9
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。