a history of science-4-第20章

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ture　as　high　as　that　of　the　native　of　the　South；　who　lives　in　so　different　a　medium。　　This　fact；　when　its　true　significance　is　perceived；　proves　that　the　heat　given　off　to　the　surrounding　medium　is　restored　within　the　body　with　great　rapidity。　This　compensation　takes　place　more　rapidly　in　winter　than　in　summer；　at　the　pole　than　at　the　equator。

〃Now　in　different　climates　the　quantity　of　oxygen　introduced　into　the　system　of　respiration；　as　has　been　already　shown；　varies　according　to　the　temperature　of　the　external　air；　the　quantity　of　inspired　oxygen　increases　with　the　loss　of　heat　by　external　cooling；　and　the　quantity　of　carbon　or　hydrogen　necessary　to　combine　with　this　oxygen　must　be　increased　in　like　ratio。　It　is　evident　that　the　supply　of　heat　lost　by　cooling　is　effected　by　the　mutual　action　of　the　elements　of　the　food　and　the　inspired　oxygen；　which　combine　together。　　To　make　use　of　a　familiar；　but　not　on　that　account　a　less　just　illustration；　the　animal　body　acts；　in　this　respect；　as　a　furnace；　which　we　supply　with　fuel。　It　signifies　nothing　what　intermediate　forms　food　may　assume；　what　changes　it　may　undergo　in　the　body；　the　last　change　is　uniformly　the　conversion　of　carbon　into　carbonic　acid　and　of　its　hydrogen　into　water；　the　unassimilated　nitrogen　of　the　food；　along　with　the　unburned　or　unoxidized　carbon；　is　expelled　in　the　excretions。　In　order　to　keep　up　in　a　furnace　a　constant　temperature；　we　must　vary　the　supply　of　fuel　according　to　the　external　temperaturethat　is；　according　to　the　supply　of　oxygen。

〃In　the　animal　body　the　food　is　the　fuel；　with　a　proper　supply　of　oxygen　we　obtain　the　heat　given　out　during　its　oxidation　or　combustion。〃＇3＇


BLOOD　CORPUSCLES；　MUSCLES；　AND　GLANDS

Further　researches　showed　that　the　carriers　of　oxygen；　from　the　time　of　its　absorption　in　the　lungs　till　its　liberation　in　the　ultimate　tissues；　are　the　red　corpuscles；　whose　function　had　been　supposed　to　be　the　mechanical　one　of　mixing　of　the　blood。　　It　transpired　that　the　red　corpuscles　are　composed　chiefly　of　a　substance　which　Kuhne　first　isolated　in　crystalline　form　in　1865；　and　which　was　named　haemoglobina　substance　which　has　a　marvellous　affinity　for　oxygen；　seizing　on　it　eagerly　at　the　lungs　vet　giving　it　up　with　equal　readiness　when　coursing　among　the　remote　cells　of　the　body。　When　freighted　with　oxygen　it　becomes　oxyhaemoglobin　and　is　red　in　color；　when　freed　from　its　oxygen　it　takes　a　purple　hue；　hence　the　widely　different　appearance　of　arterial　and　venous　blood；　which　so　puzzled　the　early　physiologists。

This　proof　of　the　vitally　important　role　played　by　the　red…blood　corpuscles　led；　naturally；　to　renewed　studies　of　these　infinitesimal　bodies。　It　was　found　that　they　may　vary　greatly　in　number　at　different　periods　in　the　life　of　the　same　individual；　proving　that　they　may　be　both　developed　and　destroyed　in　the　adult　organism。　　Indeed；　extended　observations　left　no　reason　to　doubt　that　the　process　of　corpuscle　formation　and　destruction　may　be　a　perfectly　normal　onethat；　in　short；　every　red…blood　corpuscle　runs　its　course　and　dies　like　any　more　elaborate　organism。　They　are　formed　constantly　in　the　red　marrow　of　bones；　and　are　destroyed　in　the　liver；　where　they　contribute　to　the　formation　of　the　coloring　matter　of　the　bile。　　Whether　there　are　other　seats　of　such　manufacture　and　destruction　of　the　corpuscles　is　not　yet　fully　determined。　Nor　are　histologists　agreed　as　to　whether　the　red…blood　corpuscles　themselves　are　to　be　regarded　as　true　cells；　or　merely　as　fragments　of　cells　budded　out　from　a　true　cell　for　a　special　purpose；　but　in　either　case　there　is　not　the　slightest　doubt　that　the　chief　function　of　the　red　corpuscle　is　to　carry　oxygen。

