the circulation of the blood-第2章

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the　enjoyment　of　a　highly　developed　civilisation。　　But　of　what
knowledge　they　may　have　possessed　beyond　this　we　know　nothing；　and　in
tracing　back　the　springs　of　the　origin　of　everything　that　we　call
〃modern　science〃　（which　is　not　merely　knowing；　but　knowing
systematically；　and　with　the　intention　and　endeavour　to　find　out　the
causal　connection　of　things）I　say　that　when　we　trace　back　the
different　lines　of　all　the　modern　sciences　we　come　at　length　to　one
epoch　and　to　one　countrythe　epoch　being　about　the　fourth　and　fifth
centuries　before　Christ；　and　the　country　being　ancient　Greece。　　It　is
there　that　we　find　the　commencement　and　the　root　of　every　branch　of
physical　science　and　of　scientific　method。　　If　we　go　back　to　that　time
we　have　in　the　works　attributed　to　Aristotle；　who　flourished　between
300　and　400　years　before　Christ；　a　sort　of　encyclopaedia　of　the
scientific　knowledge　of　that　dayand　a　very　marvellous　collection　of；
in　many　respects；　accurate　and　precise　knowledge　it　is。　　But；　so　far　as
regards　this　particular　topic；　Aristotle；　it　must　be　confessed；　has　not
got　very　far　beyond　common　knowledge。　　He　knows　a　little　about　the
structure　of　the　heart。　　I　do　not　think　that　his　knowledge　is　so
inaccurate　as　many　people　fancy；　but　it　does　not　amount　to　much。　　A　very
few　years　after　his　time；　however；　there　was　a　Greek　philosopher；
Erasistratus；　who　lived　about　three　hundred　years　before　Christ；　and
who　must　have　pursued　anatomy　with　much　care；　for　he　made　the　important
discovery　that　there　are　membranous　flaps；　which　are　now　called
〃valves；〃　at　the　origins　of　the　great　vessels；　and　that　there　are
certain　other　valves　in　the　interior　of　the　heart　itself。

Fig。　1。The　apparatus　of　the　circulation；　as　at　present　known。　　The
capillary　vessels；　which　connect　the　arteries　and　veins；　are　omitted；
on　account　of　their　small　size。　　The　shading　of　the　〃venous　system〃　is
given　to　all　the　vessels　which　contain　venous　blood；　that　of　the
〃arterial　system〃　to　all　the　vessels　which　contain　arterial　blood。

I　have　here　（Fig。　1）　a　purposely　rough；　but；　so　far　as　it　goes；
accurate；　diagram　of　the　structure　of　the　heart　and　the　course　of　the
blood。　　The　heart　is　supposed　to　be　divided　into　two　portions。　　It
would　be　possible；　by　very　careful　dissection；　to　split　the　heart　down
the　middle　of　a　partition；　or　so…called　'septum'；　which　exists　in　it；
and　to　divide　it　into　the　two　portions　which　you　see　here　represented；
in　which　case　we　should　have　a　left　heart　and　a　right　heart；　quite
distinct　from　one　another。　　You　will　observe　that　there　is　a　portion　of
each　heart　which　is　what　is　called　the　ventricle。　　Now　the　ancients
applied　the　term　'heart'　simply　and　solely　to　the　ventricles。　　They　did
not　count　the　rest　of　the　heartwhat　we　now　speak　of　as　the
'auricles'as　any　part　of　the　heart　at　all；　but　when　they　spoke　of　the
heart　they　meant　the　left　and　the　right　ventricles；　and　they　described
those　great　vessels；　which　we　now　call　the　'pulmonary　veins'　and　the
'vena　cava'；　as　opening　directly　into　the　heart　itself。

What　Erasistratus　made　out　was　that；　at　the　roots　of　the　aorta　and　the
pulmonary　artery　（Fig。　　1）　there　were　valves；　which　opened　in　the
direction　indicated　by　the　arrows；　and；　on　the　other　hand；　that　at　the
junction　of　what　he　called　the　veins　with　the　heart　there　were　other
valves；　which　also　opened　again　in　the　direction　indicated　by　the
arrows。　　This　was　a　very　capital　discovery；　because　it　proved　that　if
the　heart　was　full　of　fluid；　and　if　there　were　any　means　of　causing
that　fluid　in　the　ventricles　to　move；　then　the　fluid　could　move　only　in
one　direction；　for　you　will　observe　that；　as　soon　as　the　fluid　is
compressed；　the　two　valves　between　the　ventricles　and　the　veins　will　be
shut；　and　the　fluid　will　be　obliged　to　move　into　the　arteries；　and；　if
it　tries　to　get　back　from　them　into　the　heart；　it　is　prevented　from
doing　so　by　the　valves　at　the　origin　of　the　arteries；　which　we　now　call
the　semilunar　valves　（half…moon　shaped　valves）；　so　that　it　is
impossible；　if　the　fluid　move　at　all；　that　it　should　move　in　any　other
way　than　from　the　great　veins　into　the　arteries。　　Now　that　was　a　very
remarkable　and　striking　discovery。

