Thekidneyswhentheyarepresentexistnotofactualnecessity，butasmattersofgreaterfinishandperfection。Forbytheirspecialcharactertheyaresuitedtoserveintheexcretionofthefluidwhichcollectsinthebladder。Inanimalsthereforewherethisfluidisveryabundantlyformed，theirpresenceenablesthebladdertoperformitsproperofficewithgreaterperfection。
Sincethenbothkidneysandbladderexistinanimalsforoneandthesamefunction，wemustnexttreatofthebladder，thoughinsodoingwedisregardthedueorderofsuccessioninwhichthepartsshouldbeenumerated。Fornotawordhasyetbeensaidofthemidriff，whichisoneofthepartsthatenvironthevisceraandthereforehastobeconsideredwiththem。
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Itisnoteveryanimalthathasabladder；thoseonlybeingapparentlyintendedbynaturetohaveone，whoselungcontainsblood。Tosuchitwasbutreasonablethatsheshouldgivethispart。
Forthesuperabundanceintheirlungofitsnaturalconstituentscausesthemtobethethirstiestofanimals，andmakesthemrequireamorethanordinaryquantitynotmerelyofsolidbutalsoofliquidnutriment。Thisincreasedconsumptionnecessarilyentailstheproductionofanincreasedamountofresidue；whichthusbecomestooabundanttobeconcoctedbythestomachandexcretedwithitsownresidualmatter。Theresidualfluidmustthereforeofnecessityhaveareceptacleofitsown；andthusitcomestopassthatallanimalswhoselungcontainsbloodareprovidedwithabladder。Thoseanimals，ontheotherhand，thatarewithoutalungofthischaracter，andthateitherdrinkbutsparinglyowingtotheirlungbeingofaspongytexture，orneverimbibefluidatallfordrinking’ssakebutonlyasnutriment，insectsforinstanceandfishes，andthataremoreovercladwithfeathersorscalesorscalyplates-alltheseanimals，owingtothesmallamountoffluidwhichtheyimbibe，andowingalsotosuchresidueastheremaybebeingconvertedintofeathersandthelike，areinvariablywithoutabladder。TheTortoises，whicharecomprisedamonganimalswithscalyplates，formtheonlyexception；andthisismerelyduetotheimperfectdevelopmentoftheirnaturalconformation；theexplanationofthematterbeingthatinthesea-tortoisesthelungisflesh-likeandcontainsblood，resemblingthelungoftheox，andthatintheland-tortoisesitisofdisproportionatelylargesize。Moreover，inasmuchasthecoveringwhichinveststhemisdenseandshell-like，sothatthemoisturecannotexhalethroughtheporousflesh，asitdoesinbirdsandinsnakesandotheranimalswithscalyplates，suchanamountofsecretionisformedthatsomespecialpartisrequiredtoreceiveandholdit。Thisthenisthereasonwhytheseanimals，aloneoftheirkind，haveabladder，thesea-tortoisealargeone，theland-tortoisesanextremelysmallone。
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Whathasbeensaidofthebladderisequallytrueofthekidneys。
Forthesealsoarewantinginallanimalsthatarecladwithfeathersorwithscalesorwithscale-likeplates；theseaandlandtortoisesformingtheonlyexception。Insomeofthebirds，however，thereareflattenedkidneylikebodies，asthoughthefleshallottedtotheformationofthekidneys，unabletofindonesingleplaceofsufficientsize，hadbeenscatteredoverseveral。
TheEmyshasneitherbladdernorkidneys。Forthesoftnessofitsshellallowsofthereadytranspirationoffluid；andforthisreasonneitheroftheorgansmentionedexistsinthisanimal。Allotheranimals，however，whoselungcontainsbloodare，asbeforesaid，providedwithkidneys。Fornatureusestheseorgansfortwoseparatepurposes，namelyfortheexcretionoftheresidualfluid，andtosubservetheblood-vessels，achannelleadingtothemfromthegreatvessel。
Inthecentreofthekidneyisacavityofvariablesize。Thisisthecaseinallanimals，exceptingtheseal。Thekidneysofthisanimalaremoresolidthanthoseofanyother，andinformresemblethekidneysoftheox。Thehumankidneysareofsimilarshape；beingasitweremadeupofnumeroussmallkidneys，andnotpresentingoneunbrokensurfacelikethekidneysofsheepandotherquadrupeds。Forthisreason，shouldthekidneysofamanbeonceattackedbydisease，themaladyisnoteasilyexpelled。Foritisasthoughmanykidneyswerediseasedandnotmerelyone；whichnaturallyenhancesthedifficultiesofacure。
Theductwhichrunstothekidneyfromthegreatvesseldoesnotterminateinthecentralcavity，butisexpendedonthesubstanceoftheorgan，sothatthereisnobloodinthecavity，norisanycoagulumfoundthereafterdeath。Apairofstoutducts，voidofblood，run，onefromthecavityofeachkidney，tothebladder；andotherducts，strongandcontinuous，leadintothekidneysfromtheaorta。Thepurposeofthisarrangementistoallowthesuperfluousfluidtopassfromtheblood-vesselintothekidney，andtheresultingrenalexcretiontocollectbythepercolationofthefluidthroughthesolidsubstanceoftheorgan，initscentre，whereasageneralrulethereisacavity。（Thisbythewayexplainswhythekidneyisthemostill-savouredofalltheviscera。）Fromthecentralcavitythefluidisdischargedintothebladderbytheductsthathavebeenmentioned，havingalreadyassumedingreatdegreethecharacterofexcrementalresidue。Thebladderisasitweremooredtothekidneys；for，asalreadyhasbeenstated，itisattachedtothembystrongducts。Thesethenarethepurposesforwhichthekidneysexist，andsuchthefunctionsoftheseorgans。
