a history of science-4-第44节

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th；　and　forbade　the　young　iconoclast　the　privilege　of　further　hearings。　From　which　it　is　evident　that　the　partially　liberated　spirit　of　the　new　psychology　had　by　no　means　freed　itself　altogether；　at　the　close　of　the　first　quarter　of　the　nineteenth　century；　from　the　metaphysical　cobwebs　of　its　long　incarceration。


FUNCTIONS　OF　THE　NERVES

While　studies　of　the　brain　were　thus　being　inaugurated；　the　nervous　system；　which　is　the　channel　of　communication　between　the　brain　and　the　outside　world；　was　being　interrogated　with　even　more　tangible　results。　　The　inaugural　discovery　was　made　in　1811　by　Dr。　（afterwards　Sir　Charles）　Bell；＇1＇　the　famous　English　surgeon　and　experimental　physiologist。　It　consisted　of　the　observation　that　the　anterior　roots　of　the　spinal　nerves　are　given　over　to　the　function　of　conveying　motor　impulses　from　the　brain　outward；　whereas　the　posterior　roots　convey　solely　sensory　impulses　to　the　brain　from　without。　Hitherto　it　had　been　supposed　that　all　nerves　have　a　similar　function；　and　the　peculiar　distribution　of　the　spinal　nerves　had　been　an　unsolved　puzzle。

Bell's　discovery　was　epochal；　but　its　full　significance　was　not　appreciated　for　a　decade；　nor；　indeed；　was　its　validity　at　first　admitted。　　In　Paris；　in　particular；　then　the　court　of　final　appeal　in　all　matters　scientific；　the　alleged　discovery　was　looked　at　askance；　or　quite　ignored。　　But　in　1823　the　subject　was　taken　up　by　the　recognized　leader　of　French　physiologyFrancois　Magendiein　the　course　of　his　comprehensive　experimental　studies　of　the　nervous　system；　and　Bell's　conclusions　were　subjected　to　the　most　rigid　experimental　tests　and　found　altogether　valid。　Bell　himself；　meanwhile；　had　turned　his　attention　to　the　cranial　nerves；　and　had　proved　that　these　also　are　divisible　into　two　setssensory　and　motor。　　Sometimes；　indeed；　the　two　sets　of　filaments　are　combined　into　one　nerve　cord；　but　if　traced　to　their　origin　these　are　found　to　arise　from　different　brain　centres。　Thus　it　was　clear　that　a　hitherto　unrecognized　duality　of　function　pertains　to　the　entire　extra…cranial　nervous　system。　Any　impulse　sent　from　the　periphery　to　the　brain　must　be　conveyed　along　a　perfectly　definite　channel；　the　response　from　the　brain；　sent　out　to　the　peripheral　muscles；　must　traverse　an　equally　definite　and　altogether　different　course。　　If　either　channel　is　interruptedas　by　the　section　of　its　particular　nerve　tractthe　corresponding　message　is　denied　transmission　as　effectually　as　an　electric　current　is　stopped　by　the　section　of　the　transmitting　wire。

Experimenters　everywhere　soon　confirmed　the　observations　of　Bell　and　Magendie；　and；　as　always　happens　after　a　great　discovery；　a　fresh　impulse　was　given　to　investigations　in　allied　fields。　　Nevertheless；　a　full　decade　elapsed　before　another　discovery　of　comparable　importance　was　made。　Then　Marshall　Hall；　the　most　famous　of　English　physicians　of　his　day；　made　his　classical　observations　on　the　phenomena　that　henceforth　were　to　be　known　as　reflex　action。　　In　1832；　while　experimenting　one　day　with　a　decapitated　newt；　he　observed　that　the　headless　creature's　limbs　would　contract　in　direct　response　to　certain　stimuli。　　Such　a　response　could　no　longer　be　secured　if　the　spinal　nerves　supplying　a　part　were　severed。　Hence　it　was　clear　that　responsive　centres　exist　in　the　spinal　cord　capable　of　receiving　a　sensory　message　and　of　transmitting　a　motor　impulse　in　replya　function　hitherto　supposed　to　be　reserved　for　the　brain。　Further　studies　went　to　show　that　such　phenomena　of　reflex　action　on　the　part　of　centres　lying　outside　the　range　of　consciousness；　both　in　the　spinal　cord　and　in　the　brain　itself；　are　extremely　common；　that；　in　short；　they　enter　constantly　into　the　activities　of　every　living　organism　and　have　a　most　important　share　in　the　sum　total　of　vital　movements。　Hence；　Hall's　discovery　must　always　stand　as　one　of　the　great　mile…stones　of　the　advance　of　neurological　science。

Hall　gave　an　admirably　clear　and　interesting　account　of　his　experiments　and　conclusions　in　a　paper　before　the　Royal　Society；　〃On　the　Reflex　Functions　of　the　Medulla　Oblongata　and　the　Medulla　Spinalis；〃　from　which；　as　published　in　the　Transactions　of　the　society　for　1833；　we　may　quote　at　some　length：

〃In　the　entire　animal；　sensation　and　voluntary　motion；　functions　of　the　cerebrum；　combine　with　the　functions　of　the　medulla　oblongata　and　medulla　spinalis；　and　may　therefore　render　it　difficult　or　impossible　to　determine　those　which　are　peculiar　to　each；　if；　in　an　animal　deprived　of　the　brain；　the　spinal　marrow　or　the　nerves　supplying　the　muscles　be　stimulated；　those　muscles；　whether　voluntary　or　respiratory；　are　equally　thrown　into　contraction；　and；　it　may　be　added；　equally　in　the　complete　and　in　the　mutilated　animal；　and；　in　the　case　of　the　nerves；　equally　in　limbs　connected　with　and　detached　from　the　spinal　marrow。

