a history of science-4-第18章

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conclusively　that　digestion　would　take　place　in　the　stomach　in　the　same　manner　and　the　same　time　if　the　substance　to　be　digested　was　protected　from　the　peristalic　movements　of　the　stomach　and　subjected　to　the　action　of　the　gastric　juice　only。　He　did　this　by　introducing　the　substances　to　be　digested　into　the　stomach　in　tubes；　and　thus　protected　so　that　while　the　juices　of　the　stomach　could　act　upon　them　freely　they　would　not　be　affected　by　any　movements　of　the　organ。

Following　up　these　experiments；　he　attempted　to　show　that　digestion　could　take　place　outside　the　body　as　well　as　in　it；　as　it　certainly　should　if　it　were　a　purely　chemical　process。　He　collected　quantities　of　gastric　juice；　and　placing　it　in　suitable　vessels　containing　crushed　grain　or　flesh；　kept　the　mixture　at　about　the　temperature　of　the　body　for　several　hours。　After　repeated　experiments　of　this　kind；　apparently　conducted　with　great　care；　Reaumur　reached　the　conclusion　that　〃the　gastric　juice　has　no　more　effect　out　of　the　living　body　in　dissolving　or　digesting　the　food　than　water；　mucilage；　milk；　or　any　other　bland　fluid。〃＇3＇　Just　why　all　of　these　experiments　failed　to　demonstrate　a　fact　so　simple　does　not　appear；　but　to　Spallanzani；　at　least；　they　were　by　no　means　conclusive；　and　he　proceeded　to　elaborate　upon　the　experiments　of　Reaumur。　　He　made　his　experiments　in　scaled　tubes　exposed　to　a　certain　degree　of　heat；　and　showed　conclusively　that　the　chemical　process　does　go　on；　even　when　the　food　and　gastric　juice　are　removed　from　their　natural　environment　in　the　stomach。　In　this　he　was　opposed　by　many　physiologists；　among　them　John　Hunter；　but　the　truth　of　his　demonstrations　could　not　be　shaken；　and　in　later　years　we　find　Hunter　himself　completing　Spallanzani's　experiments　by　his　studies　of　the　post…mortem　action　of　the　gastric　juice　upon　the　stomach　walls。

That　Spallanzani's　and　Hunter's　theories　of　the　action　of　the　gastric　juice　were　not　at　once　universally　accepted　is　shown　by　an　essay　written　by　a　learned　physician　in　1834。　In　speaking　of　some　of　Spallanzani's　demonstrations；　he　writes：　〃In　some　of　the　experiments；　in　order　to　give　the　flesh　or　grains　steeped　in　the　gastric　juice　the　same　temperature　with　the　body；　the　phials　were　introduced　under　the　armpits。　But　this　is　not　a　fair　mode　of　ascertaining　the　effects　of　the　gastric　juice　out　of　the　body；　for　the　influence　which　life　may　be　supposed　to　have　on　the　solution　of　the　food　would　be　secured　in　this　case。　　The　affinities　connected　with　life　would　extend　to　substances　in　contact　with　any　part　of　the　system：　substances　placed　under　the　armpits　are　not　placed　at　least　in　the　same　circumstances　with　those　unconnected　with　a　living　animal。〃　But　just　how　this　writer　reaches　the　conclusion　that　〃the　experiments　of　Reaumur　and　Spallanzani　give　no　evidence　that　the　gastric　juice　has　any　peculiar　influence　more　than　water　or　any　other　bland　fluid　in　digesting　the　food〃＇4＇　is　difficult　to　understand。

The　concluding　touches　were　given　to　the　new　theory　of　digestion　by　John　Hunter；　who；　as　we　have　seen；　at　first　opposed　Spallanzani；　but　who　finally　became　an　ardent　champion　of　the　chemical　theory。　Hunter　now　carried　Spallanzani's　experiments　further　and　proved　the　action　of　the　digestive　fluids　after　death。　For　many　years　anatomists　had　been　puzzled　by　pathological　lesion　of　the　stomach；　found　post　mortem；　when　no　symptoms　of　any　disorder　of　the　stomach　had　been　evinced　during　life。　Hunter　rightly　conceived　that　these　lesions　were　caused　by　the　action　of　the　gastric　juice；　which；　while　unable　to　act　upon　the　living　tissue；　continued　its　action　chemically　after　death；　thus　digesting　the　walls　of　the　stomach　in　which　it　had　been　formed。　　And；　as　usual　with　his　observations；　be　turned　this　discovery　to　practical　use　in　accounting　for　certain　phenomena　of　digestion。　　The　following　account　of　the　stomach　being　digested　after　death　was　written　by　Hunter　at　the　desire　of　Sir　John　Pringle；　when　he　was　president　of　the　Royal　Society；　and　the　circumstance　which　led　to　this　is　as　follows：　〃I　was　opening；　in　his　presence；　the　body　of　a　patient　of　his　own；　where　the　stomach　was　in　part　dissolved；　which　appeared　to　him　very　unaccountable；　as　there　had　been　no　previous　symptom　that　could　have　led　him　to　suspect　any　disease　in　the　stomach。　I　took　that　opportunity　of　giving　him　my　ideas　respecting　it；　and　told　him　that　I　had　long　been　making　experiments　on　digestion；　and　considered　this　as　one　of　the　facts　which　proved　a　converting　power　in　the　gastric　juice。　。　。　。　There　are　a　great　many　powers　in　nature　which　the　living　principle　does　not　enable　the　animal　matter；　with　which　it　is　combined；　to　resistviz。；　the　mechanical　and　most　of　the　strongest　chemical　solvents。　It　renders　it；　however；　capable　of　resisting　the　powers　of　fermentation；　digestion；　and　perhaps　several　others；　which　are　well　known　to　act　on　the　same　matter　when　deprived　of　the　living　principle　and　entirely　to　decompose　it。　　〃