Inallanimalsthathavekidneys，thatontherightisplacedhigherthanthatontheleft。Forinasmuchasmotioncommencesfromtheright，andtheorgansonthissideareinconsequencestrongerthanthoseontheleft，theymustallpushupwardsinadvanceoftheiroppositefellows；asmaybeseeninthefactthatmenevenraisetherighteyebrowmorethantheleft，andthattheformerismorearchedthanthelatter。Therightkidneybeingthusdrawnupwardsisinallanimalsbroughtintocontactwiththeliver；fortheliverliesontherightside。
Ofalltheviscerathekidneysarethosethathavethemostfat。
Thisisinthefirstplacetheresultofnecessity，becausethekidneysarethepartsthroughwhichtheresidualmatterspercolate。
Forthebloodwhichisleftbehindafterthisexcretion，beingofpurequality，isofeasyconcoction，andthefinalresultofthoroughblood-concoctionislardandsuet。Forjustasacertainamountoffireisleftintheashesofsolidsubstancesaftercombustion，soalsodoesaremnantoftheheatthathasbeendevelopedremaininfluidsafterconcoction；andthisisthereasonwhyoilymatterislight，andfloatsonthesurfaceofotherfluids。Thefatisnotformedinthekidneysthemselves，thedensityoftheirsubstanceforbiddingthis，butisdepositedabouttheirexternalsurface。Itconsistsoflardorofsuet，accordingastheanimal’sfatisoftheformerorlattercharacter。Thedifferencebetweenthesetwokindsoffathasalreadybeensetforthinotherpassages。Theformation，then，offatinthekidneysistheresultofnecessity；being，asexplained，aconsequenceofthenecessaryconditionswhichaccompanythepossessionofsuchorgans。Butatthesametimethefathasafinalcause，namelytoensurethesafetyofthekidneys，andtomaintaintheirnaturalheat。Forplaced，astheseorgansare，closetothesurface，theyrequireagreatersupplyofheatthanotherparts。
Forwhilethebackisthicklycoveredwithflesh，soastoformashieldfortheheartandneighbouringviscera，theloins，inaccordancewitharulethatappliestoallbendings，aredestituteofflesh；andfatisthereforeformedasasubstituteforit，sothatthekidneysmaynotbewithoutprotection。Thekidneys，moreover，bybeingfatarethebetterenabledtosecreteandconcocttheirfluid；forfatishot，anditisheatthateffectsconcoction。
Such，then，arethereasonswhythekidneysarefat。Butinallanimalstherightkidneyislessfatthanitsfellow。Thereasonforthisis，thatthepartsontherightsidearenaturallymoresolidandmoresuitedformotionthanthoseontheleft。Butmotionisantagonistictofat，forittendstomeltit。
Animalsthen，asageneralrule，deriveadvantagefromtheirkidneysbeingfat；andthefatisoftenveryabundantandextendsoverthewholeoftheseorgans。But，shouldthelikeoccurinthesheep，deathensues。Beitskidneys，however，asfatastheymay，theyareneversofatbutthatsomepart，ifnotinbothatanyrateintherightone，isleftfree。Thereasonwhysheeparetheonlyanimalsthatsufferinthismanner，orsuffermorethanothers，isthatinanimalswhosefatiscomposedoflardthisisoffluidconsistency，sothatthereisnotthesamechanceintheircaseofwindgettingshutinandcausingmischief。Butitistosuchanenclosureofwindthatrotisdue。Andthuseveninmen，thoughitisbeneficialtothemtohavefatkidneys，yetshouldtheseorgansbecomeover-fatanddiseased，deadlypainsensue。Astothoseanimalswhosefatconsistsofsuet，innoneisthesuetsodenseasinthesheep，neitherisitnearlysoabundant；forofallanimalsthereisnoneinwhichthekidneysbecomesosoongorgedwithfatasinthesheep。Rot，then，isproducedbythemoistureandthewindgettingshutupinthekidneys，andisamaladythatcarriesoffsheepwithgreatrapidity。
Forthediseaseforthwithreachestheheart，passingthitherbytheaortaandthegreatvessel，theductswhichconnectthesewiththekidneysbeingofunbrokencontinuity。
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Wehavenowdealtwiththeheartandthelung，asalsowiththeliver，spleen，andkidneys。Thelatterareseparatedfromtheformerbythemidriffor，assomecallit，thePhrenes。Thisdividesofftheheartandlung，and，asalreadysaid，iscalledPhrenesinsanguineousanimals，allofwhichhaveamidriff，justastheyallhaveaheartandaliver。Fortheyrequireamidrifftodividetheregionoftheheartfromtheregionofthestomach，sothatthecentrewhereinabidesthesensorysoulmaybeundisturbed，andnotbeoverwhelmed，directlyfoodistaken，byitsup-steamingvapourandbytheabundanceofheatthensuperinduced。Foritwastoguardagainstthisthatnaturemadeadivision，constructingthemidriffasakindofpartition-wallandfence，andsoseparatedthenoblerfromthelessnobleparts，inallcaseswhereaseparationofupperfromlowerispossible。Fortheupperpartisthemorehonourable，andisthatforthesakeofwhichtherestexists；whilethelowerpartexistsforthesakeoftheupperandconstitutesthenecessaryelementinthebody，inasmuchasitistherecipientofthefood。
