbothoftheepiglottisandofthetongue，that，whilethefoodisbeinggroundtoapulpinthemouth，thetongueveryrarelygetscaughtbetweentheteeth；and，whilethefoodispassingovertheepiglottisseldomdoesaparticleofitslipintothewindpipe。
Theanimalswhichhavebeenmentionedashavingnoepiglottisowethisdeficiencytothedrynessoftheirfleshandtothehardnessoftheirskin。Foranepiglottismadeofsuchmaterialswouldnotadmitofeasymotion。Itwould，indeed，takealongertimetoshutdownanepiglottismadeofthepeculiarfleshoftheseanimals，andshapedlikethatofthosewithhairyskins，thantobringtheedgesofthewindpipeitselfintocontactwitheachother。
Thusmuchthenastothereasonwhysomeanimalshaveanepiglottiswhileothershavenone，andthusmuchalsoastoitsuse。
Itisacontrivanceofnaturetoremedytheviciouspositionofthewindpipeinfrontoftheoesophagus。Thatpositionistheresultofnecessity。Foritisinthefrontandcentreofthebodythattheheartissituated，inwhichwesayistheprincipleoflifeandthesourceofallmotionandsensation。（Forsensationandmotionareexercisedinthedirectionwhichwetermforwards，anditisonthisveryrelationthatthedistinctionofbeforeandbehindisfounded。）
Butwheretheheartis，thereandsurroundingitisthelung。Nowinspiration，whichoccursforthesakeofthelungandforthesakeoftheprinciplewhichhasitsseatintheheart，iseffectedthroughthewindpipe。Sincethentheheartmustofnecessitylieintheveryfrontplaceofall，itfollowsthatthelarynxalsoandthewindpipemustofnecessitylieinfrontoftheoesophagus。Fortheyleadtothelungandheart，whereastheoesophagusleadstothestomach。Anditisauniversallawthat，asregardsaboveandbelow，frontandback，rightandleft，thenoblerandmorehonourablepartinvariablyisplaceduppermost，infront，andontheright，ratherthanintheoppositepositions，unlesssomemoreimportantobjectstandsintheway。
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Wehavenowdealtwiththeneck，theoesophagus，andthewindpipe，andhavenexttotreatoftheviscera。Thesearepeculiartosanguineousanimals，someofwhichhaveallofthem，othersonlyapart，whilenobloodlessanimalshaveanyatall。Democritusthenseemstohavebeenmistakeninthenotionheformedoftheviscera，if，thatistosay，hefanciedthatthereasonwhynonewerediscoverableinbloodlessanimalswasthattheseanimalsweretoosmalltoallowthemtobeseen。For，insanguineousanimals，bothheartandliverarevisibleenoughwhenthebodyisonlyjustformed，andwhileitisstillextremelysmall。Forthesepartsaretobeseenintheeggsometimesasearlyasthethirdday，beingthennobiggerthanapoint；andarevisiblealsoinabortedembryos，whilestillexcessivelyminute。Moreover，astheexternalorgansarenotpreciselyalikeinallanimals，buteachcreatureisprovidedwithsuchasaresuitedtoitsspecialmodeoflifeandmotion，soisitwiththeinternalparts，thesealsodifferingindifferentanimals。Viscera，then，arepeculiartosanguineousanimals；
andthereforeareeachandallformedfromsanguineousmaterial，asisplainlytobeseeninthenew-bornyoungoftheseanimals。Forinsuchthevisceraaremoresanguineous，andofgreaterbulkinproportiontothebody，thanatanylaterperiodoflife，itbeingintheearlieststageofformationthatthenatureofthematerialanditsabundancearemostconspicuous。Thereisaheart，then，inallsanguineousanimals，andthereasonforthishasalreadybeengiven。Forthatsanguineousanimalsmustnecessarilyhavebloodisself-evident。
And，asthebloodisfluid，itisalsoamatterofnecessitythatthereshallbeareceptacleforit；anditisapparentlytomeetthisrequirementthatnaturehasdevisedtheblood-vessels。These，again，mustnecessarilyhaveoneprimarysource。Foritispreferablethatthereshallbeonesuch，whenpossible，ratherthanseveral。Thisprimarysourceofthevesselsistheheart。Forthevesselsmanifestlyissuefromitanddonotgothroughit。Moreover，beingasitishomogeneous，ithasthecharacterofablood-vessel。
