on the gait of animals-第5部分

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are　opposed　in　this　respect　to　the　upper；　because　the　first　joints　are



opposites；　the　shoulder　bending　forwards；　the　hip　backwards；　wherefore



also　the　ankle　bends　backwards；　and　the　wrist　of　the　hand　forwards。



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　　This　is　the　way　then　the　limbs　bend；　and　for　the　reasons　given。



But　the　hind　limbs　move　criss…cross　with　the　fore　limbs；　after　the　off



fore　they　move　the　near　hind；　then　the　near　fore；　and　then　the　off



hind。　The　reason　is　that　（a）　if　they　moved　the　forelegs　together　and



first；　the　animal　would　be　wrenched；　and　the　progression　would　be　a



stumbling　forwards　with　the　hind　parts　as　it　were　dragged　after。



Again；　that　would　not　be　walking　but　jumping；　and　it　is　hard　to　make　a



continuous　change　of　place；　jumping　all　the　time。　Here　is　evidence



of　what　I　say；　even　as　it　is；　all　horses　that　move　in　this　way　soon



begin　to　refuse；　for　example　the　horses　in　a　religious　procession。　For



these　reasons　the　fore　limbs　and　the　hind　limbs　move　in　this



separate　way。　Again；　（b）　if　they　moved　both　the　right　legs　first　the



weight　would　be　outside　the　supporting　limbs　and　they　would　fall。　If



then　it　is　necessary　to　move　in　one　or　other　of　these　ways　or



criss…cross　fashion；　and　neither　of　these　two　is　satisfactory；　they



must　move　criss…cross；　for　moving　in　the　way　we　have　said　they



cannot　possibly　experience　either　of　these　untoward　results。　And



this　is　why　horses　and　such…like　animals　stand　still　with　their　legs



put　forward　criss…cross；　not　with　the　right　or　the　left　put　forward



together　at　once。　In　the　same　fashion　animals　with　more　than　four　legs



make　their　movements；　if　you　take　two　consecutive　pairs　of　legs　the



hind　move　criss…cross　with　the　forelegs；　you　can　see　this　if　you　watch



them　moving　slowly。　Even　crabs　move　in　this　way；　and　they　are



polypods。　They；　too；　always　move　criss…cross　in　whichever　direction



they　are　making　progress。　For　in　direction　this　animal　has　a



movement　all　its　own；　it　is　the　only　animal　that　moves　not　forwards；



but　obliquely。　Yet　since　forwards　is　a　distinction　relative　to　the



line　of　vision；　Nature　has　made　its　eyes　able　to　conform　to　its　limbs；



for　its　eyes　can　move　themselves　obliquely；　and　therefore　after　a



fashion　crabs　are　no　exception　but　in　this　sense　move　forwards。



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　　Birds　bend　their　legs　in　the　same　way　as　quadrupeds。　For　their



natural　construction　is　broadly　speaking　nearly　the　same。　That　is；



in　birds　the　wings　are　a　substitute　for　the　forelegs；　and　so　they



are　bent　in　the　same　way　as　the　forelegs　of　a　quadruped；　since　when



they　move　to　progress　the　natural　beginning　of　change　is　from　the



wings　（as　in　quadrupeds　from　the　forelegs）。　Flight　in　fact　is　their



appropriate　movement。　And　so　if　the　wings　be　cut　off　a　bird　can



neither　stand　still　nor　go　forwards。



　　Again；　the　bird　though　a　biped　is　not　erect；　and　has　the　forward



parts　of　the　body　lighter　than　the　hind；　and　so　it　is　necessary　（or　at



least　preferable　for　the　standing　posture）　to　have　the　thigh　so　placed



below　the　body　as　it　actually　is；　I　mean　growing　towards　the　back。



If　then　it　must　have　this　situation　the　flexion　of　the　leg　must　be



backwards；　as　in　the　hind　legs　of　quadrupeds。　The　reasons　are　the　same



as　those　given　in　the　case　of　viviparous　quadrupeds。



　　If　now　we　survey　generally　birds　and　winged　insects；　and　animals



which　swim　in　a　watery　medium；　all　I　mean　that　make　their　progress



in　water　by　dint　of　organs　of　movement；　it　is　not　difficult　to　see



that　it　is　better　to　have　the　attachment　of　the　parts　in　question



oblique　to　the　frame；　exactly　as　in　fact　we　see　it　to　be　both　in　birds



and　insects。　And　this　same　arrangement　obtains　also　among　fishes。



Among　birds　the　wings　are　attached　obliquely；　so　are　the　fins　in　water



animals；　and　the　feather…like　wings　of　insects。　In　this　way　they



divide　the　air　or　water　most　quickly　and　with　most　force　and　so　effect



their　movement。　For　the　hinder　parts　in　this　way　would　follow　forwards



as　they　are　carried　along　in　the　yielding　medium；　fish　in　the　water；



birds　in　the　air。



　　Of　oviparous　quadrupeds　all　those　that　live　in　holes；　like



crocodiles；　lizards；　spotted　lizards；　freshwater　tortoises；　and



turtles；　have　their　legs　attached　obliquely　as　their　whole　body



sprawls　over　the　ground；　and　bend　them　obliquely。　The　reason　is　that



this　is　useful　for　ease　in　creeping　into　holes；　and　for　sitting　upon



their　eggs　and　guarding　them。　And　as　they　are　splayed　outwards　they



must　of　necessity　tuck　in　their　thighs　and　put　them　under　them　in



order　to　achieve　the　lifting　of　the　whole　body。　In　view　of　this　they



cannot　bend　them　otherwise　than　outwards。



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　　We　have　already　stated　the　fact　that　non…sanguineous　animals　with



