Table Of ContentFOREWORD
The Posterior Cranial Fossa: Microsurgical Anatomy
and Surgical Approaches
This work has grown out of my personal desire to im- TABLE 1. ResidentsandFellowsWhoHaveWorkedinDr.
prove the care of my patients. It represents a lifelong Rhoton’sMicrosurgeryLaboratory
attempt to gain an understanding of the anatomy and
Name Location
intricaciesofthebrainthatwouldimprovethesafety,gentle-
ness,andaccuracyofsurgeryformypatients.Duringcollege, HajimeArai Tokyo,Japan
I planned to pursue a career in social work but, during a AllenS.Boyd,Jr. Memphis,Tennessee
course on psychology and the brain, I became captivated by RobertBuza Salem,Oregon
ChristopherC.Carver Salinas,California
the possibility of serving humanity through a career in neu-
EvandrodeOliveira Sa˜oPaulo,Brazil
rosurgery.Duringmedicalschool,Ibeganworkinginaneu-
W.FrankEmmons Olympia,Washington
roscience laboratory in my spare time and, at the end of my
J.PaulFerguson Rome,Georgia
residency,Icompletedafellowshipinneuroanatomy.Itwas AndrewD.Fine Gainesville,Florida
duringthisfellowshipthatIrealizedthepotentialforgreater BrandonFradd Gainesville,Florida
knowledge about microneurosurgical anatomy to improve KiyotakaFujii Fukuoka,Japan
the care of my patients. This volume, a distillation of our HirohikoGibo Nagano,Japan
JohnL.Grant Portsmouth,Virginia
studies of the posterior fossa, represents nearly 40 years of
KristinnGudmundsson Reykjavik,Iceland
workandstudyinwhichmorethan50residentsandfellows
DavidG.Hardy Cambridge,England
have participated, resulting in several hundred publications. FrankS.Harris Temple,Texas
It has been gratifying to view the role of our fellows and TsutomoHitotsumatsu Fukuoka,Japan
trainees in spreading this knowledge to other countries and TakuyaInoue Fukuoka,Japan
around the world and to see the benefits of neurosurgeons TooruInoue Fukuoka,Japan
ChangJinKim Seoul,SouthKorea
applyingthisknowledgetoimprovesurgeryfortheirpatients
ToshiroKatsuta Fukuoka,Japan
(Table1).EspeciallygratifyinghasbeentheassociationwithDrs.
ShigeakiKobayashi Matsumoto,Japan
KatsutoshiKitamura,MasashiFukui,andToshioMatsushimain
WilliamLineaweaver Stanford,California
Fukuoka,Japan,andDrs.EvandrodeOliveira,HelderTedeschi, J.RichardLister Peoria,Illinois
and Hung Wen in Sa˜o Paulo, Brazil. It is to the fellows and QingLiangLiu Beijing,China
associates in the microsurgery laboratory that this volume is JackE.Maniscalco Tampa,Florida
dedicated.Specialthanksgotoourmedicalillustrators,David RichardG.Martin Huntsville,Alabama
HaruoMatsuno Fukuoka,Japan
Peace and Robin Barry, who have worked with us for 2
ToshioMatsushima Fukuoka,Japan
decades; to Ron Smith, who has directed the microsurgery
J.RobertMozingo (Deceased)
laboratory for many years; and to Laura Dickinson and Fran HiroshiMuratani Fukuoka,Japan
Johnson,whohavelaboredovertheseandearliermanuscripts. AntonioC.M.Mussi Sa˜oPaulo,Brazil
Inthebeginning,nearly40yearsago,evenwithmicrosur- ShinjiNagata Fukuoka,Japan
gicaltechniques,ourdissectionswerecrudebycurrentstan- YoshihiroNatori Fukuoka,Japan
KazunariOka Fukuoka,Japan
dards,withphotographsneedingtoberetouchedtobringout
MichioOno Tokyo,Japan
the facets of anatomy important in achieving a satisfactory
T.GlennPait LittleRock,Arkansas
outcome at surgery. Over the years, as we have learned to
WayneS.Paullus Amarillo,Texas
expose fine neural structures, the display of microsurgical DavidPerlmutter Sarasota,Florida
anatomy has become more vividly accurate and beautiful WadeH.Renn Valdosta,Georgia