Againitspositionisthatofaprimaryordominatingpart。Fornature，whennoothermoreimportantpurposestandsinherway，placesthemorehonourablepartinthemorehonourableposition；andtheheartliesaboutthecentreofthebody，butratherinitsupperthanitslowerhalf，andalsomoreinfrontthanbehind。Thisismostevidentinthecaseofman，buteveninotheranimalsthereisatendencyinthehearttoassumeasimilarposition，inthecentreofthenecessarypartofthebody，thatistosayofthepartwhichterminatesintheventforexcrement。Forthelimbsvaryinpositionindifferentanimals，andarenottobecountedwiththepartswhicharenecessaryforlife。Forlifecanbemaintainedevenwhentheyareremoved；whileitisself-evidentthattheadditionofthemtoananimalisnotdestructiveofit。
Therearesomewhosaythatthevesselscommenceinthehead。Inthistheyareclearlymistaken。Forinthefirstplace，accordingtotheirrepresentation，therewouldbemanysourcesforthevessels，andthesescattered；andsecondly，thesesourceswouldbeinaregionthatismanifestlycold，asisshownbyitsintoleranceofchill，whereastheregionoftheheartisasmanifestlyhot。Again，asalreadysaid，thevesselscontinuetheircoursethroughtheotherviscera，butnovesselspreadsthroughtheheart。Fromthisitisquiteevidentthattheheartisapartofthevesselsandtheirorigin；andforthisitiswellsuitedbyitsstructure。Foritscentralpartconsistsofadenseandhollowsubstance，andismoreoverfullofblood，asthoughthevesselstookthencetheirorigin。Itishollowtoserveforthereceptionoftheblood，whileitswallisdense，thatitmayservetoprotectthesourceofheat。Forhere，andherealoneinallthevisceraandindeedinallthebody，thereisbloodwithoutblood-vessels，thebloodelsewherebeingalwayscontainedwithinvessels。Noristhisbutconsistentwithreason。Forthebloodisconveyedintothevesselsfromtheheart，butnonepassesintotheheartfromwithout。Forinitselfitconstitutestheoriginandfountain，orprimaryreceptacle，oftheblood。Itishowever，fromdissectionsandfromobservationsontheprocessofdevelopmentthatthetruthofthesestatementsreceivesitsclearestdemonstration。Fortheheartisthefirstofallthepartstobeformed；andnosoonerisitformedthanitcontainsblood。Moreover，themotionsofpainandpleasure，andgenerallyofallsensation，plainlyhavetheirsourceintheheart，andfindinittheirultimatetermination。This，indeed，reasonwouldleadustoexpect。Forthesourcemust，when。everpossible，beone；and，ofallplaces，thebestsuitedforasourceisthecentre。Forthecentreisone，andisequallyoralmostequallywithinreachofeverypart。Again，asneithertheblooditself，noryetanypartwhichisbloodless，isendowedwithsensation，itisplainthatthatpartwhichfirsthasblood，andwhichholdsitasitwereinareceptacle，mustbetheprimarysourceofsensation。Andthatthispartistheheartisnotonlyarationalinference，butalsoevidenttothesenses。Fornosooneristheembryoformed，thanitsheartisseeninmotionasthoughitwerealivingcreature，andthisbeforeanyoftheotherparts，itbeing，asthusshown，thestarting-pointoftheirnatureinallanimalsthathaveblood。A
furtherevidenceofthetruthofwhathasbeenstatedisthefactthatnosanguineousanimaliswithoutaheart。Fortheprimarysourceofbloodmustofnecessitybepresentinthemall。Itistruethatsanguineousanimalsnotonlyhaveaheartbutalsoinvariablyhavealiver。Butnoonecouldeverdeemthelivertobetheprimaryorganeitherofthewholebodyoroftheblood。Forthepositioninwhichitisplacedisfarfrombeingthatofaprimaryordominatingpart；and，moreover，inthemostperfectlyfinishedanimalsthereisanotherpart，thespleen，whichasitwerecounterbalancesit。Stillfurther，thelivercontainsnospaciousreceptacleinitssubstance，asdoestheheart；butitsbloodisinavesselasinalltheotherviscera。Thevessel，moreover，extendsthroughit，andnovesselwhatsoeveroriginatesinit；foritisfromtheheartthatallthevesselstaketheirrise。Sincethenoneorotherofthesetwopartsmustbethecentralsource，andsinceitisnottheliverwhichissuch，itfollowsofnecessitythatitistheheartwhichisthesourceoftheblood，asalsotheprimaryorganinotherrespects。Forthedefinitivecharacteristicofananimalisthepossessionofsensation；
