Subsequentgenerations，however，putthisinterpretationuponthetheory，conceivingthevariousspheresasactualcrystallinebodies。Itisdifficulttoimaginejusthowthevariousepicyclesweresupposedtorevolvewithoutinterferingwiththemajorspheres，butperhapsthisisnogreaterdifficultythanispresentedbytheallegedpropertiesoftheether，whichphysicistsofto-dayacceptasatleastaworkinghypothesis。Weshallseelateronhowfirmlytheconceptionofconcentriccrystallinesphereswasheldto，andthatnorealchallengewasevergiventhattheoryuntilthediscoverywasmadethatcometshaveanorbitthatmustnecessarilyintersectthespheresofthevariousplanets。
Ptolemy’ssystemofgeographyineightbooks，foundedonthatofMarinusofTyre，wasscarcelylesscelebratedthroughouttheMiddleAgesthantheAlmagest。Itcontainedlittle，however，thatneedconcernushere，beingratheranelaborationofthedoctrinestowhichwehavealreadysufficientlyreferred。NoneofPtolemy’soriginalmanuscriptshascomedowntous，butthereisanallegedfifth-centurymanuscriptattributedtoAgathadamonofAlexandriawhichhaspeculiarinterestbecauseitcontainsaseriesoftwenty-sevenelaboratelycoloredmapsthataresupposedtobederivedfrommapsdrawnupbyPtolemyhimself。Inthesemapstheseaiscoloredgreen，themountainsredordarkyellow，andthelandwhite。Ptolemyassumedthatadegreeattheequatorwas500stadiainsteadof604stadiainlength。Wearenotinformedastothegroundsonwhichthisassumptionwasmade，butithasbeensuggestedthattheerrorwasatleastpartiallyinstrumentalinleadingtooneverycuriousresult。\"TakingtheparallelofRhodes，\"saysDonaldson，[5]\"hecalculatedthelongitudesfromtheFortunateIslandstoCattigaraorthewestcoastofBorneoat180degrees，conceivingthistobeone-halfthecircumferenceoftheglobe。Therealdistanceisonly125
degreesor127degrees，sothathismeasurementiswrongbyonethirdofthewhole，one-sixthfortheerrorinthemeasurementofadegreeandone-sixthfortheerrorsinmeasuringthedistancegeometrically。Theseerrors，owingtotheauthorityattributedtothegeographyofPtolemyintheMiddleAges，producedaconsequenceofthegreatestimportance。TheyreallyledtothediscoveryofAmerica。ForthedesignofColumbustosailfromthewestofEuropetotheeastofAsiawasfoundedonthesuppositionthatthedistancewaslessbyonethirdthanitreallywas。\"Thisviewisperhapsatriflefanciful，sincethereisnothingtosuggestthatthecourageofColumbuswouldhavebalkedatthegreaterdistance，andsincetheprotestsofthesailors，whichnearlythwartedhisefforts，weremadelongbeforethedistanceasestimatedbyPtolemyhadbeencovered；neverthelessitisinterestingtorecallthatthegreatgeographicaldoctrines，uponwhichColumbusmustchieflyhavebasedhisarguments，hadbeenbeforetheworldinanauthoritativeformpracticallyunheededformorethantwelvehundredyears，awaitingachampionwithcourageenoughtoputthemtothetest。
Thereisoneotherfieldofscientificinvestigationtowhichwemustgivebriefattentionbeforeleavingtheantiqueworld。Thisisthefieldofphysiologyandmedicine。InconsideringitweshallhavetodowiththeverylastgreatscientistoftheAlexandrianschool。ThiswasClaudiusGalenus，commonlyknownasGalen，amanwhosefamewasdestinedtoeclipsethatofallotherphysiciansofantiquityexceptHippocrates，andwhosedoctrinesweretohavethesameforceintheirfieldthroughouttheMiddleAgesthatthedoctrinesofAristotlehadforphysicalscience。
ButbeforewetakeupGalen’sspecificlabors，itwillbewelltoinquirebrieflyastothestateofmedicalartandscienceintheRomanworldatthetimewhenthelastgreatphysicianofantiquitycameuponthescene。