Sofar，then，ashasyetbeenstated，therelationsbetweenthesetwoordersofpartsaredeterminedbyafinalcause。Wehave，however，toinquirewhethernecessitymaynotalsohaveashareinthematter；anditmustbeadmittedthatthesemutualrelationscouldnotfromtheverybeginninghavepossiblybeenotherthantheyare。
Forheterogeneouspartscanbemadeupoutofhomogeneousparts，eitherfromapluralityofthem，orfromasingleone，asisthecasewithsomeoftheviscerawhich，varyinginconfiguration，areyet，tospeakbroadly，formedfromasinglehomogeneoussubstance；butthathomogeneoussubstancesshouldbeformedoutofacombinationofheterogeneouspartsisclearlyanimpossibility。Forthesecauses，then，somepartsofanimalsaresimpleandhomogeneous，whileothersarecompositeandheterogeneous；anddividingthepartsintotheactiveorexecutiveandthesensitive，eachoneoftheformeris，asbeforesaid，heterogeneous，andeachoneofthelatterhomogeneous。
Foritisinhomogeneouspartsalonethatsensationcanoccur，asthefollowingconsiderationsshow。
Eachsenseisconfinedtoasingleorderofsensibles，anditsorganmustbesuchastoadmittheactionofthatkindororder。Butitisonlythatwhichisendowedwithapropertyinpossethatisactedonbythatwhichhasthelikepropertyinesse，sothatthetwoarethesameinkind，andifthelatterissinglesoalsoistheformer。
Thusitisthatwhilenophysiologistseverdreamofsayingofthehandorfaceorothersuchpartthatoneisearth，anotherwater，anotherfire，theycoupleeachseparatesense-organwithaseparateelement，assertingthisonetobeairandthatothertobefire。
Sensation，then，isconfinedtothesimpleorhomogeneousparts。
But，asmightreasonablybeexpected，theorganoftouch，thoughstillhomogeneous，isyettheleastsimpleofallthesense-organs。Fortouchmorethananyothersenseappearstobecorrelatedtoseveraldistinctkindsofobjects，andtorecognizemorethanonecategoryofcontrasts，heatandcold，forinstance，solidityandfluidity，andothersimilaroppositions。Accordingly，theorganwhichdealswiththesevariedobjectsisofallthesense-organsthemostcorporeal，beingeithertheflesh，orthesubstancewhichinsomeanimalstakestheplaceofflesh。
Nowastherecannotpossiblybeananimalwithoutsensation，itfollowsasanecessaryconsequencethateveryanimalmusthavesomehomogeneousparts；forthesealonearecapableofsensation，theheterogeneouspartsservingfortheactivefunctions。Again，asthesensoryfaculty，themotorfaculty，andthenutritivefacultyarealllodgedinoneandthesamepartofthebody，aswasstatedinaformertreatise，itisnecessarythatthepartwhichistheprimaryseatoftheseprinciplesshallontheonehand，initscharacterofgeneralsensoryrecipient，beoneofthesimpleparts；andontheotherhandshall，initsmotorandactivecharacter，beoneoftheheterogeneousparts。Forthisreasonitistheheartwhichinsanguineousanimalsconstitutesthiscentralpart，andinbloodlessanimalsitisthatwhichtakestheplaceofaheart。Fortheheart，liketheotherviscera，isoneofthehomogeneousparts；for，ifcutup，itspiecesarehomogeneousinsubstancewitheachother。Butitisatthesametimeheterogeneousinvirtueofitsdefiniteconfiguration。Andthesameistrueoftheotherso-calledviscera，whichareindeedformedfromthesamematerialastheheart。Foralltheseviscerahaveasanguineouscharacterowingtotheirbeingsituateduponvascularductsandbranches。Forjustasastreamofwaterdepositsmud，sothevariousviscera，theheartexcepted，are，asitwere，depositsfromthestreamofbloodinthevessels。Andastotheheart，theverystarting-pointofthevessels，andtheactualseatoftheforcebywhichthebloodisfirstfabricated，itisbutwhatonewouldnaturallyexpect，thatoutoftheselfsamenutrimentofwhichitistherecipientitsownpropersubstanceshallbeformed。
Such，then，arethereasonswhythevisceraareofsanguineousaspect；
andwhyinonepointofviewtheyarehomogeneous，inanotherheterogeneous。
2
Ofthehomogeneouspartsofanimals，somearesoftandfluid，othershardandsolid；andoftheformersomearefluidpermanently，othersonlysolongastheyareinthelivingbody。Suchareblood，serum，lard，suet，marrow，semen，bile，milkwhenpresent，flesh，andtheirvariousanalogues。Forthepartsenumeratedarenottobefoundinallanimals，someanimalsonlyhavingpartsanalogoustothem。Ofthehardandsolidhomogeneouspartsbone，fish-spine，sinew，blood-vessel，areexamples。Thelastofthesepointstoasub-divisionthatmaybemadeintheclassofhomogeneousparts。Forinsomeofthemthewholeandaportionofthewholeinonesensearedesignatedbythesameterm-as，forexample，isthecasewithblood-vesselandbitofblood-vessel-whileinanothersensetheyarenot；butaportionofaheterogeneouspart，suchasface，innosensehasthesamedesignationasthewhole。
Thefirstquestiontobeaskediswhatarethecausestowhichthesehomogeneouspartsowetheirexistence？Thecausesarevarious；andthiswhetherthepartsbesolidorfluid。Thusonesetofhomogeneouspartsrepresentthematerialoutofwhichtheheterogeneouspartsareformed；foreachseparateorganisconstructedofbones，sinews，flesh，andthelike；whichareeitheressentialelementsinitsformation，orcontributetotheproperdischargeofitsfunction。Asecondsetarethenutrimentofthefirst，andareinvariablyfluid，forallgrowthoccursattheexpenseoffluidmatter；whileathirdsetaretheresidueofthesecond。Such，forinstance，arethefaecesand，inanimalsthathaveabladder，theurine；theformerbeingthedregsofthesolidnutriment，thelatterofthefluid。
