Thistheniswhathesaysofrespiration。But，aswesaid，allanimalsthatevidentlyrespiredosobymeansofthewindpipe，whentheybreatheeitherthroughthemouthorthroughthenostrils。
Hence，ifitisofthiskindofrespirationthatheistalking，wemustaskhowittallieswiththeexplanationgiven。Butthefactsseemtobequiteopposed。Thechestisraisedinthemannerofaforge—bellowswhenthebreathisdrawnin—itisquitereasonablethatitshouldbeheatwhichraisesupandthatthebloodshouldoccupythehotregion—butitcollapsesandsinksdown，likethebellowsoncemore，whenthebreathisletout。Thedifferenceisthatinabellowsitisnotbythesamechannelthattheairistakeninandletout，butinbreathingitis。
But，ifEmpedoclesisaccountingonlyforrespirationthroughthenostrils，heismuchinerror，forthatdoesnotinvolvethenostrilsalone，butpassesbythechannelbesidetheuvulawheretheextremityoftheroofofthemouthis，someoftheairgoingthiswaythroughtheaperturesofthenostrilsandsomethroughthemouth，bothwhenitentersandwhenitpassesout。Suchthenisthenatureandmagnitudeofthedifficultiesbesettingthetheoriesofotherwritersconcerningrespiration。
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Wehavealreadystatedthatlifeandthepresenceofsoulinvolveacertainheat。Noteventhedigestingprocesstowhichisduethenutritionofanimalsoccursapartfromsoulandwarmth，foritistofirethatinallcaseselaborationisdue。Itisforthisreason，precisely，thattheprimarynutritivesoulalsomustbelocatedinthatpartofthebodyandinthatdivisionofthisregionwhichistheimmediatevehicleofthisprinciple。Theregioninquestionisintermediatebetweenthatwherefoodentersandthatwhereexcrementisdischarged。Inbloodlessanimalsithasnoname，butinthesanguineousclassthisorganiscalledtheheart。Thebloodconstitutesthenutrimentfromwhichtheorgansoftheanimalaredirectlyformed。Likewisethebloodvesselsmusthavethesameoriginatingsource，sincetheoneexistsfortheother’sbehoof—asavesselorreceptacleforit。Insanguineousanimalstheheartisthestarting—pointoftheveins；theydonottraverseit，butarefoundtostretchoutfromit，asdissectionsenableustosee。
Nowtheotherpsychicalfacultiescannotexistapartfromthepowerofnutrition（thereasonhasalreadybeenstatedinthetreatiseOntheSoul），andthisdependsonthenaturalfire，bytheunionwithwhichNaturehassetitaglow。Butfire，aswehavealreadystated，isdestroyedintwoways，eitherbyextinctionorbyexhaustion。Itsuffersextinctionfromitsopposites。Henceitcanbeextinguishedbythesurroundingcoldbothwheninmassand（thoughmorespeedily）whenscattered。Nowthiswayofperishingisduetoviolenceequallyinlivingandinlifelessobjects，forthedivisionofananimalbyinstrumentsandconsequentcongelationbyexcessofcoldcausedeath。Butexhaustionisduetoexcessofheat；
ifthereistoomuchheatcloseathandandthethingburningdoesnothaveafreshsupplyoffueladdedtoit，itgoesoutbyexhaustion，notbytheactionofcold。Hence，ifitisgoingtocontinueitmustbecooled，forcoldisapreventiveagainstthisformofextinction。
15
Someanimalsoccupythewater，othersliveonland，and，thatbeingso，inthecaseofthosewhichareverysmallandbloodlesstherefrigerationduetothesurroundingwaterorairissufficienttopreventdestructionfromthiscause。Havinglittleheat，theyrequirelittlecoldtocombatit。Hencetoosuchanimalsarealmostallshort—lived，for，beingsmall，theyhavelessscopefordeflectiontowardseitherextreme。Butsomeinsectsarelonger—livedthoughbloodless，likealltheothers），andthesehaveadeepindentationbeneaththewaist，inordertosecurecoolingthroughthemembrane，whichthereisthinner。Theyarewarmeranimalsandhencerequiremorerefrigeration，andsucharebees（someofwhichliveaslongassevenyears）andallthatmakeahummingnoise，likewasps，cockchafers，andcrickets。Theymakeasoundasifofpantingbymeansofair，for，inthemiddlesectionitself，theairwhichexistsinternallyandisinvolvedintheirconstruction，causingarisingandfallingmovement，producesfrictionagainstthemembrane。