a history of science-4-第45节

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f　equilibrium　or　balanced　muscular　actiona　function　not；　I　think；　hitherto　recognized　by　physiologists。

The　three　kinds　of　muscular　motion　hitherto　known　may　be　distinguished　in　another　way。　　The　muscles　of　voluntary　motion　and　of　respiration　may　be　excited　by　stimulating　the　nerves　which　supply　them；　in　any　part　of　their　course；　whether　at　their　source　as　a　part　of　the　medulla　oblongata　or　the　medulla　spinalis　or　exterior　to　the　spinal　canal：　the　muscles　of　involuntary　motion　are　chiefly　excited　by　the　actual　contact　of　stimuli。　　In　the　case　of　the　reflex　function　alone　the　muscles　are　excited　by　a　stimulus　acting　mediately　and　indirectly　in　a　curved　and　reflex　course；　along　superficial　subcutaneous　or　submucous　nerves　proceeding　from　the　medulla。　The　first　three　of　these　causes　of　muscular　motion　may　act　on　detached　limbs　or　muscles。　　The　last　requires　the　connection　with　the　medulla　to　be　preserved　entire。

〃All　the　kinds　of　muscular　motion　may　be　unduly　excited；　but　the　reflex　function　is　peculiar　in　being　excitable　in　two　modes　of　action；　not　previously　subsisting　in　the　animal　economy；　as　in　the　case　of　sneezing；　coughing；　vomiting；　etc。　The　reflex　function　also　admits　of　being　permanently　diminished　or　augmented　and　of　taking　on　some　other　morbid　forms；　of　which　I　shall　treat　hereafter。

〃Before　I　proceed　to　the　details　of　the　experiments　upon　which　this　disposition　rests；　it　may　be　well　to　point　out　several　instances　in　illustration　of　the　various　sources　of　and　the　modes　of　muscular　action　which　have　been　enumerated。　None　can　be　more　familiar　than　the　act　of　swallowing。　Yet　how　complicated　is　the　act！　　The　apprehension　of　the　food　by　the　teeth　and　tongue；　etc。；　is　voluntary；　and　cannot；　therefore；　take　place　in　an　animal　from　which　the　cerebrum　is　removed。　The　transition　of　food　over　the　glottis　and　along　the　middle　and　lower　part　of　the　pharynx　depends　upon　the　reflex　action：　it　can　take　place　in　animals　from　which　the　cerebrum　has　been　removed　or　the　ninth　pair　of　nerves　divided；　but　it　requires　the　connection　with　the　medulla　oblongata　to　be　preserved　entirely；　and　the　actual　contact　of　some　substance　which　may　act　as　a　stimulus：　　it　is　attended　by　the　accurate　closure　of　the　glottis　and　by　the　contraction　of　the　pharynx。　The　completion　of　the　act　of　deglutition　is　dependent　upon　the　stimulus　immediately　impressed　upon　the　muscular　fibre　of　the　oesophagus；　and　is　the　result　of　excited　irritability。

〃However　plain　these　observations　may　have　made　the　fact　that　there　is　a　function　of　the　nervous　muscular　system　distinct　from　sensation；　from　the　voluntary　and　respiratory　motions；　and　from　irritability；　it　is　right；　in　every　such　inquiry　as　the　present；　that　the　statements　and　reasonings　should　be　made　with　the　experiment；　as　it　were；　actually　before　us。　It　has　already　been　remarked　that　the　voluntary　and　respiratory　motions　are　spontaneous；　not　necessarily　requiring　the　agency　of　a　stimulus。　If；　then；　an　animal　can　be　placed　in　such　circumstances　that　such　motions　will　certainly　not　take　place；　the　power　of　moving　remaining；　it　may　be　concluded　that　volition　and　the　motive　influence　of　respiration　are　annihilated。　Now　this　is　effected　by　removing　the　cerebrum　and　the　medulla　oblongata。　These　facts　are　fully　proved　by　the　experiments　of　Legallois　and　M。　Flourens；　and　by　several　which　I　proceed　to　detail；　for　the　sake　of　the　opportunity　afforded　by　doing　so　of　stating　the　arguments　most　clearly。

〃I　divided　the　spinal　marrow　of　a　very　lively　snake　between　the　second　and　third　vertebrae。　　The　movements　of　the　animal　were　immediately　before　extremely　vigorous　and　unintermitted。　From　the　moment　of　the　division　of　the　spinal　marrow　it　lay　perfectly　tranquil　and　motionless；　with　the　exception　of　occasional　gaspings　and　slight　movements　of　the　head。　It　became　quite　evident　that　this　state　of　quiescence　would　continue　indefinitely　were　the　animal　secured　from　all　external　impressions。

〃Being　now　stimulated；　the　body　began　to　move　with　great　activity；　and　continued　to　do　so　for　a　considerable　time；　each　change　of　position　or　situation　bringing　some　fresh　part　of　the　surface　of　the　animal　into　contact　with　the　table　or　other　objects　and　renewing　the　application　of　stimulants。

〃At　length　the　animal　became　again　quiescent；　and　being　carefully　protected　from　all　external　impressions　it　moved　no　more；　but　died　in　the　precise　position　and　form　which　it　had　last　assumed。

