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。