Thewayinwhichtheymovethisregionislikethemotionduetothelungsinanimalsthatbreathetheouterair，ortothegillsinfishes。Whatoccursiscomparabletothesuffocationofarespiringanimalbyholdingitsmouth，forthenthelungcausesaheavingmotionofthiskind。Inthecaseoftheseanimalsthisinternalmotionisnotsufficientforrefrigeration，butininsectsitis。Itisbyfrictionagainstthemembranethattheyproducethehummingsound，aswesaid，inthewaythatchildrendobyblowingthroughtheholesofareedcoveredbyafinemembrane。Itisthusthatthesingingcricketstooproducetheirsong；theypossessgreaterwarmthandareindentedatthewaist，butthesonglessvarietyhavenofissurethere。
Animalsalsowhicharesanguineousandpossessalung，thoughthatcontainslittlebloodandisspongy，caninsomecases，owingtothelatterfact，livealongtimewithoutbreathing；forthelung，containinglittlebloodorfluid，canrisealongway：itsownmotioncanforalongtimeproducesufficientrefrigeration。Butatlastitceasestosuffice，andtheanimaldiesofsuffocationifitdoesnotrespire—aswehavealreadysaid。Forofexhaustionthatkindwhichisdestructionduetolackofrefrigerationiscalledsuffocation，andwhatsoeveristhusdestroyedissaidtobesuffocated。
Wehavealreadystatedthatamonganimalsinsectsdonotrespire，andthefactisopentoobservationinthecaseofevensmallcreatureslikefliesandbees，fortheycanswimaboutinafluidforalongtimeifitisnottoohotortoocold。Yetanimalswithlittlestrengthtendtobreathemorefrequently。These，however，dieofwhatiscalledsuffocationwhenthestomachbecomesfilledandtheheatinthecentralsegmentisdestroyed。Thisexplainsalsowhytheyreviveafterbeingamongashesforatime。
Againamongwater—animalsthosethatarebloodlessremainalivelongerinairthanthosethathavebloodandadmitthesea—water，as，forexample，fishes。Sinceitisasmallquantityofheattheypossess，theairisforalongtimeadequateforthepurposesofrefrigerationinsuchanimalsasthecrustaceaandthepolyps。Itdoesnothoweversuffice，owingtotheirwantofheat，tokeepthemfinallyinlife，formostfishesalsolivethoughamongearth，yetinamotionlessstate，andaretobefoundbydigging。Forallanimalsthathavenolungatallorhaveabloodlessonerequirelessrefrigeration。
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Concerningthebloodlessanimalswehavedeclaredthatinsomecasesitisthesurroundingair，inothersfluid，thataidsthemaintenanceoflife。Butinthecaseofanimalspossessingbloodandheart，allwhichhavealungadmittheairandproducethecoolingeffectbybreathinginandout。Allanimalshavealungthatareviviparousandaresointernally，notexternallymerely（theSelachiaareviviparous，butnotinternally），andoftheoviparousclassthosethathavewings，e。g。birds，andthosewithscales，e。g。
tortoises，lizards，andsnakes。Theformerclasshavealungchargedwithblood，butinthemostpartofthelatteritisspongy。Hencetheyemployrespirationmoresparinglyasalreadysaid。Thefunctionisfoundalsoinallthatfrequentandpasstheirlifeinthewater，e。g。theclassofwater—snakesandfrogsandcrocodilesandhemydes，bothsea—andland—tortoises，andseals。
Alltheseandsimilaranimalsbothbringforthonlandandsleeponshoreor，whentheydosointhewater，keeptheheadabovethesurfaceinordertorespire。Butallwithgillsproducerefrigerationbytakinginwater；theSelachiaandallotherfootlessanimalshavegills。Fisharefootless，andthelimbstheyhavegettheirname（pterugion）fromtheirsimilaritytowings（pterux）。Butofthosewithfeetoneonly，sofarasobserved，hasgills。Itiscalledthetadpole。
Noanimalyethasbeenseentopossessbothlungsandgills，andthereasonforthisisthatthelungisdesignedforthepurposeofrefrigerationbymeansoftheair（itseemstohavederiveditsname（pneumon）fromitsfunctionasareceptacleofthebreath（pneuma）），whilegillsarerelevanttorefrigerationbywater。Nowforonepurposeoneorganisadaptedandonesinglemeansofrefrigerationissufficientineverycase。Hence，sinceweseethatNaturedoesnothinginvain，andifthereweretwoorgansonewouldbepurposeless，thisisthereasonwhysomeanimalshavegills，otherslungs，butnonepossessboth。