Eventheindividualhomogeneouspartspresentvariations，whichareintendedineachcasetorenderthemmoreserviceablefortheirpurpose。Thevariationsofthebloodmaybeselectedtoillustratethis。Fordifferentbloodsdifferintheirdegreesofthinnessorthickness，ofclearnessorturbidity，ofcoldnessorheat；andthiswhetherwecomparethebloodsfromdifferentpartsofthesameindividualorthebloodsofdifferentanimals。For，intheindividual，allthedifferencesjustenumerateddistinguishthebloodoftheupperandofthelowerhalvesofthebody；and，dealingwithclasses，onesectionofanimalsissanguineous，whiletheotherhasnoblood，butonlysomethingresemblingitinitsplace。Asregardstheresultsofsuchdifferences，thethickerandthehotterbloodis，themoreconduciveisittostrength，whileinproportiontoitsthinnessanditscoldnessisitssuitabilityforsensationandintelligence。Alikedistinctionexistsalsointhefluidwhichisanalogoustoblood。Thisexplainshowitisthatbeesandothersimilarcreaturesareofamoreintelligentnaturethanmanysanguineousanimals；andthat，ofsanguineousanimals，thosearethemostintelligentwhosebloodisthinandcold。Noblestofallarethosewhosebloodishot，andatthesametimethinandclear。Forsucharesuitedalikeforthedevelopmentofcourageandofintelligence。Accordingly，theupperpartsaresuperiorintheserespectstothelower，themalesuperiortothefemale，andtherightsidetotheleft。Aswiththebloodsoalsowiththeotherparts，homogeneousandheterogeneousalike。Forherealsosuchvariationsasoccurmustbeheldeithertoberelatedtotheessentialconstitutionandmodeoflifeoftheseveralanimals，or，inothercases，tobemerelymattersofslightlybetterorslightlyworse。Twoanimals，forinstance，mayhaveeyes。Butinonetheseeyesmaybeoffluidconsistency，whileintheothertheyarehard；andinonetheremaybeeyelids，intheothernosuchappendages。Insuchacase，thefluidconsistencyandthepresenceofeyelids，whichareintendedtoaddtotheaccuracyofvision，aredifferencesofdegree。Astowhyallanimalsmustofnecessityhavebloodorsomethingofasimilarcharacter，andwhatthenatureofbloodmaybe，thesearematterswhichcanonlybeconsideredwhenwehavefirstdiscussedhotandcold。Forthenaturalpropertiesofmanysubstancesarereferabletothesetwoelementaryprinciples；
anditisamatteroffrequentdisputewhatanimalsorwhatpartsofanimalsarehotandwhatcold。Forsomemaintainthatwateranimalsarehotterthansuchasliveonland，assertingthattheirnaturalheatcounterbalancesthecoldnessoftheirmedium；andagain，thatbloodlessanimalsarehotterthanthosewithblood，andfemalesthanmales。Parmenides，forinstance，andsomeothersdeclarethatwomenarehotterthanmen，andthatitisthewarmthandabundanceoftheirbloodwhichcausestheirmenstrualflow，whileEmpedoclesmaintainstheoppositeopinion。Again，comparingthebloodandthebile，somespeakoftheformerashotandofthelatterascold，whileothersinvertthedescription。Iftherebethisendlessdisputingabouthotandcold，whichofallthingsthataffectoursensesarethemostdistinct，whatarewetothinkastoourothersensoryimpressions？
Theexplanationofthedifficultyappearstobethattheterm’hotter’isusedinseveralsenses；sothatdifferentstatements，thoughinverbalcontradictionwitheachother，mayyetallbemoreorlesstrue。Thereought，then，tobesomeclearunderstandingastothesenseinwhichnaturalsubstancesaretobetermedhotorcold，solidorfluid。Foritappearsmanifestthatthesearepropertiesonwhichevenlifeanddeatharelargelydependent，andthattheyaremoreoverthecausesofsleepandwaking，ofmaturityandoldage，ofhealthanddisease；whilenosimilarinfluencebelongstoroughnessandsmoothness，toheavinessandlightness，nor，inshort，toanyothersuchpropertiesofmatter。Thatthisshouldbesoisbutinaccordancewithrationalexpectation。Forhotandcold，solidandfluid，aswasstatedinaformertreatise，arethefoundationsofthephysicalelements。
Isthenthetermhotusedinonesenseorinmany？Toanswerthiswemustascertainwhatspecialeffectisattributedtoahottersubstance，andiftherebeseveralsuch，howmanythesemaybe。Abodythenisinonesensesaidtobehotterthananother，ifitimpartagreateramountofheattoanobjectincontactwithit。Inasecondsense，thatissaidtobehotterwhichcausesthekeenersensationwhentouched，andespeciallyifthesensationbeattendedwithpain。
Thiscriterion，however，wouldseemsometimestobeafalseone；foroccasionallyitistheidiosyncrasyoftheindividualthatcausesthesensationtobepainful。Again，oftwothings，thatisthehotterwhichthemorereadilymeltsafusiblesubstance，orsetsonfireaninflammableone。Again，oftwomassesofoneandthesamesubstance，thelargerissaidtohavemoreheatthanthesmaller。