than we had imagined at the onset, and it has enhanced the SaranS.Rosner Hawthorne,NewYork
accuracy and beauty of our surgery. We hope that it will do NaokatsuSaeki Chiba,Japan
ShujiSakata Fukuoka,Japan
thesameforourreaders.Weplantoproduceasecondissue
EduardoSeoane BuenosAires,Argentina
onthecerebrumandsupratentorialareasin2002forthe25th
Xiang-enShi Beijing,China
anniversary of Neurosurgery, which I had some role in birth- RyusuiTanaka Tokyo,Japan
ing,asPresidentoftheCongressofNeurologicalSurgeons22 HelderTedeschi Sa˜oPaulo,Brazil
yearsago. ErdenerTimurkaynak Ankara,Turkey
In the early development of neurosurgery, approaches to HungT.Wen Sa˜oPaulo,Brazil
the posterior fossa were directed largely via the occipital C.J.Whang Seoul,SouthKorea
IsaoYamamoto Yokohama,Japan
squamaandlessfrequentlyviathesubtemporaltranstentorial
ArnoldA.Zeal Jacksonville,Florida
route.Withthedevelopmentofmicrosurgeryandcranialbase
Neurosurgery,Vol.47,No.3,September2000Supplement S5
S6 Rhoton
surgery, it became possible to work in long, narrow expo- neurosurgery at the University of Florida. These gifts have
sures, thus setting the stage for opening virtually all of the endowed the following chairs and professorships: the R.D.
cranial base through carefully placed windows exposing KeeneFamilyChair,theC.M.andK.E.OverstreetChair,the
small and selective parts of the posterior fossa. These devel- Mark Overstreet Chair, the Albert E. and Birdie W. Einstein
opments led to approaches to the posterior fossa via the Chair, the James and Newton Eblen Chair, the Dunspaugh-
temporalboneaswellassetthestageforapproachesdirected DaltonChair,theEdwardShedWellsChair,theRobertZ.and
via the anterior and middle cranial base. In this volume, we Nancy J. Greene Chair, the L.D. Hupp Chair, the William
haveattemptednotonlytodisplaythebrainandcranialbase MerzProfessorship,andtheAlbertL.Rhoton,Jr.Chairman’s
inthebestviewsforunderstandingtheanatomy,butalsoto Professorship. The most recent of these is the $4 million gift
show the anatomy as exposed in opening multiple surgical establishingtheAlbertL.Rhoton,Jr.NeurosurgeryProfessor-
routes to the posterior fossa. For those wanting even greater ship,heldbyWilliamA.Friedman,whohasfollowedmeas
detail than displayed in this volume, our prior works, pub-
Chairman of Neurosurgery. The efforts of the numerous cli-
lishedlargelyinNeurosurgeryandtheJournalofNeurosurgery,
nicians and scientists recruited, as a result of the Endowed
canbeconsulted.
Chairs,contributedgreatlytothefoundingoftheUniversity
Thisworkhasbeensustainedbynumerousprivatecontri-
ofFloridaBrainInstitute,whereourstudiesofmicrosurgical
butionstoourdepartmentandtheUniversityofFlorida.Most
anatomyarebeingcompleted.Withthisvolume,wejoinour
prominentamongthesehasbeentheR.D.Keenefamily,who
donors in their aspiration to improve the lives of those un-
madethefirst$1milliongifttotheUniversityofFlorida,agift
dergoingbrainsurgerythroughouttheworld.
that has supported our work for many years. Their gift was
followed by additional endowments, totaling $16 million, AlbertL.Rhoton,Jr.
which support many aspects of education and research in Gainesville,Florida
CranialcavitydrawingbyLeonardodaVincicapturesthegrowingsenseofascienceofproporationsforRenaissance
artists.Inadditiontoservingasanatomicalspecimens,hisdrawingsremainconsummateexamplesofdraftsmanship.
Courtesy,Dr.EdwinTodd,Pasadena,California.(AlsoseepagesS193andS286.)
Neurosurgery,Vol.47,No.3,September2000Supplement
CHAPTER 1
Cerebellum and Fourth Ventricle
Albert L. Rhoton, Jr., M.D.