Thatportionofthemidriffwhichisneartheribsisfleshierandstrongerthantherest，butthecentralparthasmoreofamembranouscharacter；forthisstructureconducesbesttoitsstrengthanditsextensibility。Nowthatthemidriff，whichisakindofoutgrowthfromthesidesofthethorax，actsasascreentopreventheatmountingupfrombelow，isshownbywhathappens，shouldit，owingtoitsproximitytothestomach，attractthencethehotandresidualfluid。Forwhenthisoccursthereensuesforthwithamarkeddisturbanceofintellectandofsensation。ItisindeedbecauseofthisthatthemidriffiscalledPhrenes，asthoughithadsomeshareintheprocessofthinking（Phronein）。inreality，however，ithasnopartwhatsoeveritselfinthematter，but，lyingincloseproximitytoorgansthathave，itbringsaboutthemanifestchangesofintelligenceinquestionbyactinguponthem。Thistooexplainswhyitscentralpartisthin。Forthoughthisisinsomemeasuretheresultofnecessity，inasmuchasthoseportionsofthefleshywholewhichlienearesttotheribsmustnecessarilybefleshierthantherest，yetbesidesthisthereisafinalcause，namelytogiveitassmallaproportionofhumouraspossible；for，haditbeenmadeoffleshthroughout，itwouldhavebeenmorelikelytoattractandholdalargeamountofthis。Thatheatingofitaffectssensationrapidlyandinanotablemannerisshownbythephenomenaoflaughing。Forwhenmenaretickledtheyarequicklyseta-laughing，becausethemotionquicklyreachesthispart，andheatingitthoughbutslightlyneverthelessmanifestlysodisturbsthementalactionastooccasionmovementsthatareindependentofthewill。Thatmanaloneisaffectedbyticklingisduefirstlytothedelicacyofhisskin，andsecondlytohisbeingtheonlyanimalthatlaughs。Fortobetickledistobesetinlaughter，thelaughterbeingproducedsuchamotionasmentionedoftheregionofthearmpit。
Itissaidalsothatwhenmeninbattlearewoundedanywherenearthemidriff，theyareseentolaugh，owingtotheheatproducedbythewound。Thismaypossiblybethecase。Atanyrateitisastatementmadebymuchmorecrediblepersonsthanthosewhotellthestoryofthehumanhead，howitspeaksafteritiscutoff。Forsosomeassert，andevencallinHomertosupportthem，representinghimasalludingtothiswhenhewrote，’Hisheadstillspeakingrolledintothedust，’
insteadof’Theheadofthespeaker’。SofullywasthepossibilityofsuchanoccurrenceacceptedinCaria，thatoneofthatcountrywasactuallybroughttotrialunderthefollowingcircumstances。ThepriestofZeusHoplosmioshadbeenmurdered；butasyetithadnotbeenascertainedwhowastheassassin；whencertainpersonsassertedthattheyhadheardthemurderedman’shead，whichhadbeenseveredfromthebody，repeatseveraltimesthewords，’Cercidasslewmanonmam。’SearchwasthereuponmadeandamanofthosepartswhoborethenameofCercidashuntedoutandputuponhistrial。Butitisimpossiblethatanyoneshouldutterawordwhenthewindpipeisseveredandnomotionanylongerderivedfromthelung。Moreover，amongtheBarbarians，whereheadsarechoppedoffwithgreatrapidity，nothingofthekindhaseveryetoccurred。Why，again，doesnotthelikeoccurinthecaseofotheranimalsthanman？Forthatnoneofthemshouldlaugh，whentheirmidriffiswounded，isbutwhatonewouldexpect；fornoanimalbutmaneverlaughs。So，too，thereisnothingirrationalinsupposingthatthetrunkmayrunforwardstoacertaindistanceaftertheheadhasbeencutseeingthatbloodlessanimalsatanyratecanlive，andthatforaconsiderabletime，afterdecapitation，ashasbeensetforthandexplainedinotherpassages。
Thepurposes，then，forwhichthevisceraseverallyexisthavenowbeenstated。Itisofnecessityupontheinnerterminationsofthevesselsthattheyaredeveloped；forhumour，andthatofabloodycharacter，cannotbutexudeatthesepoints，anditisofthis，solidifiedandcoagulated，thatthesubstanceofthevisceraisformed。Thustheyareofabloodycharacter，andinsubstanceresembleeachotherwhiletheydifferfromotherparts。
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Thevisceraareenclosedeachinamembrane。Fortheyrequiresomecoveringtoprotectthemfrominjury，andrequire，moreover，thatthiscoveringshallbelight。Tosuchrequirementsmembraneiswelladapted；foritiscloseintexturesoastoformagoodprotection，destituteoffleshsoasneithertoattracthumournorretainit，andthinsoastobelightandnotaddtotheweightofthebody。Ofthemembranesthosearethestoutestandstrongestwhichinvesttheheartandthebrain；asisbutconsistentwithreason。Forthesearethepartswhichrequiremostprotection，seeingthattheyarethemaingoverningpowersoflife，andthatitistogoverningpowersthatguardisdue。
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Someanimalshavealltheviscerathathavebeenenumerated；
othershaveonlysomeofthem。Inwhatkindofanimalsthislatteristhecase，andwhatistheexplanation，hasalreadybeenstated。
Moreover，theself-sameviscerapresentdifferencesindifferentpossessors。Fortheheartisnotpreciselyalikeinallanimalsthathaveone；nor，infact，isanyviscuswhatsoever。Thustheliverisinsomeanimalssplitintoseveralparts，whileinothersitiscomparativelyundivided。Suchdifferencesinitsformpresentthemselvesevenamongthosesanguineousanimalsthatareviviparous，butaremoremarkedinfishesandintheoviparousquadrupeds，andthiswhetherwecomparethemwitheachotherorwiththeVivipara。
Asforbirds，theirliververynearlyresemblesthatoftheVivipara；forinthem，asinthese，itisofapureandblood-likecolour。Thereasonofthisisthatthebodyinboththeseclassesofanimalsadmitsofthefreestexhalation，sothattheamountoffoulresidualmatterwithinisbutsmall。HenceitisthatsomeoftheViviparaarewithoutanygall-bladderatall。Forthelivertakesalargeshareinmaintainingthepurityofcompositionandthehealthinessofthebody。Fortheseareconditionsthatdependfinallyandinthemainupontheblood，andthereismorebloodintheliverthaninanyoftheotherviscera，theheartonlyexcepted。Ontheotherhand，theliverofoviparousquadrupedsandfishesinclines，asarule，toayellowhue，andthereareevensomeoftheminwhichitisentirelyofthisbadcolour，inaccordancewiththebadcompositionoftheirbodiesgenerally。Such，forinstance，isthecaseinthetoad，thetortoise，andothersimilaranimals。