If　the　oxygen　is　taken　to　the　ultimate　cells　before　combining　with　the　combustibles　it　is　to　consume；　it　goes　without　saying　that　these　combustibles　themselves　must　be　carried　there　also。　Nor　could　it　be　in　doubt　that　the　chiefest　of　these　ultimate　tissues；　as　regards；　quantity　of　fuel　required；　are　the　muscles。　A　general　and　comprehensive　view　of　the　organism　includes；　then；　digestive　apparatus　and　lungs　as　the　channels　of　fuel…supply；　blood　and　lymph　channels　as　the　transportation　system；　and　muscle　cells；　united　into　muscle　fibres；　as　the　consumption　furnaces；　where　fuel　is　burned　and　energy　transformed　and　rendered　available　for　the　purposes　of　the　organism；　supplemented　by　a　set　of　excretory　organs；　through　which　the　waste　productsthe　ashesare　eliminated　from　the　system。

But　there　remain；　broadly　speaking；　two　other　sets　of　organs　whose　size　demonstrates　their　importance　in　the　economy　of　the　organism；　yet　whose　functions　are　not　accounted　for　in　this　synopsis。　These　are　those　glandlike　organs；　such　as　the　spleen；　which　have　no　ducts　and　produce　no　visible　secretions；　and　the　nervous　mechanism；　whose　central　organs　are　the　brain　and　spinal　cord。　　What　offices　do　these　sets　of　organs　perform　in　the　great　labor…specializing　aggregation　of　cells　which　we　call　a　living　organism？

As　regards　the　ductless　glands；　the　first　clew　to　their　function　was　given　when　the　great　Frenchman　Claude　Bernard　（the　man　of　whom　his　admirers　loved　to　say；　〃He　is　not　a　physiologist　merely；　he　is　physiology　itself〃）　discovered　what　is　spoken　of　as　the　glycogenic　function　of　the　liver。　The　liver　itself；　indeed；　is　not　a　ductless　organ；　but　the　quantity　of　its　biliary　output　seems　utterly　disproportionate　to　its　enormous　size；　particularly　when　it　is　considered　that　in　the　case　of　the　human　species　the　liver　contains　normally　about　one…fifth　of　all　the　blood　in　the　entire　body。　Bernard　discovered　that　the　blood　undergoes　a　change　of　composition　in　passing　through　the　liver。　　The　liver　cells　（the　peculiar　forms　of　which　had　been　described　by　Purkinje；　Henle；　and　Dutrochet　about　1838）　have　the　power　to　convert　certain　of　the　substances　that　come　to　them　into　a　starchlike　compound　called　glycogen；　and　to　store　this　substance　away　till　it　is　needed　by　the　organism。　　This　capacity　of　the　liver　cells　is　quite　independent　of　the　bile…making　power　of　the　same　cells；　hence　the　discovery　of　this　glycogenic　function　showed　that　an　organ　may　have　more　than　one　pronounced　and　important　specific　function。　But　its　chief　importance　was　in　giving　a　clew　to　those　intermediate　processes　between　digestion　and　final　assimilation　that　are　now　known　to　be　of　such　vital　significance　in　the　economy　of　the　organism。

In　the　forty　odd　years　that　have　elapsed　since　this　pioneer　observation　of　Bernard；　numerous　facts　have　come　to　light　showing　the　extreme　importance　of　such　intermediate　alterations　of　food…supplies　in　the　blood　as　that　performed　by　the　liver。　It　has　been　shown　that　the　pancreas；　the　spleen；　the　thyroid　gland；　the　suprarenal　capsules　are　absolutely　essential；　each　in　its　own　way；　to　the　health　of　the　organism；　through　metabolic　changes　which　they　alone　seem　capable　of　performing；　and　it　is　suspected　that　various　other　tissues；　including　even　the　muscles　themselves；　have　somewhat　similar　metabolic　capacities　in　addition　to　their　recognized　functions。　But　so　extremely　intricate　is　the　chemistry　of　the　substances　involved　that　in　no　single　case　has　the　exact　nature　of　the　metabolisms　wrought　by　these　organs　been　fully　made　out。　　Each　is　in　its　way　a　chemical　laboratory　indispensable　to　the　right　conduct　of　the　organism；　but　the　precise　nature　of　its　operations　remains　inscrutable。　The　vast　importance　of　the　operations　of　these　intermediate　organs　is　unquestioned。