Again，oftwobodies，thatissaidtobethehotterwhichtakesthelongertimeincooling，asalsowecallthatwhichisrapidlyheatedhotterthanthatwhichislongaboutit；asthoughtherapidityimpliedproximityandthisagainsimilarityofnature，whilethewantofrapidityimplieddistanceandthisagaindissimilarityofnature。Thetermhotterisusedtheninallthevarioussensesthathavebeenmentioned，andperhapsinstillmore。Nowitisimpossibleforonebodytobehotterthananotherinallthesedifferentfashions。Boilingwaterforinstance，thoughitismorescaldingthanflame，yethasnopowerofburningormeltingcombustibleorfusiblematter，whileflamehas。Soagainthisboilingwaterishotterthanasmallfire，andyetgetscoldmorerapidlyandcompletely。
Forinfactfireneverbecomescold；whereaswaterinvariablydoesso。
Boilingwater，again，ishottertothetouchthanoil；yetitgetscoldandsolidmorerapidlythanthisotherfluid。Blood，again，ishottertothetouchthaneitherwateroroil，andyetcoagulatesbeforethem。Iron，again，andstonesandothersimilarbodiesarelongeringettingheatedthanwater，butwhenonceheatedburnothersubstanceswithamuchgreaterintensity。Anotherdistinctionisthis。
Insomeofthebodieswhicharecalledhottheheatisderivedfromwithout，whileinothersitbelongstothebodiesthemselves；anditmakesamostimportantdifferencewhethertheheathastheformerorthelatterorigin。Fortocallthatoneoftwobodiesthehotter，whichispossessedofheat，wemayalmostsay，accidentallyandnotofitsownessence，isverymuchthesamethingasif，findingthatsomemaninafeverwasamusician，oneweretosaythatmusiciansarehotterthanhealthymen。Ofthatwhichishotperseandthatwhichishotperaccidens，theformeristheslowertocool，whilenotrarelythelatteristhehottertothetouch。Theformeragainisthemoreburningofthetwo-flame，forinstance，ascomparedwithboilingwater-whilethelatter，astheboilingwater，whichishotperaccidens，isthemoreheatingtothetouch。Fromallthisitisclearthatitisnosimplemattertodecidewhichoftwobodiesisthehotter。Forthefirstmaybethehotterinonesense，thesecondthehotterinanother。Indeedinsomeofthesecasesitisimpossibletosaysimplyevenwhetherathingishotornot。Fortheactualsubstratummaynotitselfbehot，butmaybehotwhencoupledwitbheatasanattribute，aswouldbethecaseifoneattachedasinglenametohotwaterorhotiron。Itisafterthismannerthatbloodishot。Insuchcases，inthose，thatis，inwhichthesubstratumowesitsheattoanexternalinfluence，itisplainthatcoldisnotamereprivation，butanactualexistence。
Thereisnoknowingbutthatevenfiremaybeanotherofthesecases。Forthesubstratumoffiremaybesmokeorcharcoal，andthoughtheformeroftheseisalwayshot，smokebeinganuprisingvapour，yetthelatterbecomescoldwhenitsflameisextinguished，asalsowouldoilandpinewoodundersimilarcircumstances。Butevensubstancesthathavebeenburntnearlyallpossesssomeheat，cinders，forexample，andashes，thedejectionsalsoofanimals，and，amongtheexcretions，bile；becausesomeresidueofheathasbeenleftinthemaftertheircombustion。Itisinanothersensethatpinewoodandfatsubstancesarehot；namely，becausetheyrapidlyassumetheactualityoffire。
Heatappearstocausebothcoagulationandmelting。Nowsuchthingsasareformedmerelyofwateraresolidifiedbycold，whilesuchasareformedofnothingbuteartharesolidifiedbyfire。Hotsubstancesagainaresolidifiedbycold，and，whentheyconsistchieflyofearth，theprocessofsolidificationisrapid，andtheresultingsubstanceisinsoluble；but，whentheirmainconstituentiswater，thesolidmatterisagainsoluble。Whatkindsofsubstances，however，admitofbeingsolidified，andwhatarethecausesofsolidification，arequestionsthathavealreadybeendealtwithmorepreciselyinanothertreatise。
Inconclusion，then，seeingthatthetermshotandhotterareusedinmanydifferentsenses，andthatnoonesubstancecanbehotterthanothersinallthesesenses，wemust，whenweattributethischaractertoanobject，addsuchfurtherstatementsasthatthissubstanceishotterperse，thoughthatotherisoftenhotterperaccidens；oragain，thatthissubstanceispotentiallyhot，thatotheractuallyso；oragain，thatthissubstanceishotterinthesenseofcausingagreaterfeelingofheatwhentouched，whilethatotherishotterinthesenseofproducingflameandburning。Thetermhotbeingusedinallthesevarioussenses，itplainlyfollowsthatthetermcoldwillalsobeusedwithlikeambiguity。
Somuchthenastothesignificationofthetermshotandcold，hotterandcolder。
3
Innaturalsequencewehavenexttotreatofsolidandfluid。
Thesetermsareusedinvarioussenses。Sometimes，forinstance，theydenotethingsthatarepotentially，atothertimesthingsthatareactually，solidorfluid。Iceforexample，oranyothersolidifiedfluid，isspokenofasbeingactuallyandaccidentallysolid，whilepotentiallyandessentiallyitisfluid。Similarlyearthandashesandthelike，whenmixedwithwater，areactuallyandaccidentallyfluid，butpotentiallyandessentiallyaresolid。Nowseparatetheconstituentsinsuchamixtureandyouhaveontheonehandthewaterycomponentstowhichitsfluiditywasdue，andthesearebothactuallyandpotentiallyfluid，andontheotherhandtheearthycomponents，andtheseareineverywaysolid；anditistobodiesthataresolidinthiscompletemannerthattheterm’solid’ismostproperlyandabsolutelyapplicable。Soalsotheoppositeterm’fluld’isstrictlyandabsolutelyapplicabletothatonlywhichisbothpotentiallyandactuallyfluid。Thesameremarkappliesalsotohotbodiesandtocold。
