第28章

Thecommongroundofallthesevariouslinesofinvestigationsofpathologist，anatomist，physiologist，physicist，andpsychologistis，clearly，thecentralnervoussystem——thespinalcordandthebrain。Theimportanceofthesestructuresasthefociofnervousandmentalactivitieshasbeenrecognizedmoreandmorewitheachnewaccretionofknowledge，andtheeffortstofathomthesecretsoftheirintimatestructurehasbeenunceasing。Fortheearlierstudents，onlythecrudemethodsofgrossdissectionsandmicroscopicalinspectionwereavailable。Thesecouldrevealsomething，butofcoursetheinnersecretswereforthekeenerinsightofthemicroscopistalone。Andevenforhimthetaskofinvestigationwasfarfromfacile，forthecentralnervoustissuesarethemostdelicateandfragile，andonmanyaccountsthemostdifficultofmanipulationofanyinthebody。

Specialmethods，therefore，wereneededforthisessay，andbrainhistologyhasprogressedbyfitfulimpulses，eachforwardjetmarkingtheintroductionofsomeingeniousimprovementofmechanicaltechnique，whichplacedanewweaponinthehandsoftheinvestigators。

Theverybeginningwasmadein1824byRolando，whofirstthoughtofcuttingchemicallyhardenedpiecesofbraintissuesintothinsectionsformicroscopicalexamination——thebasalstructureuponwhichalmostallthelateradvanceshavebeenconducted。Mullerpresentlydiscoveredthatbichromateofpotassiuminsolutionmakesthebestoffluidsforthepreliminarypreservationandhardeningofthetissues。Stilling，in1842，perfectedthemethodbyintroducingthecustomofcuttingaseriesofconsecutivesectionsofthesametissue，inordertotracenervetractsandestablishspacialrelations。Thenfromtimetotimemechanicalingenuityaddedfreshdetailsofimprovement。Itwasfoundthatpiecesofhardenedtissueofextremedelicacycanbemadebettersubjecttomanipulationbybeingimpregnatedwithcollodionorcelloidineandembeddedinparaffine。Latterlyithasbecomeusualtocutsectionsalsofromfreshtissues，unchangedbychemicals，byfreezingthemsuddenlywithvaporizedetheror，better，carbonicacid。Bythesemethods，andwiththeaidofperfectedmicrotomes，theworkerofrecentperiodsavailshimselfofsectionsofbraintissuesofatenuousnesswhichtheearlyinvestigatorscouldnotapproach。

Butmoreimportanteventhanthecuttingofthinsectionsistheprocessofmakingthedifferentpartsofthesectionvisible，onetissuedifferentiatedfromanother。Thethinsection，astheearlyworkersexaminedit，waspracticallycolorless，andeventhecrudestdetailsofitsstructureweremadeoutwithextremedifficulty。Remakdid，indeed，managetodiscoverthatthebraintissueiscellular，asearlyas1833，andEhrenberginthesameyearsawthatitisalsofibrillar，butbeyondthisnogreatadvancewasmadeuntil1858，whenasuddenimpulsewasreceivedfromanewprocessintroducedbyGerlach。Theprocessitselfwasmostsimple，consistingessentiallyofnothingmorethanthetreatmentofamicroscopicalsectionwithasolutionofcarmine。

Buttheresultwaswonderful，forwhensuchasectionwasplacedunderthelensitnolongerappearedhomogeneous。Sprinkledthroughitssubstancewereseenirregularbodiesthathadtakenonabeautifulcolor，whilethematrixinwhichtheywereembeddedremainedunstained。Inaword，thecentralnervecellhadsprungsuddenlyintoclearview。