〃The　operation　of　all　these　various　causes　may　be　designated　centric；　as　taking　place　AT；　or　at　least　in　a　direction　FROM；　central　parts　of　the　nervous　system。　　But　there　is　another　function　the　phenomena　of　which　are　of　a　totally　different　order　and　obey　totally　different　laws；　being　excited　by　causes　in　a　situation　which　is　EXCENTRIC　in　the　nervous　systemthat　is；　distant　from　the　nervous　centres。　This　mode　of　action　has　not；　I　think；　been　hitherto　distinctly　understood　by　physiologists。

〃Many　of　the　phenomena　of　this　principle　of　action；　as　they　occur　in　the　limbs；　have　certainly　been　observed。　　But；　in　the　first　place；　this　function　is　by　no　means　confined　to　the　limbs；　for；　while　it　imparts　to　each　muscle　its　appropriate　tone；　and　to　each　system　of　muscles　its　appropriate　equilibrium　or　balance；　it　performs　the　still　more　important　office　of　presiding　over　the　orifices　and　terminations　of　each　of　the　internal　canals　in　the　animal　economy；　giving　them　their　due　form　and　action；　and；　in　the　second　place；　in　the　instances　in　which　the　phenomena　of　this　function　have　been　noticed；　they　have　been　confounded；　as　I　have　stated；　with　those　of　sensation　and　volition；　or；　if　they　have　been　distinguished　from　these；　they　have　been　too　indefinitely　denominated　instinctive；　or　automatic。　I　have　been　compelled；　therefore；　to　adopt　some　new　designation　for　them；　and　I　shall　now　give　the　reasons　for　my　choice　of　that　which　is　given　in　the　title　of　this　paper'Reflex　Functions。'

〃This　property　is　characterized　by　being　EXCITED　in　its　action　and　REFLEX　in　its　course：　　in　every　instance　in　which　it　is　exerted　an　impression　made　upon　the　extremities　of　certain　nerves　is　conveyed　to　the　medulla　oblongata　or　the　medulla　spinalis；　and　is　reflected　along　the　nerves　to　parts　adjacent　to；　or　remote　from；　that　which　has　received　the　impression。

〃It　is　by　this　reflex　character　that　the　function　to　which　I　have　alluded　is　to　be　distinguished　from　every　other。　There　are；　in　the　animal　economy；　four　modes　of　muscular　action；　of　muscular　contraction。　　The　first　is　that　designated　VOLUNTARY：　volition；　originated　in　the　cerebrum　and　spontaneous　in　its　acts；　extends　its　influence　along　the　spinal　marrow　and　the　motor　nerves　in　a　DIRECT　LINE　to　the　voluntary　muscles。　The　SECOND　is　that　of　RESPIRATION：　　like　volition；　the　motive　influence　in　respiration　passes　in　a　DIRECT　LINE　from　one　point　of　the　nervous　system　to　certain　muscles；　but　as　voluntary　motion　seems　to　originate　in　the　cerebrum；　so　the　respiratory　motions　originate　in　the　medulla　oblongata：　like　the　voluntary　motions；　the　motions　of　respirations　are　spontaneous；　they　continue；　at　least；　after　the　eighth　pair　of　nerves　have　been　divided。　　The　THIRD　kind　of　muscular　action　in　the　animal　economy　is　that　termed　involuntary：　　it　depends　upon　the　principle　of　irritability　and　requires　the　IMMEDIATE　application　of　a　stimulus　to　the　nervo…muscular　fibre　itself。　These　three　kinds　of　muscular　motion　are　well　known　to　physiologists；　and　I　believe　they　are　all　which　have　been　hitherto　pointed　out。　There　is；　however；　a　FOURTH；　which　subsists；　in　part；　after　the　voluntary　and　respiratory　motions　have　ceased；　by　the　removal　of　the　cerebrum　and　medulla　oblongata；　and　which　is　attached　to　the　medulla　spinalis；　ceasing　itself　when　this　is　removed；　and　leaving　the　irritability　undiminished。　In　this　kind　of　muscular　motion　the　motive　influence　does　not　originate　in　any　central　part　of　the　nervous　system；　but　from　a　distance　from　that　centre；　it　is　neither　spontaneous　in　its　action　nor　direct　in　its　course；　it　is；　on　the　contrary；　EXCITED　by　the　application　of　appropriate　stimuli；　which　are　not；　however；　applied　immediately　to　the　muscular　or　nervo…muscular　fibre；　but　to　certain　membraneous　parts；　whence　the　impression　is　carried　through　the　medulla；　REFLECTED　and　reconducted　to　the　part　impressed；　or　conducted　to　a　part　remote　from　it　in　which　muscular　contraction　is　effected。

〃The　first　three　modes　of　muscular　action　are　known　only　by　actual　movements　of　muscular　contractions。　　But　the　reflex　function　exists　as　a　continuous　muscular　action；　as　a　power　presiding　over　organs　not　actually　in　a　state　of　motion；　preserving　in　some；　as　the　glottis；　an　open；　in　others；　as　the　sphincters；　a　closed　form；　and　in　the　limbs　a　due　degree　of　equilibrium　or　balanced　muscular　actiona　function　not；　I　think；　hitherto　recognized　by　physiologis