Thesedistinctions，then，beinglaiddown，itisplainthatbloodisessentiallyhotinsofarasthatheatisconnotedinitsname；justasifboilingwaterweredenotedbyasingleterm，boilingwouldbeconnotedinthatterm。Butthesubstratumofblood，thatwhichitisinsubstancewhileitisbloodinform，isnothot。Bloodtheninacertainsenseisessentiallyhot，andinanothersenseisnotso。
Forheatisincludedinthedefinitionofblood，justaswhitenessisincludedinthedefinitionofawhiteman，andsofarthereforebloodisessentiallyhot。Butsofarasbloodbecomeshotfromsomeexternalinfluence，itisnothotessentially。
Aswithhotandcold，soalsoisitwithsolidandfluid。Wecanthereforeunderstandhowsomesubstancesarehotandfluidsolongastheyremaininthelivingbody，butbecomeperceptiblycoldandcoagulatesosoonastheyareseparatedfromit；whileothersarehotandconsistentwhileinthebody，butwhenwithdrawnunderachangetotheoppositecondition，andbecomecoldandfluid。Oftheformerbloodisanexample，ofthelatterbile；forwhilebloodsolidifieswhenthusseparated，yellowbileunderthesamecircumstancesbecomesmorefluid。Wemustattributetosuchsubstancesthepossessionofoppositepropertiesinagreaterorlessdegree。
Inwhatsense，then，thebloodishotandinwhatsensefluid，andhowfaritpartakesoftheoppositeproperties，hasnowbeenfairlyexplained。Nowsinceeverythingthatgrowsmusttakenourishment，andnutrimentinallcasesconsistsoffluidandsolidsubstances，andsinceitisbytheforceofheatthattheseareconcoctedandchanged，itfollowsthatalllivingthings，animalsandplantsalike，mustonthisaccount，ifonnoother，haveanaturalsourceofheat。Thisnaturalheat，moreover，mustbelongtomanyparts，seeingthattheorgansbywhichthevariouselaborationsofthefoodareeffectedaremanyinnumber。Forfirstofallthereisthemouthandthepartsinsidethemouth，onwhichthefirstshareinthedutyclearlydevolves，insuchanimalsatleastasliveonfoodwhichrequiresdisintegration。Themouth，however，doesnotactuallyconcoctthefood，butmerelyfacilitatesconcoction；forthesubdivisionofthefoodintosmallbitsfacilitatestheactionofheatuponit。Afterthemouthcometheupperandthelowerabdominalcavities，andhereitisthatconcoctioniseffectedbytheaidofnaturalheat。Again，justasthereisachannelfortheadmissionoftheunconcoctedfoodintothestomach，namelythemouth，andinsomeanimalstheso-calledoesophagus，whichiscontinuouswiththemouthandreachestothestomach，somusttherealsobeotherandmorenumerouschannelsbywhichtheconcoctedfoodornutrimentshallpassoutofthestomachandintestinesintothebodyatlarge，andtowhichthesecavitiesshallserveasakindofmanger。Forplantsgettheirfoodfromtheearthbymeansoftheirroots；andthisfoodisalreadyelaboratedwhentakenin，whichisthereasonwhyplantsproducenoexcrement，theearthanditsheatservingtheminthesteadofastomach。Butanimals，withscarcelyanexception，andconspicuouslyallsuchasarecapableoflocomotion，areprovidedwithastomachalsac，whichisasitwereaninternalsubstitutefortheearth。Theymustthereforehavesomeinstrumentwhichshallcorrespondtotherootsofplants，withwhichtheymayabsorbtheirfoodfromthissac，sothattheproperendofthesuccessivestagesofconcoctionmayatlastbeattained。Themouththen，itsdutydone，passesoverthefoodtothestomach，andtheremustnecessarilybesomethingtoreceiveitinturnfromthis。Thissomethingisfurnishedbythebloodvessels，whichrunthroughoutthewholeextentofthemesenteryfromitslowestpartrightuptothestomach。AdescriptionofthesewillbefoundinthetreatisesonAnatomyandNaturalHistory。Nowasthereisareceptaclefortheentiremattertakenasfood，andalsoareceptacleforitsexcrementalresidue，andagainathirdreceptacle，namelythevessels，whichserveassuchfortheblood，itisplainthatthisbloodmustbethefinalnutritivematerialinsuchanimalsashaveit；whileinbloodlessanimalsthesameisthecasewiththefluidwhichrepresentstheblood。Thisexplainswhytheblooddiminishesinquantitywhennofoodistaken，andincreaseswhenmuchisconsumed，andalsowhyitbecomeshealthyandunhealthyaccordingasthefoodisoftheoneortheothercharacter。Thesefacts，then，andothersofalikekind，makeitplainthatthepurposeofthebloodinsanguineousanimalsistosubservethenutritionofthebody。Theyalsoexplainwhynomoresensationisproducedbytouchingthebloodthanbytouchingoneoftheexcretionsorthefood，whereaswhenthefleshistouchedsensationisproduced。Forthebloodisnotcontinuousnorunitedbygrowthwiththeflesh，butsimplylieslooseinitsreceptacle，thatisintheheartandvessels。Themannerinwhichthepartsgrowattheexpenseoftheblood，andindeedthewholequestionofnutrition，willfindamoresuitableplaceforexpositioninthetreatiseonGeneration，andinotherwritings。Forourpresentpurposeallthatneedbesaidisthatthebloodexistsforthesakeofnutrition，thatisthenutritionoftheparts；andwiththismuchletusthereforecontentourselves。
4
