on the relations of man to the lower animals-第8部分

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connecting　them　together；　as　what　is　called　the　'great　commissure'　or
'corpus　callosum。'　　The　subject　requires　careful　re…investigation；　but
if　the　currently　received　statements　are　correct；　the　appearance　of　the
'corpus　callosum'　in　the　placental　mammals　is　the　greatest　and　most
sudden　modification　exhibited　by　the　brain　in　the　whole　series　of
vertebrated　animalsit　is　the　greatest　leap　anywhere　made　by　Nature　in
her　brain　work。　　For　the　two　halves　of　the　brain　being　once　thus　knit
together；　the　progress　of　cerebral　complexity　is　traceable　through　a
complete　series　of　steps　from　the　lowest　Rodent；　or　Insectivore；　to
Man；　and　that　complexity　consists；　chiefly；　in　the　disproportionate
development　of　the　cerebral　hemispheres　and　of　the　cerebellum；　but
especially　of　the　former；　in　respect　to　the　other　parts　of　the　brain。

In　the　lower　placental　mammals；　the　cerebral　hemispheres　leave　the
proper　upper　and　posterior　face　of　the　cerebellum　completely　visible；
when　the　brain　is　viewed　from　above；　but；　in　the　higher　forms；　the
hinder　part　of　each　hemisphere；　separated　only　by　the　tentorium　（p。
281）　from　the　anterior　face　of　the　cerebellum；　inclines　backwards　and
downwards；　and　grows　out；　as　the　so…called　〃posterior　lobe；〃　so　as　at
length　to　overlap　and　hide　the　cerebellum。　　In　all　Mammals；　each
cerebral　hemisphere　contains　a　cavity　which　is　termed　the　'ventricle；'
and　as　this　ventricle　is　prolonged；　on　the　one　hand；　forwards；　and　on
the　other　downwards；　into　the　substance　of　the　hemisphere；　it　is　said
to　have　two　horns　or　'cornua；　an　'anterior　cornu；'　and　a　'descending
cornu。'　　When　the　posterior　lobe　is　well　developed；　a　third　prolongation
of　the　ventricular　cavity　extends　into　it；　and　is　called　the　〃posterior
cornu。〃

In　the　lower　and　smaller　forms　of　placental　Mammals　the　surface　of　the
cerebral　hemispheres　is　either　smooth　or　evenly　rounded；　or　exhibits　a
very　few　grooves；　which　are　technically　termed　'sulci；'separating
ridges　or　'convolutions'　of　the　substance　of　the　brain；　and　the　smaller
species　of　all　orders　tend　to　a　similar　smoothness　of　brain。　　But；　in
the　higher　orders；　and　especially　the　larger　members　of　these　orders；
the　grooves；　or　sulci；　become　extremely　numerous；　and　the　intermediate
convolutions　proportionately　more　complicated　in　their　meanderings；
until；　in　the　Elephant；　the　Porpoise；　the　higher　Apes；　and　Man；　the
cerebral　surface　appears　a　perfect　labyrinth　of　tortuous　foldings。

Where　a　posterior　lobe　exists　and　presents　its　customary　cavitythe
posterior　cornuit　commonly　happens　that　a　particular　sulcus　appears
upon　the　inner　and　under　surface　of　the　lobe；　parallel　with　and　beneath
the　floor　of　the　cornuwhich　is；　as　it　were；　arched　over　the　roof　of
the　sulcus。　　It　is　as　if　the　groove　had　been　formed　by　indenting　the
floor　of　the　posterior　horn　from　without　with　a　blunt　instrument；　so
that　the　floor　should　rise　as　a　convex　eminence。　　Now　this　eminence　is
what　has　been　termed　the　'Hippocampus　minor；'　the　'Hippocampus　major'
being　a　larger　eminence　in　the　floor　of　the　descending　cornu。　　What　may
be　the　functional　importance　of　either　of　these　structures　we　know　not。