andthefirstsensorypartisthatwhichfirsthasblood；thatistosayistheheart，whichisthesourceofbloodandthefirstofthepartstocontainit。
Theapexoftheheartispointedandmoresolidthantherestoftheorgan。Itliesagainstthebreast，andentirelyintheanteriorpartofthebody，inordertopreventthatregionfromgettingchilled。Forinallanimalsthereiscomparativelylittlefleshoverthebreast，whereasthereisamoreabundantcoveringofthatsubstanceontheposteriorsurface，sothattheheathasinthebackasufficientamountofprotection。Inallanimalsbutmantheheartisplacedinthecentreofthepectoralregion；butinmanitinclinesalittletowardstheleft，sothatitmaycounterbalancethechillinessofthatside。Fortheleftsideiscolderinman，ascomparedwiththeright，thaninanyotheranimal。Ithasbeenstatedinanearliertreatisethateveninfishestheheartholdsthesamepositionasinotheranimals；andthereasonhasbeengivenwhyitappearsnottodoso。Theapexoftheheart，itistrue，isinthemturnedtowardsthehead，butthisinfishesisthefrontaspect，foritisthedirectioninwhichtheirmotionoccurs。
Theheartagainisabundantlysuppliedwithsinews，asmightreasonablybeexpected。Forthemotionsofthebodycommencefromtheheart，andarebroughtaboutbytractionandrelaxation。Thehearttherefore，which，asalreadysaid，’asitwerealivingcreatureinsideitspossessor，requiressomesuchsubservientandstrengtheningparts。
Innoanimalsdoestheheartcontainabone，certainlyinnoneofthosethatwehaveourselvesinspected，withtheexceptionofthehorseandacertainkindofox。Intheseexceptionalcasestheheart，owingtoitslargebulk，isprovidedwithaboneasasupport；justasthebonesserveassupportsforthebodygenerally。
Inanimalsofgreatsizethehearthasthreecavities；insmalleranimalsithastwo；andinallhasatleastone，for，asalreadystated，theremustbesomeplaceinthehearttoserveasareceptacleforthefirstblood；which，ashasbeenmentionedmorethanonce，isformedinthisorgan。Butinasmuchasthemainblood-vesselsaretwoinnumber，namelytheso-calledgreatvesselandtheaorta，eachofwhichistheoriginofothervessels；inasmuch，moreover，asthesetwovesselspresentdifferences，hereaftertobediscussed，whencomparedwitheachother，itisofadvantagethattheyalsoshallthemselveshavedistinctorigins。Thisadvantagewillbeobtainedifeachsidehaveitsownblood，andthebloodofonesidebekeptseparatefromthatoftheother。Forthisreasontheheart，wheneveritispossible，hastworeceptacles。Andthispossibilityexistsinthecaseoflargeanimals，forinthemtheheart，asthebodygenerally，isoflargesize。Againitisstillbetterthatthereshallbethreecavities，sothatthemiddleandoddonemayserveasacentrecommontobothsides。Butthisrequiresthehearttobeofgreatermagnitude，sothatitisonlyinthelargestheartsthattherearethreecavities。
Ofthesethreecavitiesitistherightthathasthemostabundantandthehottestblood，andthisexplainswhythelimbsalsoontherightsideofthebodyarewarmerthanthoseontheleft。Theleftcavityhastheleastbloodofall，andthecoldest；whileinthemiddlecavitytheblood，asregardsquantityandheat，isintermediatetotheothertwo，beinghoweverofpurerqualitythaneither。Foritbehovesthesupremeparttobeastranquilaspossible，andthistranquillitycanbeensuredbythebloodbeingpure，andofmoderateamountandwarmth。