Whatarecalledfibresarefoundinthebloodofsomeanimalsbutnotofall。Therearenone，forinstance，inthebloodofdeerandofroes；andforthisreasonthebloodofsuchanimalsasthesenevercoagulates。Foronepartofthebloodconsistsmainlyofwaterandthereforedoesnotcoagulate，thisprocessoccurringonlyintheotherandearthyconstituent，thatistosayinthefibres，whilethefluidpartisevaporating。
Someatanyrateoftheanimalswithwaterybloodhaveakeenerintellectthanthosewhosebloodisofanearthiernature。Thisisduenottothecoldnessoftheirblood，butrathertoitsthinnessandpurity；neitherofwhichqualitiesbelongstotheearthymatter。Forthethinnerandpureritsfluidis，themoreeasilyaffectedisananimal’ssensibility。Thusitisthatsomebloodlessanimals，notwithstandingtheirwantofblood，areyetmoreintelligentthansomeamongthesanguineouskinds。Suchforinstance，asalreadysaid，isthecasewiththebeeandthetribeofants，andwhateverotheranimalstheremaybeofalikenature。Atthesametimetoogreatanexcessofwatermakesanimalstimorous。Forfearchillsthebody；sothatinanimalswhoseheartcontainssowateryamixturethewayispreparedfortheoperationofthisemotion。Forwateriscongealedbycold。Thisalsoexplainswhybloodlessanimalsare，asageneralrule，moretimorousthansuchashaveblood，sothattheyremainmotionless，whenfrightened，anddischargetheirexcretions，andinsomeinstanceschangecolour。Suchanimals，ontheotherhand，ashavethickandabundantfibresintheirbloodareofamoreearthynature，andofacholerictemperament，andliabletoburstsofpassion。Forangerisproductiveofheat；andsolids，whentheyhavebeenmadehot，giveoffmoreheatthanfluids。Thefibrestherefore，beingearthyandsolid，areturnedintosomanyhotembersintheblood，liketheembersinavapour-bath，andcauseebullitioninthefitsofpassion。
Thisexplainswhybullsandboarsaresocholericandsopassionate。
Fortheirbloodisexceedinglyrichinfibres，andthebull’satanyratecoagulatesmorerapidlythanthatofanyotheranimal。Ifthesefibres，thatistosayiftheearthyconstituentsofwhichwearespeaking，aretakenoutoftheblood，thefluidthatremainsbehindwillnolongercoagulate；justasthewateryresidueofmudwillnotcoagulateafterremovaloftheearth。Butifthefibresareleftthefluidcoagulates，asalsodoesmud，undertheinfluenceofcold。Forwhentheheatisexpelledbythecold，thefluid，ashasbeenalreadystated，passesoffwithitbyevaporation，andtheresidueisdriedupandsolidified，notbyheatbutbycold。Solong，however，asthebloodisinthebody，itiskeptfluidbyanimalheat。
Thecharacterofthebloodaffectsboththetemperamentandthesensoryfacultiesofanimalsinmanyways。Thisisindeedwhatmightreasonablybeexpected，seeingthatthebloodisthematerialofwhichthewholebodyismade。Fornutrimentsuppliesthematerial，andthebloodistheultimatenutriment。Itmakesthenaconsiderabledifferencewhetherthebloodbehotorcold，thinorthick，turbidorclear。
Thewaterypartofthebloodisserum；anditiswatery，eitherowingtoitsnotbeingyetconcocted，orowingtoitshavingbecomecorrupted；sothatonepartoftheserumistheresultantofanecessaryprocess，whileanotherpartismaterialintendedtoservefortheformationoftheblood。
5
Thedifferencesbetweenlardandsuetcorrespondtodifferencesofblood。Forbotharebloodconcoctedintotheseformsasaresultofabundantnutrition，beingthatsurplusbloodthatisnotexpendedonthefleshypartofthebody，andisofaneasilyconcoctedandfattycharacter。Thisisshownbytheunctuousaspectofthesesubstances；
forsuchunctuousaspectinfluidsisduetoacombinationofairandfire。Itfollowsfromwhathasbeensaidthatnonon-sanguineousanimalshaveeitherlardorsuet；fortheyhavenoblood。Amongsanguineousanimalsthosewhosebloodisdensehavesuetratherthanlard。Forsuetisofanearthynature，thatistosay，itcontainsbutasmallproportionofwaterandischieflycomposedofearth；andthisitisthatmakesitcoagulate，justasthefibrousmatterofbloodcoagulates，orbrothswhichcontainsuchfibrousmatter。Thusitisthatinthosehornedanimalsthathavenofrontteethintheupperjawthefatconsistsofsuet。Fortheveryfactthattheyhavehornsandhuckle-bonesshowsthattheircompositionisrichinthisearthyelement；forallsuchappurtenancesaresolidandearthyincharacter。
Ontheotherhandinthosehornlessanimalsthathavefrontteethinbothjaws，andwhosefeetaredividedintotoes，thereisnosuet，butinitsplacelard；andthis，notbeingofanearthycharacter，neithercoagulatesnordriesupintoafriablemass。
Bothlardandsuetwhenpresentinmoderateamountarebeneficial；
fortheycontributetohealthandstrength，whiletheyarenohindrancetosensation。Butwhentheyarepresentingreatexcess，theyareinjuriousanddestructive。Forwerethewholebodyformedofthemitwouldperish。Forananimalisananimalinvirtueofitssensorypart，thatisinvirtueofitsflesh，orofthesubstanceanalogoustoflesh。Buttheblood，asbeforestated，isnotsensitive；