As　if　to　demonstrate；　by　a　striking　example；　the　impossibility　of
erecting　any　cerebral　barrier　between　man　and　the　apes；　Nature　has
provided　us；　in　the　latter　animals；　with　an　almost　complete　series　of
gradations　from　brains　little　higher　than　that　of　a　Rodent；　to　brains
little　lower　than　that　of　Man。　　And　it　is　a　remarkable　circumstance
that　though；　so　far　as　our　present　knowledge　extends；　there　'is'　one
true　structural　break　in　the　series　of　forms　of　Simian　brains；　this
hiatus　does　not　lie　between　Man　and　the　man…like　apes；　but　between　the
lower　and　the　lowest　Simians；　or；　in　other　words；　between　the　old　and
new　world　apes　and　monkeys；　and　the　Lemurs。　　Every　Lemur　which　has　yet
been　examined；　in　fact；　has　its　cerebellum　partially　visible　from
above；　and　its　posterior　lobe；　with　the　contained　posterior　cornu　and
hippocampus　minor；　more　or　less　rudimentary。　　Every　Marmoset；　American
monkey；　old…world　monkey；　Baboon；　or　Man…like　ape；　on　the　contrary；　has
its　cerebellum　entirely　hidden；　posteriorly；　by　the　cerebral　lobes；　and
possesses　a　large　posterior　cornu；　with　a　well…developed　hippocampus
minor。

FIG。　20。Drawings　of　the　internal　casts　of　a　Man's　and　of　a
Chimpanzee's　skull；　of　the　same　absolute　length；　and　placed　in
corresponding　positions。　　'A'。　Cerebrum；　'B'。　Cerebellum。　　The　former
drawing　is　taken　from　a　cast　in　the　Museum　of　the　Royal　College　of
Surgeons；　the　latter　from　the　photograph　of　the　cast　of　a　Chimpanzee's
skull；　which　illustrates　the　paper　by　Mr。　Marshall　'On　the　Brain　of　the
Chimpanzee'　in　the　'Natural　History　Review'　for　July；　1861。　The　sharper
definition　of　the　lower　edge　of　the　cast　of　the　cerebral　chamber　in　the
Chimpanzee　arises　from　the　circumstance　that　the　tentorium　remained　in
that　skull　and　not　in　the　Man's。　The　cast　more　accurately　represents
the　brain　in　Chimpanzee　than　in　the　Man；　and　the　great　backward
projection　of　the　posterior　lobes　of　the　cerebrum　of　the　former；　beyond
the　cerebellum；　is　conspicuous。

In　many　of　these　creatures；　such　as　the　Saimiri　（'Chrysothrix'）；　the
cerebral　lobes　overlap　and　extend　much　further　behind　the　cerebellum；
in　proportion；　than　they　do　in　man　（Fig。　16）and　it　is　quite　certain
that；　in　all；　the　cerebellum　is　completely　covered　behind；　by
well…developed　posterior　lobes。　　The　fact　can　be　verified　by　every　one
who　possesses　the　skull　of　any　old　or　new　world　monkey。　　For；　inasmuch
as　the　brain　in　all　mammals　completely　fills　the　cranial　cavity；　it　is
obvious　that　a　cast　of　the　interior　of　the　skull　will　reproduce　the
general　form　of　the　brain；　at　any　rate　with　such　minute　and；　for　the
present　purpose；　utterly　unimportant　differences　as　may　result　from　the
absence　of　the　enveloping　membranes　of　the　brain　in　the　dry　skull。　　But
if　such　a　cast　be　made　in　plaster；　and　compared　with　a　similar　cast　of
the　interior　of　a　human　skull；　it　will　be　obvious　that　the　cast　of　the
cerebral　chamber；　representing　the　cerebrum　of　the　ape；　as　completely
covers　over　and　overlaps　the　cast　of　the　cerebellar　chamber；
representing　the　cerebellum；　as　it　does　in　the　man　（Fig。　20）。　　A
careless　observer；　forgetting　that　a　soft　structure　like　the　brain
loses　its　proper　shape　the　moment　it　is　taken　out　of　the　skull；　may
indeed　mistake　the　uncovered　condition　of　the　cerebellum　of　an　extracted
and　distorted　brain　for　the　natural　relations　of　the　parts；　but　his
error　must　become　patent　even　to　himself　if　he　try　to　replace　the　brain
within　the　cranial　chamber。　To　suppose　that　the　cerebellum　of　an　ape　is
naturally　uncovered　behind　is　a　miscomprehension　comparable　only　to
that　of　one　who　should　imagine　that　a　man's　lungs　always　occupy　but　a
small　portion　of　the　thoracic　cavitybecause　they　do　so　when　the　chest
is　opened；　and　their　elasticity　is　no　longer　neutralized　by　the
pressure　of　the　air。

