Andersons pediatric cardiology 79
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Therightfibroustrigoneisthencontinuouswiththemembranousseptum,the
conjoinedstructurebeingknownasthecentralfibrousbody.Fromthetwo
fibroustrigones,cordsoffibroustissueextendaroundtheorificesofthemitral
valve.Itisrare,however,tofindthesecordsencirclingtheentiretyofthevalvar
orificetoprovideuniformsupportfortheattachmentsofthemuralleafletofthe
mitralvalve.Whenwellformed,thefibroustissuefrequentlytakestheformofa
shortfibrousstrip,ratherthanacircularcord.17Often,thefibroustissuefades
outcompletelyatvarioussitesaroundthering,withtheatrialandventricular
musclemassesthenbeingseparatedfromeachotherintheselocationsbythe
fibrofattytissueoftheleftatrioventriculargroove.Themitralvalvarleafletsthen
takeoriginfromtheventricularmyocardium,ratherthanfromafibrousskeleton.
Thisarrangementistheruleratherthantheexceptionintherightatrioventricular
junction,whereitisusuallythefibrofattytissuesoftheatrioventriculargroove
thatservetoinsulatetheatrialfromtheventricularmusculature.Takentogether,
therefore,theso-calledfibrousskeletonofthehumanheartispoorlyformed,
beingafirmstructureonlywithintheaorticroot.
ConductionTissues
Theconductiontissuesaresmallareasofspecializedmyocardiumthatoriginate
anddisseminatethecardiacimpulse.Althoughitisonlyrarelypossibleto
visualizethetissuesdirectly,theirsitesaresufficientlyconstantforaccurate
anatomiclandmarkstobeestablishedasaguidetotheirlocation.
Thecardiacimpulseisgeneratedinthesinusnode.Thissmallcigar-shaped
structureislocatedinthemajorityofindividualssubepicardiallywithinthe
terminalgroove,beingpositionedinferiortothecrestoftheatrialappendage
(Fig.2.28).
FIG.2.28 Usuallocationofthesinusnode(greenoval),whichliesinthe
terminalgrooveatthesuperiorsinuatrialjunction.Inaboutone-tenthof
individuals,thenodecrossesthecrestoftherightatrialappendagein
horseshoefashion,asshownintheinset.ICV,Inferiorcavalvein;SCV,
superiorcavalvein.
Inaboutone-tenthofindividuals,thenodeextendsacrossthecrestofthe
appendagetositlikeahorseshoe,withonelimbintheterminalgrooveandthe
otherintheinteratrialgroove.Theimpulsefromthesinusnodeisconductedat
thenodalmarginsintoworkingatrialmyocardium,andisthencarriedthrough
theatrialwallstowardtheatrioventricularnode.Muchwaswritteninthepast
concerningthepresenceofinternodalatrialtracts.Therearenonarrowand
insulatedtractsofcardiomyocytesjoiningthecellsofthesinusnodewiththose
oftheatrioventricularnodethatareinanywayanalogoustotheinsulated
ventricularconductionpathways.Pathwaysofpreferentialconductiondoexist
throughtheterminalcrest,thesinusseptum,andaroundthemarginsoftheoval
fossa.Themorerapidspreadofconductionthroughtheseareasissimplya
consequenceofthemoreorderedpackingofthecardiomyocyteswithinthese
prominentmuscularbundles.
Theatrioventricularnode,surroundedonmostsidesbyshortzonesof
transitionalcells,isfoundattheapexofthetriangleofKoch.Thisimportant
landmarkisdelineatedbythetendonofTodaro,theattachmentoftheseptal
leafletofthetricuspidvalve,andtheorificeofthecoronarysinus(Fig.2.29).
FIG.2.29 Septalsurfaceoftherightatrium,withtheendocardium
removedtoshowthedirectionoftheaggregatedcardiomyocytesthat