asthereforeisneitherlardnorsuet，seeingthattheyarenothingbutconcoctedblood。Werethenthewholebodycomposedofthesesubstances，itwouldbeutterlywithoutsensation。Suchanimals，again，asareexcessivelyfatagerapidly。Forsomuchoftheirbloodisusedinformingfat，thattheyhavebutlittleleft；andwhenthereisbutlittlebloodthewayisalreadyopenfordecay。Fordecaymaybesaidtobedeficiencyofblood，thescantinessofwhichrendersitliable，likeallbodiesofsmallbulk，tobeinjuriouslyaffectedbyanychanceexcessofheatorcold。Forthesamereasonfatanimalsarelessprolificthanothers。Forthatpartofthebloodwhichshouldgotoformsemenandseedisusedupintheproductionoflardandsuet，whicharenothingbutconcoctedblood；sothatintheseanimalsthereiseithernoreproductiveexcretionatall，oronlyascantyamount。
6
Somuchthenofbloodandserum，andoflardandsuet。Eachofthesehasbeendescribed，andthepurposestoldforwhichtheyseverallyexist。Themarrowalsoisofthenatureofblood，andnot，assomethink，thegerminalforceofthesemen。Thatthisisthecaseisquiteevidentinveryyounganimals。Forintheembryothemarrowoftheboneshasablood-likeappearance，whichisbutnatural，seeingthatthepartsareallconstructedoutofblood，andthatitisonbloodthattheembryoisnourished。But，astheyounganimalgrowsupandripensintomaturity，themarrowchangesitscolour，justasdotheexternalpartsandtheviscera。Forthevisceraalsoinanimals，solongastheyareyoung，haveeachandallablood-likelook，owingtothelargeamountofthisfluidwhichtheycontain。
Theconsistencyofthemarrowagreeswiththatofthefat。Forwhenthefatconsistsoflard，thenthemarrowalsoisunctuousandlard-like；butwhenthebloodisconvertedbyconcoctionintosuet，anddoesnotassumetheformoflard，thenthemarrowalsohasasuetycharacter。Inthoseanimals，therefore，thathavehornsandarewithoutupperfrontteeth，themarrowhasthecharacterofsuet；whileittakestheformoflardinthosethathavefrontteethinbothjaws，andthatalsohavethefootdividedintotoes。Whathasbensaidhardlyappliestothespinalmarrow。Foritisnecessarythatthisshallbecontinuousandextendwithoutbreakthroughthewholebackbone，inasmuchasthisboneconsistsofseparatevertebrae。Butwerethespinalmarroweitherofunctuousfatorofsuet，itcouldnotholdtogetherinsuchacontinuousmassasitdoes，butwouldeitherbetoofluidortoofrangible。
Therearesomeanimalsthatcanhardlybesaidtohaveanymarrow。
Thesearethosewhosebonesarestrongandsolid，asisthecasewiththelion。Forinthisanimalthemarrowissoutterlyinsignificantthattheboneslookasthoughtheyhadnoneatall。
However，asitisnecessarythatanimalsshallhavebonesorsomethinganalogoustothem，suchasthefish-spinesofwater-animals，itisalsoamatterofnecessitythatsomeofthesebonesshallcontainmarrow；forthesubstancecontainedwithinthebonesisthenutrimentoutofwhichtheseareformed。Nowtheuniversalnutriment，asalreadystated，isblood；andthebloodwithinthebone，owingtotheheatwhichisdevelopedinitfromitsbeingthussurrounded，undergoesconcoction，andself-concoctedbloodissuetorlard；sothatitisperfectlyintelligiblehowthemarrowwithinthebonecomestohavethecharacterofthesesubstances。Soalsoitiseasytounderstandwhy，inthoseanimalsthathavestrongandcompactbones，someoftheseshouldbeentirelyvoidofmarrow，whiletherestcontainbutlittleofit；forherethenutrimentisspentinformingthebones。
Thoseanimalsthathavefish-spinesinplaceofboneshavenoothermarrowthanthatofthechine。Forinthefirstplacetheyhavenaturallybutasmallamountofblood；andsecondlytheonlyhollowfish-spineisthatofthechine。Inthisthenmarrowisformed；
thisbeingtheonlyspineinwhichthereisspaceforit，and，moreover，beingtheonlyonewhichowingtoitsdivisionintopartsrequiresaconnectingbond。Thistooisthereasonwhythemarrowofthechine，asalreadymentioned，issomewhatdifferentfromthatofotherbones。For，havingtoactthepartofaclasp，itmustbeofglutinouscharacter，andatthesametimesinewysoastoadmitofstretching。
Suchthenarethereasonsfortheexistenceofmarrow，inthoseanimalsthathaveany，andsuchitsnature。Itisevidentlythesurplusofthesanguineousnutrimentapportionedtothebonesandfish-spines，whichhasundergoneconcoctionowingtoitsbeingenclosedwithinthem。
7
Fromthemarrowwepassoninnaturalsequencetothebrain。Fortherearemanywhothinkthatthebrainitselfconsistsofmarrow，andthatitformsthecommencementofthatsubstance，becausetheyseethatthespinalmarrowiscontinuouswithit。Inrealitythetwomaybesaidtobeutterlyoppositetoeachotherincharacter。Forofallthepartsofthebodythereisnonesocoldasthebrain；
whereasthemarrowisofahotnature，asisplainlyshownbyitsfatandunctuouscharacter。Indeedthisistheveryreasonwhythebrainandspinalmarrowarecontinuouswitheachother。For，wherevertheactionofanypartisinexcess，naturesocontrivesastosetbyitanotherpartwithanexcessofcontraryaction，sothattheexcessesofthetwomaycounterbalanceeachother。Nowthatthemarrowishotisclearlyshownbymanyindications。Thecoldnessofthebrainisalsomanifestenough。Forinthefirstplaceitiscoldeventothetouch；and，secondly，ofallthefluidpartsofthebodyitisthedriestandtheonethathastheleastblood；forinfactithasnobloodatallinitspropersubstance。Thisbrainisnotresidualmatter，noryetisitoneofthepartswhichareanatomicallycontinuouswitheachother；butithasacharacterpeculiartoitself，asmightindeedbeexpected。Thatithasnocontinuitywiththeorgansofsenseisplainfromsimpleinspection，andisstillmoreclearlyshownbythefact，that，whenitistouched，nosensationisproduced；