Thespleen，again，variesindifferentanimals。Forinthosethathavehornsandclovenhoofs，suchasthegoat，thesheep，andthelike，itisofaroundedform；exceptingwhenincreasedsizehascausedsomepartofittoextenditsgrowthlongitudinally，ashashappenedinthecaseoftheox。Ontheotherhand，itiselongatedinallpolydactylousanimals。Such，forinstance，isthecaseinthepig，inman，andinthedog。Whileinanimalswithsolidhoofsitisofaformintermediatetothesetwo，beingbroadinonepart，narrowinanother。Such，forexample，isitsshapeinthehorse，themule，andtheass。
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Thevisceradifferfromthefleshnotonlyintheturgidaspectoftheirsubstance，butalsoinposition；fortheyliewithinthebody，whereasthefleshisplacedontheoutside。Theexplanationofthisisthatthesepartspartakeofthecharacterofblood-vessels，andthatwhiletheformerexistforthesakeofthevessels，thelattercannotexistwithoutthem。
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Belowthemidriffliesthestomach，placedattheendoftheoesophaguswhenthereisone，andinimmediatecontiguitywiththemouthwhentheoesophagusiswanting。Continuouswiththisstomachiswhatiscalledthegut。Thesepartsarepresentinallanimals，forreasonsthatareself-evident。Foritisamatterofnecessitythatananimalshallreceivetheincomingfood；andnecessaryalsothatitshalldischargethesamewhenitsgoodnessisexhausted。Thisresidualmatter，again，mustnotoccupythesameplaceastheyetunconcoctednutriment。Forastheingressoffoodandthedischargeoftheresidueoccuratdistinctperiods，soalsomusttheynecessarilyoccurindistinctplaces。Thustheremustbeonereceptaclefortheingoingfoodandanotherfortheuselessresidue，andbetweenthese，therefore，apartinwhichthechangefromoneconditiontotheothermaybeeffected。These，however，arematterswhichwillbemoresuitablysetforthwhenwecometodealwithGenerationandNutrition。Whatwehaveatpresenttoconsiderarethevariationspresentedbythestomachanditssubsidiaryparts。Forneitherinsizenorinshapearethesepartsuniformlyalikeinallanimals。Thusthestomachissingleinallsuchsanguineousandviviparousanimalsashaveteethinfrontofbothjaws。Itissinglethereforeinallthepolydactylouskinds，suchasman，dog，lion，andtherest；inallthesolid-hoofedanimalsalso，suchashorse，mule，ass；andinallthosewhich，likethepig，thoughtheirhoofiscloven，yethavefrontteethinbothjaws。When，however，ananimalisoflargesize，andfeedsonsubstancesofsothornyandligneousacharacterastobedifficultofconcoction，itmayinconsequencehaveseveralstomachs，asforinstanceisthecasewiththecamel。Asimilarmultiplicityofstomachsexistsalsointhehornedanimals；thereasonbeingthathorn-bearinganimalshavenofrontteethintheupperjaw。Thecamelalso，thoughithasnohorns，isyetwithoutupperfrontteeth。Theexplanationofthisisthatitismoreessentialforthecameltohaveamultiplestomachthantohavetheseteeth。Itsstomach，then，isconstructedlikethatofanimalswithoutupperfrontteeth，and，itsdentalarrangementsbeingsuchastomatchitsstomach，theteethinquestionarewanting。Theywouldindeedbeofnoservice。Itsfood，moreover，beingofathornycharacter，anditstonguenecessarilymadeofafleshysubstance，natureusestheearthymatterwhichissavedfromtheteethtogivehardnesstothepalate。Thecamelruminateslikethehornedanimals，becauseitsmultiplestomachresemblestheirs。Forallanimalsthathavehorns，thesheepforinstance，theox，thegoat，thedeer，andthelike，haveseveralstomachs。Forsincethemouth，owingtoitslackofteeth，onlyimperfectlyperformsitsofficeasregardsthefood，thismultiplicityofstomachsisintendedtomakeupforitsshortcomings；theseveralcavitiesreceivingthefoodonefromtheotherinsuccession；thefirsttakingtheunreducedsubstances，thesecondthesamewhensomewhatreduced，thethirdwhenreductioniscomplete，andthefourthwhenthewholehasbecomeasmoothpulp。Suchisthereasonwhythereisthismultiplicityofpartsandcavitiesinanimalswithsuchdentition。Thenamesgiventotheseveralcavitiesarethepaunch，thehoneycombbag，themanyplies，andthereed。Howthesepartsarerelatedtoeachother，inpositionandinshape，mustbelookedforinthetreatisesonAnatomyandtheResearchesconcerningAnimals。
Birdsalsopresentvariationsinthepartwhichactsasarecipientofthefood；andthereasonforthesevariationsisthesameasintheanimalsjustmentioned。Forhereagainitisbecausethemouthfailstoperformitsofficeandfailsevenmorecompletely-forbirdshavenoteethatall，noranyinstrumentwhatsoeverwithwhichtocomminuteorgrinddowntheirfood-itis，Isay，becauseofthis，thatinsomeofthemwhatiscalledthecropprecedesthestomachanddoestheworkofthemouth；whileinotherstheoesophagusiseitherwidethroughoutorapartofitbulgesjustbeforeitentersthestomach，soastoformapreparatorystore-housefortheunreducedfood；orthestomachitselfhasaprotuberanceinsomepart，orisstrongandfleshy，soastobeabletostoreupthefoodforaconsiderableperiodandtoconcoctit，inspiteofitsnothavingbeengroundintoapulp。Fornatureretrievestheinefficiencyofthemouthbyincreasingtheefficiencyandheatofthestomach。Otherbirdsthereare，such，namely，ashavelonglegsandliveinmarshes，thathavenoneoftheseprovisions，butmerelyanelongatedoesophagus。Theexplanationofthisistobefoundinthemoistcharacteroftheirfood。Forallthesebirdsfeedonsubstanceseasyofreduction，andtheirfoodbeingmoistandnotrequiringmuchconcoction，theirdigestivecavitiesareofacorrespondingcharacter。
