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Thereisaresemblancebetweentheosseousandthevascularsystems；
foreachhasacentralpartinwhichitbegins，andeachformsacontinuouswhole。Fornoboneinthebodyexistsasaseparatethinginitself，buteachiseitheraportionofwhatmaybeconsideredacontinuouswhole，oratanyrateislinkedwiththerestbycontactandbyattachments；sothatnaturemayuseadjoiningboneseitherasthoughtheywereactuallycontinuousandformedasinglebone，or，forpurposesofflexure，asthoughtheyweretwoanddistinct。Andsimilarlynoblood-vesselhasinitselfaseparateindividuality；
buttheyallformpartsofonewhole。Foranisolatedbone，ifsuchtherewere，wouldinthefirstplacebeunabletoperformtheofficeforthesakeofwhichbonesexist；for，wereitdiscontinuousandseparatedfromtherestbyagap，itwouldbeperfectlyunabletoproduceeitherflexureorextension；noronlyso，butitwouldactuallybeinjurious，actinglikeathornoranarrowlodgedintheflesh。Similarlyifavesselwereisolated，andnotcontinuouswiththevascularcentre，itwouldbeunabletoretainthebloodwithinitinaproperstate。Foritisthewarmthderivedfromthiscentrethathindersthebloodfromcoagulating；indeedtheblood，whenwithdrawnfromitsinfluence，becomesmanifestlyputrid。Nowthecentreororiginoftheblood-vesselsistheheart，andthecentreororiginofthebones，inallanimalsthathavebones，iswhatiscalledthechine。Withthisalltheotherbonesofthebodyareincontinuity；foritisthechinethatholdstogetherthewholelengthofananimalandpreservesitsstraightness。Butsinceitisnecessarythatthebodyofananimalshallbendduringlocomotion，thischine，whileitisoneinvirtueofthecontinuityofitsparts，yetitsdivisionintovertebraeismadetoconsistofmanysegments。Itisfromthischinethatthebonesofthelimbs，insuchanimalsashavetheseparts，proceed，andwithittheyarecontinuous，beingfastenedtogetherbythesinewswherethelimbsadmitofflexure，andhavingtheirextremitiesadaptedtoeachother，eitherbytheonebeinghollowedandtheotherrounded，orbybothbeinghollowedandincludingbetweenthemahucklebone，asaconnectingbolt，soastoallowofflexureandextension。Forwithoutsomesucharrangementthesemovementswouldbeutterlyimpossible，oratanyratewouldbeperformedwithgreatdifficulty。Therearesomejoints，again，inwhichthelowerendoftheoneboneandtheupperendoftheotherarealikeinshape。Inthesecasesthebonesareboundtogetherbysinews，andcartilaginouspiecesareinterposedinthejoint，toserveasakindofpadding，andpreventthetwoextremitiesfromgratingagainsteachother。
Roundaboutthebones，andattachedtothembythinfibrousbands，growthefleshyparts，forthesakeofwhichthebonesthemselvesexist。Forjustasanartist，whenheismouldingananimaloutofclayorothersoftsubstance，takesfirstsomesolidbodyasabasis，androundthismouldstheclay，soalsohasnatureactedinfashioningtheanimalbodyoutofflesh。Thuswefindallthefleshyparts，withoneexception，supportedbybones，whichserve，whenthepartsareorgansofmotion，tofacilitateflexure，and，whenthepartsaremotionless，actasaprotection。Theribs，forexample，whichenclosethechestareintendedtoensurethesafetyoftheheartandneighbouringviscera。Theexceptionofwhichmentionwasmadeisthebelly。Thewallsofthisareinallanimalsdevoidofbones；inorderthattheremaybenohindrancetotheexpansionwhichnecessarilyoccursinthispartafterameal，nor，infemales，anyinterferencewiththegrowthofthefoetus，whichislodgedhere。
Nowthebonesofviviparousanimals，ofsuch，thatis，asarenotmerelyexternallybutalsointernallyviviparous，varybutverylittlefromeachotherinpointofstrength，whichinallofthemisconsiderable。FortheViviparaintheirbodilyproportionsarefaraboveotheranimals，andmanyofthemoccasionallygrowtoanenormoussize，asisthecaseinLibyaandinhotanddrycountriesgenerally。Butthegreaterthebulkofananimal，thestronger，thebigger，andtheharder，arethesupportswhichitrequires；andcomparingthebiganimalswitheachother，thisrequirementwillbemostmarkedinthosethatlivealifeofrapine。Thusitisthatthebonesofmalesareharderthanthoseoffemales；andthebonesofflesh-eaters，thatgettheirfoodbyfighting，areharderthanthoseofHerbivora。OfthistheLionisanexample；forsohardareitsbones，that，whenstruck，theygiveoffsparks，asthoughtheywerestones。ItmaybementionedalsothattheDolphin，inasmuchasitisviviparous，isprovidedwithbonesandnotwithfish-spines。
Inthosesanguineousanimals，ontheotherhand，thatareoviparous，thebonespresentsuccessiveslightvariationsofcharacter。ThusinBirdstherearebones，butthesearenotsostrongasthebonesoftheVivipara。ThencometheOviparousfishes，wherethereisnobone，butmerelyfish-spine。IntheSerpentstootheboneshavethecharacteroffish-spine，exceptingintheverylargespecies，wherethesolidfoundationofthebodyrequirestobestronger，inorderthattheanimalitselfmaybestrong，thesamereasonprevailingasinthecaseoftheVivipara。Lastly，intheSelachia，astheyarecalled，thefish-spinesarereplacedbycartilage。Foritisnecessarythatthemovementsoftheseanimalsshallbeofanundulatingcharacter；