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Everyanimalinordertoexistrequiresnutriment，inordertopreventitselffromdying，refrigeration；andsoNatureemploysthesameorganforbothpurposes。For，asinsomecasesthetongueservesbothfordiscerningtastesandforspeech，soinanimalswithlungsthemouthisemployedbothinworkingupthefoodandinthepassageofthebreathoutwardsandinwards。Inlunglessandnon—respiringanimalsitisemployedinworkingupthefood，whileinthoseofthemthatrequirerefrigerationitisthegillsthatarecreatedforthispurpose。
Weshallstatefurtheronhowitisthattheseorganshavethefacultyofproducingrefrigeration。Buttopreventtheirfoodfromimpedingtheseoperationsthereisasimilarcontrivanceintherespiringanimalsandinthosethatadmitwater。Atthemomentofrespirationtheydonottakeinfood，forotherwisesuffocationresultsowingtothefood，whetherliquidordry，slippinginthroughthewindpipeandlyingonthelung。Thewindpipeissituatedbeforetheoesophagus，throughwhichfoodpassesintowhatiscalledthestomach，butinquadrupedswhicharesanguineousthereis，asitwere，alidoverthewindpipe—theepiglottis。Inbirdsandoviparousquadrupedsthiscoveringisabsent，butitsofficeisdischargedbyacontractionofthewindpipe。Thelatterclasscontractthewindpipewhenswallowingtheirfood；theformerclosedowntheepiglottis。Whenthefoodhaspassed，theepiglottisisintheonecaseraised，andintheotherthewindpipeisexpanded，andtheairenterstoeffectrefrigeration。Inanimalswithgillsthewaterisfirstdischargedthroughthemandthenthefoodpassesinthroughthemouth；theyhavenowindpipeandhencecantakenoharmfromliquidlodginginthisorgan，onlyfromitsenteringthestomach。
Forthesereasonstheexpulsionofwaterandtheseizingoftheirfoodisrapid，andtheirteetharesharpandinalmostallcasesarrangedinasaw—likefashion，fortheyaredebarredfromchewingtheirfood。
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Amongwater—animalsthecetaceansmaygiverisetosomeperplexity，thoughtheytoocanberationallyexplained。
Examplesofsuchanimalsaredolphinsandwhales，andallothersthathaveablowhole。Theyhavenofeet，yetpossessalungthoughadmittingthesea—water。Thereasonforpossessingalungisthatwhichwehavenowstated[refrigeration]；theadmissionofwaterisnotforthepurposeofrefrigeration。Thatiseffectedbyrespiration，fortheyhavealung。Hencetheysleepwiththeirheadoutofthewater，anddolphins，atanyrate，snore。Further，iftheyareentangledinnetstheysoondieofsuffocationowingtolackofrespiration，andhencetheycanbeseentocometothesurfaceowingtothenecessityofbreathing。But，sincetheyhavetofeedinthewater，theymustadmitit，anditisinordertodischargethisthattheyallhaveablow—hole；afteradmittingthewatertheyexpelitthroughtheblow—holeasthefishesdothroughthegills。Thepositionoftheblow—holeisanindicationofthis，foritleadstononeoftheorganswhicharechargedwithblood；butitliesbeforethebrainandthencedischargeswater。
Itisfortheverysamereasonthatmolluscsandcrustaceansadmitwater—ImeansuchanimalsasCarabiandCarcini。Fornoneoftheseisrefrigerationanecessity，forineverycasetheyhavelittleheatandarebloodless，andhencearesufficientlycooledbythesurroundingwater。Butinfeedingtheyadmitwater，andhencemustexpelitinordertopreventitsbeingswallowedsimultaneouslywiththefood。Thuscrustaceans，liketheCarciniandCarabi，dischargewaterthroughthefoldsbesidetheirshaggyparts，whilecuttlefishandthepolypsemployforthispurposethehollowabovethehead。
Thereis，however，amorepreciseaccountoftheseintheHistoryofAnimals。
Thusithasbeenexplainedthatthecauseoftheadmissionofthewaterisrefrigeration，andthefactthatanimalsconstitutedforalifeinwatermustfeedinit。
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Anaccountmustnextbegivenofrefrigerationandthemannerinwhichitoccursinrespiringanimalsandthosepossessedofgills。