DepartmentofNeurologicalSurgery,UniversityofFlorida,Gainesville,Florida
Keywords:Cerebellarartery,Cranialnerve,Fourthventricle,Intracranialvein,Microsurgicalanatomy
Theposteriorcranialfossa,thelargestanddeepestofthe Thissectiononthecerebellumandfourthventriclewillbegin
threecranialfossae,containsthemostcomplexintracra- at the cerebellar surfaces and progress to the deeper neural
nial anatomy. Here, in approximately one-eighth the structures.
intracranial space, are found the pathways regulating con-
sciousness, vital autonomic functions, and motor activities
andsensoryreceptionforthehead,body,andextremities,in
CEREBELLAR SURFACES
additiontothecentersforcontrollingbalanceandgait.Only
2ofthe12pairsofcranialnervesarelocatedentirelyoutside The cortical surfaces are divided on the basis of the struc-
theposteriorfossa;the10otherpairshaveasegmentwithin turestheyface,oralongwhichtheymaybeexposed,tomake
the posterior fossa (22, 25) (Fig. 1.1). The posterior fossa is this description more readily applicable to the operative set-
strategically situated at the outlet of the cerebrospinal fluid ting(Fig.1.2).Thefirstsurface,thetentorialsurface,facesthe
flow from the ventricular system. The arterial relationships tentoriumandisretractedinasupracerebellarapproach;the
areespeciallycomplex,withthevertebralandbasilararteries secondsurface,thesuboccipitalsurface,islocatedbelowand
having relatively inaccessible segments deep in front of the between the lateral and sigmoid sinuses and is exposed in a
brainstem and the major cerebellar arteries coursing in rela- suboccipital craniectomy; and the third surface, the petrosal
tion to multiple sets of cranial nerves before reaching the surface, faces forward toward the posterior surface of the
cerebellum(9,10,18,19). petrous bone and is retracted to expose the cerebellopontine
The posterior fossa extends from the tentorial incisura, angle.Eachofthesurfaceshasthevermisinthemidlineand
throughwhichitcommunicateswiththesupratentorialspace, the hemispheres laterally and is divided by a major fissure
to the foramen magnum, through which it communicates named on the basis of the surface that it divides. The hemi-
with the spinal canal. It is surrounded by the occipital, tem- sphericlobulesformingeachofthethreesurfacescommonly
poral,parietal,andsphenoidbones(Fig.1.1).Itisboundedin overlapontoandformapartoftheadjacentsurfaces(22).The
frontbythedorsumsellae,theposteriorpartofthesphenoid fissures dividing the three cortical surfaces are to be distin-
body,andtheclivalpartoftheoccipitalbone;behindbythe guished from the fissures between the cerebellum and the
lowerportionofthesquamosalpartoftheoccipitalbone;and brainstem.
oneachsidebythepetrousandmastoidpartsofthetemporal
bone, the lateral part of the occipital bone, and above and
Tentorial surface
behind by a small part of the mastoid angle of the parietal
bone. Its intracranial surface is penetrated by the jugular The tentorial surface faces and conforms to the lower sur-
foramen,internalacousticmeatus,hypoglossalcanal,theves- faceofthetentorium(Figs.1.2–1.4).Theanteromedialpartof
tibularandcochlearaqueducts,andseveralvenousemissary this surface, the apex, formed by the anterior vermis, is the
foramina,allofwhichwillbeexploredingreaterdetail.The highest point on the cerebellum. This surface slopes down-
uppersurfaceofthecerebellumisseparatedfromthesupra- wardfromitsanteromedialtoitsposterolateraledge.Onthe
tentorialspacebythetentoriumcerebelli.Optimizinganop- tentorial surface, the transition from the vermis to the hemi-
erative approach to the posterior fossa requires an under- spheresissmoothandnotmarkedbythedeepfissuresonthe
standingoftherelationshipsofthecerebellum,cranialnerves, suboccipital surface between the vermis and hemispheres.
brainstem, the cerebellar arteries, veins, and peduncles, and Deepnotches,theanteriorandposteriorcerebellarincisurae,
the complex fissures between the cerebellum and brainstem. groove the anterior and posterior edges of the tentorial sur-
The relationships of the fourth ventricle to the cerebellar face in the midline. The brainstem fits into the anterior cere-
surfaces and the fissures through which the ventricle is ap- bellar incisura and the falx cerebelli fits into the posterior
proachedsurgicallyareamongthemostcomplexinthebrain. incisura(Fig.1.2).