Fishesareprovidedwithteeth，whichinalmostallofthemareofthesharpinterfittingkind。Forthereisbutonesmallsectioninwhichitisotherwise。OfthesethefishcalledScarus（Parrot-fish）
isanexample。Andthisisprobablythereasonwhythisfishapparentlyruminates，thoughnootherfishesdoso。Forthosehornedanimalsthathavenofrontteethintheupperjawalsoruminate。
Infishestheteethareallsharp；sothattheseanimalscandividetheirfood，thoughimperfectly。Foritisimpossibleforafishtolingerorspendtimeintheactofmastication，andthereforetheyhavenoteeththatareflatorsuitableforgrinding；forsuchteethwouldbetonopurpose。Theoesophagusagaininsomefishesisentirelywanting，andintherestisbutshort。Inorder，however，tofacilitatetheconcoctionofthefood，someofthem，astheCestreus（mullet），haveafleshystomachresemblingthatofabird；
whilemostofthemhavenumerousprocessescloseagainstthestomach，toserveasasortofantechamberinwhichthefoodmaybestoredupandundergoputrefactionandconcoction。Thereiscontrastbetweenfishesandbirdsinthepositionoftheseprocesses。Forinfishestheyareplacedclosetothestomach；whileinbirds，ifpresentatall，theyarelowerdown，neartheendofthegut。SomeoftheViviparaalsohaveprocessesconnectedwiththelowerpartofthegutwhichservethesamepurposeasthatstatedabove。
Thewholetribeoffishesisofgluttonousappetite，owingtothearrangementsforthereductionoftheirfoodbeingveryimperfect，andmuchofitconsequentlypassingthroughthemwithoutundergoingconcoction；and，ofall，thosearethemostgluttonousthathaveastraightintestine。Forasthepassageoffoodinsuchcasesisrapid，andtheenjoymentderivedfromitinconsequencebutbrief，itfollowsofnecessitythatthereturnofappetiteisalsospeedy。
Ithasalreadybeenmentionedthatinanimalswithfrontteethinbothjawsthestomachisofsmallsize。Itmaybeclassedprettynearlyalwaysunderoneorotheroftwoheadings，namelyasresemblingthestomachofthedog，orasresemblingthestomachofthepig。Inthepigthestomachislargerthaninthedog，andpresentscertainfoldsofmoderatesize，thepurposeofwhichistolengthenouttheperiodofconcoction；whilethestomachofthedogisofsmallsize，notmuchlargerincalibrethanthegut，andsmoothontheinternalsurface。
Notmuchlarger，Isay，thanthegut；forinallanimalsafterthestomachcomesthegut。This，likethestomach，presentsnumerousmodifications。Forinsomeanimalsitisuniform，whenuncoiled，andalikethroughout，whileinothersitdiffersindifferentportions。
Thusinsomecasesitiswiderintheneighbourhoodofthestomach，andnarrowertowardstheotherend；andthisexplainsbythewaywhydogshavetostrainsomuchindischargingtheirexcrement。Butinmostanimalsitistheupperportionthatisthenarrowerandthelowerthatisofgreaterwidth。
Ofgreaterlengththaninotheranimals，andmuchconvoluted，aretheintestinesofthosethathavehorns。Theseintestines，moreover，asalsothestomach，areofamplervolume，inaccordancewiththelargersizeofthebody。Foranimalswithhornsare，asarule，animalsofnosmallbulk，becauseofthethoroughelaborationwhichtheirfoodundergoes。Thegut，exceptinthoseanimalswhereitisstraight，invariablywidensoutaswegetfartherfromthestomachandcometowhatiscalledthecolon，andtoakindofcaecaldilatation。Afterthisitagainbecomesnarrowerandconvoluted。
Thensucceedsastraightportionwhichrunsrightontothevent。Thisventisknownastheanus，andisinsomeanimalssurroundedbyfat，inothersnotso。Allthesepartshavebeensocontrivedbynatureastoharmonizewiththevariousoperationsthatrelatetothefoodanditsresidue。For，astheresidualfoodgetsfartheronandlowerdown，thespacetocontainitenlarges，allowingittoremainstationaryandundergoconversion。Thusisitinthoseanimalswhich，owingeithertotheirlargesize，ortotheheatofthepartsconcerned，requiremorenutriment，andconsumemorefodderthantherest。
Neitherisitwithoutapurpose，that，justasanarrowergutsucceedstotheupperstomach，soalsodoestheresidualfood，whenitsgoodnessisthoroughlyexhausted，passfromthecolonandtheamplespaceofthelowerstomachintoanarrowerchannelandintothespiralcoil。Forsonaturecanregulateherexpenditureandpreventtheexcrementalresiduefrombeingdischargedallatonce。
Inallsuchanimals，however，ashavetobecomparativelymoderateintheiralimentation，thelowerstomachpresentsnowideandroomyspaces，thoughtheirgutisnotstraight，buthasanumberofconvolutions。Foramplitudeofspacecausesdesireforamplefood，andstraightnessoftheintestinecausesquickreturnofappetite。Andthusitisthatallanimalswhosefoodreceptaclesareeithersimpleorspaciousareofgluttonoushabits，thelattereatingenormouslyatameal，theformermakingmealsatshortintervals。
Again，sincethefoodintheupperstomach，havingjustbeenswallowed，mustofnecessitybequitefresh，whilethatwhichhasreachedthelowerstomachmusthavehaditsjuicesexhaustedandresembledung，itfollowsofnecessitythattheremustalsobesomeintermediatepart，inwhichthechangemaybeeffected，andwherethefoodwillbeneitherperfectlyfreshnoryetdung。Andthusitisthat，inallsuchanimalsaswearenowconsidering，thereisfoundwhatiscalledthejejunum；whichisapartofthesmallgut，ofthegut，thatis，whichcomesnexttothestomach。Forthisjejunumliesbetweentheuppercavitywhichcontainstheyetunconcoctedfoodandthelowercavitywhichholdstheresidualmatter，whichbythetimeithasgotherehasbecomeworthless。Thereisajejunuminalltheseanimals，butitisonlyplainlydiscernibleinthoseoflargesize，andthisonlywhentheyhaveabstainedfromfoodforacertaintime。