Wehavealreadysaidthatallanimalswithlungsrespire。Thereasonwhysomecreatureshavethisorgan，andwhythosehavingitneedrespiration，isthatthehigheranimalshaveagreaterproportionofheat，foratthesametimetheymusthavebeenassignedahighersoulandtheyhaveahighernaturethanplants。Hencetoothosewithmostbloodandmostwarmthinthelungareofgreatersize，andanimalinwhichthebloodinthelungispurestandmostplentifulisthemosterect，namelyman；andthereasonwhyhealonehashisupperpartdirectedtotheupperpartoftheuniverseisthathepossessessuchalung。Hencethisorganasmuchasanyothermustbeassignedtotheessenceoftheanimalbothinmanandinothercases。
Thisthenisthepurposeofrefrigeration。Asfortheconstrainingandefficientcause，wemustbelievethatitcreatedanimalslikethis，justasitcreatedmanyothersalsonotofthisconstitution。
Forsomehaveagreaterproportionofearthintheircomposition，likeplants，andothers，e。g。aquaticanimals，containalargeramountofwater；whilewingedandterrestrialanimalshaveanexcessofairandfirerespectively。Itisalwaysintheregionpropertotheelementpreponderatingintheschemeoftheirconstitutionthatthingsexist。
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Empedoclesistheninerrorwhenhesaysthatthoseanimalswhichhavethemostwarmthandfireliveinthewatertocounterbalancetheexcessofheatintheirconstitution，inorderthat，sincetheyaredeficientincoldandfluid，theymaybekeptinlifebythecontrarycharacteroftheregiontheyoccupy；forwaterhaslessheatthanair。Butitiswhollyabsurdthatthewater—animalsshouldineverycaseoriginateondryland，andafterwardschangetheirplaceofabodetothewater；fortheyarealmostallfootless。He，however，whendescribingtheiroriginalstructuresaysthat，thoughoriginatingondryland，theyhaveabandoneditandmigratedtothewater。Butagainitisevidentthattheyarenotwarmerthanland—animals，forinsomecasestheyhavenobloodatall，inotherslittle。
Thequestion，however，astowhatsortsofanimalsshouldbecalledwarmandwhatcold，hasineachspecialcasereceivedconsideration。ThoughinonerespectthereisreasonintheexplanationwhichEmpedoclesaimsatestablishing，yethisaccountisnotcorrect。Excessinabodilystateiscuredbyasituationorseasonofoppositecharacter，buttheconstitutionisbestmaintainedbyanenvironmentakintoit。Thereisadifferencebetweenthematerialofwhichanyanimalisconstitutedandthestatesanddispositionsofthatmaterial。Forexample，ifnatureweretoconstituteathingofwaxorofice，shewouldnotpreserveitbyputtingitinahotplace，fortheopposingqualitywouldquicklydestroyit，seeingthatheatdissolvesthatwhichcoldcongeals。
Again，athingcomposedofsaltornitrewouldnotbetakenandplacedinwater，forfluiddissolvesthatofwhichtheconsistencyisduetothehotandthedry。
Henceifthefluidandthedrysupplythematerialforallbodies，itisreasonablethatthingsthecompositionofwhichisduetothefluidandthecoldshouldhaveliquidfortheirmedium[and，iftheyarecold，theywillexistinthecold]，whilethatwhichisduetothedrywillbefoundinthedry。Thustreesgrownotinwaterbutondryland。Butthesametheorywouldrelegatethemtothewater，onaccountoftheirexcessofdryness，justasitdoesthethingsthatareexcessivelyfiery。Theywouldmigratethithernotonaccountofitscoldbutowingtoitsfluidity。
Thusthenaturalcharacterofthematerialofobjectsisofthesamenatureastheregioninwhichtheyexist；theliquidisfoundinliquid，thedryonland，thewarminair。Withregard，however，tostatesofbody，acoldsituationhas，ontheotherhand，abeneficialeffectonexcessofheat，andawarmenvironmentonexcessofcold，fortheregionreducestoameantheexcessinthebodilycondition。Theregionsappropriatetoeachmaterialandtherevolutionsoftheseasonswhichallexperiencesupplythemeanswhichmustbesoughtinordertocorrectsuchexcesses；but，whilestatesofthebodycanbeopposedincharactertotheenvironment，thematerialofwhichitiscomposedcanneverbeso。This，then，isasufficientexplanationofwhyitisnotowingtotheheatintheirconstitutionthatsomeanimalsareaquatic，othersterrestrial，asEmpedoclesmaintains，andofwhysomepossesslungsandothersdonot。