andthisagainrequirestheframeworkthatsupportsthebodytobemadeofapliableandnotofabrittlesubstance。Moreover，intheseSelachianaturehasusedalltheearthymatterontheskin；andsheisunabletoallottomanydifferentpartsoneandthesamesuperfluityofmaterial。Eveninviviparousanimalsmanyofthebonesarecartilaginous。Thishappensinthosepartswhereitistotheadvantageofthesurroundingfleshthatitssolidbaseshallbesoftandmucilaginous。Such，forinstance，isthecasewiththeearsandnostrils；forinprojectingparts，suchasthese，brittlesubstanceswouldsoongetbroken。Cartilageandboneareindeedfundamentallythesamething，thedifferencesbetweenthembeingmerelymattersofdegree。Thusneithercartilagenorbone，whenoncecutoff，growsagain。Nowthecartilagesoftheselandanimalsarewithoutmarrow，thatiswithoutanydistinctlyseparatemarrow。Forthemarrow，whichinbonesisdistinctlyseparate，isheremixedupwiththewholemass，andgivesasoftandmucilaginousconsistencetothecartilage。ButintheSelachiathechine，thoughitiscartilaginous，yetcontainsmarrow；forhereitstandsinthesteadofabone。
Verynearlyresemblingthebonestothetoucharesuchpartsasnails，hoofs，whethersolidorcloven，horns，andthebeaksofbirds，allofwhichareintendedtoserveasmeansofdefence。Fortheorganswhicharemadeoutofthesesubstances，andwhicharecalledbythesamenamesasthesubstancesthemselves，theorganhoof，forinstance，andtheorganhorn，arecontrivancestoensurethepreservationoftheanimalstowhichtheyseverallybelong。Inthisclasstoomustbereckonedtheteeth，whichinsomeanimalshavebutasinglefunction，namelythemasticationofthefood，whileinotherstheyhaveanadditionaloffice，namelytoserveasweapons；asisthecasewithallanimalsthathavesharpinterfittingteethorthathavetusks。Allthesepartsarenecessarilyofsolidandearthycharacter；forthevalueofaweapondependsonsuchproperties。Theirearthycharacterexplainshowitisthatallsuchpartsaremoredevelopedinfour-footedviviparathaninman。Forthereisalwaysmoreearthinthecompositionoftheseanimalsthaninthatofthehumanbody。However，notonlyallthesepartsbutsuchothersasarenearlyconnectedwiththem，skinforinstance，bladder，membrane，hairs，feathers，andtheiranalogues，andanyothersimilarpartsthattheremaybe，willbeconsideredfartheronwiththeheterogeneousparts。Thereweshallinquireintothecauseswhichproducethem，andintotheobjectsoftheirpresenceseverallyinthebodiesofanimals。For，aswiththeheterogeneousparts，sowiththese，itisfromaconsiderationoftheirfunctionsthatalonewecanderiveanyknowledgeofthem。Thereasonfordealingwiththematallinthispartofthetreatise，andclassifyingthemwiththehomogeneousparts，isthatunderoneandthesamenameareconfoundedtheentireorgansandthesubstancesofwhichtheyarecomposed。Butofallthesesubstancesfleshandboneformthebasis。Semenandmilkwerealsopassedoverwhenwewereconsideringthehomogeneousfluids。ForthetreatiseonGenerationwillaffordamoresuitableplacefortheirexamination，seeingthattheformerofthetwoistheveryfoundationofthethinggenerated，whilethelatterisitsnourishment。
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Letusnowmake，asitwere，afreshbeginning，andconsidertheheterogeneousparts，takingthosefirstwhicharethefirstinimportance。Forinallanimals，atleastinalltheperfectkinds，therearetwopartsmoreessentialthantherest，namelythepartwhichservesfortheingestionoffood，andthepartwhichservesforthedischargeofitsresidue。Forwithoutfoodgrowthandevenexistenceisimpossible。Interveningagainbetweenthesetwopartsthereisinvariablyathird，inwhichislodgedthevitalprinciple。
Asforplants，thoughtheyalsoareincludedbyusamongthingsthathavelife，yetaretheywithoutanypartforthedischargeofwasteresidue。Forthefoodwhichtheyabsorbfromthegroundisalreadyconcocted，andtheygiveoffasitsequivalenttheirseedsandfruits。
Plants，again，inasmuchastheyarewithoutlocomotion，presentnogreatvarietyintheirheterogeneousparts。For，wherethefunctionsarebutfew，fewalsoaretheorgansrequiredtoeffectthem。Theconfigurationofplantsisamatterthenforseparateconsideration。
Animals，however，thatnotonlylivebutfeel，presentagreatermultiformityofparts，andthisdiversityisgreaterinsomeanimalsthaninothers，beingmostvariedinthosetowhosesharehasfallennotmerelifebutlifeofhighdegree。Nowsuchananimalisman。
Forofalllivingbeingswithwhichweareacquaintedmanalonepartakesofthedivine，oratanyratepartakesofitinafullermeasurethantherest。Forthisreason，then，andalsobecausehisexternalpartsandtheirformsaremorefamiliartousthanthoseofotheranimals，wemustspeakofmanfirst；andthisthemorefitly，becauseinhimalonedothenaturalpartsholdthenaturalposition；
hisupperpartbeingturnedtowardsthatwhichisupperintheuniverse。For，ofallanimals，manalonestandserect。
