a history of science-4-第54章

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in　the　frontal　lobe；　and　here　and　there　other　observers　had　made　tentatives　in　the　same　direction。　Boillard　had　even　followed　the　matter　up　with　pertinacity；　but　the　world　was　not　ready　to　listen　to　him。　　Now；　however；　in　the　half…decade　that　followed　Broca's　announcements；　interest　rose　to　fever…beat；　and　through　the　efforts　of　Broca；　Boillard；　and　numerous　others　it　was　proved　that　a　veritable　centre　having　a　strange　domination　over　the　memory　of　articulate　words　has　its　seat　in　the　third　convolution　of　the　frontal　lobe　of　the　cerebrum；　usually　in　the　left　hemisphere。　That　part　of　the　brain　has　since　been　known　to　the　English…speaking　world　as　the　convolution　of　Broca；　a　name　which；　strangely　enough；　the　discoverer's　compatriots　have　been　slow　to　accept。

This　discovery　very　naturally　reopened　the　entire　subject　of　brain　localization。　　It　was　but　a　short　step　to　the　inference　that　there　must　be　other　definite　centres　worth　the　seeking；　and　various　observers　set　about　searching　for　them。　　In　1867　a　clew　was　gained　by　Eckhard；　who；　repeating　a　forgotten　experiment　by　Haller　and　Zinn　of　the　previous　century；　removed　portions　of　the　brain　cortex　of　animals；　with　the　result　of　producing　convulsions。　But　the　really　vital　departure　was　made　in　1870　by　the　German　investigators　Fritsch　and　Hitzig；　who；　by　stimulating　definite　areas　of　the　cortex　of　animals　with　a　galvanic　current；　produced　contraction　of　definite　sets　of　muscles　of　the　opposite　side　of　the　body。　These　most　important　experiments；　received　at　first　with　incredulity；　were　repeated　and　extended　in　1873　by　Dr。　David　Ferrier；　of　London；　and　soon　afterwards　by　a　small　army　of　independent　workers　everywhere；　prominent　among　whom　were　Franck　and　Pitres　in　France；　Munck　and　Goltz　in　Germany；　and　Horsley　and　Schafer　in　England。　　The　detailed　results；　naturally　enough；　were　not　at　first　all　in　harmony。　　Some　observers；　as　Goltz；　even　denied　the　validity　of　the　conclusions　in　toto。　But　a　consensus　of　opinion；　based　on　multitudes　of　experiments；　soon　placed　the　broad　general　facts　for　which　Fritsch　and　Hitzig　contended　beyond　controversy。　　It　was　found；　indeed；　that　the　cerebral　centres　of　motor　activities　have　not　quite　the　finality　at　first　ascribed　to　them　by　some　observers；　since　it　may　often　happen　that　after　the　destruction　of　a　centre；　with　attending　loss　of　function；　there　may　be　a　gradual　restoration　of　the　lost　function；　proving　that　other　centres　have　acquired　the　capacity　to　take　the　place　of　the　one　destroyed。　　There　are　limits　to　this　capacity　for　substitution；　however；　and　with　this　qualification　the　definiteness　of　the　localization　of　motor　functions　in　the　cerebral　cortex　has　become　an　accepted　part　of　brain　physiology。

Nor　is　such　localization　confined　to　motor　centres。　Later　experiments；　particularly　of　Ferrier　and　of　Munck；　proved　that　the　centres　of　vision　are　equally　restricted　in　their　location；　this　time　in　the　posterior　lobes　of　the　brain；　and　that　hearing　has　likewise　its　local　habitation。　Indeed；　there　is　every　reason　to　believe　that　each　form　of　primary　sensation　is　based　on　impressions　which　mainly　come　to　a　definitely　localized　goal　in　the　brain。　　But　all　this；　be　it　understood；　has　no　reference　to　the　higher　forms　of　intellection。　All　experiment　has　proved　futile　to　localize　these　functions；　except　indeed　to　the　extent　of　corroborating　the　familiar　fact　of　their　dependence　upon　the　brain；　and；　somewhat　problematically；　upon　the　anterior　lobes　of　the　cerebrum　in　particular。　But　this　is　precisely　what　should　be　expected；　for　the　clearer　insight　into　the　nature　of　mental　processes　makes　it　plain　that　in　the　main　these　alleged　〃faculties〃　are　not　in　themselves　localized。　Thus；　for　example；　the　〃faculty〃　of　language　is　associated　irrevocably　with　centres　of　vision；　of　hearing；　and　of　muscular　activity；　to　go　no　further；　and　only　becomes　possible　through　the　association　of　these　widely　separated　centres。　The　destruction　of　Broca's　centre；　as　was　early　discovered；　does　not　altogether　deprive　a　patient　of　his　knowledge　of　language。　He　may　be　totally　unable　to　speak　（though　as　to　this　there　are　all　degrees　of　variation）；　and　yet　may　comprehend　what　is　said　to　him；　and　be　able　to　read；　think；　and　even　write　correctly。　Thus　it　appears　that　Broca's　centre　is　peculiarly　bound　up　with　the　capacity　for　articulate　speech；　but　is　far　enough　from　being　the　seat　of　the　faculty　of　language　in　its　entirety。