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Theexplanationoftheadmissionofairandrespirationinthoseanimalsinwhichalungisfound，andespeciallyinthoseinwhichitisfullofblood，istobefoundinthefactthatitisofaspongynatureandfulloftubes，andthatitisthemostfullychargedwithbloodofallthevisceralorgans。Allanimalswithafull—bloodedlungrequirerapidrefrigerationbecausethereislittlescopefordeviationfromthenormalamountoftheirvitalfire；theairalsomustpenetrateallthroughitonaccountofthelargequantityofbloodandheatitcontains。Butboththeseoperationscanbeeasilyperformedbyair，for，beingofasubtlenature，itpenetrateseverywhereandthatrapidly，andsoperformsitscoolingfunction；butwaterhastheoppositecharacteristics。
Thereasonwhyanimalswithafull—bloodedlungrespiremostishencemanifest；themoreheatthereis，thegreateristheneedforrefrigeration，andatthesametimebreathcaneasilypasstothesourceofheatintheheart。
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Inordertounderstandthewayinwhichtheheartisconnectedwiththelungbymeansofpassages，wemustconsultbothdissectionsandtheaccountintheHistoryofAnimals。Theuniversalcauseoftheneedwhichtheanimalhasforrefrigeration，istheunionofthesoulwithfirethattakesplaceintheheart。Respirationisthemeansofeffectingrefrigeration，ofwhichthoseanimalsmakeusethatpossessalungaswellasaheart。Butwhenthey，asforexamplethefishes，whichonaccountoftheiraquaticnaturehavenolung，possessthelatterorganwithouttheformer，thecoolingiseffectedthroughthegillsbymeansofwater。Forocularevidenceastohowtheheartissituatedrelativelytothegillswemustemploydissections，andforprecisedetailswemustrefertoNaturalHistory。Asasummarizingstatement，however，andforpresentpurposes，thefollowingistheaccountofthematter。
Itmightappearthatthehearthasnotthesamepositioninterrestrialanimalsandfishes，butthepositionreallyisidentical，fortheapexoftheheartisinthedirectioninwhichtheyinclinetheirheads。Butitistowardsthemouthinfishesthattheapexoftheheartpoints，seeingthattheydonotinclinetheirheadsinthesamedirectionasland—animalsdo。Nowfromtheextremityoftheheartatubeofasinewy，arterialcharacterrunstothecentrewherethegillsalljoin。Thisthenisthelargestofthoseducts，butoneithersideoftheheartothersalsoissueandruntotheextremityofeachgill，andbymeansoftheceaselessflowofwaterthroughthegills，effectthecoolingwhichpassestotheheart。
Insimilarfashionasthefishmovetheirgills，respiringanimalswithrapidactionraiseandletfallthechestaccordingasthebreathisadmittedorexpelled。Ifairislimitedinamountandunchangedtheyaresuffocated，foreithermedium，owingtocontactwiththeblood，rapidlybecomeshot。Theheatofthebloodcounteractstherefrigerationand，whenrespiringanimalscannolongermovethelungaquaticanimalstheirgills，whetherowingtodiscaseoroldage，theirdeathensues。
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Tobebornandtodiearecommontoallanimals，buttherearespecificallydiversewaysinwhichthesephenomenaoccur；ofdestructiontherearedifferenttypes，thoughyetsomethingiscommontothemall。Thereisviolentdeathandagainnaturaldeath，andtheformeroccurswhenthecauseofdeathisexternal，thelatterwhenitisinternal，andinvolvedfromthebeginningintheconstitutionoftheorgan，andnotanaffectionderivedfromaforeignsource。Inthecaseofplantsthenamegiventothisiswithering，inanimalssenility。Deathanddecaypertaintoallthingsthatarenotimperfectlydeveloped；totheimperfectalsotheymaybeascribedinnearlythesamebutnotanidenticalsense。UndertheimperfectIclasseggsandseedsofplantsastheyarebeforetherootappears。