Inman，then，theheadisdestituteofflesh；thisbeingthenecessaryconsequenceofwhathasalreadybeenstatedconcerningthebrain。Thereare，indeed，somewhoholdthatthelifeofman-wouldbelongerthanitis，werehisheadmoreabundantlyfurnishedwithflesh；andtheyaccountfortheabsenceofthissubstancebysayingthatitisintendedtoaddtotheperfectionofsensation。Forthebraintheyasserttobetheorganofsensation；andsensation，theysay，cannotpenetratetopartsthataretoothicklycoveredwithflesh。Butneitherpartofthisstatementistrue。Onthecontrary，weretheregionofthebrainthicklycoveredwithflesh，theverypurposeforwhichanimalsareprovidedwithabrainwouldbedirectlycontravened。Forthebrainwoulditselfbeheatedtoexcessandsounabletocoolanyotherpart；and，astotheotherhalfoftheirstatement，thebraincannotbethecauseofanyofthesensations，seeingthatitisitselfasutterlywithoutfeelingasanyoneoftheexcretions。Thesewritersseethatcertainofthesensesarelocatedinthehead，andareunabletodiscernthereasonforthis；theyseealsothatthebrainisthemostpeculiarofalltheanimalorgans；andoutofthesefactstheyformanargument，bywhichtheylinksensationandbraintogether。Ithas，however，alreadybeenclearlysetforthinthetreatiseonSensation，thatitistheregionoftheheartthatconstitutesthesensorycentre。Therealsoitwasstatedthattwoofthesenses，namelytouchandtaste，aremanifestlyinimmediateconnexionwiththeheart；andthatasregardstheotherthree，namelyhearing，sight，andthecentrallyplacedsenseofsmell，itisthecharacteroftheirsense-organswhichcausesthemtobelodgedasaruleinthehead。Visionissoplacedinallanimals。Butsuchisnotinvariablythecasewithhearingorwithsmell。Forfishesandthelikehearandsmell，andyethavenovisibleorgansforthesesensesinthehead；afactwhichdemonstratestheaccuracyoftheopinionheremaintained。Nowthatvision，wheneveritexists，shouldbeintheneighbourhoodofthebrainisbutwhatonewouldrationallyexpect。Forthebrainisfluidandcold，andvisionisofthenatureofwater，waterbeingofalltransparentsubstancestheonemosteasilyconfined。Moreoveritcannotbutnecessarilybethatthemoreprecisesenseswillhavetheirprecisionrenderedstillgreaterifministeredtobypartsthathavethepurestblood。
Forthemotionoftheheatofblooddestroyssensoryactivity。Forthesereasonstheorgansoftheprecisesensesarelodgedinthehead。
Itisnotonlytheforepartoftheheadthatisdestituteofflesh，butthehindpartalso。For，inallanimalsthathaveahead，itisthisheadwhichmorethananyotherpartrequirestobeheldup。
But，weretheheadheavilyladenwithflesh，thiswouldbeimpossible；
fornothingsoburdenedcanbeheldupright。Thisisanadditionalproofthattheabsenceoffleshfromtheheadhasnoreferencetobrainsensation。Forthereisnobraininthehinderpartofthehead，andyetthisisasmuchwithoutfleshasisthefront。
Insomeanimalshearingaswellasvisionislodgedintheregionofthehead。Noristhiswithoutarationalexplanation。Forwhatiscalledtheemptyspaceisfullofair，andtheorganofhearingis，aswesay，ofthenatureofair。Nowtherearechannelswhichleadfromtheeyestotheblood-vesselsthatsurroundthebrain；andsimilarlythereisachannelwhichleadsbackagainfromeachearandconnectsitwiththehinderpartofthehead。Butnopartthatiswithoutbloodisendowedwithsensation，asneitheristheblooditself，butonlysomeoneofthepartsthatareformedofblood。
Thebraininallanimalsthathaveoneisplacedinthefrontpartofthehead；becausethedirectioninwhichsensationactsisinfront；andbecausetheheart，fromwhichsensationproceeds，isinthefrontpartofthebody；andlastlybecausetheinstrumentsofsensationaretheblood-containingparts，andthecavityintheposteriorpartoftheskullisdestituteofblood-vessels。
Astothepositionofthesense-organs，theyhavebeenarrangedbynatureinthefollowingwell-orderedmanner。Theorgansofhearingaresoplacedastodividethecircumferenceoftheheadintotwoequalhalves；fortheyhavetohearnotonlysoundswhicharedirectlyinlinewiththemselves，butsoundsfromallquarters。Theorgansofvisionareplacedinfront，becausesightisexercisedonlyinastraightline，andmovingaswedoinaforwarddirectionitisnecessarythatweshouldseebeforeus，inthedirectionofourmotion。Lastly，theorgansofsmellareplacedwithgoodreasonbetweentheeyes。Forasthebodyconsistsoftwoparts，arighthalfandaleft，soalsoeachorganofsenseisdouble。Inthecaseoftouchthisisnotapparent，thereasonbeingthattheprimaryorganofthissenseisnotthefleshoranalogouspart，butliesinternally。Inthecaseoftaste，whichismerelyamodificationoftouchandwhichisplacedinthetongue，thefactismoreapparentthaninthecaseoftouch，butstillnotsomanifestasinthecaseoftheothersenses。
However，evenintasteitisevidentenough；forinsomeanimalsthetongueisplainlyforked。Thedoublecharacterofthesensationsis，however，moreconspicuousintheotherorgansofsense。Fortherearetwoearsandtwoeyes，andthenostrils，thoughjoinedtogether，arealsotwo。Weretheselatterotherwisedisposed，andseparatedfromeachotherasaretheears，neithertheynorthenoseinwhichtheyareplacedwouldbeabletoperformtheiroffice。Forinsuchanimalsashavenostrilsolfactioniseffectedbymeansofinspiration，andtheorganofinspirationisplacedinfrontandinthemiddleline。Thisisthereasonwhynaturehasbroughtthetwonostrilstogetherandplacedthemasthecentralofthethreesense-organs，settingthemsidebysideonalevelwitheachother，toavailthemselvesoftheinspiratorymotion。Inotheranimalsthanmanthearrangementofthesesense-organsisalsosuchasisadaptedineachcasetothespecialrequirements。
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Forinstance，inquadrupedstheearsstandoutfreelyfromtheheadandaresettoallappearanceabovetheeyes。Notthattheyareinrealityabovetheeyes；buttheyseemtobeso，becausetheanimaldoesnotstanderect，buthasitsheadhungdownwards。Thisbeingtheusualattitudeoftheanimalwheninmotion，itisofadvantagethatitsearsshallbehighupandmovable；forbyturningthemselvesabouttheycanthebettertakeinsoundsfromeveryquarter。