Intheheartofanimalsthereisalsoakindofjoint-likedivision，somethinglikethesuturesoftheskull。Thisisnot，however，attributabletotheheartbeingformedbytheunionofseveralpartsintoacompoundwhole，butisrather，asalreadysaid，theresultofajoint-likedivision。Thesejointingsaremostdistinctinanimalsofkeensensibility，andlesssointhosethatareofdullerfeeling，inswineforinstance。Differentheartsdifferalsofromeachotherintheirsizes，andintheirdegreesoffirmness；andthesedifferencessomehowextendtheirinfluencetothetemperamentsoftheanimals。Forinanimalsoflowsensibilitytheheartishardanddenseintexture，whileitissofterinsuchasareendowedwithkeenerfeeling。Soalsowhentheheartisoflargesizetheanimalistimorous，whileitismorecourageousiftheorganbesmallerandofmoderatebulk。Forintheformerthebodilyaffectionwhichresultsfromterroralreadypre-exists；forthebulkoftheheartisoutofallproportiontotheanimal’sheat，whichbeingsmallisreducedtoinsignificanceinthelargespace，andthusthebloodismadecolderthanitwouldotherwisebe。
Theheartisoflargesizeinthehare，thedeer，themouse，thehyena，theass，theleopard，themarten，andinprettynearlyallotheranimalsthateitheraremanifestlytimorous，orbetraytheircowardicebytheirspitefulness。
Whathasbeensaidoftheheartasawholeisnolesstrueofitscavitiesandoftheblood-vessels；thesealsoifoflargesizebeingcold。Forjustasafireofequalsizegiveslessheatinalargeroomthaninasmallone，soalsodoestheheatinalargecavityoralargeblood-vessel，thatisinalargereceptacle，havelesseffectthaninasmallone。Moreover，allhotbodiesarecooledbymotionsexternaltothemselves，andthemorespaciousthecavitiesandvesselsare，thegreatertheamountofspirittheycontain，andthemorepotentitsaction。Thusitisthatnoanimalthathaslargecavitiesinitsheart，orlargeblood-vessels，iseverfat，thevesselsbeingindistinctandthecavitiessmallinallormostfatanimals。
Theheartagainistheonlyoneoftheviscera，andindeedtheonlypartofthebody，thatisunabletotolerateanyseriousaffection。Thisisbutwhatmightreasonablybeexpected。For，iftheprimaryordominantpartbediseased，thereisnothingfromwhichtheotherpartswhichdependuponitcanderivesuccour。Aproofthattheheartisthusunabletotolerateanymorbidaffectionisfurnishedbythefactthatinnosacrificialvictimhasiteverbeenseentobeaffectedwiththosediseasesthatareobservableintheotherviscera。Forthekidneysarefrequentlyfoundtobefullofstones，andgrowths，andsmallabscesses，asalsoaretheliver，thelung，andmorethanallthespleen。Therearealsomanyothermorbidconditionswhichareseentooccurintheseparts，thosewhichareleastliabletosuchbeingtheportionofthelungwhichisclosetothewindpipe，andtheportionoftheliverwhichliesaboutthejunctionwiththegreatblood-vessel。Thisagainadmitsofarationalexplanation。Foritisinthesepartsthatthelungandliveraremostcloselyincommunionwiththeheart。Ontheotherhand，whenanimalsdienotbysacrificebutfromdisease，andfromaffectionssuchasarementionedabove，theyarefoundondissectiontohavemorbidaffectionsoftheheart。
Thusmuchoftheheart，itsnature，andtheendandcauseofitsexistenceinsuchanimalsashaveit。
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Induesequencewehavenexttodiscusstheblood-vessels，thatistosaythegreatvesselandtheaorta。Foritisintothesetwothatthebloodfirstpasseswhenitquitstheheart；andalltheothervesselsarebutoffshootsfromthem。Nowthatthesevesselsexistonaccountofthebloodhasalreadybeenstated。Foreveryfluidrequiresareceptacle，andinthecaseofthebloodthevesselsarethatreceptacle。Letusnowexplainwhythesevesselsaretwo，andwhytheyspringfromonesinglesource，andextendthroughoutthewholebody。