A　consideration　of　the　functions　of　that　other　set　of　organs　known　collectively　as　the　nervous　system　is　reserved　for　a　later　chapter。



VI。　THEORIES　OF　ORGANIC　EVOLUTION

GOETHE　AND　THE　METAMORPHOSIS　OF　PARTS

When　Coleridge　said　of　Humphry　Davy　that　he　might　have　been　the　greatest　poet　of　his　time　had　he　not　chosen　rather　to　be　the　greatest　chemist；　it　is　possible　that　the　enthusiasm　of　the　friend　outweighed　the　caution　of　the　critic。　　But　however　that　may　be；　it　is　beyond　dispute　that　the　man　who　actually　was　the　greatest　poet　of　that　time　might　easily　have　taken　the　very　highest　rank　as　a　scientist　had　not　the　muse　distracted　his　attention。　　Indeed；　despite　these　distractions；　Johann　Wolfgang　von　Goethe　achieved　successes　in　the　field　of　pure　science　that　would　insure　permanent　recognition　for　his　name　had　he　never　written　a　stanza　of　poetry。　Such　is　the　versatility　that　marks　the　highest　genius。

It　was　in　1790　that　Goethe　published　the　work　that　laid　the　foundations　of　his　scientific　reputationthe　work　on　the　Metamorphoses　of　Plants；　in　which　he　advanced　the　novel　doctrine　that　all　parts　of　the　flower　are　modified　or　metamorphosed　leaves。

〃Every　one　who　observes　the　growth　of　plants；　even　superficially；〃　wrote　Goethe；　〃will　notice　that　certain　external　parts　of　them　become　transformed　at　times　and　go　over　into　the　forms　of　the　contiguous　parts；　now　completely；　now　to　a　greater　or　less　degree。　　Thus；　for　example；　the　single　flower　is　transformed　into　a　double　one　when；　instead　of　stamens；　petals　are　developed；　which　are　either　exactly　like　the　other　petals　of　the　corolla　in　form；　and　color　or　else　still　bear　visible　signs　of　their　origin。

〃When　we　observe　that　it　is　possible　for　a　plant　in　this　way　to　take　a　step　backward；　we　shall　give　so　much　the　more　heed　to　the　regular　course　of　nature　and　learn　the　laws　of　transformation　according　to　which　she　produces　one　part　through　another；　and　displays　the　most　varying　forms　through　the　modification　of　one　single　organ。

〃Let　us　first　direct　our　attention　to　the　plant　at　the　moment　when　it　develops　out　of　the　seed…kernel。　The　first　organs　of　its　upward　growth　are　known　by　the　name　of　cotyledons；　they　have　also　been　called　seed…leaves。

〃They　often　appear　shapeless；　filled　with　new　matter；　and　are　just　as　thick　as　they　are　broad。　　Their　vessels　are　unrecognizable　and　are　hardly　to　be　distinguished　from　the　mass　of　the　whole；　they　bear　almost　no　resemblance　to　a　leaf；　and　we　could　easily　be　misled　into　regarding　them　as　special　organs。　　Occasionally；　however；　they　appear　as　real　leaves；　their　vessels　are　capable　of　the　most　minute　development；　their　similarity　to　the　following　leaves　does　not　permit　us　to　take　them　for　special　organs；　but　we　recognize　them　instead　to　be　the　first　leaves　of　the　stalk。

〃The　cotyledons　are　mostly　double；　and　there　is　an　observation　to　be　made　here　which　will　appear　still　more　important　as　we　proceedthat　is；　that　the　leaves　of　the　first　node　are　often　paired；　even　when　the　following　leaves　of　the　stalk　stand　alternately　upon　it。　Here　we　see　an　approximation　and　a　joining　of　parts　which　nature　afterwards　separates　and　places　at　a　distance　from　one　another。　It　is　still　more　remarkable　when　the　cotyledons　take　the　form　of　many　little　leaves　gathered　about　an　axis；　and　the　stalk　which　grows　gradually　from　their　midst　produces　the　following　leaves　arranged　around　it　singly　in　a　whorl。　This　may　be　observed　very　exactly　in　the　growth　of　the　pinus　species。　Here　a　corolla　of　needles　forms　at　the　same　time　a　calyx；　and　we　shall　have　occasion　to　remember　the　present　case　in　connection　with　similar　phenomena　later。

〃On　the　other　hand；　we　observe　that　even　the　cotyledons　which　are　most　like　a　leaf　when　compared　with　the　following　leaves　of　the　stalk　are　always　more　undeveloped　or　les