And　the　error　is　the　less　excusable；　as　it　must　become　apparent　to　every
one　who　examines　a　section　of　the　skull　of　any　ape　above　a　Lemur；
without　taking　the　trouble　to　make　a　cast　of　it。　For　there　is　a　very
marked　groove　in　every　such　skull；　as　in　the　human　skullwhich
indicates　the　line　of　attachment　of　what　is　termed　the　'tentorium'a
sort　of　parchment…like　shelf；　or　partition；　which；　in　the　recent　state；
is　interposed　between　the　cerebrum　and　cerebellum；　and　prevents　the
former　from　pressing　upon　the　latter。　（See　Fig。　16。）

This　groove；　therefore；　indicates　the　line　of　separation　between　that
part　of　the　cranial　cavity　which　contains　the　cerebrum；　and　that　which
contains　the　cerebellum；　and　as　the　brain　exactly　fills　the　cavity　of
the　skull；　it　is　obvious　that　the　relations　of　these　two　parts　of　the
cranial　cavity　at　once　informs　us　of　the　relations　of　their　contents。
Now　in　man；　in　all　the　old…world；　and　in　all　the　new…world　Simiae；　with
one　exception；　when　the　face　is　directed　forwards；　this　line　of
attachment　of　the　tentorium；　or　impression　for　the　lateral　sinus；　as　it
is　technically　called；　is　nearly　horizontal；　and　the　cerebral　chamber
invariably　overlaps　or　projects　behind　the　cerebellar　chamber。　　In　the
Howler　Monkey　or　'Mycetes'　（see　Fig。　16）；　the　line　passes　obliquely
upwards　and　backwards；　and　the　cerebral　overlap　is　almost　nil；　while　in
the　Lemurs；　as　in　the　lower　mammals；　the　line　is　much　more　inclined　in
the　same　direction；　and　the　cerebellar　chamber　projects　considerably
beyond　the　cerebral。

When　the　gravest　errors　respecting　points　so　easily　settled　as　this
question　respecting　the　posterior　lobes　can　be　authoritatively
propounded；　it　is　no　wonder　that　matters　of　observation；　of　no　very
complex　character；　but　still　requiring　a　certain　amount　of　care；　should
have　fared　worse。　　Any　one　who　cannot　see　the　posterior　lobe　in　an
ape's　brain　is　not　likely　to　give　a　very　valuable　opinion　respecting
the　posterior　cornu　or　the　hippocampus　minor。　　If　a　man　cannot　see　a
church；　it　is　preposterous　to　take　his　opinion　about　its　altar…piece　or
painted　windowso　that　I　do　not　feel　bound　to　enter　upon　any
discussion　of　these　points；　but　content　myself　with　assuring　the　reader
that　the　posterior　cornu　and　the　hippocampus　minor；　have　now　been
seenusually；　at　least　as　well　developed　as　in　man；　and　often
betternot　only　in　the　Chimpanzee；　the　Orang；　and　the　Gibbon；　but　in
all　the　genera　of　the　old　world　baboons　and　monkeys；　and　in　most　of　the
new　world　forms；　including　the　Marmosets。*

　　　　＇Footnote＇　*See　the　note　at　the　end　of　this　essay　for　a
　　　　succinct　history　of　the　controversy　to　which　allusion　is
　　　　here　made。

FIG。　21。Drawings　of　the　cerebral　hemispheres　of　a　Man　and　of　a
Chimpanzee　of　the　same　length；　in　order　to　show　the　relative
proportions　of　the　parts：　the　former　taken　from　a　specimen；　which　Mr。
Flower；　Conservator　of　the　Museum　of　the　Royal　College　of　Surgeons；　was
good　enough　to　dissect　for　me；　the　latter；　from　the　photograph　of　a
similarly　dissected　Chimpanzee's　brain；　given　in　Mr。　Marshall's　paper
above　referred　to。　　'a'；　posterior　lobe；　'b'；　lateral　ventricle；　'c'；
posterior　cornu；　'x'；　the　hippocampus　minor。

In　fact；　all　the　abundant　and　trustworthy　evidence　（consisting　of　the