Thereason，then，whythesetwovesselscoalesceintoonecentre，andspringfromonesource，isthatthesensorysoulisinallanimalsactuallyone；andthisone-nessofthesensorysouldeterminesacorrespondingone-nessofthepartinwhichitprimarilyabides。Insanguineousanimalsthisone-nessisnotonlyactualbutpotential，whereasinsomebloodlessanimalsitisonlyactual。Where，however，thesensorysoulislodged，therealsoandintheselfsameplacemustnecessarilybethesourceofheat；and，again，wherethisistherealsomustbethesourceoftheblood，seeingthatitthencederivesitswarmthandfluidity。Thus，then，intheonenessofthepartinwhichislodgedtheprimesourceofsensationandofheatisinvolvedtheone-nessofthesourceinwhichthebloodoriginates；andthis，again，explainswhytheblood-vesselshaveonecommonstarting-point。
Thevessels，again，aretwo，becausethebodyofeverysanguineousanimalthatiscapableoflocomotionisbilateral；forinallsuchanimalsthereisadistinguishablebeforeandbehind，arightandleft，anaboveandbelow。Nowasthefrontismorehonourableandofhighersupremacythanthehinderaspect，soalsoandinlikedegreeisthegreatvesselsuperiortotheaorta。Forthegreatvesselisplacedinfront，whiletheaortaisbehind；theformeragainisplainlyvisibleinallsanguineousanimals，whilethelatterisinsomeindistinctandinsomenotdiscernibleatall。
Lastly，thereasonforthevesselsbeingdistributedthroughouttheentirebodyisthatinthem，orinpartsanalogoustothem，iscontainedtheblood，orthefluidwhichinbloodlessanimalstakestheplaceofblood，andthatthebloodoranalogousfluidisthematerialfromwhichthewholebodyismade。Nowastothemannerinwhichanimalsarenourished，andastothesourcefromwhichtheyobtainnutrimentandastothewayinwhichtheyabsorbthisfromthestomach，thesearematterswhichmaybemoresuitablyconsideredandexplainedinthetreatiseonGeneration。Butinasmuchasthepartsare，asalreadysaid，formedoutoftheblood，itisbutrationalthattheflowofthebloodshouldextend，asitdoes，throughoutthewholeofthebody。Forsinceeachpartisformedofblood，eachmusthavebloodaboutandinitssubstance。
Togiveanillustrationofthis。Thewater-coursesingardensaresoconstructedastodistributewaterfromonesinglesourceorfountintonumerouschannels，whichdivideandsubdividesoastoconveyittoallparts；and，again，inhouse-buildingstonesarethrowndownalongthewholeground-planofthefoundationwalls；becausethegarden-plantsintheonecasegrowattheexpenseofthewater，andthefoundationwallsintheotherarebuiltoutofthestones。Nowjustafterthesamefashionhasnaturelaiddownchannelsfortheconveyanceofthebloodthroughoutthewholebody，becausethisbloodisthematerialoutofwhichthewholefabricismade。Thisbecomesveryevidentinbodiesthathaveundergonegreatemaciation。
Forinsuchthereisnothingtobeseenbuttheblood-vessels；justaswhenfig-leavesorvine-leavesorthelikehavedriedup，thereisnothingleftofthembuttheirvessels。Theexplanationofthisisthattheblood，orfluidwhichtakesitsplace，ispotentiallybodyandflesh，orsubstanceanalogoustoflesh。Nowjustasinirrigationthelargestdykesarepermanent，whilethesmallestaresoonfilledupwithmudanddisappear，againtobecomevisiblewhenthedepositofmudceases；soalsodothelargestblood-vesselsremainpermanentlyopen，whilethesmallestareconvertedactuallyintoflesh，thoughpotentiallytheyarenowhitlessvesselsthanbefore。
Thistooexplainswhy，solongasthefleshofananimalisinitsintegrity，bloodwillflowfromanypartofitwhatsoeverthatiscut，thoughnovessel，howeversmall，bevisibleinit。Yettherecanbenoblood，unlesstherebeablood-vessel。Thevesselsthenarethere，butareinvisibleowingtotheirbeingcloggedup，justasthedykesforirrigationareinvisibleuntiltheyhavebeenclearedofmud。
Astheblood-vesselsadvance，theybecomegraduallysmallerandsmaller，untilatlasttheirtubesaretoofinetoadmittheblood。