Amostinterestingbodyitproved，thisnervecell，organglioncell，asitcametobecalled。Itwasseentobeexceedinglyminuteinsize，requiringhighpowersofthemicroscopetomakeitvisible。Itexistsinalmostinfinitenumbers，not，however，scatteredatrandomthroughthebrainandspinalcord。Onthecontrary，itisconfinedtothoseportionsofthecentralnervousmasseswhichtothenakedeyeappeargrayincolor，beingaltogetherwantinginthewhitesubstancewhichmakesupthechiefmassofthebrain。Eveninthegraymatter，thoughsometimesthicklydistributed，theganglioncellsareneverinactualcontactonewithanother；theyalwayslieembeddedinintercellulartissues，whichcametobeknown，followingVirchow，astheneuroglia。

Eachganglioncellwasseentobeirregularincontour，andtohavejuttingoutfromittwosetsofminutefibres，onesetrelativelyshort，indefinitelynumerous，andbranchingineverydirection；theothersetlimitedinnumber，sometimesevensingle，andstartingoutdirectlyfromthecellasifbentonalongerjourney。Thenumerousfilamentscametobeknownasprotoplasmicprocesses；theotherfibrewasnamed，afteritsdiscoverer，theaxiscylinderofDeiters。Itwasanaturalinference，thoughnotclearlydemonstrableinthesections，thatthesefilamentousprocessesaretheconnectinglinksbetweenthedifferentnervecellsandalsothechannelsofcommunicationbetweennervecellsandtheperipheryofthebody。Thewhitesubstanceofbrainandcord，apparently，ismadeupofsuchconnectingfibres，thusbringingthedifferentganglioncellseverywhereintocommunicationonewithanother。

Intheattempttotracetheconnectingnervetractsthroughthiswhitesubstancebyeithermacroscopicalormicroscopicalmethods，mostimportantaidisgivenbyamethodoriginatedbyWallerin1852。Earlierthanthat，in1839，Nassehaddiscoveredthataseverednervecorddegeneratesinitsperipheralportions。Wallerdiscoveredthateverynervefibre，sensoryormotor，hasanervecelltoorfromwhichitleads，whichdominatesitsnutrition，sothatitcanonlyretainitsvitalitywhileitsconnectionwiththatcellisintact。Suchcellshenamedtrophiccentres。

Certaincellsoftheanteriorpartofthespinalcord，forexample，arethetrophiccentresofthespinalmotornerves。

Othertrophiccentres，governingnervetractsinthespinalcorditself，areinthevariousregionsofthebrain。ItoccurredtoWallerthatbydestroyingsuchcentres，orbyseveringtheconnectionatvariousregionsbetweenanervoustractanditstrophiccentre，sharplydefinedtractscouldbemadetodegenerate，andtheirlocationcouldsubsequentlybeaccuratelydefined，asthedegeneratedtissuestakeonachangedaspect，bothtomacroscopicalandmicroscopicalobservation。Recognitionofthisprinciplethusgavetheexperimenteranewweaponofgreatefficiencyintracingnervousconnections。Moreover，thesameprinciplehaswideapplicationincaseofthehumansubjectindisease，suchasthelesionofnervetractsorthedestructionofcentresbylocalizedtumors，byembolisms，orbytraumatisms。

Allthesevariousmethodsofanatomicalexaminationcombinetomaketheconclusionalmostunavoidablethatthecentralganglioncellsaretheveritable"centres"ofnervousactivitytowhichsomanyotherlinesofresearchhavepointed。Theconclusionwasstrengthenedbyexperimentsofthestudentsofmotorlocalization，whichshowedthattheveritablecentresoftheirdiscoverylie，demonstrably，inthegraycortexofthebrain，notinthewhitematter。Butthefullproofcamefrompathology。Atthehandsofamultitudeofobserversitwasshownthatincertainwell-knowndiseasesofthespinalcord，withresultingparalysis，itistheganglioncellsthemselvesthatarefoundtobedestroyed。Similarly，inthecaseofsufferersfromchronicinsanities，withmarkeddementia，theganglioncellsofthecortexofthebrainarefoundtohaveundergonedegeneration。Thebrainsofpareticsinparticularshowsuchdegeneration，instrikingcorrespondencewiththeirmentaldecadence。Thepositionoftheganglioncellastheultimatecentreofnervousactivitieswasthusplacedbeyonddispute。