In　a　similar　way；　most　of　the　supposed　isolated　〃faculties〃　of　higher　intellection　appear；　upon　clearer　analysis；　as　complex　aggregations　of　primary　sensations；　and　hence　necessarily　dependent　upon　numerous　and　scattered　centres。　Some　〃faculties；〃　as　memory　and　volition；　may　be　said　in　a　sense　to　be　primordial　endowments　of　every　nerve　celleven　of　every　body　cell。　　Indeed；　an　ultimate　analysis　relegates　all　intellection；　in　its　primordial　adumbrations；　to　every　particle　of　living　matter。　But　such　refinements　of　analysis；　after　all；　cannot　hide　the　fact　that　certain　forms　of　higher　intellection　involve　a　pretty　definite　collocation　and　elaboration　of　special　sensations。　Such　specialization；　indeed；　seems　a　necessary　accompaniment　of　mental　evolution。　　That　every　such　specialized　function　has　its　localized　centres　of　co…ordination；　of　some　such　significance　as　the　demonstrated　centres　of　articulate　speech；　can　hardly　be　in　doubtthough　this；　be　it　understood；　is　an　induction；　not　as　yet　a　demonstration。　　In　other　words；　there　is　every　reason　to　believe　that　numerous　〃centres；〃　in　this　restricted　sense；　exist　in　the　brain　that　have　as　yet　eluded　the　investigator。　Indeed；　the　current　conception　regards　the　entire　cerebral　cortex　as　chiefly　composed　of　centres　of　ultimate　co…ordination　of　impressions；　which　in　their　cruder　form　are　received　by　more　primitive　nervous　tissuesthe　basal　ganglia；　the　cerebellum　and　medulla；　and　the　spinal　cord。

This；　of　course；　is　equivalent　to　postulating　the　cerebral　cortex　as　the　exclusive　seat　of　higher　intellection。　This　proposition；　however；　to　which　a　safe　induction　seems　to　lead；　is　far　afield　from　the　substantiation　of　the　old　conception　of　brain　localization；　which　was　based　on　faulty　psychology　and　equally　faulty　inductions　from　few　premises。　The　details　of　Gall's　system；　as　propounded　by　generations　of　his　mostly　unworthy　followers；　lie　quite　beyond　the　pale　of　scientific　discussion。　　Yet；　as　I　have　said；　a　germ　of　truth　was　therethe　idea　of　specialization　of　cerebral　functionsand　modern　investigators　have　rescued　that　central　conception　from　the　phrenological　rubbish　heap　in　which　its　discoverer　unfortunately　left　it　buried。


THE　MINUTE　STRUCTURE　OF　THE　BRAIN

The　common　ground　of　all　these　various　lines　of　investigations　of　pathologist；　anatomist；　physiologist；　physicist；　and　psychologist　is；　clearly；　the　central　nervous　systemthe　spinal　cord　and　the　brain。　The　importance　of　these　structures　as　the　foci　of　nervous　and　mental　activities　has　been　recognized　more　and　more　with　each　new　accretion　of　knowledge；　and　the　efforts　to　fathom　the　secrets　of　their　intimate　structure　has　been　unceasing。　For　the　earlier　students；　only　the　crude　methods　of　gross　dissections　and　microscopical　inspection　were　available。　These　could　reveal　something；　but　of　course　the　inner　secrets　were　for　the　keener　insight　of　the　microscopist　alone。　And　even　for　him　the　task　of　investigation　was　far　from　facile；　for　the　central　nervous　tissues　are　the　most　delicate　and　fragile；　and　on　many　accounts　the　most　difficult　of　manipulation　of　any　in　the　body。

Special　methods；　therefore；　were　needed　for　this　essay；　and　brain　histology　has　progressed　by　fitful　impulses；　each　forward　jet　marking　the　introduction　of　some　ingenious　improvement　of　mechanical　technique；　which　placed　a　new　weapon　in　the　hands　of　the　investigators。

The　very　beginning　was　made　in　1824　by　Rolando；　who　first　thought　of　cutting　chemically　hardened　pieces　of　brain　tissues　into　thin　sections　for　microscopical　examinationthe　basal　structure　upon　which　almost　all　the　later　advances　have　been　conducted。　Muller　presently　discovered　that　bichromate　of　potassium　in　solution　makes　the　best　of　fluids　for　the　preliminary　preservation　and　hardening　of　the　tissues。　　Stilling；　in　1842；　perfected　th