a history of science-4-第51章

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nge　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　exactly　the　opposite　effect。　The　only　tenable　explanation　seemed　to　be　that　this　particular　impulse　must　arrest　or　inhibit　the　action　of　the　impulses　that　ordinarily　cause　the　heart　muscles　to　contract。　But　the　idea　of　such　inhibition　of　one　impulse　by　another　was　utterly　novel　and　at　first　difficult　to　comprehend。　Gradually；　however；　the　idea　took　its　place　in　the　current　knowledge　of　nerve　physiology；　and　in　time　it　came　to　be　understood　that　what　happens　in　the　case　of　the　heart　nerve…supply　is　only　a　particular　case　under　a　very　general；　indeed　universal；　form　of　nervous　action。　　Growing　out　of　Bernard's　initial　discovery　came　the　final　understanding　that　the　entire　nervous　system　is　a　mechanism　of　centres　subordinate　and　centres　superior；　the　action　of　the　one　of　which　may　be　counteracted　and　annulled　in　effect　by　the　action　of　the　other。　　This　applies　not　merely　to　such　physical　processes　as　heart…beats　and　arterial　contraction　and　relaxing；　but　to　the　most　intricate　functionings　which　have　their　counterpart　in　psychical　processes　as　well。　Thus　the　observation　of　the　inhibition　of　the　heart's　action　by　a　nervous　impulse　furnished　the　point　of　departure　for　studies　that　led　to　a　better　understanding　of　the　modus　operandi　of　the　mind's　activities　than　had　ever　previously　been　attained　by　the　most　subtle　of　psychologists。


PSYCHO…PHYSICS

The　work　of　the　nerve　physiologists　had　thus　an　important　bearing　on　questions　of　the　mind。　　But　there　was　another　company　of　workers　of　this　period　who　made　an　even　more　direct　assault　upon　the　〃citadel　of　thought。〃　A　remarkable　school　of　workers　had　been　developed　in　Germany；　the　leaders　being　men　who；　having　more　or　less　of　innate　metaphysical　bias　as　a　national　birthright；　had　also　the　instincts　of　the　empirical　scientist；　and　whose　educational　equipment　included　a　profound　knowledge　not　alone　of　physiology　and　psychology；　but　of　physics　and　mathematics　as　well。　These　men　undertook　the　novel　task　of　interrogating　the　relations　of　body　and　mind　from　the　standpoint　of　physics。　　They　sought　to　apply　the　vernier　and　the　balance；　as　far　as　might　be；　to　the　intangible　processes　of　mind。

The　movement　had　its　precursory　stages　in　the　early　part　of　the　century；　notably　in　the　mathematical　psychology　of　Herbart；　but　its　first　definite　output　to　attract　general　attention　came　from　the　master…hand　of　Hermann　Helmholtz　in　1851。　It　consisted　of　the　accurate　measurement　of　the　speed　of　transit　of　a　nervous　impulse　along　a　nerve　tract。　　To　make　such　measurement　had　been　regarded　as　impossible；　it　being　supposed　that　the　flight　of　the　nervous　impulse　was　practically　instantaneous。　But　Helmholtz　readily　demonstrated　the　contrary；　showing　that　the　nerve　cord　is　a　relatively　sluggish　message…bearer。　According　to　his　experiments；　first　performed　upon　the　frog；　the　nervous　〃current〃　travels　less　than　one　hundred　feet　per　second。　Other　experiments　performed　soon　afterwards　by　Helmholtz　himself；　and　by　various　followers；　chief　among　whom　was　Du　Bois…Reymond；　modified　somewhat　the　exact　figur