〃It　requires　a　little　manoeuvre　to　perform　this　experiment　successfully：　the　motions　of　the　animal　must　be　watched　and　slowly　and　cautiously　arrested　by　opposing　some　soft　substance；　as　a　glove　or　cotton　wool；　they　are　by　this　means　gradually　lulled　into　quiescence。　The　slightest　touch　with　a　hard　substance；　the　slightest　stimulus；　will；　on　the　other　hand；　renew　the　movements　on　the　animal　in　an　active　form。　But　that　this　phenomenon　does　not　depend　upon　sensation　is　further　fully　proved　by　the　facts　that　the　position　last　assumed；　and　the　stimuli；　may　be　such　as　would　be　attended　by　extreme　or　continued　pain；　if　the　sensibility　were　undestroyed：　　in　one　case　the　animal　remained　partially　suspended　over　the　acute　edge　of　the　table；　in　others　the　infliction　of　punctures　and　the　application　of　a　lighted　taper　did　not　prevent　the　animal；　still　possessed　of　active　powers　of　motion；　from　passing　into　a　state　of　complete　and　permanent　quiescence。〃


In　summing　up　this　long　paper　Hall　concludes　with　this　sentence：　〃The　reflex　function　appears　in　a　word　to　be　the　COMPLEMENT　of　the　functions　of　the　nervous　system　hitherto　known。〃＇2＇

All　these　considerations　as　to　nerve　currents　and　nerve　tracts　becoming　stock　knowledge　of　science；　it　was　natural　that　interest　should　become　stimulated　as　to　the　exact　character　of　these　nerve　tracts　in　themselves；　and　all　the　more　natural　in　that　the　perfected　microscope　was　just　now　claiming　all　fields　for　its　own。　A　troop　of　observers　soon　entered　upon　the　study　of　the　nerves；　and　the　leader　here；　as　in　so　many　other　lines　of　microscopical　research；　was　no　other　than　Theodor　Schwann。　　Through　his　efforts；　and　with　the　invaluable　aid　of　such　other　workers　as　Remak；　Purkinje；　Henle；　Muller；　and　the　rest；　all　the　mystery　as　to　the　general　characteristics　of　nerve　tracts　was　cleared　away。　It　came　to　be　known　that　in　its　essentials　a　nerve　tract　is　a　tenuous　fibre　or　thread　of　protoplasm　stretching　between　two　terminal　points　in　the　organism；　one　of　such　termini　being　usually　a　cell　of　the　brain　or　spinal　cord；　the　other　a　distribution…point　at　or　near　the　peripheryfor　example；　in　a　muscle　or　in　the　skin。　Such　a　fibril　may　have　about　it　a　protective　covering；　which　is　known　as　the　sheath　of　Schwann；　but　the　fibril　itself　is　the　essential　nerve　tract；　and　in　many　cases；　as　Remak　presently　discovered；　the　sheath　is　dispensed　with；　particularly　in　case　of　the　nerves　of　the　so…called　sympathetic　system。

This　sympathetic　system　of　ganglia　and　nerves；　by…the…bye；　had　long　been　a　puzzle　to　the　physiologists。　　Its　ganglia；　the　seeming　centre　of　the　system；　usually　minute　in　size　and　never　very　large；　are　found　everywhere　through　the　organism；　but　in　particular　are　gathered　into　a　long　double　chain　which　lies　within　the　body　cavity；　outside　the　spinal　column；　and　represents　the　sole　nervous　system　of　the　non…vertebrated　organisms。　Fibrils　from　these　ganglia　were　seen　to　join　the　cranial　and　spinal　nerve　fibrils　and　to　accompany　them　everywhere；　but　what　special　function　they　subserved　was　long　a　mere　matter　of　conjecture　and　led　to　many　absurd　speculations。　　Fact　was　not　substituted　for　conjecture　until　about　the　year　1851；　when　the　great　Frenchman　Claude　Bernard　conclusively　proved　that　at　least　one　chief　function　of　the　sympathetic　fibrils　is　to　cause　contraction　of　the　walls　of　the　arterioles　of　the　system；　thus　regulating　the　blood…supply　of　any　given　part。　Ten　years　earlier　Henle　had　demonstrated　the　existence　of　annular　bands　of　muscle　fibres　in　the　arterioles；　hitherto　a　much…mooted　question；　and　several　tentative　explanations　of　the　action　of　these　fibres　had　been　made；　particularly　by　the　brothers　Weber；　by　Stilling；　who；　as　early　as　1840；　had　ventured　to　speak　of　〃vaso…motor〃　nerves；　and　by　Schiff；　who　was　hard　upon　the　same　track　at　the　time　of　Bernard's　discovery。　But　a　clear　light　was　not　thrown　on　the　subject　until　Bernard's　experiments　were　made　in　1851。　　The　experiments　were　soon　after　confirmed　and　extended　by　Brown…Sequard；　Waller；　Budge；　and　numerous　others；　and　henceforth　physiologists　felt　that　they　understood　how　the　blood…supply　of　any　given　part　is　regulated　by　the　nervous　system。

In　reality；　however；　they　had　learned　only　half　the　story；　as　Bernard　himself　proved　only　a　few　years　later　by　opening　up　a　new　and　quite　unsuspected　chapter。　　While　experimenting　in　1858　he　discovered　that　there　are　certain　nerves　supplying　the　heart　which；　if　stimulated；　cause　that　organ　to　relax　and　cease　beating。　　As　the　heart　is　essentially　nothing　more　than　an　aggregation　of　muscles；　this　phenomenon　was　utterly　puzzling　and　without　precedent　in　the　experience　of　physiologists。　An　impulse　travelling　along　a　motor　nerve　had　been　supposed　to　be　able　to　cause　a　muscular　contraction　and　to　do　nothing　else；　yet　here　such　an　impulse　had　e