Itisalwaystosomelackofheatthatdeathisdue，andinperfectcreaturesthecauseisitsfailureintheorgancontainingthesourceofthecreature’sessentialnature。Thismemberissituate，ashasbeensaid，atthejunctionoftheupperandlowerparts；inplantsitisintermediatebetweentherootandthestem，insanguineousanimalsitistheheart，andinthosethatarebloodlessthecorrespondingpartoftheirbody。Butsomeoftheseanimalshavepotentiallymanysourcesoflife，thoughinactualitytheypossessonlyone。Thisiswhysomeinsectslivewhendivided，andwhy，evenamongsanguineousanimals，allwhosevitalityisnotintenseliveforalongtimeafterthehearthasbeenremoved。Tortoises，forexample，dosoandmakemovementswiththeirfeet，solongastheshellisleft，afacttobeexplainedbythenaturalinferiorityoftheirconstitution，asitisininsectsalso。
Thesourceoflifeislosttoitspossessorswhentheheatwithwhichitisboundupisnolongertemperedbycooling，for，asI
haveoftenremarked，itisconsumedbyitself。Hencewhen，owingtolapseoftime，thelungintheoneclassandthegillsintheothergetdriedup，theseorgansbecomehardandearthyandincapableofmovement，andcannotbeexpandedorcontracted。Finallythingscometoaclimax，andthefiregoesoutfromexhaustion。
Henceasmalldisturbancewillspeedilycausedeathinoldage。
Littleheatremains，forthemostofithasbeenbreathedawayinthelongperiodoflifepreceding，andhenceanyincreaseofstrainontheorganquicklycausesextinction。Itisjustasthoughtheheartcontainedatinyfeebleflamewhichtheslightestmovementputsout。
Henceinoldagedeathispainless，fornoviolentdisturbanceisrequiredtocausedeath，andthereisanentireabsenceoffeelingwhenthesoul’sconnexionissevered。Alldiseaseswhichhardenthelungbyformingtumoursorwasteresidues，orbyexcessofmorbidheat，ashappensinfevers，acceleratethebreathingowingtotheinabilityofthelungtomovefareitherupwardsordownwards。
Finally，whenmotionisnolongerpossible，thebreathisgivenoutanddeathensues。
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Generationistheinitialparticipation，mediatedbywarmsubstance，inthenutritivesoul，andlifeisthemaintenanceofthisparticipation。Youthistheperiodofthegrowthoftheprimaryorganofrefrigeration，oldageofitsdecay，whiletheinterveningtimeistheprimeoflife。
Aviolentdeathordissolutionconsistsintheextinctionorexhaustionofthevitalheat（foreitherofthesemaycausedissolution），whilenaturaldeathistheexhaustionoftheheatowingtolapseoftime，andoccurringattheendoflife。Inplantsthisistowither，inanimalstodie。Death，inoldage，istheexhaustionduetoinabilityonthepartoftheorgan，owingtooldage，toproducerefrigeration。Thisthenisouraccountofgenerationandlifeanddeath，andthereasonfortheiroccurrenceinanimals。
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Itishencealsoclearwhyrespiringanimalsaresuffocatedinwaterandfishesinair。Foritisbywaterinthelatterclass，byairintheformerthatrefrigerationiseffected，andeitherofthesemeansofperformingthefunctionisremovedbyachangeofenvironment。
Thereisalsotobeexplainedineithercasethecauseofthecauseofthemotionofthegillsandofthelungs，theriseandfallofwhicheffectstheadmissionandexpulsionofthebreathorofwater。Thefollowing，moreover，isthemanneroftheconstitutionoftheorgan。
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Inconnexionwiththehearttherearethreephenomena，which，thoughapparentlyofthesamenature，arereallynotso，namelypalpitation，pulsation，andrespiration。
Palpitationistherushingtogetherofthehotsubstanceintheheartowingtothechillinginfluenceofresidualorwasteproducts。
Itoccurs，forexample，intheailmentknownas’spasms’andinotherdiseases。Itoccursalsoinfear，forwhenoneisafraidtheupperpartsbecomecold，andthehotsubstance，fleeingaway，byitsconcentrationintheheartproducespalpitation。Itiscrushedintososmallaspacethatsometimeslifeisextinguished，andtheanimalsdieofthefrightandmorbiddisturbance。