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Inbirds，ontheotherhand，therearenoears，butonlytheauditorypassages。Thisisbecausetheirskinishardandbecausetheyhavefeathersinsteadofhairs，sothattheyhavenotgotthepropermaterialfortheformationofears。Exactlythesameisthecasewithsuchoviparousquadrupedsasarecladwithscalyplates，andthesameexplanationappliestothem。Thereisalsooneoftheviviparousquadrupeds，namelytheseal，thathasnoearsbutonlytheauditorypassages。Theexplanationofthisisthattheseal，thoughaquadruped，isaquadrupedofstuntedformation。
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Men，andBirds，andQuadrupeds，viviparousandoviparousalike，havetheireyesprotectedbylids。IntheViviparatherearetwoofthese；andbothareusedbytheseanimalsnotonlyinclosingtheeyes，butalsointheactofblinking；whereastheoviparousquadrupeds，andtheheavy-bodiedbirdsaswellassomeothers，useonlythelowerlidtoclosetheeye；whilebirdsblinkbymeansofamembranethatissuesfromthecanthus。Thereasonfortheeyesbeingthusprotectedisthatnaturehasmadethemoffluidconsistency，inordertoensurekeennessofvision。Forhadtheybeencoveredwithhardskin，theywould，itistrue，havebeenlessliabletogetinjuredbyanythingfallingintothemfromwithout，buttheywouldnothavebeensharp-sighted。Itisthentoensurekeennessofvisionthattheskinoverthepupilisfineanddelicate；whilethelidsaresuperaddedasaprotectionfrominjury。Itisasastillfurthersafeguardthatalltheseanimalsblink，andmanmostofall；thisaction（whichisnotperformedfromdeliberateintentionbutfromanaturalinstinct）servingtokeepobjectsfromfallingintotheeyes；andbeingmorefrequentinmanthanintherestoftheseanimals，becauseofthegreaterdelicacyofhisskin。Theselidsaremadeofarollofskin；anditisbecausetheyaremadeofskinandcontainnofleshthatneitherthey，northesimilarlyconstructedprepuce，uniteagainwhenoncecut。
Astotheoviparousquadrupeds，andsuchbirdsasresembletheminclosingtheeyewiththelowerlid，itisthehardnessoftheskinoftheirheadswhichmakesthemdoso。Forsuchbirdsashaveheavybodiesarenotmadeforflight；andsothematerialswhichwouldotherwisehavegonetoincreasethegrowthofthefeathersaredivertedthence，andusedtoaugmentthethicknessoftheskin。
Birdsthereforeofthiskindclosetheeyewiththelowerlid；whereaspigeonsandthelikeusebothupperandlowerlidsforthepurpose。Asbirdsarecoveredwithfeathers，sooviparousquadrupedsarecoveredwithscalyplates；andtheseinalltheirformsareharderthanhairs，sothattheskinalsotowhichtheybelongisharderthantheskinofhairyanimals。Intheseanimals，then，theskinontheheadishard，andsodoesnotallowoftheformationofanuppereyelid，whereaslowerdowntheintegumentisofaflesh-likecharacter，sothatthelowerlidcanbethinandextensible。
Theactofblinkingisperformedbytheheavy-bodiedbirdsbymeansofthemembranealreadymentioned，andnotbythislowerlid。
Forinblinkingrapidmotionisrequired，andsuchisthemotionofthismembrane，whereasthatofthelowerlidisslow。Itisfromthecanthusthatisnearesttothenostrilsthatthemembranecomes。Foritisbettertohaveonestarting-pointfornictitationthantwo；
andinthesebirdsthisstarting-pointisthejunctionofeyeandnostrils，ananteriorstarting-pointbeingpreferabletoalateralone。Oviparousquadrupedsdonotblinkinlikemannerasthebirds；
for，livingastheydoontheground，theyarefreefromthenecessityofhavingeyesoffluidconsistencyandofkeensight，whereastheseareessentialrequisitesforbirds，inasmuchastheyhavetousetheireyesatlongdistances。Thistooexplainswhybirdswithtalons，thathavetosearchforpreybyeyefromaloft，andthereforesoartogreaterheightsthanotherbirds，aresharpsighted；whilecommonfowlsandthelike，thatliveonthegroundandarenotmadeforflight，havenosuchkeennessofvision。Forthereisnothingintheirmodeoflifewhichimperativelyrequiresit。
FishesandInsectsandthehard-skinnedCrustaceapresentcertaindifferencesintheireyes，butsofarresembleeachotherasthatnoneofthemhaveeyelids。Asforthehard-skinnedCrustaceaitisutterlyoutofthequestionthattheyshouldhaveany；foraneyelid，tobeofuse，requirestheactionoftheskintoberapid。
Theseanimalsthenhavenoeyelidsand，indefaultofthisprotection，theireyesarehard，justasthoughthelidwereattachedtothesurfaceoftheeye，andtheanimalsawthroughit。Inasmuch，however，assuchhardnessmustnecessarilybluntthesharpnessofvision，naturehasendowedtheeyesofInsects，andstillmorethoseofCrustacea，withmobility（justasshehasgivensomequadrupedsmovableears），inorderthattheymaybeabletoturntothelightandcatchitsrays，andsoseemoreplainly。Fishes，however，haveeyesofafluidconsistency。Foranimalsthatmovemuchabouthavetousetheirvisionatconsiderabledistances。Ifnowtheyliveonland，theairinwhichtheymoveistransparentenough。Butthewaterinwhichfishesliveisahindrancetosharpsight，thoughithasthisadvantageovertheair，thatitdoesnotcontainsomanyobjectstoknockagainsttheeyes。Theriskofcollisionbeingthussmall，nature，whomakesnothinginvain，hasgivennoeyelidstofishes，whiletocounterbalancetheopacityofthewatershehasmadetheireyesoffluidconsistency。