But　it　is　not　given　to　any　man　to　be　altogether　right　（that　is　a
reflection　which　it　is　very　desirable　for　every　man　who　has　had　the
good　luck　to　be　nearly　right　once；　always　to　bear　in　mind）；　and
Erasistratus；　while　he　made　this　capital　and　important　discovery；　made　a
very　capital　and　important　error　in　another　direction；　although　it　was
a　very　natural　error。　　If；　in　any　animal　which　is　recently　killed；　you
open　one　of　those　pulsating　trunks　which　I　referred　to　a　short　time
ago；　you　will　find；　as　a　general　rule；　that　it　either　contains　no　blood
at　all　or　next　to　none；　but　that；　on　the　contrary；　it　is　full　of　air。
Very　naturally；　therefore；　Erasistratus　came　to　the　conclusion　that
this　was　the　normal　and　natural　state　of　the　arteries；　and　that　they
contained　air。　　We　are　apt　to　think　this　a　very　gross　blunder；　but；　to
anybody　who　is　acquainted　with　the　facts　of　the　case；　it　is；　at　first
sight；　an　exceedingly　natural　conclusion。　Not　only　so；　but　Erasistratus
might　have　very　justly　imagined　that　he　had　seen　his　way　to　the　meaning
of　the　connection　of　the　left　side　of　the　heart　with　the　lungs；　for　we
find　that　what　we　now　call　the　pulmonary　vein　is　connected　with　the
lungs；　and　branches　out　in　them　（Fig。　1）。　Finding　that　the　greater　part
of　this　system　of　vessels　was　filled　with　air　after　death；　this　ancient
thinker　very　shrewdly　concluded　that　its　real　business　was　to　receive
air　from　the　lungs；　and　to　distribute　that　air　all　through　the　body；　so
as　to　get　rid　of　the　grosser　humours　and　purify　the　blood。　　That　was　a
very　natural　and　very　obvious　suggestion；　and　a　highly　ingenious　one；
though　it　happened　to　be　a　great　error。　　You　will　observe　that　the　only
way　of　correcting　it　was　to　experiment　upon　living　animals；　for　there
is　no　other　way　in　which　this　point　could　be　settled。

Fig。2；The　Course　of　the　Blood　according　to　Galen　（A。D。　170）。

And　hence　we　are　indebted；　for　the　correction　of　the　error　of
Erasistratus；　to　one　of　the　greatest　experimenters　of　ancient　or　modern
times；　Claudius　Galenus；　who　lived　in　the　second　century　after　Christ。
I　say　it　was　to　this　man　more　than　any　one　else；　because　he　knew　that
the　only　way　of　solving　physiological　problems　was　to　examine　into　the
facts　in　the　living　animal。　And　because　Galen　was　a　skilful　anatomist；
and　a　skilful　experimenter；　he　was　able　to　show　in　what　particulars
Erasistratus　had　erred；　and　to　build　up　a　system　of　thought　upon　this
subject　which　was　not　improved　upon　for　fully　1；300　years。　　I　have
endeavoured；　in　Fig。　2；　to　make　clear　to　you　exactly　what　it　was　he
tried　to　establish。　　You　will　observe　that　this　diagram　is　practically
the　same　as　that　given　in　Fig。　1；　only　simplified。　　The　same　facts　may
be　looked　upon　by　different　people　from　different　points　of　view。　Galen
looked　upon　these　facts　from　a　very　different　point　of　view　from　that
which　we　ourselves　occupy；　but；　so　far　as　the　facts　are　concerned；　they
were　the　same　for　him　as　for　us。　　Well　then；　the　first　thing　that　Galen
did　was　to　make　out　experimentally　that；　during　life；　the　arteries　are
not　full　of　air；　but　that　they　are　full　of　blood。　　And　he　describes　a
great　variety　of　experiments　which　he　made　upon　living　animals　with　the
view　of　proving　this　point；　which　he　did　prove　effectually　and　for　all
time；　and　that　you　will　observe　was　the　only　way　of　settling　the
matter。　　Furthermore；　he　demonstrated　that　the　cavities　of　the　left
side　of　the　heartwhat　we　now　call　the　left　auricle　and　the　left
ventricleare；　like　the　arteries；　full　of　blood　during　life；　and　that
that　blood　was　of　the　scarlet　kindarterialised；　or　as　he　called　it
〃pneumatised；〃　blood。　　It　was　known　before；　that　the　pulmonary　artery；
the　right　ventricle；　and　the　veins；　contain　the　darker　kind　of　blood；
which　was　thence　called　venous。　　Having　proved　that　the　whole　of　the
left　side　of　the　heart；　during　life；　is　full　of　scarlet　arterial　blood；
Galen's　next　point　was　to　inquire　into　the　mode　of　communication
between　the　arteries　and　veins。　　It