Neurosurgery,Vol.47,No.3,September2000Supplement S7
S8 Rhoton
directedanteriorlyabovetheoriginoftheposteriorrootofthe
trigeminal nerve. The posterior border between the tentorial
and the suboccipital surfaces also has a lateral and a medial
part. The lateral part (the posterolateral margin) is parallel
and adjacent to the lateral sinus and separates the hemi-
sphericpartofthesuboccipitalandtentorialsurfaces,andthe
shortmedialpart(theposteromedialmargin)facestheposte-
rior cerebellar incisura and separates the vermic part of the
two surfaces. The lateral angle, formed by the junction of
theanterolateralandposterolateralmargins,islocatedatthe
junction of sigmoid, lateral, and superior petrosal sinuses.
Veinsoftenconvergeontheanteriorandlateralangles.
The hemispheric part of the tentorial surface includes the
quadrangular, simple, and superior semilunar lobules, and
the vermian part includes the culmen, declive, and folium.
Thevermianandtherelatedhemisphericpartsfromaboveto
below in sequence are the culmen and the quadrangular
lobule,thedecliveandthesimplelobule,andthefoliumand
thesuperiorsemilunarlobule.Thetentorialsurfaceisdivided
atthesiteofitsmajorfissure,thetentorialfissure,intoante-
rior and posterior parts. This fissure, located between the
quadrangularandthesimplelobulesonthehemisphereand
theculmenandthedecliveonthevermis,hasalsobeencalled
the primary fissure. The postclival fissure separates the sim-
pleandthesuperiorsemilunarlobules.Theinterfolialfissures
onthissurfacepassanterolaterallyfromthemidlineandare
continuouswiththefissuresonthesuperiorhalfofthepetro-
salsurface.
Suboccipital surface
FIGURE 1.1. A,superiorviewoftheposteriorcranialfossa.
Theosseuswallsoftheposteriorfossaareformedbythe The suboccipital surface, located below and between the
occipital,temporal,andsphenoidbones.Thefossais lateralandsigmoidsinuses,isthemostcomplexofthethree
boundedinfrontbythedorsumsellaeandposteriorpartof surfaces(Figs.1.2and1.5).Operativeapproachestothefourth
thesphenoidboneandtheclivalpartoftheoccipitalbone; ventricle and most cerebellar tumors are commonly directed
behindbythelowerportionofthesquamosalpartofthe aroundorthroughthissurface.Ithasadeepverticaldepres-
occipitalbone;andoneachsidebythepetrousandmastoid sion,theposteriorcerebellarincisura,whichcontainsafoldof
partsofthetemporalbone,andthelateralpartoftheoccipi- dura, the falx cerebelli. The vermis is folded into and forms
talbone.Onesmallpartabovethetemporalboneisformed the cortical surface within this incisura. The lateral walls of
bytheinferiorangleoftheparietalbone.B,nervesand theincisuraareformedbythemedialaspectsofthecerebellar
arteriesoftheposteriorfossa.Only2ofthe12pairsofcra- hemispheres.Deepclefts,thevermohemisphericfissures,sep-
nialnervescourseentirelyoutsidetheposteriorfossa.The arate the vermis from the hemispheres. The vermian surface
tentorium,whichisattachedalongthepetrousridges,roofs within the incisura has a diamond shape. The upper half of
theposteriorfossa.A.,artery;Ac.,acoustic;A.I.C.A.,antero- thediamond-shapedformationhasapyramidalshapeandis
inferiorcerebellarartery;Bas.,basilar;CN,cranialnerve; calledthepyramid.Thefoliumandthetuber,superiortothe
For.,foramen;Int.,internal;Jug.,jugular;Occip.,occipital; pyramid,formtheapexofthesuboccipitalpartofthevermis.
P.C.A.,posteriorcerebralartery;P.I.C.A.,posteroinferiorcer- The lower half of the diamond-shaped formation, the uvula,
ebellarartery;S.C.A.,superiorcerebellarartery;Temp.,tem- projects downward between the tonsils, thus mimicking the
poral;Tent.,tentorial;Vert.,vertebral. situation in the oropharynx. The rostromedial margin of the
tonsils borders the tapering edges of the uvula. The nodule,
The anterior border, separating the tentorial and petrosal thelowermostsubdivisionofthevermis,ishiddendeeptothe
surfaces, has a lateral part (the anterolateral margin) that is uvula. The strip of vermis within the incisura is broadest at
paralleltothesuperiorpetrosalsinusandseparatesthehemi- the junction of the pyramid and uvula. Inferiorly, the poste-
spheric part of the tentorial and petrosal surfaces, and a rior cerebellar incisura is continuous with the vallecula cer-
medialpart(theanteromedialmargin)thatfacesthemidbrain ebelli,acleftbetweenthetonsilsthatleadsthroughthefora-
and forms the posterior border of the fissure between the menofMagendieintothefourthventricle.
