a history of science-4-第55章

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uids　for　the　preliminary　preservation　and　hardening　of　the　tissues。　　Stilling；　in　1842；　perfected　the　method　by　introducing　the　custom　of　cutting　a　series　of　consecutive　sections　of　the　same　tissue；　in　order　to　trace　nerve　tracts　and　establish　spacial　relations。　　Then　from　time　to　time　mechanical　ingenuity　added　fresh　details　of　improvement。　It　was　found　that　pieces　of　hardened　tissue　of　extreme　delicacy　can　be　made　better　subject　to　manipulation　by　being　impregnated　with　collodion　or　celloidine　and　embedded　in　paraffine。　Latterly　it　has　become　usual　to　cut　sections　also　from　fresh　tissues；　unchanged　by　chemicals；　by　freezing　them　suddenly　with　vaporized　ether　or；　better；　carbonic　acid。　By　these　methods；　and　with　the　aid　of　perfected　microtomes；　the　worker　of　recent　periods　avails　himself　of　sections　of　brain　tissues　of　a　tenuousness　which　the　early　investigators　could　not　approach。

But　more　important　even　than　the　cutting　of　thin　sections　is　the　process　of　making　the　different　parts　of　the　section　visible；　one　tissue　differentiated　from　another。　　The　thin　section；　as　the　early　workers　examined　it；　was　practically　colorless；　and　even　the　crudest　details　of　its　structure　were　made　out　with　extreme　difficulty。　　Remak　did；　indeed；　manage　to　discover　that　the　brain　tissue　is　cellular；　as　early　as　1833；　and　Ehrenberg　in　the　same　year　saw　that　it　is　also　fibrillar；　but　beyond　this　no　great　advance　was　made　until　1858；　when　a　sudden　impulse　was　received　from　a　new　process　introduced　by　Gerlach。　　The　process　itself　was　most　simple；　consisting　essentially　of　nothing　more　than　the　treatment　of　a　microscopical　section　with　a　solution　of　carmine。　But　the　result　was　wonderful；　for　when　such　a　section　was　placed　under　the　lens　it　no　longer　appeared　homogeneous。　Sprinkled　through　its　substance　were　seen　irregular　bodies　that　had　taken　on　a　beautiful　color；　while　the　matrix　in　which　they　were　embedded　remained　unstained。　　In　a　word；　the　central　nerve　cell　had　sprung　suddenly　into　clear　view。

A　most　interesting　body　it　proved；　this　nerve　cell；　or　ganglion　cell；　as　it　came　to　be　called。　　It　was　seen　to　be　exceedingly　minute　in　size；　requiring　high　powers　of　the　microscope　to　make　it　visible。　It　exists　in　almost　infinite　numbers；　not；　however；　scattered　at　random　through　the　brain　and　spinal　cord。　　On　the　contrary；　it　is　confined　to　those　portions　of　the　central　nervous　masses　which　to　the　naked　eye　appear　gray　in　color；　being　altogether　wanting　in　the　white　substance　which　makes　up　the　chief　mass　of　the　brain。　Even　in　the　gray　matter；　though　sometimes　thickly　distributed；　the　ganglion　cells　are　never　in　actual　contact　one　with　another；　they　always　lie　embedded　in　intercellular　tissues；　which　came　to　be　known；　following　Virchow；　as　the　neuroglia。

Each　ganglion　cell　was　seen　to　be　irregular　in　contour；　and　to　have　jutting　out　from　it　two　sets　of　minute　fibres；　one　set　relatively　short；　indefinitely　numerous；　and　branching　in　every　direction；　the　other　set　limited　in　number；　sometimes　even　single；　and　starting　out　directly　from　the　cell　as　if　bent　on　a　longer　journey。　The　numerous　filaments　came　to　be　known　as　protoplasmic　processes；　the　other　fibre　was　named；　after　its　discoverer；　the　axis　cylinder　of　Deiters。　　It　was　a　natural　inference；　though　not　clearly　demonstrable　in　the　sections；　that　these　filamentous　processes　are　the　connecting　links　between　the　different　nerve　cells　and　also　the　channels　of　communication　between　nerve　cells　and　the　periphery　of　the　body。　The　white　substance　of　brain　and　cord；　apparently；　is　made　up　of　such　connecting　fibres；　thus　bringing　the　different　ganglion　cells　everywhere　into　communication　one　with　another。