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Allanimalsthathavehairsonthebodyhavelashesontheeyelids；butbirdsandanimalswithscale-likeplates，beinghairless，havenone。TheLibyanostrich，indeed，formsanexception；for，thoughabird，itisfurnishedwitheyelashes。Thisexception，however，willbeexplainedhereafter。Ofhairyanimals，manalonehaslashesonbothlids。Forinquadrupedsthereisagreaterabundanceofhaironthebackthanontheundersideofthebody；whereasinmanthecontraryisthecase，andthehairismoreabundantonthefrontsurfacethanontheback。Thereasonforthisisthathairisintendedtoserveasaprotectiontoitspossessor。Now，inquadrupeds，owingtotheirinclinedattitude，theunderoranteriorsurfacedoesnotrequiresomuchprotectionastheback，andisthereforeleftcomparativelybald，inspiteofitsbeingthenoblerofthetwosides。
Butinman，owingtohisuprightattitude，theanteriorandposteriorsurfacesofthebodyareonanequalityasregardsneedofprotection。Naturethereforehasassignedtheprotectivecoveringtothenoblerofthetwosurfaces；forinvariablyshebringsaboutthebestarrangementofsuchasarepossible。Thisthenisthereasonthatthereisnolowereyelashinanyquadruped；thoughinsomeafewscatteredhairssproutoutunderthelowerlid。Thisalsoisthereasonthattheyneverhavehairintheaxillae，noronthepubes，asmanhas。Theirhair，then，insteadofbeingcollectedintheseparts，iseitherthicklysetoverthewholedorsalsurface，asisthecaseforinstanceindogs，or，sometimes，formsamane，asinhorsesandthelike，orasinthemalelionwherethemaneisstillmoreflowingandample。So，again，wheneverthereisatailofanylength，naturedecksitwithhair，withlonghairifthestemofthetailbeshort，asinhorses，withshorthairifthestembelong，regardalsobeinghadtotheconditionoftherestofthebody。Fornatureinvariablygivestoonepartwhatshesubtractsfromanother。
Thuswhenshehascoveredthegeneralsurfaceofananimal’sbodywithanexcessofhair，sheleavesadeficiencyintheregionofthetail。This，forinstance，inthecasewithbears。
Noanimalhassomuchhairontheheadasman。This，inthefirstplace，isthenecessaryresultofthefluidcharacterofhisbrain，andofthepresenceofsomanysuturesinhisskull。Forwhereverthereisthemostfluidandthemostheat，therealsomustnecessarilyoccurthegreatestoutgrowth。But，secondly，thethicknessofthehairinthisparthasafinalcause，beingintendedtoprotectthehead，bypreservingitfromexcessofeitherheatorcold。Andasthebrainofmanislargerandmorefluidthanthatofanyotheranimal，itrequiresaproportionatelygreateramountofprotection。Forthemorefluidasubstanceis，themorereadilydoesitgetexcessivelyheatedorexcessivelychilled，whilesubstancesofanoppositecharacterarelessliabletosuchinjuriousaffections。
These，however，arematterswhichbytheircloseconnexionwitheyelasheshaveledustodigressfromourrealtopic，namelythecausetowhichtheselashesowetheirexistence。Wemustthereforedeferanyfurtherremarkswemayhavetomakeonthesematterstilltheproperoccasionarisesandthenreturntotheirconsideration。
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Botheyebrowsandeyelashesexistfortheprotectionoftheeyes；
theformerthattheymayshelterthem，liketheeavesofahouse，fromanyfluidsthattrickledownfromthehead；thelattertoactlikethepalisadeswhicharesometimesplacedinfrontofenclosures，andkeepoutanyobjectswhichmightotherwisegetin。Thebrowsareplacedoverthejunctionoftwobones，whichisthereasonthatinoldagetheyoftenbecomesobushyastorequirecutting。Thelashesaresetattheterminationsofsmallblood-vessels。Forthevesselscometoanendwheretheskinitselfterminates；and，inallplaceswheretheseendingsoccur，theexudationofmoistureofacorporealcharacternecessitatesthegrowthofhairs，unlesstherebesomeoperationofnaturewhichinterferes，bydivertingthemoisturetoanotherpurpose。
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Viviparousquadrupeds，asarule，presentnogreatvarietyofformintheorganofsmell。Inthoseofthem，however，whosejawsprojectforwardsandtapertoanarrowend，soastoformwhatiscalledasnout，thenostrilsareplacedinthisprojection，therebeingnootheravailableplan；while，intherest，thereisamoredefinitedemarcationbetweennostrilsandjaws。Butinnoanimalisthispartsopeculiarasintheelephant，whereitattainsanextraordinaryandstrength。Fortheelephantusesitsnostrilasahand；thisbeingtheinstrumentwithwhichitconveysfood，fluidandsolidalike，toitsmouth。Withit，too，ittearsuptrees，coilingitroundtheirstems。Infactitappliesitgenerallytothepurposesofahand。Fortheelephanthasthedoublecharacterofalandanimal，andofonethatlivesinswamps。Seeingthenthatithastogetitsfoodfromthewater，andyetmustnecessarilybreathe，inasmuchasitisalandanimalandhasblood；seeing，also，thatitsexcessiveweightpreventsitfrompassingrapidlyfromwatertoland，assomeothersanguineousviviparathatbreathecando，itbecomesnecessarythatitshallbesuitedalikeforlifeinthewaterandforlifeondryland。justthenasdiversaresometimesprovidedwithinstrumentsforrespiration，throughwhichtheycandrawairfromabovethewater，andthusmayremainforalongtimeunderthesea，soalsohaveelephantsbeenfurnishedbynaturewiththeirlengthenednostril；and，whenevertheyhavetotraversethewater，theyliftthisupabovethesurfaceandbreathethroughit。Fortheelephant’sproboscis，asalreadysaid，isanostril。Nowitwouldhavebeenimpossibleforthisnostriltohavetheformofaproboscis，haditbeenhardandincapableofbending。Foritsverylengthwouldthenhavepreventedtheanimalfromsupplyingitselfwithfood，beingasgreatanimpedimentastheofcertainoxen，thataresaidtobeobligedtowalkbackwardswhiletheyaregrazing。Itisthereforesoftandflexible，and，beingsuch，ismade，inadditiontoitsownproperfunctions，toservetheofficeofthefore-feet；natureinthisfollowingherwontedplanofusingoneandthesamepartforseveralpurposes。Forinpolydactylousquadrupedsthefore-feetareintendednotmerelytosupporttheweightofthebody，buttoserveashands。