midbrain and cerebellum. The anterior angle formed by the The hemispheric portion of the suboccipital surface is
junction of the anterolateral and anteromedial margins is formed by the superior and inferior semilunar and biventral
Neurosurgery,Vol.47,No.3,September2000Supplement
Cerebellum and Fourth Ventricle S9
FIGURE 1.2. Tentorial,suboccipital,andpetrosalcerebellar
surfaces.A,thetentorialsurfacefacesthelowersurfaceof
thetentorium.Theanteriorvermisisthemostsuperiorpart
ofthetentorialsurface.Thissurfaceslopesdownwardtoits
posteriorandlateralmargins.Thevermiansubdivisionsof
thissurfacearesuperiortotheircorrespondinghemispheric
parts.Theclassicalnomenclatureappliedtothevermian
andhemisphericsubdivisionsofthetentorialsurfaceis
listedontheright,andoursimplifiednomenclatureislisted
ontheleft.Theculmenandquadrangularlobules
correspondtotheanteriorpartofthetentorialsurface,and
thedeclive,simplelobules,andpartofthesuperior
semilunarlobulescorrespondtotheposteriorpartofthe
tentorialsurface.Thefissureseparatingthetentorialsurface
intoanteriorandposteriorpartsisreferredtoasthe
tentorialfissureinournomenclature,butistheprimary
fissureinoldernomenclature.Thisfissureseparatesthe
hemisphericsurfacebetweenthequadrangularandsimple
lobulesandthevermisbetweenthedecliveandculmen.The
anteriorpartofthesuperiorsurfaceofthecerebellum
surroundstheposteriorhalfofthemidbraintoformthe
cerebellomesencephalicfissure.B,suboccipitalsurface.The
suboccipitalsurfaceislocatedbelowandbetweenthe
sigmoidandlateralsinusesandisthesurfacethatisexposed
inawidebilateralsuboccipitalcraniectomy.Theclassical
nomenclatureappliedtothissurfaceisshownontheright,
andoursimplifiednomenclatureisontheleft.Thevermis
sitsinalargemediandepression,theposteriorcerebellar
incisura,betweenthecerebellarhemispheres.Accordingto
classicalnomenclature,theportionsofthevermiswithinthe
incisurafromabovetobelowarethefolium,tuber,pyramid,
anduvula.Thepartsofthehemisphericsurfacefromabove
tobelowarethesuperiorandinferiorsemilunarand
biventrallobulesandthetonsils.Theselobulesextend
beyondthesuboccipitalsurfacetotheothersurfacesofthe
cerebellum.Theprebiventralfissuresbetweentheinferior
semilunarandthebiventrallobulesseparatethehemispheres
intosuperiorandinferiorparts,andtheprepyramidalfissure
betweenthepyramidandtuberseparatesthevermisinto
superiorandinferiorparts.Werefertotheunionofthe
prebiventralandtheprepyramidalfissuresthatdividethe
suboccipitalsurfaceintosuperiorandinferiorpartsas
thesuboccipitalfissure.Frombelowtoabovethe
correspondingvermianandhemisphericpartsaretheuvula
andthetonsils,thepyramidandthebiventrallobules,the
tuberandinferiorsemilunarlobules,andthefoliumandthesuperiorsemilunarlobules.Thepetrosal(horizontal)fissure,the
mostprominentfissureonthepetrosalsurface,extendsontothesuboccipitalsurfaceanddividesthesuperiorhalfofthe
suboccipitalsurfacebetweenthesuperiorandinferiorsemilunarlobules.Thecerebellomedullaryfissureextendssuperiorly
betweenthecerebellumandmedulla.C,petrosalsurface.Thepetrosalsurfacefacesforwardtowardthepetroustemporal
boneandisthesurfacethatisretractedtosurgicallyexposethecerebellopontineangle.Theclassicalnomenclatureapplied
tothissurfaceisshownontheright,andoursimplifiednomenclatureisontheleft.Thepetrosalfissuredividesthepetrosal
surfaceintosuperiorandinferiorparts.Thesuperiorpartisformedbythequadrangular,simple,andasmallpartofthe
superiorsemilunarlobules.Theinferiorpartisformedbytheinferiorsemilunarandbiventrallobulesandthetonsil.The
cerebellopontinefissuresareV-shapedfissuresformedwherethecerebellumwrapsaroundtheponsandthemiddlecerebellar
peduncles.Thesefissureshaveasuperiorandaninferiorlimb,whichmeetatalateralapex.Thepetrosalfissureextendslaterally
fromtheapexofthecerebellopontinefissures.Ant.,anterior;Cer.Med.,cerebellomedullary;Cer.Pon.,cerebellopontine;CN,
cranialnerve;Fiss.,fissure;Horiz.,horizontal;Inf.,inferior;Pet.,petrosal;Post.,posterior;Quad.,quadrangular;Suboccip.,
suboccipital;Sup.,superior;Tent.,tentorial.