In　the　attempt　to　trace　the　connecting　nerve　tracts　through　this　white　substance　by　either　macroscopical　or　microscopical　methods；　most　important　aid　is　given　by　a　method　originated　by　Waller　in　1852。　Earlier　than　that；　in　1839；　Nasse　had　discovered　that　a　severed　nerve　cord　degenerates　in　its　peripheral　portions。　Waller　discovered　that　every　nerve　fibre；　sensory　or　motor；　has　a　nerve　cell　to　or　from　which　it　leads；　which　dominates　its　nutrition；　so　that　it　can　only　retain　its　vitality　while　its　connection　with　that　cell　is　intact。　　Such　cells　he　named　trophic　centres。　Certain　cells　of　the　anterior　part　of　the　spinal　cord；　for　example；　are　the　trophic　centres　of　the　spinal　motor　nerves。　Other　trophic　centres；　governing　nerve　tracts　in　the　spinal　cord　itself；　are　in　the　various　regions　of　the　brain。　It　occurred　to　Waller　that　by　destroying　such　centres；　or　by　severing　the　connection　at　various　regions　between　a　nervous　tract　and　its　trophic　centre；　sharply　defined　tracts　could　be　made　to　degenerate；　and　their　location　could　subsequently　be　accurately　defined；　as　the　degenerated　tissues　take　on　a　changed　aspect；　both　to　macroscopical　and　microscopical　observation。　Recognition　of　this　principle　thus　gave　the　experimenter　a　new　weapon　of　great　efficiency　in　tracing　nervous　connections。　Moreover；　the　same　principle　has　wide　application　in　case　of　the　human　subject　in　disease；　such　as　the　lesion　of　nerve　tracts　or　the　destruction　of　centres　by　localized　tumors；　by　embolisms；　or　by　traumatisms。

All　these　various　methods　of　anatomical　examination　combine　to　make　the　conclusion　almost　unavoidable　that　the　central　ganglion　cells　are　the　veritable　〃centres〃　of　nervous　activity　to　which　so　many　other　lines　of　research　have　pointed。　The　conclusion　was　strengthened　by　experiments　of　the　students　of　motor　localization；　which　showed　that　the　veritable　centres　of　their　discovery　lie；　demonstrably；　in　the　gray　cortex　of　the　brain；　not　in　the　white　matter。　　But　the　full　proof　came　from　pathology。　At　the　hands　of　a　multitude　of　observers　it　was　shown　that　in　certain　well…known　diseases　of　the　spinal　cord；　with　resulting　paralysis；　it　is　the　ganglion　cells　themselves　that　are　found　to　be　destroyed。　Similarly；　in　the　case　of　sufferers　from　chronic　insanities；　with　marked　dementia；　the　ganglion　cells　of　the　cortex　of　the　brain　are　found　to　have　undergone　degeneration。　The　brains　of　paretics　in　particular　show　such　degeneration；　in　striking　correspondence　with　their　mental　decadence。　The　position　of　the　ganglion　cell　as　the　ultimate　centre　of　nervous　activities　was　thus　placed　beyond　dispute。

Meantime；　general　acceptance　being　given　the　histological　scheme　of　Gerlach；　according　to　which　the　mass　of　the　white　substance　of　the　brain　is　a　mesh…work　of　intercellular　fibrils；　a　proximal　idea　seemed　attainable　of　the　way　in　which　the　ganglionic　activities　are　correlated；　and；　through　association；　built　up；　so　to　speak；　into　the　higher　mental　processes。　Such　a　conception　accorded　beautifully　with　the　ideas　of　the　associationists；　who　had　now　become　dominant　in　psychology。　But　one　standing　puzzle　attended　this　otherwise　satisfactory　correlation　of　anatomical　observations　and　psychic　analyses。　It　was　this：　　Since；　according　to　the　histologist；　the　intercellular　fibres；　along　which　impulses　are　conveyed；　connect　each　brain　cell；　directly　or　indirectly；　with　every　other　brain　cell　in　an　endless　mesh…work；　how　is　it　possible　that　various　sets　of　cells　may　at　times　be　shut　off　from　one　another？　Such　isolation　must　take　place；　for　all　normal　ideation　depends　for　its　integrity　quite　as　much　upon　the　shutting…out　of　the　great　mass　of　associations　as　upon　the　inclusion　of　certain　other　associations。　For　example；　a　student　in　solving　a　mathematical　problem　must　for　the　moment　become　quite　oblivious　to　the　special　associations　that　have　to　do　with　geography；　natural　history；　and　the　like。　But　does　histology　give　any　clew　to　the　way　in　which　such　isolation　may　be　effected？

Attempts　were　made　to　find　an　answer　through　consideration　of　the　very　peculiar　character　of　the　blood…supply　in　the　brain。　Here；　as　nowhere　else；　the　terminal　twigs　of　the　arteries　are　arranged　in　closed　systems；　not　anastomosing　freely　with　neighboring　systems。　Clearly；　then；　a　restricted　area　of　the　brain　may；　through　the　controlling　influence　of　the　vasomotor　nerves；　be　flushed　with　arterial　blood　while　neighboring　parts　remain　relatively　anaemic。　And　since　vital　activities　unquestionably　depend　in　part　upon　the　supply　of　arterial　blood；　this　peculiar　arrangement　of