Butinelephants，thoughtheymustbereckonedpolydactylous，astheirfoothasneitherclovennorsolidhoof，thefore-feet，owingtothegreatsizeandweightofthebody，arereducedtotheconditionofmeresupports；andindeedtheirslowmotionandunfitnessforbendingmakethemuselessforanyotherpurpose。Anostril，then，isgiventotheelephantforrespiration，astoeveryotheranimalthathasalung，andislengthenedoutandendowedwithitspowerofcoilingbecausetheanimalhastoremainforconsiderableperiodsoftimeinthewater，andisunabletopassthencetodrygroundwithanyrapidity。Butasthefeetareshornoftheirfulloffice，thissamepartisalso，asalreadysaid，madebynaturetosupplytheirplace，andgivesuchhelpasotherwisewouldberenderedbythem。
Astoothersanguineousanimals，theBirds，theSerpents，andtheOviparousquadrupeds，inallofthemtherearethenostril-holes，placedinfrontofthemouth；butinnonearethereanydistinctlyformednostrils，nothinginfactwhichcanbecallednostrilsexceptfromafunctionalpointofview。Abirdatanyratehasnothingwhichcanproperlybecalledanose。Foritsso-calledbeakisasubstituteforjaws。Thereasonforthisistobefoundinthenaturalconformationofbirds。Fortheyarewingedbipeds；andthismakesitnecessarythattheirheadsandneckshallbeoflightweight；justasitmakesitnecessarythattheirbreastshallbenarrow。Thebeakthereforewithwhichtheyareprovidedisformedofabone-likesubstance，inorderthatitmayserveasaweaponaswellasfornutritivepurposes，butismadeofnarrowdimensionstosuitthesmallsizeofthehead。Inthisbeakareplacedtheolfactorypassages。
Buttherearenonostrils；forsuchcouldnotpossiblybeplacedthere。
Asforthoseanimalsthathavenorespiration，ithasalreadybeenexplainedwhyitisthattheyarewithoutnostrils，andperceiveodourseitherthroughgills，orthroughablowhole，or，iftheyareinsects，bythehypozoma；andhowthepowerofsmellingdepends，liketheirmotion，upontheinnatespiritoftheirbodies，whichinallofthemisimplantedbynatureandnotintroducedfromwithout。
Underthenostrilsarethelips，insuchsanguineousanimals，thatis，ashaveteeth。Forinbirds，asalreadyhasbeensaid，thepurposesofnutritionanddefencearefulfilledbyabonelikebeak，whichformsacompoundsubstituteforteethandlips。Forsupposingthatoneweretocutoffaman’slips，unitehisupperteethtogether，andsimilarlyhisunderones，andthenweretolengthenoutthetwoseparatepiecesthusformed，narrowingthemoneithersideandmakingthemprojectforwards，supposing，Isay，thistobedone，weshouldatoncehaveabird-likebeak。
Theuseofthelipsinallanimalsexceptmanistopreserveandguardtheteeth；andthusitisthatthedistinctnesswithwhichthelipsareformedisindirectproportiontothedegreeofnicetyandperfectionwithwhichtheteetharefashioned。Inmanthelipsaresoftandflesh-likeandcapableofseparatingfromeachother。Theirpurpose，asinotheranimals，istoguardtheteeth，buttheyaremoreespeciallyintendedtoserveahigheroffice，contributingincommonwithotherpartstoman’sfacultyofspeech。Forjustasnaturehasmademan’stongueunlikethatofotheranimals，and，inaccordancewithwhatIhavesaidishernotuncommonpractice，hasuseditfortwodistinctoperations，namelyfortheperceptionofsavoursandforspeech，soalsohassheactedwithregardtothelips，andmadethemservebothforspeechandfortheprotectionoftheteeth。Forvocalspeechconsistsofcombinationsoftheletters，andmostofthesewouldbeimpossibletopronounce，werethelipsnotmoist，northetonguesuchasitis。Forsomelettersareformedbyclosuresofthelipsandothersbyapplicationsofthetongue。Butwhatarethedifferencespresentedbytheseandwhatthenatureandextentofsuchdifferences，arequestionstowhichanswersmustbesoughtfromthosewhoareversedinmetricalscience。Itwasnecessarythatthetwopartswhichwearediscussingshould，inconformitywiththerequirements，beseverallyadaptedtofulfiltheofficementionedabove，andbeofappropriatecharacter。Thereforearetheymadeofflesh，andfleshissofterinmanthaninanyotheranimal，thereasonforthisbeingthatofallanimalsmanhasthemostdelicatesenseoftouch。
17
Thetongueisplacedunderthevaultedroofofthemouth。Inlandanimalsitpresentsbutlittlediversity。Butinotheranimalsitisvariable，andthiswhetherwecomparethemasaclasswithsuchasliveonland，orcomparetheirseveralspecieswitheachother。Itisinmanthatthetongueattainsitsgreatestdegreeoffreedom，ofsoftness，andofbreadth；theobjectofthisbeingtorenderitsuitableforitsdoublefunction。Foritssoftnessfitsitfortheperceptionofsavours，asensewhichismoredelicateinmanthaninanyotheranimal，softnessbeingmostimpressionablebytouch，ofwhichsensetasteisbutavariety。Thissamesoftnessagain，togetherwithitsbreadth，adaptsitforthearticulationoflettersandforspeech。Forthesequalities，combinedwithitsfreedomfromattachment，arethosewhichsuititbestforadvancingandretiringineverydirection。Thatthisissoisplain，ifweconsiderthecaseofthosewhoaretongue-tiedinhoweverslightadegree。Fortheirspeechisindistinctandlisping；thatistosaytherearecertainletterswhichtheycannotpronounce。Inbeingbroadiscomprisedthepossibilityofbecomingnarrow；forinthegreatthesmallisincluded，butnotthegreatinthesmall。