Thisfluidcanthereforenolongerfinditswaythroughthem，thoughtheystillgivepassagetothehumourwhichwecallsweat；andespeciallysowhenthebodyisheated，andthemouthsofthesmallvesselsaredilated。Instances，indeed，arenotunknownofpersonswhoinconsequenceofacachecticstatehavesecretedsweatthatresembledblood，theirbodyhavingbecomelooseandflabby，andtheirbloodwatery，owingtotheheatinthesmallvesselshavingbeentooscantyforitsconcoction。For，aswasbeforesaid，everycompoundofearthandwater-andbothnutrimentandbloodaresuch-becomesthickerfromconcoction。Theinabilityoftheheattoeffectconcoctionmaybedueeithertoitsbeingabsolutelysmallinamount，ortoitsbeingsmallinproportiontothequantityoffood，whenthishasbeentakenexcess。Thisexcessagainmaybeoftwokinds，eitherquantitativeorqualitative；forallsubstancesarenotequallyamenabletoconcoction。
Thewidestpassagesinthebodyareofallpartsthemostliabletohaemorrhage；sothatbleedingoccursnotinfrequentlyfromthenostrils，thegums，andthefundament，occasionallyalsofromthemouth。Suchhaemorrhagesareofapassivekind，andnotviolentasarethosefromthewindpipe。
Thegreatvesselandtheaorta，whichaboveliesomewhatapart，lowerdownexchangepositions，andbysodoinggivecompactnesstothebody。Forwhentheyreachthepointwherethelegsdiverge，theyeachsplitintotwo，andthegreatvesselpassesfromthefronttotherear，andtheaortafromthereartothefront。Bythistheycontributetotheunityofthewholefabric。Forasinplaitedworkthepartsholdmorefirmlytogetherbecauseoftheinterweaving，soalsobytheinterchangeofpositionbetweentheblood-vesselsaretheanteriorandposteriorpartsofthebodymorecloselyknittogether。Asimilarexchangeofpositionoccursalsointheupperpartofthebody，betweenthevesselsthathaveissuedfromtheheart。
ThedetailshoweverofthemutualrelationsofthedifferentvesselsmustbelookedforinthetreatisesonAnatomyandtheResearchesconcerningAnimals。
Somuch，then，asconcernstheheartandtheblood-vessels。Wemustnowpassontotheothervisceraandapplythesamemethodofinquirytothem。
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Thelung，then，isanorganfoundinalltheanimalsofacertainclass，becausetheyliveonland。Fortheremustofnecessitybesomemeansorotheroftemperingtheheatofthebody；andinsanguineousanimals，astheyareofanespeciallyhotnature，thecoolingagencymustbeexternal，whereasinthebloodlesskindstheinnatespiritissufficientofitselfforthepurpose。Theexternalcoolingagentmustbeeitherairorwater。Infishestheagentiswater。Fishesthereforeneverhavealung，buthavegillsinitsplace，aswasstatedinthetreatiseonRespiration。Butanimalsthatbreathearecooledbyair。Thesethereforeareallprovidedwithalung。
Alllandanimalsbreathe，andevensomewateranimals，suchasthewhale，thedolphin，andallthespoutingCetacea。Formanyanimalsliehalf-waybetweenterrestrialandaquatic；somethatareterrestrialandthatinspireairbeingneverthelessofsuchabodilyconstitutionthattheyabideforthemosttimeinthewater；andsomethatareaquaticpartakingsolargelyofthelandcharacter，thatrespirationconstitutesforthemthemanconditionoflife。
Theorganofrespirationisthelung。Thisderivesitsmotionfromtheheart；butitisitsownlargesizeandspongytexturethataffordsamplitudeofspaceforentranceofthebreath。Forwhenthelungrisesupthebreathstreamsin，andisagainexpelledwhenthelungcollapses。Ithasbeensaidthatthelungexistsasaprovisiontomeetthejumpingoftheheart。Butthisisoutofthequestion。Formanispracticallytheonlyanimalwhoseheartpresentsthisphenomenonofjumping，inasmuchashealoneisinfluencedbyhopeandanticipationofthefuture。Moreover，inmostanimalsthelungisseparatedfromtheheartbyaconsiderableintervalandliesaboveit，sothatitcancontributenothingtomitigateanyjumping。