inwhichrespectitresemblesthebloodofanimalsandtheirexcrement。Thepurposeofitspresenceinanimalsisnolessthanthepreservationofthewholebody。Forsomewritersassertthatthesoulisfireorsomesuchforce。This，however，isbutaroughandinaccurateassertion；anditwouldperhapsbebettertosaythatthesoulisincorporateinsomesubstanceofafierycharacter。Thereasonforthisbeingsoisthatofallsubstancesthereisnonesosuitableforministeringtotheoperationsofthesoulasthatwhichispossessedofheat。Fornutritionandtheimpartingofmotionareofficesofthesoul，anditisbyheatthatthesearemostreadilyeffected。Tosaythenthatthesoulisfireismuchthesamethingastoconfoundtheaugerorthesawwiththecarpenterorhiscraft，simplybecausetheworkiswroughtbythetwoinconjunction。Sofarthenthismuchisplain，thatallanimalsmustnecessarilyhaveacertainamountofheat。Butasallinfluencesrequiretobecounterbalanced，sothattheymaybereducedtomoderationandbroughttothemean（forinthemean，andnotineitherextreme，liesthetrueandrationalposition），naturehascontrivedthebrainasacounterpoisetotheregionoftheheartwithitscontainedheat，andhasgivenittoanimalstomoderatethelatter，combininginitthepropertiesofearthandwater。Forthisreasonitis，thateverysanguineousanimalhasabrain；whereasnobloodlesscreaturehassuchanorgan，unlessindeeditbe，asthePoulp，byanalogy。Forwherethereisnoblood，thereinconsequencethereisbutlittleheat。
Thebrain，then，temperstheheatandseethingoftheheart。Inorder，however，thatitmaynotitselfbeabsolutelywithoutheat，butmayhaveamoderateamount，branchesrunfrombothblood-vessels，thatistosayfromthegreatvesselandfromwhatiscalledtheaorta，andendinthemembranewhichsurroundsthebrain；whileatthesametime，inordertopreventanyinjuryfromtheheat，theseencompassingvessels，insteadofbeingfewandlarge，arenumerousandsmall，andtheirbloodscantyandclear，insteadofbeingabundantandthick。
Wecannowunderstandwhydefluxionshavetheirorigininthehead，andoccurwheneverthepartsaboutthebrainhavemorethanadueproportionofcoldness。Forwhenthenutrimentsteamsupwardsthroughtheblood-vessels，itsrefuseportionischilledbytheinfluenceofthisregion，andformsdefluxionsofphlegmandserum。Wemustsuppose，tocomparesmallthingswithgreat，thatthelikehappenshereasoccursintheproductionofshowers。Forwhenvapoursteamsupfromtheearthandiscarriedbytheheatintotheupperregions，sosoonasitreachesthecoldairthatisabovetheearth，itcondensesagainintowaterowingtotherefrigeration，andfallsbacktotheearthasrain。These，however，arematterswhichmaybesuitablyconsideredinthePrinciplesofDiseases，sofarasnaturalphilosophyhasanythingtosaytothem。
Itisthebrainagain-or，inanimalsthathavenobrain，thepartanalogoustoit-whichisthecauseofsleep。Foreitherbychillingthebloodthatstreamsupwardsafterfood，orbysomeothersimilarinfluences，itproducesheavinessintheregioninwhichitlies（whichisthereasonwhydrowsypersonshangthehead），andcausestheheattoescapedownwardsincompanywiththeblood。Itistheaccumulationofthisinexcessinthelowerregionthatproducescompletesleep，takingawaythepowerofstandinguprightfromthoseanimalstowhomthatpostureisnatural，andfromtherestthepowerofholdingupthehead。These，however，arematterswhichhavebeenseparatelyconsideredinthetreatisesonSensationandonSleep。
Thatthebrainisacompoundofearthandwaterisshownbywhatoccurswhenitisboiled。For，whensotreated，itturnshardandsolid，inasmuchasthewaterisevaporatedbytheheat，andleavestheearthypartbehind。Justthesameoccurswhenpulseandotherfruitsareboiled。Forthesealsoarehardenedbytheprocess，becausethewaterwhichentersintotheircompositionisdrivenoffandleavestheearth，whichistheirmainconstituent，behind。
Ofallanimals，manhasthelargestbraininproportiontohissize；
anditislargerinmenthaninwomen。Thisisbecausetheregionoftheheartandofthelungishotterandricherinbloodinmanthaninanyotheranimal；andinmenthaninwomen。Thisagainexplainswhyman，aloneofanimals，standserect。Fortheheat，overcominganyoppositeinclination，makesgrowthtakeitsownlineofdirection，whichisfromthecentreofthebodyupwards。Itisthenasacounterpoisetohisexcessiveheatthatinman’sbrainthereisthissuperabundantfluidityandcoldness；anditisagainowingtothissuperabundancethatthecranialbone，whichsomecalltheBregma，isthelasttobecomesolidified；solongdoesevaporationcontinuetooccurthroughitundertheinfluenceofheat。Manistheonlysanguineousanimalinwhichthistakesplace。Man，again，hasmoresuturesinhisskullthananyotheranimal，andthemalemorethanthefemale。Theexplanationisagaintobefoundinthegreatersizeofthebrain，whichdemandsfreeventilation，proportionatetoitsbulk。Forifthebrainbeeithertoofluidortoosolid，itwillnotperformitsoffice，butintheonecasewillfreezetheblood，andintheotherwillnotcoolitatall；andthuswillcausedisease，madness，anddeath。Forthecardiacheatandthecentreoflifeismostdelicateinitssympathies，andisimmediatelysensitivetotheslightestchangeoraffectionofthebloodontheoutersurfaceofthebrain。