Neurosurgery,Vol.47,No.3,September2000Supplement
S10 Rhoton
FIGURE 1.3. Tentorialsurfaceandcerebellomesencephalicfissure.A,thetentorialsurfacefacesthetentorium,whichhas
beenremoved.Thesurfaceslopesdownwardfromtheapextotheposteriorandlateralmargins.Theupperpartofthetento-
rialsurfacesurroundstheposteriorhalfofthemidbrainandformstheposteriorlipofthecerebellomesencephalicfissure.
Theanteriorcerebellarincisura,thenotchwherethebrainstemfitsintotheanteriorpartofthetentorialsurface,islocated
anteriorlyandtheposteriorcerebellarincisura,thenotchwherethefalxcerebellifitsintothecerebellum,islocatedposteri-
orly.B,enlargedviewofthecerebellomesencephalicfissure,whichextendsdownwardbetweenthemidbrainandthecere-
bellum.Thesuperficialpartoftheposteriorlipisformedbytheculmeninthemidlineandthequadrangularlobulelaterally.
Thequadrigeminalcisternextendscaudallyfromthepinealintothecerebellomesencephalicfissure.C,theculmenhasbeen
removedtoexposethecentrallobuleanditswings,whichformpartoftheposteriorlipofthecerebellomesencephalicfis-
sure.D,thecentrallobuleanditswings,thelingula,thesuperiormedullaryvelum,andmedialpartofthesuperiorcerebellar
peduncleshavebeenremovedtoexposethefourthventricle.Thelowerhalfoftheroofisformedinthemidlinebythe
Neurosurgery,Vol.47,No.3,September2000Supplement
Cerebellum and Fourth Ventricle S11
lobules and the tonsils, and the vermic portion is formed by the superior pole faces the uvula medially and the biventral
the folium, tuber, pyramid, and uvula. The vermian and the lobulelaterally.
relatedhemisphericpartsfromabovetobelowarethefolium
andthesuperiorsemilunarlobules,thetuberandtheinferior Petrosal surface
semilunarlobules,thepyramidandthebiventrallobules,and
Thepetrosaloranteriorsurfacefacestheposteriorsurface
theuvulaandthetonsils.
of the petrous bones, the brainstem, and the fourth ventricle
Thesuboccipitalsurfaceisdividedatitsmajorfissure,the
(Figs. 1.2 and 1.7). The lateral or hemispheric part of the
suboccipitalfissure,intosuperiorandinferiorparts.Thesub-
petrosalsurfacerestsagainstthepetrousboneandisretracted
occipital fissure has a vermian and a hemispheric part. The
toexposethecerebellopontineangle.Themedianorvermian
vermian part of this fissure, the prepyramidal fissure, sepa-
part of the petrosal surface has a deep longitudinal furrow,
ratesthetuberandthepyramid,andthehemisphericpart,the
theanteriorcerebellarincisura,thatwrapsaroundtheposte-
prebiventral fissure, separates the biventral and the inferior
rior surface of the brainstem and fourth ventricle. The right
semilunar lobules. The prebiventral and prepyramidal fis-
andlefthalvesofthepetrosalsurfacesarenotconnectedfrom
sures are continuous at the vermohemispheric junction, and
sidetosidebyacontinuousstripofvermis,asarethesuboc-
together they form the suboccipital fissure. The petrosal fis-
cipital and tentorial surfaces, because of the interposition of
sure, the major fissure on the petrosal surface, extends from
thefourthventriclebetweenthesuperiorandinferiorpartof
the petrosal surface onto the suboccipital surface, and sepa-
thevermis.Thevermalcomponentsrostraltothefourthven-
ratesthesuperiorandinferiorsemilunarlobuleslaterallyand
triclearethelingula,thecentrallobule,andtheculmen,and
the folium and the tuber medially. The tonsillobiventral fis-
those caudal to the fourth ventricle are the nodule and the
sureseparatesthetonsilandthebiventrallobule.