Thelungdiffersmuchindifferentanimals。Forinsomeitisoflargesizeandcontainsblood；whileinothersitissmallerandofspongytexture。Intheviviparaitislargeandrichinblood，becauseoftheirnaturalheat；whileintheoviparaitissmallanddrybutcapableofexpandingtoavastextentwheninflated。Amongterrestrialanimals，theoviparousquadrupeds，suchaslizards，tortoises，andthelike，havethiskindoflung；and，amonginhabitantsoftheair，theanimalsknownasbirds。Forinallthesethelungisspongy，andlikefoam。Foritismembranousandcollapsesfromalargebulktoasmallone，asdoesfoamwhenitrunstogether。Inthistooliestheexplanationofthefactthattheseanimalsarelittleliabletothirstanddrinkbutsparingly，andthattheyareabletoremainforaconsiderabletimeunderwater。For，inasmuchastheyhavebutlittleheat，theverymotionofthelung，airlikeandvoid，sufficesbyitselftocoolthemforaconsiderableperiod。
Theseanimals，speakinggenerally，arealsodistinguishedfromothersbytheirsmallerbulk。Forheatpromotesgrowth，andabundanceofbloodisasureindicationofheat。Heat，again，tendstomakethebodyerect；andthusitisthatmanisthemosterectofanimals，andtheviviparamoreerectthanotherquadrupeds。Fornoviviparousanimal，beitapodousorbeitpossessedoffeet，issogiventocreepintoholesasaretheovipara。
Thelung，then，existsforrespiration；andthisisitsuniversaloffice；butinoneorderofanimalsitisbloodlessandhasthestructuredescribedabove，tosuitthespecialrequirementsThereis，however，noonetermtodenoteallanimalsthathavealung；nodesignation，thatis，likethetermBird，applicabletothewholeofacertainclass。Yetthepossessionofalungisapartoftheiressence，justasmuchasthepresenceofcertaincharactersconstitutestheessenceofabird。
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Oftheviscerasomeappeartobesingle，astheheartandlung；
otherstobedouble，asthekidneys；whileofathirdkinditisdoubtfulinwhichclasstheyshouldbereckoned。Fortheliverandthespleenwouldseemtoliehalf-waybetweenthesingleandthedoubleorgans。Fortheymayberegardedeitherasconstitutingeachasingleorgan，orasapairoforgansresemblingeachotherincharacter。
Inreality，however，alltheorgansaredouble。Thereasonforthisisthatthebodyitselfisdouble，consistingoftwohalves，whicharehowevercombinedtogetherunderonesupremecentre。Forthereisanupperandalowerhalf，afrontandarear，arightsideandaleft。
Thisexplainswhyitisthateventhebrainandtheseveralorgansofsensetendinallanimalstoconsistoftwoparts；andthesameexplanationappliestotheheartwithitscavities。ThelungagaininOviparaisdividedtosuchanextentthattheseanimalslookasthoughtheyhadactuallytwolungs。Astothekidneys，noonecanoverlooktheirdoublecharacter。Butwhenwecometotheliverandthespleen，anyonemightfairlybeindoubt。Thereasonofthisis，that，inanimalsthatnecessarilyhaveaspleen，thisorganissuchthatitmightbetakenforakindofbastardliver；whileinthoseinwhichaspleenisnotanactualnecessitybutismerelypresent，asitwere，bywayoftoken，inanextremelyminuteform，theliverplainlyconsistsoftwoparts；ofwhichthelargertendstolieontherightsideandthesmallerontheleft。NotbutwhattherearesomeevenoftheOviparainwhichthisconditioniscomparativelyindistinctlymarked；while，ontheotherhand，therearesomeViviparainwhichtheliverismanifestlydividedintotwoparts。Examplesofsuchdivisionarefurnishedbytheharesofcertainregions，whichhavetheappearanceofhavingtwolivers，andbythecartilaginousandsomeotherfishes。
Itisthepositionoftheliverontherightsideofthebodythatisthemaincausefortheformationofthespleen；theexistenceofwhichthusbecomestoacertainextentamatterofnecessityinallanimals，thoughnotofverystringentnecessity。