Forthenalonecanonehitontheexactperiodwhenthefoodlieshalf-waybetweentheupperandlowercavities；aperiodwhichisveryshort，forthetimeoccupiedinthetransitionoffoodisbutbrief。Infemalesthisjejunummayoccupyanypartwhatsoeveroftheupperintestine，butinmalesitcomesjustbeforethecaecumandthelowerstomach。
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Whatisknownasrennetisfoundinallanimalsthathaveamultiplestomach，andinthehareamonganimalswhosestomachissingle。Intheformertherennetneitheroccupiesthelargepaunch，northehoneycombbag，northeterminalreed，butisfoundinthecavitywhichseparatesthisterminalonefromthetwofirst，namelyintheso-calledmanyplies。Itisthethickcharacteroftheirmilkwhichcausesalltheseanimalstohaverennet；whereasinanimalswithasinglestomachthemilkisthin，andconsequentlynorennetisformed。Itisthisdifferenceinthicknesswhichmakesthemilkofhornedanimalscoagulate，whilethatofanimalswithouthornsdoesnot。Rennetformsintheharebecauseitfeedsonherbagethathasjuicelikethatofthefig；forjuiceofthiskindcoagulatesthemilkinthestomachofthesucklings。WhyitisinthemanypliesthatrennetisformedinanimalswithmultiplestomachshasbeenstatedintheProblems。
BookIV
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THEaccountwhichhasnowbeengivenoftheviscera，thestomach，andtheotherseveralpartsholdsequallygoodnotonlyfortheoviparousquadrupeds，butalsoforsuchapodousanimalsastheSerpents。Thesetwoclassesofanimalsareindeednearlyakin，aserpentresemblingalizardwhichhasbeenlengthenedoutanddeprivedofitsfeet。Fishes，again，resemblethesetwogroupsinalltheirparts，exceptingthat，whilethese，beinglandanimals，havealung，fisheshavenolung，butgillsinitsplace。Noneoftheseanimals，exceptingthetortoise，asalsonofish，hasaurinarybladder。Forowingtothebloodlessnessoftheirlung，theydrinkbutsparingly；
andsuchfluidastheyhaveisdivertedtothescalyplates，asinbirdsitisdivertedtothefeathers，andthustheycometohavethesamewhitematteronthesurfaceoftheirexcrementasweseeonthatofbirds。Forinanimalsthathaveabladder，itsexcretionwhenvoidedthrowsdownadepositofearthybrineinthecontainingvessel。Forthesweetandfreshelements，beinglight，areexpendedontheflesh。
AmongtheSerpents，thesamepeculiarityattachestovipers，asamongfishesattachestoSelachia。Forboththeseandvipersareexternallyviviparous，butpreviouslyproduceovainternally。
Thestomachinalltheseanimalsissingle，justasitissingleinallotheranimalsthathaveteethinfrontofbothjaws；andtheirvisceraareexcessivelysmall，asalwayshappenswhenthereisnobladder。Inserpentsthesevisceraare，moreover，differentlyshapedfromthoseofotheranimals。For，aserpent’sbodybeinglongandnarrow，itscontentsareasitweremouldedintoasimilarform，andthuscometobethemselveselongated。
Allanimalsthathavebloodpossessanomentum，amesentery，intestineswiththeirappendages，and，moreover，adiaphragmandaheart；andall，exceptingfishes，alungandawindpipe。Therelativepositions，moreover，ofthewindpipeandtheoesophagusarepreciselysimilarinthemall；andthereasonisthesameashasalreadybeengiven。
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Almostallsanguineousanimalshaveagall-bladder。Insomethisisattachedtotheliver，inothersseparatedfromthatorganandattachedtotheintestines，beingapparentlyinthelattercasenolessthanintheformeranappendageofthelowerstomach。Itisinfishesthatthisismostclearlyseen。Forallfisheshaveagall-bladder；andinmostofthemitisattachedtotheintestine，beinginsome，asintheAmia，unitedwiththis，likeaborder，alongitswholelength。ItissimilarlyplacedinmostserpentsTherearethereforenogoodgroundsfortheviewentertainedbysomewriters，thatthegallexistsforthesakeofsomesensoryaction。Fortheysaythatitsuseistoaffectthatpartofthesoulwhichislodgedintheneighbourhoodoftheliver，vexingthispartwhenitiscongealed，andrestoringittocheerfulnesswhenitagainflowsfree。Butthiscannotbe。Forinsomeanimalsthereisabsolutelynogall-bladderatall——inthehorse，forinstance，themule，theass，thedeer，andtheroe；andinothers，asthecamel，thereisnodistinctbladder，butmerelysmallvesselsofabiliarycharacter。
Again，thereisnosuchorganintheseal，nor，ofpurelysea-animals，inthedolphin。Evenwithinthelimitsofthesamegenus，someanimalsappeartohaveandotherstobewithoutit。Such，forinstance，isthecasewithmice；suchalsowithman。Forinsomeindividualsthereisadistinctgall-bladderattachedtotheliver，whileinothersthereisnogall-bladderatall。Thisexplainshowtheexistenceofthispartinthewholegenushasbeenamatterofdispute。Foreachobserver，accordingashehasfounditpresentorabsentintheindividualcaseshehasexamined，hassupposedittobepresentorabsentinthewholegenus。Thesamehasoccurredinthecaseofsheepandofgoats。Fortheseanimalsusuallyhaveagall-bladder；