Hunter　concludes　his　paper　with　the　following　paragraph：　〃These　appearances　throw　considerable　light　on　the　principle　of　digestion；　and　show　that　it　is　neither　a　mechanical　power；　nor　contractions　of　the　stomach；　nor　heat；　but　something　secreted　in　the　coats　of　the　stomach；　and　thrown　into　its　cavity；　which　there　animalizes　the　food　or　assimilates　it　to　the　nature　of　the　blood。　The　power　of　this　juice　is　confined　or　limited　to　certain　substances；　especially　of　the　vegetable　and　animal　kingdoms；　and　although　this　menstruum　is　capable　of　acting　independently　of　the　stomach；　yet　it　is　indebted　to　that　viscus　for　its　continuance。＇5＇


THE　FUNCTION　OF　RESPIRATION

It　is　a　curious　commentary　on　the　crude　notions　of　mechanics　of　previous　generations　that　it　should　have　been　necessary　to　prove　by　experiment　that　the　thin；　almost　membranous　stomach　of　a　mammal　has　not　the　power　to　pulverize；　by　mere　attrition；　the　foods　that　are　taken　into　it。　　However；　the　proof　was　now　for　the　first　time　forthcoming；　and　the　question　of　the　general　character　of　the　function　of　digestion　was　forever　set　at　rest。　Almost　simultaneously　with　this　great　advance；　corresponding　progress　was　made　in　an　allied　field：　　the　mysteries　of　respiration　were　at　last　cleared　up；　thanks　to　the　new　knowledge　of　chemistry。　The　solution　of　the　problem　followed　almost　as　a　matter　of　course　upon　the　advances　of　that　science　in　the　latter　part　of　the　century。　Hitherto　no　one　since　Mayow；　of　the　previous　century；　whose　flash　of　insight　had　been　strangely　overlooked　and　forgotten；　had　even　vaguely　surmised　the　true　function　of　the　lungs。　The　great　Boerhaave　had　supposed　that　respiration　is　chiefly　important　as　an　aid　to　the　circulation　of　the　blood；　his　great　pupil；　Haller；　had　believed　to　the　day　of　his　death　in　1777　that　the　main　purpose　of　the　function　is　to　form　the　voice。　No　genius　could　hope　to　fathom　the　mystery　of　the　lungs　so　long　as　air　was　supposed　to　be　a　simple　element；　serving　a　mere　mechanical　purpose　in　the　economy　of　the　earth。

But　the　discovery　of　oxygen　gave　the　clew；　and　very　soon　all　the　chemists　were　testing　the　air　that　came　from　the　lungsDr。　Priestley；　as　usual；　being　in　the　van。　　His　initial　experiments　were　made　in　1777；　and　from　the　outset　the　problem　was　as　good　as　solved。　Other　experimenters　confirmed　his　results　in　all　their　essentialsnotably　Scheele　and　Lavoisier　and　Spallanzani　and　Davy。　　It　was　clearly　established　that　there　is　chemical　action　in　the　contact　of　the　air　with　the　tissue　of　the　lungs；　that　some　of　the　oxygen　of　the　air　disappears；　and　that　carbonic…acid　gas　is　added　to　the　inspired　air。　　It　was　shown；　too；　that　the　blood；　having　come　in　contact　with　the　air；　is　changed　from　black　to　red　in　color。　These　essentials　were　not　in　dispute　from　the　first。　　But　as　to　just　what　chemical　changes　caused　these　results　was　the　subject　of　controversy。　Whether；　for　example；　oxygen　is　actually　absorbed　into　the　blood；　or　whether　it　merely　unites　with　carbon　given　off　from　the　blood；　was　long　in　dispute。

Each　of　the　main　disputants　was　biased　by　his　own　particular　views　as　to　the　moot　points　of　chemistry。　　Lavoisier；　for　example；　believed　oxygen　gas　to　be　composed　of　a　metal　oxygen　combined　with　the　alleged　element　heat；　Dr。　Priestley　thought　it　a　co