Meantime，generalacceptancebeinggiventhehistologicalschemeofGerlach，accordingtowhichthemassofthewhitesubstanceofthebrainisamesh-workofintercellularfibrils，aproximalideaseemedattainableofthewayinwhichtheganglionicactivitiesarecorrelated，and，throughassociation，builtup，sotospeak，intothehighermentalprocesses。Suchaconceptionaccordedbeautifullywiththeideasoftheassociationists，whohadnowbecomedominantinpsychology。Butonestandingpuzzleattendedthisotherwisesatisfactorycorrelationofanatomicalobservationsandpsychicanalyses。Itwasthis:Since，accordingtothehistologist，theintercellularfibres，alongwhichimpulsesareconveyed，connecteachbraincell，directlyorindirectly，witheveryotherbraincellinanendlessmesh-work，howisitpossiblethatvarioussetsofcellsmayattimesbeshutofffromoneanother？Suchisolationmusttakeplace，forallnormalideationdependsforitsintegrityquiteasmuchupontheshutting-outofthegreatmassofassociationsasupontheinclusionofcertainotherassociations。Forexample，astudentinsolvingamathematicalproblemmustforthemomentbecomequiteoblivioustothespecialassociationsthathavetodowithgeography，naturalhistory，andthelike。Butdoeshistologygiveanyclewtothewayinwhichsuchisolationmaybeeffected？

Attemptsweremadetofindananswerthroughconsiderationoftheverypeculiarcharacteroftheblood-supplyinthebrain。Here，asnowhereelse，theterminaltwigsofthearteriesarearrangedinclosedsystems，notanastomosingfreelywithneighboringsystems。Clearly，then，arestrictedareaofthebrainmay，throughthecontrollinginfluenceofthevasomotornerves，beflushedwitharterialbloodwhileneighboringpartsremainrelativelyanaemic。Andsincevitalactivitiesunquestionablydependinpartuponthesupplyofarterialblood，thispeculiararrangementofthevascularmechanismmayveryproperlybesupposedtoaidinthelocalizedactivitiesofthecentralnervousganglia。Butthisexplanationleftmuchtobedesired——inparticularwhenitisrecalledthatallhigherintellectionmustinallprobabilityinvolvemultitudesofwidelyscatteredcentres。

Nobetterexplanationwasforthcoming，however，untiltheyear1889，whenofasuddenthemysterywasclearedawaybyafreshdiscovery。NotlongbeforethistheItalianhistologistDr。

CamilleGolgihaddiscoveredamethodofimpregnatinghardenedbraintissueswithasolutionofnitrateofsilver，withtheresultofstainingthenervecellsandtheirprocessesalmostinfinitelybetterthanwaspossiblebythemethodsofGerlach，orbyanyofthemultiformmethodsthatotherworkershadintroduced。Nowforthefirsttimeitbecamepossibletotracethecellularprolongationsdefinitelytotheirtermini，forthefinerfibrilshadnotbeenrenderedvisiblebyanypreviousmethodoftreatment。Golgihimselfprovedthatthesetoffibrilsknownasprotoplasmicprolongationsterminatebyfreeextremities，andhavenodirectconnectionwithanycellsavetheonefromwhichtheyspring。Heshowedalsothattheaxiscylindersgiveoffmultitudesoflateralbranchesnothithertosuspected。Butherehepaused，missingtherealimportofthediscoveryofwhichhewashardonthetrack。ItremainedfortheSpanishhistologistDr。S。RamonyCajaltofollowuptheinvestigationbymeansofanimprovedapplicationofGolgi’smethodofstaining，andtodemonstratethattheaxiscylinders，togetherwithalltheircollateralbranches，thoughsometimesextendingtoagreatdistance，yetfinallyterminate，liketheothercellprolongations，inarborescentfibrilshavingfreeextremities。Inaword，itwasshownthateachcentralnervecell，withitsfibrillaroffshoots，isanisolatedentity。