but，whileinsomelocalitiesitissoenormouslybigastoappearamonstrosity，asisthecaseinNaxos，inothersitisaltogetherwanting，asisthecaseinacertaindistrictbelongingtotheinhabitantsofChalcisinEuboea。Moreover，thegall-bladderinfishesisseparated，asalreadymentioned，byaconsiderableintervalfromtheliver。NolessmistakenseemstobetheopinionofAnaxagorasandhisfollowers，thatthegall-bladderisthecauseofacutediseases，inasmuchasitbecomesover-full，andspirtsoutitsexcessontothelung，theblood-vessels，andtheribs。For，almostinvariably，thosewhosufferfromtheseformsofdiseasearepersonswhohavenogall-bladderatall，aswouldbequiteevidentweretheytobedissected。Moreover，thereisnokindofcorrespondencebetweentheamountofbilewhichispresentinthesediseasesandtheamountwhichisexuded。Themostprobableopinionisthat，asthebilewhenitispresentinanyotherpartofthebodyisamereresiduumoraproductofdecay，soalsowhenitispresentintheregionoftheliveritisequallyexcrementalandhasnofurtheruse；justasisthecasewiththedejectionsofthestomachandintestines。Forthougheventheresiduaareoccasionallyusedbynatureforsomeusefulpurpose，yetwemustnotinallcasesexpecttofindsuchafinalcause；forgrantedtheexistenceinthebodyofthisorthatconstituent，withsuchandsuchproperties，manyresultsmustensuemerelyasnecessaryconsequencesoftheseproperties。Allanimals，then，whoseishealthyincompositionandsuppliedwithnonebutsweetblood，areeitherentirelywithoutagall-bladderonthisorgan，orhavemerelysmallbile-containingvessels；oraresomewithandsomewithoutsuchparts。Thusitisthattheliverinanimalsthathavenogall-bladderis，asarule，ofgoodcolourandsweet；andthat，whenthereisagall-bladder，thatpartoftheliverissweetestwhichliesimmediatelyunderneathit。But，whenanimalsareformedofbloodlesspureincomposition，thebileservesfortheexcretionofitsimpureresidue。Fortheverymeaningofexcrementisthatitistheoppositeofnutriment，andofbitterthatitistheoppositeofsweet；andhealthybloodissweet。Sothatitisevidentthatthebile，whichisbitter，cannothaveanyuse，butmustsimplybeapurifyingexcretion。Itwasthereforenobadsayingofoldwritersthattheabsenceofagall-bladdergavelonglife。Insosayingtheyhadinminddeerandanimalswithsolidhoofs。
Forsuchhavenogall-bladderandlivelong。Butbesidesthesethereareotheranimalsthathavenogall-bladder，thoughthoseoldwritershadnotnoticedthefact，suchasthecamelandthedolphin；
andthesealsoare，asithappens，long-lived。Seeing，indeed，thattheliverisnotonlyuseful，butanecessaryandvitalpartinallanimalsthathaveblood，itisbutreasonablethatonitscharactershoulddependthelengthortheshortnessoflife。Norlessreasonableisitthatthisorganandnoneothershouldhavesuchanexcretionasthebile。Fortheheart，unableasitistostandanyviolentaffection，wouldbeutterlyintolerantoftheproximityofsuchafluid；and，astotherestoftheviscera，noneexceptingtheliverarenecessarypartsofananimal。Itistheliverthereforethatalonehasthisprovision。Inconclusion，whereverweseebilewemusttakeittobeexcremental。Fortosupposethatithasonecharacterinthispart，anotherinthat，wouldbeasgreatanabsurdityastosupposemucusorthedejectionsofthestomachtovaryincharacteraccordingtolocalityandnottobeexcrementalwhereverfound。
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Somuchthenofthegall-bladder，andofthereasonswhysomeanimalshaveone，whileothershavenot。Wehavestilltospeakofthemesenteryandtheomentum；fortheseareassociatedwiththepartsalreadydescribedandcontainedinthesamecavity。Theomentum，then，isamembranecontainingfat；thefatbeingsuetorlard，accordingasthefatoftheanimalgenerallyisoftheformerorlatterdescription。Whatkindsofanimalsaresodistinguishedhasbeenalreadysetforthinanearlierpartofthistreatise。Thismembrane，alikeinanimalsthathaveasingleandinthosethathaveamultiplestomach，growsfromthemiddleofthatorgan，alongalinewhichismarkedonitlikeaseam。Thusattached，itcoverstherestofthestomachandthegreaterpartofthebowels，andthisalikeinallsanguineousanimals，whethertheyliveonlandorinwater。Nowthedevelopmentofthispartintosuchaformashasbeendescribedistheresultofnecessity。For，wheneversolidandfluidaremixedtogetherandheated，thesurfaceinvariablybecomesmembranousandskin-like。Buttheregioninwhichtheomentumliesisfullofnutrimentofsuchamixedcharacter。Moreover，inconsequenceoftheclosetextureofthemembrane，thatportionofthesanguineousnutrimentwillalonefilterintoitwhichisofagreasycharacter；
forthisportioniscomposedofthefinestparticles；andwhenithassofilteredin，itwillbeconcoctedbytheheatofthepart，andwillbeconvertedintosuetorlard，andwillnotacquireaflesh-likeorsanguineousconstitution。Thedevelopment，then，oftheomentumissimplytheresultofnecessity。Butwhenonceformed，itisusedbynatureforanend，namely，tofacilitateandtohastentheconcoctionoffood。Forallthatishotaidsconcoction；andfatishot，andtheomentumisfat。Thistooexplainswhyithangsfromthemiddleofthestomach；fortheupperpartofthestomachhasnoneedofit，beingassistedinconcoctionbytheadjacentliver。Thusmuchasconcernstheomentum。