Thereason，then，whythevisceraarebilateralis，aswehavesaid，thattherearetwosidestothebody，arightandaleft。Foreachofthesesidesaimsatsimilaritywiththeother，andsolikewisedotheirseveralviscera；andasthesides，thoughdual，areknittogetherintounity，soalsodothevisceratendtobebilateralandyetonebyunityofconstitution。
Thoseviscerawhichliebelowthediaphragmexistoneandallonaccountoftheblood-vessels；servingasabond，bywhichthesevessels，whilefloatingfreely，areyetheldinconnexionwiththebody。Forthevesselsgiveoffbrancheswhichruntothebodythroughtheoutstretchedstructures，likesomanyanchorlinesthrownoutfromaship。Thegreatvesselsendssuchbranchestotheliverandthespleen；andtheseviscera-theliverandspleenoneithersidewiththekidneysbehind-attachthegreatvesseltothebodywiththefirmnessofnails。Theaortasendssimilarbranchestoeachkidney，butnonetotheliverorspleen。
Theseviscera，then，contributeinthismannertothecompactnessoftheanimalbody。Theliverandspleenassist，moreover，intheconcoctionofthefood；forbothareofahotcharacter，owingtothebloodwhichtheycontain。Thekidneys，ontheotherhand，takepartintheseparationoftheexcretionwhichflowsintothebladder。
Theheartthenandtheliverareessentialconstituentsofeveryanimal；theliverthatitmayeffectconcoction，theheartthatitmaylodgethecentralsourceofheat。Forsomepartorothertheremustbewhich，likeahearth，shallholdthekindlingfire；andthispartmustbewellprotected，seeingthatitis，asitwere，thecitadelofthebody。
Allsanguineousanimals，then，needthesetwoparts；andthisexplainswhythesetwoviscera，andthesetwoalone，areinvariablyfoundinthemall。Insuchofthem，however，asbreathe，thereisalsoasinvariablyathird，namelythelung。Thespleen，ontheotherhand，isnotinvariablypresent；and，inthoseanimalsthathaveit，isonlypresentofnecessityinthesamesenseastheexcretionsofthebellyandofthebladderarenecessary，inthesense，thatis，ofbeinganinevitableconcomitant。Thereforeitisthatinsomeanimalsthespleenisbutscantilydevelopedasregardssize。This，forinstance，isthecaseinsuchfeatheredanimalsashaveahotstomach。Sucharethepigeon，thehawk，andthekite。Itisthecasealsoinoviparousquadrupeds，wherethespleenisexcessivelyminute，andinmanyofthescalyfishes。Thesesameanimalsarealsowithoutabladder，becausetheloosetextureoftheirfleshallowstheresidualfluidtopassthroughandtobeappliedtotheformationoffeathersandscales。Forthespleenattractstheresidualhumoursfromthestomach，andowingtoitsbloodlikecharacterisenabledtoassistintheirconcoction。Should，however，thisresidualfluidbetooabundant，ortheheatofthespleenbetooscanty，thebodybecomessicklyfromover-repletionwithnutriment。Often，too，whenthespleenisaffectedbydisease，thebellybecomeshardowingtotherefluxintoitofthefluid；justashappenstothosewhoformtoomuchurine，fortheyalsoareliabletoasimilardiversionofthefluidsintothebelly。Butinthoseanimalsthathavebutlittlesuperfluousfluidtoexcrete，suchasbirdsandfishes，thespleenisneverlarge，andinsomeexistsnomorethanbywayoftoken。Soalsointheoviparousquadrupedsitissmall，compact，andlikeakidney。Fortheirlungisspongy，andtheydrinkbutlittle，andsuchsuperfluousfluidastheyhaveisappliedtothegrowthofthebodyandtheformationofscalyplates，justasinbirdsitisappliedtotheformationoffeathers。
Ontheotherhand，insuchanimalsashaveabladder，andwhoselungcontainsblood，thespleeniswatery，bothforthereasonalreadymentioned，andalsobecausetheleftsideofthebodyismorewateryandcolderthantheright。Foreachoftwocontrarieshasbeensoplacedastogotogetherwiththatwhichisakintoitinanotherpairofcontraries。Thusrightandleft，hotandcold，arepairsofcontraries；andrightisconjoinedwithhot，afterthemannerdescribed，andleftwithcold。