Thefluidswhicharepresentintheanimalbodyatthetimeofbirthhavenownearlyallbeenconsidered。Amongstthosethatappearonlyatalaterperiodaretheresiduaofthefood，whichincludethedepositsofthebellyandalsothoseofthebladder。Besidesthesethereisthesemenandthemilk，oneortheotherofwhichmakesitsappearanceinappropriateanimals。Ofthesefluidstheexcrementalresiduaofthefoodmaybesuitablydiscussedbythemselves，whenwecometoexamineandconsiderthesubjectofnutrition。Thenwillbethetimetoexplaininwhatanimalstheyarefound，andwhatarethereasonsfortheirpresence。SimilarlyallquestionsconcerningthesemenandthemilkmaybedealtwithinthetreatiseonGeneration，fortheformerofthesefluidsistheverystarting-pointofthegenerativeprocess，andthelatterhasnoothergroundofexistencethangenerativepurposes。
8
Wehavenowtoconsidertheremaininghomogeneousparts，andwillbeginwithflesh，andwiththesubstancethat，inanimalsthathavenoflesh，takesitsplace。Thereasonforsobeginningisthatfleshformstheverybasisofanimals，andistheessentialconstituentoftheirbody。Itsrighttothisprecedencecanalsobedemonstratedlogically。Forananimalisbyourdefinitionsomethingthathassensibilityandchiefofalltheprimarysensibility，whichisthatofTouch；anditistheflesh，oranalogoussubstance，whichistheorganofthissense。Anditistheorgan，eitherinthesamewayasthepupilistheorganofsight，thatisitconstitutestheprimaryorganofthesense；oritistheorganandthemediumthroughwhichtheobjectactscombined，thatisitanswerstothepupilwiththewholetransparentmediumattachedtoit。Nowinthecaseoftheothersensesitwasimpossiblefornaturetounitethemediumwiththesense-organ，norwouldsuchajunctionhaveservedanypurpose；butinthecaseoftouchshewascompelledbynecessitytodoso。Forofallthesense-organsthatoftouchistheonlyonethathascorporealsubstance，oratanyrateitismorecorporealthananyother，anditsmediummustbecorporeallikeitself。
Itisobviousalsotosensethatitisforthesakeofthefleshthatalltheotherpartsexist。BytheotherpartsImeanthebones，theskin，thesinews，andtheblood-vessels，and，again，thehairandthevariouskindsofnails，andanythingelsetheremaybeofalikecharacter。Thusthebonesareacontrivancetogivesecuritytothesoftparts，towhichpurposetheyareadaptedbytheirhardness；
andinanimalsthathavenobonesthesameofficeisfulfilledbysomeanalogoussubstance，asbyfishspineinsomefishes，andbycartilageinothers。
Nowinsomeanimalsthissupportingsubstanceissituatedwithinthebody，whileinsomeofthebloodlessspeciesitisplacedontheoutside。ThelatteristhecaseinalltheCrustacea，astheCarcini（Crabs）andtheCarabi（PricklyLobsters）；itisthecasealsointheTestacea，asforinstanceintheseveralspeciesknownbythegeneralnameofoysters。Forinalltheseanimalsthefleshysubstanceiswithin，andtheearthymatter，whichholdsthesoftpartstogetherandkeepsthemfrominjury，isontheoutside。Fortheshellnotonlyenablesthesoftpartstoholdtogether，butalso，astheanimalisbloodlessandsohasbutlittlenaturalwarmth，surroundsit，asachaufferettedoestheembers，andkeepsinthesmoulderingheat。Similartothisseemstobethearrangementinanotheranddistincttribeofanimals，namelytheTortoises，includingtheCheloneandtheseveralkindsofEmys。ButinInsectsandinCephalopodstheplanisentirelydifferent，therebeingmoreoveracontrastbetweenthesetwothemselves。Forinneitherofthesedoesthereappeartobeanybonyorearthypart，worthyofnotice，distinctlyseparatedfromtherestofthebody。ThusintheCephalopodsthemainbulkofthebodyconsistsofasoftflesh-likesubstance，orratherofasubstancewhichisintermediatetofleshandsinew，soasnottobesoreadilydestructibleasactualflesh。Icallthissubstanceintermediatetofleshandsinew，becauseitissoftliketheformer，whileitadmitsofstretchinglikethelatter。Itscleavage，however，issuchthatitsplitsnotlongitudinally，likesinew，butintocircularsegments，thisbeingthemostadvantageouscondition，sofarasstrengthisconcerned。Theseanimalshavealsoapartinsidethemcorrespondingtothespinousbonesoffishes。Forinstance，intheCuttle-fishesthereiswhatisknownastheossepiae，andintheCalamariesthereistheso-calledgladius。InthePoulps，ontheotherhand，thereisnosuchinternalpart，becausethebody，or，asitistermedinthem，thehead，formsbutashortsac，whereasitisofconsiderablelengthintheothertwo；anditwasthislengthwhichlednaturetoassigntothemtheirhardsupport，soastoensuretheirstraightnessandinflexibility；justasshehasassignedtosanguineousanimalstheirbonesortheirfish-spines，asthecasemaybe。TocomenowtoInsects。InthesethearrangementisquitedifferentfromthatoftheCephalopods；quitedifferentalsofromthatwhichobtainsinsanguineousanimals，asindeedhasbeenalreadystated。Forinaninsectthereisnodistinctionintosoftandhardparts，butthewholebodyishard，thehardness，however，beingofsuchacharacterastobemoreflesh-likethanbone，andmoreearthyandbone-likethanflesh。Thepurposeofthisistomakethebodyoftheinsectlessliabletogetbrokenintopieces。