Whathasbeensaidexplainswhy，amongbirds，thosethataremostcapableofpronouncinglettersaresuchashavethebroadesttongues；andwhytheviviparousandsanguineousquadrupeds，wherethetongueishardandthickandnotfreeinitsmotions，haveaverylimitedvocalarticulation。Somebirdshaveaconsiderablevarietyofnotes。Thesearethesmallerkinds。Butitisthebirdswithtalonsthathavethebroadertongues。Allbirdsusetheirtonguestocommunicatewitheachother。Butsomedothisinagreaterdegreethantherest；sothatinsomecasesitevenseemsasthoughactualinstructionwereimpartedfromonetoanotherbyitsagency。These，however，arematterswhichhavealreadybeendiscussedintheResearchesconcerningAnimals。
Astothoseoviparousandsanguineousanimalsthatlivenotintheairbutontheearth，theirtongueinmostcasesistieddownandhard，andisthereforeuselessforvocalpurposes；intheserpents，however，andinthelizardsitislongandforked，soastobesuitedfortheperceptionofsavours。Solongindeedisthispartinserpents，thatthoughsmallwhileinthemouthitcanbeprotrudedtoagreatdistance。Intheseanimalsitisforkedandhasafineandhair-likeextremity，becauseoftheirgreatlikingfordaintyfood。Forbythisarrangementtheyderiveatwofoldpleasurefromsavours，theirgustatorysensationbeingasitweredoubled。
Evensomebloodlessanimalshaveanorganthatservesfortheperceptionofsavours；andinsanguineousanimalssuchanorganisinvariablyvariablyForeveninsuchoftheseaswouldseemtoanordinaryobservertohavenothingofthekind，someofthefishesforexample，thereisakindofshabbyrepresentativeofatongue，muchlikewhatexistsinrivercrocodiles。Inmostofthesecasestheapparentabsenceofthepartcanberationallyexplainedonsomegroundorother。Forinthefirstplacetheinteriorofthemouthinanimalsofthischaracterisinvariablyspinous。Secondly，inwateranimalsthereisbutshortspaceoftimefortheperceptionofsavours，andastheuseofthissenseisthusofshortduration，shortenedalsoistheseparatepartwhichsubservesit。Thereasonfortheirfoodbeingsorapidlytransmittedtothestomachisthattheycannotpossiblyspendanytimeinsuckingoutthejuices；forweretheytoattempttodoso，thewaterwouldmakeitswayinduringtheprocess。Unlessthereforeonepullstheirmouthverywidelyopen，theprojectionofthispartisquiteinvisible。Theregionexposedbythusopeningthemouthisspinous；foritisformedbythecloseappositionofthegills，whichareofaspinouscharacter。
Incrocodilestheimmobilityofthelowerjawalsocontributesinsomemeasuretostuntthedevelopmentofthetongue。Forthecrocodile’stongueisadherenttothelowerjaw。Foritsupperandlowerjawsare，asitwere，inverted，itbeingtheupperjawwhichinotheranimalsistheimmovableone。Thetongue，however，onthisanimalisnotattachedtotheupperjaw，becausethatwouldinterferewiththeingestionoffood，butadherestothelowerjaw，becausethisis，asitwere，theupperonewhichhaschangeditsplace。Moreover，itisthecrocodile’slot，thoughalandanimal，tolivethelifeofafish，andthisagainnecessarilyinvolvesanindistinctformationofthepartinquestion。
Theroofofthemouthresemblesflesh，eveninmanyofthefishes；
andinsomeoftheriverspecies，asforinstanceinthefishesknownasCyprini，issoveryflesh-likeandsoftastobetakenbycarelessobserversforatongue。Thetongueoffishes，however，thoughitexistsasaseparatepart，isneverformedwithsuchdistinctnessasthis，ashasbeenalreadyexplained。Again，asthegustatorysensibilityisintendedtoserveanimalsintheselectionoffood，itisnotdiffusedequallyoverthewholesurfaceofthetongue-likeorgan，butisplacedchieflyinthetip；andforthisreasonitisthetipwhichistheonlypartofthetongueseparatedinfishesfromtherestofthemouth。Asallanimalsaresensibletothepleasurederivablefromfood，theyallfeeladesireforit。Fortheobjectofdesireisthepleasant。Thepart，however，bywhichfoodproducesthesensationisnotpreciselyalikeinallofthem，butwhileinsomeitisfreefromattachments，inothers，whereitisnotrequiredforvocalpur，poses，itisadherent。Insomeagainitishard，inotherssoftorflesh-like。ThuseventheCrustacea，theCarabiforinstanceandthelike，andtheCephalopods，suchastheSepiasandthePoulps，havesomesuchpartinsidethemouth。AsfortheInsects，someofthemhavethepartwhichservesastongueinsidethemouth，asisthecasewithants，andasisalsothecasewithmanyTestacea，whileinothersitisplacedexternally。Inthislattercaseitresemblesasting，andishollowandspongy，soastoserveatoneandthesametimeforthetastingandforthesuckingupofnutriment。
Thisisplainlytobeseeninfliesandbeesandallsuchanimals，andlikewiseinsomeoftheTestacea。InthePurpurae，forinstance，sostrongisthispartthatitenablesthemtoboreholesthroughthehardcoveringofshell-fish，ofthespiralsnails，forexample，thatareusedasbaittocatchthem。Soalsothegad-fliesandcattle-fliescanpiercethroughtheskinofman，andsomeofthemeventhroughtheskinsofotheranimals。Such，then，intheseanimalsisthenatureofthetongue，whichisthusasitwerethecounterpartoftheelephant’snostril。Forasintheelephantthenostrilisusedasaweapon，sointheseanimalsthetongueservesasasting。