Insteadofbeinginphysicalconnectionwithamultitudeofothernervecells，ithasnodirectphysicalconnectionwithanyothernervecellwhatever。

WhenDr。Cajalannouncedhisdiscovery，in1889，hisrevolutionaryclaimsnotunnaturallyamazedthemassofhistologists。Thereweresomefewofthem，however，whowerenotquiteunpreparedfortherevelation；inparticularHis，whohadhalfsuspectedtheindependenceofthecells，becausetheyseemedtodevelopfromdissociatedcentres；andForel，whobasedasimilarsuspiciononthefactthathehadneverbeenableactuallytotraceafibrefromonecelltoanother。TheseobserversthencamereadilytorepeatCajal’sexperiments。SoalsodidtheveteranhistologistKolliker，andsoonafterwardsalltheleaderseverywhere。TheresultwasapracticallyunanimousconfirmationoftheSpanishhistologist’sclaims，andwithinafewmonthsafterhisannouncementstheoldtheoryofunionofnervecellsintoanendlessmesh-workwascompletelydiscarded，andthetheoryofisolatednerveelements——thetheoryofneurons，asitcametobecalled——wasfullyestablishedinitsplace。

Astohowtheseisolatednervecellsfunctionate，Dr。Cajalgavetheclewfromtheveryfirst，andhisexplanationhasmetwithuniversalapproval。

Inthemodifiedview，thenervecellretainsitsoldpositionasthestorehouseofnervousenergy。Eachofthefilamentsjuttingoutfromthecellisheld，asbefore，tobeindeedatransmitterofimpulses，butatransmitterthatoperatesintermittently，likeatelephonewirethatisnotalways"connected，"and，likethatwire，thenervefibriloperatesbycontactandnotbycontinuity。

Underproperstimulationtheendsofthefibrilsreachout，comeincontactwithotherendfibrilsofothercells，andconducttheirdestinedimpulse。Againtheyretract，andcommunicationceasesforthetimebetweenthoseparticularcells。Meantime，byadifferentarrangementofthevariousconductors，differentsetsofcellsareplacedincommunication，differentassociationsofnervousimpulsesinduced，differenttrainsofthoughtengendered。

Eachfibrilwhenretractedbecomesanon-conductor，butwhenextendedandincontactwithanotherfibril，orwiththebodyofanothercell，itconductsitsmessageasreadilyasacontinuousfilamentcoulddo——preciselyasinthecaseofanelectricwire。

Thisconception，foundedonamosttangibleanatomicalbasis，enablesustoanswerthequestionastohowideasareisolated，andalso，asDr。Cajalpointsout，throwsnewlightonmanyothermentalprocesses。Onecanimagine，forexample，bykeepinginmindtheflexiblenerveprolongations，hownewtrainsofthoughtmaybeengenderedthroughnovelassociationsofcells；howfacilityofthoughtorofactionincertaindirectionsisacquiredthroughthehabitualmakingofcertainnerve-cellconnections；howcertainbitsofknowledgemayescapeourmemoryandrefusetobefoundforatimebecauseofatemporaryincapacityofthenervecellstomaketheproperconnections，andsoonindefinitely。

Ifonelikenseachnervecelltoacentraltelephoneoffice，eachofitsfilamentousprolongationstoatelephonewire，onecanimagineastrikinganalogybetweenthemodusoperandiofnervousprocessesandofthetelephonesystem。Theutilityofnewconnectionsatthecentraloffice，theuselessnessofthemechanismwhentheconnectionscannotbemade，the"wiresinuse"

thatretardyourmessage，perhapseventhecrossingofwires，bringingyouajangleofsoundsfardifferentfromwhatyoudesire——alltheseandamultiplicityofotherthingsthatwillsuggestthemselvestoeveryuserofthetelephonemaybeimaginedasbeingalmostludicrouslyparalleledintheoperationsofthenervousmechanism。Andthatparallel，startlingasitmayseem，isnotamerefutileimagining。Itissustainedandrenderedplausiblebyasoundsubstratumofknowledgeoftheanatomic