Thebeatingoftheheart，which，ascanbeseen，goesoncontinuously，issimilartothethrobbingofanabscess。That，however，isaccompaniedbypain，becausethechangeproducedinthebloodisunnatural，anditgoesonuntilthematterformedbyconcoctionisdischarged。Thereisasimilaritybetweenthisphenomenonandthatofboiling；forboilingisduetothevolatilizationoffluidbyheatandtheexpansionconsequentonincreaseofbulk。Butinanabscess，ifthereisnoevaporationthroughthewalls，theprocessterminatesinsuppurationduetothethickeningoftheliquid，whileinboilingitendsintheescapeofthefluidoutofthecontainingvessel。
Intheheartthebeatingisproducedbytheheatexpandingthefluid，ofwhichthefoodfurnishesaconstantsupply。Itoccurswhenthefluidrisestotheouterwalloftheheart，anditgoesoncontinuously；forthereisaconstantflowofthefluidthatgoestoconstitutetheblood，itbeingintheheartthatthebloodreceivesitsprimaryelaboration。Thatthisissowecanperceiveintheinitialstagesofgeneration，fortheheartcanbeseentocontainbloodbeforetheveinsbecomedistinct。Thisexplainswhypulsationinyouthexceedsthatinolderpeople，forintheyoungtheformationofvapourismoreabundant。
Alltheveinspulse，anddososimultaneouslywitheachother，owingtotheirconnexionwiththeheart。Theheartalwaysbeats，andhencetheyalsobeatcontinuouslyandsimultaneouslywitheachotherandwithit。
Palpitation，then，istherecoiloftheheartagainstthecompressionduetocold；andpulsationisthevolatilizationoftheheatedfluid。
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Respirationtakesplacewhenthehotsubstancewhichistheseatofthenutritiveprincipleincreases。Forit，liketherestofthebody，requiresnutrition，andmoresothanthemembers，foritisthroughitthattheyarenourished。Butwhenitincreasesitnecessarilycausestheorgantorise。Thisorganwemusttobeconstructedlikethebellowsinasmithy，forbothheartandlungsconformprettywelltothisshape。Suchastructuremustbedouble，forthenutritiveprinciplemustbesituatedinthecentreofthenaturalforce。
Thusonincreaseofbulkexpansionresults，whichnecessarilycausesthesurroundingpartstorise。Nowthiscanbeseentooccurwhenpeoplerespire；theyraisetheirchestbecausethemotiveprincipleoftheorgandescribedresidentwithinthechestcausesanidenticalexpansionofthisorgan。Whenitdilatestheouterairmustrushinasintoabellows，and，beingcold，byitschillinginfluencereducesbyextinctiontheexcessofthefire。But，astheincreaseofbulkcausestheorgantodilate，sodiminutioncausescontraction，andwhenitcollapsestheairwhichenteredmustpassoutagain。Whenitenterstheairiscold，butonissuingitiswarmowingtoitscontactwiththeheatresidentinthisorgan，andthisisspeciallythecaseinthoseanimalsthatpossessafull—bloodedlung。Thenumerouscanal—likeductsinthelung，intowhichitpasses，haveeachablood—vessellyingalongside，sothatthewholelungisthoughttobefullofblood。Theinwardpassageoftheairiscalledrespiration，theoutwardexpiration，andthisdoublemovementgoesoncontinuouslyjustsolongastheanimallivesandkeepsthisorganincontinuousmotion；itisforthisreasonthatlifeisboundupwiththepassageofthebreathoutwardsandinwards。
Itisinthesamewaythatthemotionofthegillsinfishestakesplace。Whenthehotsubstanceinthebloodthroughoutthemembersrises，thegillsrisetoo，andletthewaterpassthrough，butwhenitischilledandretreatsthroughitschannelstotheheart，theycontractandejectthewater。Continuallyastheheatintheheartrises，continuallyonbeingchilleditreturnsthitheragain。Hence，asinrespiringanimalslifeanddeathareboundupwithrespiration，sointheotheranimalsclasstheydependontheadmissionofwater。
Ourdiscussionoflifeanddeathandkindredtopicsisnowpracticallycomplete。Buthealthanddiscasealsoclaimtheattentionofthescientist，andnotmerclyofthephysician，insofarasanaccountoftheircausesisconcerned。Theextenttowhichthesetwodifferandinvestigatediverseprovincesmustnotescapeus，sincefactsshowthattheirinquiriesare，toacertainextent，atleastconterminous。Forphysiciansofcultureandrefinementmakesomementionofnaturalscience，andclaimtoderivetheirprinciplesfromit，whilethemostaccomplishedinvestigatorsintonaturegenerallypushtheirstudiessofarastoconcludewithanaccountofmedicalprinciples。