limbs　are　polypods　and　none　of　them　quadrupeds。　And　the　reason　why



their　legs；　except　the　extreme　pairs；　were　necessarily　attached



obliquely　and　had　their　flexions　upwards；　and　the　legs　themselves　were



somewhat　turned　under　（bandy…shape）　and　backwards　is　plain。　In　all



such　creatures　the　intermediate　legs　both　lead　and　follow。　If　then



they　lay　under　them；　they　must　have　had　their　flexion　both　forwards



and　backwards；　on　account　of　leading；　forwards；　and　on　account　of



following；　backwards。　Now　since　they　have　to　do　both；　for　this



reason　their　limbs　are　turned　under　and　bent　obliquely；　except　the　two



extreme　pairs。　（These　two　are　more　natural　in　their　movement；　the



front　leading　and　the　back　following。）　Another　reason　for　this　kind　of



flexion　is　the　number　of　their　legs；　arranged　in　this　way　they　would



interfere　less　with　one　another　in　progression　and　not　knock　together。



But　the　reason　that　they　are　bandy　is　that　all　of　them　or　most　of　them



live　in　holes；　for　creatures　living　so　cannot　possibly　be　high　above



the　ground。



　　But　crabs　are　in　nature　the　oddest　of　all　polypods；　they　do　not



progress　forwards　except　in　the　sense　explained　above；　they　are　the



only　animals　which　have　more　than　one　pair　of　leading　limbs。　The



explanation　of　this　is　the　hardness　of　their　limbs；　and　the　fact



that　they　use　them　not　for　swimming　but　for　walking；　they　always



keep　on　the　ground。　However；　the　flexion　of　the　limbs　of　all



polypods　is　oblique；　like　that　of　the　quadrupeds　which　live　in



holes…for　example　lizards　and　crocodiles　and　most　of　the　oviparous



quadrupeds。　And　the　explanation　is　that　some　of　them　in　their　breeding



periods；　and　some　all　their　life；　live　in　holes。



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　　Now　the　rest　have　bandy　legs　because　they　are　soft…skinned；　but



the　crayfish　is　hard…skinned　and　its　limbs　are　for　swimming　and　not



for　walking　（and　so　are　not　bandy）。　Crabs；　too；　have　their　limbs



bent　obliquely；　but　not　bandy　like　oviparous　quadrupeds　and



non…sanguineous　polypods；　because　their　limbs　have　a　hard　and



shell…like　skin；　although　they　don't　swim　but　live　in　holes；　they　live



in　fact　on　the　ground。　Moreover；　their　shape　is　like　a　disk；　as



compared　with　the　crayfish　which　is　elongated；　and　they　haven't　a　tail



like　the　crayfish；　a　tail　is　useful　to　the　crayfish　for　swimming；



but　the　crab　is　not　a　swimming　creature。　Further；　it　alone　has　its



side　equivalent　to　a　hinder　part；　because　it　has　many　leading　feet。



The　explanation　of　this　is　that　its　flexions　are　not　forward　nor　its



legs　turned　in　under　（bandy）。　We　have　given　above　the　reason　why　its



legs　are　not　turned　in　under；　that　is　the　hardness　and　shell…like



character　of　its　integument。



　　For　these　reasons　then　it　must　lead　off　with　more　than　one　limb；　and



move　obliquely；　obliquely；　because　the　flexion　is　oblique；　and　with



more　than　one　limb；　because　otherwise　the　limbs　that　were　still



would　have　got　in　the　way　of　those　that　were　moving。



　　Fishes　of　the　flat　kind　swim　with　their　heads　twisted；　as　one…eyed



men　walk；　they　have　their　natural　shape　distorted。　Web…footed　birds



swim　with　their　feet；　because　they　breath　the　air　and　have　lungs



they　are　bipeds；　but　because　they　have　their　home　in　the　water　they



are　webbed；　by　this　arrangement　their　feet　serve　them　instead　of　fins。



They　have　their　legs　too；　not　like　the　rest　of　birds　in　the　centre



of　their　body；　but　rather　set　back。　Their　legs　are　short；　and　being



set　back　are　serviceable　for　swimming。　The　reason　for　their　having



short　legs　is　that　nature　has　added　to　their　feet　by　subtracting



from　the　length　of　their　limbs；　instead　of　length　she　gives



stoutness　to　the　legs　and　breadth　to　the　feet。　Broad　feet　are　more



useful　than　long　for　pushing　away　the　water　when　they　are　swimming。



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　　There　is　reason；　too；　for　winged　creatures　having　feet；　but　fish



none。　The　former　have　their　home　in　the　dry　medium；　and　cannot



remain　always