Inallotheranimalsthetongueagreeswithdescriptionalreadygiven。
BookIII
1
WEhavenexttoconsidertheteeth，andwiththesethemouth，thatisthecavitywhichtheyencloseandform。Theteethhaveoneinvariableoffice，namelythereductionoffood；butbesidesthisgeneralfunctiontheyhaveotherspecialones，andthesedifferindifferentgroups。Thusinsomeanimalstheteethserveasweapons；butthiswithadistinction。Forthereareoffensiveweaponsandtherearedefensiveweapons；andwhileinsomeanimals，asthewildCarnivora，theteethanswerbothpurposes，inmanyothers，bothwildanddomesticated，theyserveonlyfordefence。Inmantheteethareadmirablyconstructedfortheirgeneraloffice，thefrontonesbeingsharp，soastocutthefoodintobits，andthehinderonesbroadandflat，soastogrindittoapulp；whilebetweentheseandseparatingthemarethedog-teeth，which，inaccordancewiththerulethatthemeanpartakesofbothextremes，shareinthecharactersofthoseoneitherside，beingbroadinonepartbutsharpinanother。Similardistinctionsofshapearepresentedbytheteethofotheranimals，withtheexceptionofthosewhoseteethareoneandallofthesharpkind。Inman，however，thenumberandthecharacterevenofthesesharpteethhavebeenmainlydeterminedbytherequirementsofspeech。Forthefrontteethofmancontributeinmanywaystotheformationofletter-sounds。
Insomeanimals，however，theteeth，asalreadysaid，servemerelyforthereductionoffood。When，besidesthis，theyserveasoffensiveanddefensiveweapons，theymayeitherbeformedintotusks，asforinstanceisthecaseinswine，ormaybesharp-pointedandinterlockwiththoseoftheoppositejaw，inwhichcasetheanimalissaidtobesaw-toothed。Theexplanationofthislatterarrangementisasfollows。
Thestrengthofsuchananimalisinitsteeth，andthesedependfortheirefficiencyontheirsharpness。Inorder，then，topreventtheirgettingbluntedbymutualfriction，suchofthemasserveforweaponsfitintoeachother’sinterspaces，andaresokeptinpropercondition。Noanimalthathassharpinterfittingteethisatthesametimefurnishedwithtusks。Fornaturenevermakesanythingsuperfluousorinvain。Shegives，therefore，tuskstosuchanimalsasstrikeinfighting，andserratedteethtosuchasbite。Sows，forinstance，havenotusks，andaccordinglysowsbiteinsteadofstriking。
Ageneralprinciplemustherebenoted，whichwillbefoundapplicablenotonlyinthisinstancebutinmanyothersthatwilloccurlateron。Natureallotseachweapon，offensiveanddefensivealike，tothoseanimalsalonethatcanuseit；or，ifnottothemalone，totheminamoremarkeddegree；andsheallotsitinitsmostperfectstatetothosethatcanuseitbest；andthiswhetheritbeasting，oraspur，orhorns，ortusks，orwhatitmayofalikekind。
Thusasmalesarestrongerandmorecholericthanfemales，itisinmalesthatsuchpartsasthosejustmentionedarefound，eitherexclusively，asinsomespecies，ormorefullydeveloped，asinothers。Forthoughfemalesareofcourseprovidedwithsuchpartsasarenolessnecessarytothemthantomales，theparts，forinstance，whichsubservenutrition，theyhaveeventheseinaninferiordegree，andthepartswhichanswernosuchnecessarypurposetheydonotpossessatall。Thisexplainswhystagshavehorns，whiledoeshavenone；whythehornsofcowsaredifferentfromthoseofbulls，and，similarly，thehornsofewesfromthoseoframs。Itexplainsalsowhythefemalesareoftenwithoutspursinspecieswherethemalesareprovidedwiththem，andaccountsforsimilarfactsrelatingtoallothersuchparts。
Allfisheshaveteethoftheserratedform，withthesingleexceptionofthefishknownastheScarus。Inmanyofthemthereareteethevenonthetongueandontheroofofthemouth。Thereasonforthisisthat，livingastheydointhewater，theycannotbutallowthisfluidtopassintothemouthwiththefood。Thefluidthusadmittedtheymustnecessarilydischargeagainwithoutdelay。Forweretheynottodoso，buttoretainitforatimewhiletrituratingthefood，thewaterwouldrunintotheirdigestivecavities。Theirteeththereforeareallsharp，beingadaptedonlyforcutting，andarenumerousandsetinmanyparts，thattheirabundancemayserveinlieuofanygrindingfaculty，tomincethefoodintosmallbits。Theyarealsocurved，becausethesearealmosttheonlyweaponswhichfishespossess。
Inalltheseofficesoftheteeththemouthalsotakesitspart；butbesidesthesefunctionsitissubservienttorespiration，inallsuchanimalsasbreatheandarecooledbyexternalagency。Fornature，asalreadysaid，usesthepartswhicharecommontoallanimalsformanyspecialpurposes，andthisofherownaccord。Thusthemouthhasoneuniversalfunctioninallanimalsalike，namelyitsalimentaryoffice；butinsome，besidesthis，thespecialdutyofservingasaweaponisattachedtoit；inothersthatofministeringtospeech；andagaininmany，thoughnotinall，theofficeofrespiration。Allthesefunctionsarethrownbynatureupononesingleorgan，theconstructionofwhichshevariessoastosuitthevariationsofoffice。
Thereforeitisthatinsomeanimalsthemouthiscontracted，whileinothersitisofwidedimensions。Thecontractedformbelongstosuchanimalsasusethemouthmerelyfornutritive，respiratory，andvocalpurposes；whereasinsuchasuseitasameansofdefenceithasawidegape。Thisisitsinvariableforminsuchanimalsasaresaw-toothed。Forseeingthattheirmodeofwarfareconsistsinbiting，itisadvantageoustothemthattheirmouthshallhaveawideopening；
forthewideritopens，thegreaterwillbetheextentofthebite，andthemorenumerouswillbetheteethcalledintoplay。