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Theso-calledmesenteryisalsoamembrane；andextendscontinuouslyfromthelongstretchofintestinetothegreatvesselandtheaorta。Initarenumerousandclose-packedvessels，whichrunfromtheintestinestothegreatvesselandtotheaorta。Theformationofthismembraneweshallfindtobetheresultofnecessity，asisthatoftheother[similar]parts。What，however，isthefinalcauseofitsexistenceinsanguineousanimalsismanifestonreflection。Foritisnecessarythatanimalsshallgetnutrimentfromwithout；and，again，thatthisshallbeconvertedintotheultimatenutriment，whichisthendistributedassustenancetothevariousparts；thisultimatenutrimentbeing，insanguineousanimals，whatwecallblood，andhaving，inbloodlessanimals，nodefinitename。Thisbeingso，theremustbechannelsthroughwhichthenutrimentshallpass，asitwerethroughroots，fromthestomachintotheblood-vessels。Nowtherootsofplantsareintheground；forthencetheirnutrimentisderived。Butinanimalsthestomachandintestinesrepresentthegroundfromwhichthenutrimentistobetaken。Themesentery，then，isanorgantocontaintheroots；andtheserootsarethevesselsthattraverseit。Thisthenisthefinalcauseofitsexistence。Buthowitabsorbsnutriment，andhowthatportionofthefoodwhichentersintothevesselsisdistributedbythemtothevariouspartsofthebody，arequestionswhichwillbeconsideredwhenwecometodealwiththegenerationandnutritionofanimals。
Theconstitutionofsanguineousanimals，sofarasthepartsasyetmentionedareconcerned，andthereasonsforsuchconstitution，havenowbeensetforth。Innaturalsequenceweshouldnextgoontotheorgansofgeneration，asyetundescribed，onwhichdependthedistinctionsofmaleandfemale。But，inasmuchasweshallhavetodealspeciallywithgenerationhereafter，itwillbemoreconvenienttodefertheconsiderationofthesepartstothatoccasion。
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VerydifferentfromtheanimalswehaveasyetconsideredaretheCephalopodaandtheCrustacea。Forthesehaveabsolutelynoviscerawhatsoever；asisindeedthecasewithallbloodlessanimals，inwhichareincludedtwoothergenera，namelytheTestaceaandtheInsects。
Forinnoneofthemdoesthematerialoutofwhichvisceraareformedexist。Noneofthem，thatis，haveblood。Thecauseofthisliesintheiressentialconstitution。Forthepresenceofbloodinsomeanimals，itsabsencefromothers，mustbeincludedintheconceptionwhichdeterminestheirrespectiveessences。Moreover，intheanimalswearenowconsidering，noneofthosefinalcauseswillbefoundtoexistwhichinsanguineousanimalsdeterminethepresenceofviscera。Fortheyhavenobloodvesselsnorurinarybladder，nordotheybreathe；theonlypartthatitisnecessaryforthemtohavebeingthatwhichisanalogoustoaheart。Forinallanimalstheremustbesomecentralandcommandingpartofthebody，tolodgethesensoryportionofthesoulandthesourceoflife。Theorgansofnutritionarealsoofnecessitypresentinthemall。Theydiffer，however，incharacterbecauseofdifferencesofthehabitatsinwhichtheygettheirsubsistence。
IntheCephalopodatherearetwoteeth，enclosingwhatiscalledthemouth；andinsidethismouthisaflesh-likesubstancewhichrepresentsatongueandservesforthediscriminationofpleasantandunpleasantfood。TheCrustaceahaveteethcorrespondingtothoseoftheCephalopoda，namelytheiranteriorteeth，andalsohavethefleshyrepresentativeofatongue。Thislatterpartisfound，moreover，inallTestacea，andserves，asinsanguineousanimals，forgustatorysensations。SimilarlyprovidedalsoaretheInsects。Forsomeofthese，suchastheBeesandtheFlies，have，asalreadydescribed，theirproboscisprotrudingfromthemouth；whilethoseothersthathavenosuchinstrumentinfronthaveapartwhichactsasatongueinsidethemouth。Such，forinstance，isthecaseintheAntsandthelike。Asforteeth，someinsectshavethem，theBeesandtheAntsforinstance，thoughinasomewhatmodifiedform，whileothersthatliveonfluidnutrimentarewithoutthem。Forinmanyinsectstheteetharenotmeanttodealwiththefood，buttoserveasweapons。
InsomeTestacea，aswassaidinthefirsttreatise，theorganwhichiscalledthetongueisofconsiderablestrength；andintheCochli（Sea-snails）therearealsotwoteeth，justasintheCrustacea。ThemouthintheCephalopodaissucceededbyalonggullet。Thisleadstoacrop，likethatofabird，anddirectlycontinuouswiththisisthestomach，fromwhichagutrunswithoutwindingstothevent。TheSepiasandthePoulpsresembleeachothercompletely，sofarasregardstheshapeandconsistencyoftheseparts。ButnotsotheTeuthides（Calamaries）。Here，asintheothergroupstherearethetwostomach-likereceptacles；butthefirstofthesecavitieshaslessresemblancetoacrop，andinneitheristheform[ortheconsistency]
thesameasintheotherkinds，thewholebodyindeedbeingmadeofasofterkindofflesh。
TheobjectofthisarrangementofthepartsinquestionisthesameintheCephalopodaasinBirds；forthesealsoareallunabletomasticatetheirfood；andthereforeitisthatacropprecedestheirstomach。
Forpurposesofdefence，andtoenablethemtoescapefromtheirfoes，theCephalopodahavewhatiscalledtheirink。Thisiscontainedinamembranouspouch，whichisattachedtothebodyandprovidedwithaterminaloutletjustatthepointwherewhatistermedthefunnelgivesissuetotheresiduaofthestomach。Thisfunnelisplacedontheventralsurfaceoftheanimal。AllCephalopodaalikehavethischaracteristicink，butchiefofalltheSepia，whereitismoreabundantthanintherest。Whentheanimalisdisturbedandfrighteneditusesthisinktomakethesurroundingwaterblackandturbid，andso，asitwere，putsashieldinfrontofitsbody。