uvula. The hemispheric surfaces are formed by the wings of
Thetonsils,themostprominentstructureblockingaccessto
thecentrallobuleandtheanteriorsurfacesofthequadrangu-
the caudal part of the fourth ventricle, are a hemispheric
lar, simple, biventral, and superior and inferior semilunar
component(Figs.1.5and1.6).Eachtonsilisanovoidstructure
lobules,thetonsils,andtheflocculi.Thevermianandrelated
in the inferomedial part of the suboccipital surface that is
hemisphericpartsarethecentrallobuleandthewingsofthe
attachedtotheremainderofthecerebellumalongitssupero-
centrallobule,theculmenandthequadrangularlobules,the
lateral border by a white matter bundle called the tonsillar
nodule and the flocculi, and the uvula and the tonsils. The
peduncle. The remaining tonsillar surfaces are free surfaces.
major fissure on this surface, the petrosal fissure, also called
The inferior pole and posterior surface face the cisterna
thehorizontalfissure,splitsthepetrosalsurfaceintosuperior
magna and are visible inferomedial to the remainder of the
and inferior parts and extends onto the suboccipital surface
suboccipital surface. The lateral surface of each tonsil is cov-
betweenthesuperiorandinferiorsemilunarlobules.
ered by, but is separated from, the biventral lobule by a
narrow cleft, except superiorly at the level of the tonsillar
peduncle.Themedial,anterior,andsuperiorsurfacesallface THE FOURTH VENTRICLE AND THE
otherneuralstructures,butareseparatedfromthembynar- CEREBELLAR-BRAINSTEM FISSURES
row fissures. The anterior surface of each tonsil faces and is
Fourth ventricle
separated from the posterior surface of the medulla by the
cerebellomedullaryfissure.Themedialsurfacesofthetonsils Thefourthventricleisabroad,tent-shapedmidlinecavity
face each other across a narrow cleft, the vallecula, which located between the cerebellum and the brainstem. It is con-
leads into the fourth ventricle. The ventral aspect of the su- nectedrostrallythroughtheaqueductwiththethirdventricle,
periorpoleofeachtonsilfacesthethreestructures(telacho- caudallythroughtheforamenofMagendiewiththecisterna
roidea, inferior medullary velum, and nodule) forming the magna, and laterally through the foramina of Luschka with
lowerhalfoftheroofofthefourthventricle.Thesuperiorpole the cerebellopontine angles. Most of the cranial nerves arise
is separated from the surrounding structures by a posterior nearitsfloor.Ithasaroof,afloor,andtwolateralrecesses.It
extension of the cerebellomedullary fissure, called either the isventraltothecerebellum,dorsaltotheponsandmedulla,
telovelotonsillarorsupratonsillarcleft.Theposterioraspectof andmedialtothecerebellarpeduncles.
(cid:138)
noduleandlaterallybytheinferiormedullaryvelum,whichpasseslaterallyabove,butisseparatedfromtherostralpoleof
thetonsilsbythecerebellomedullaryfissure.E,someofthemiddlepedunclehasbeenremovedtoexposethechoroidplexus
extendingthroughthelateralrecessintothecerebellopontineanglebelowthefacialandvestibulocochlearnerves.F,oblique
viewofthelowerhalfoftheroofformedbytheinferiormedullaryvelumandthetelachoroideainwhichthechoroidplexus
arises.Theinferiormedullaryvelumarisesonthesurfaceofthenoduleandextendslaterallytoblendintotheflocculusand,
withtheflocculusandnodule,formstheflocculonodularlobeofthecerebellum.A.I.C.A.,anteroinferiorcerebellarartery;
Cent.,central;Cer.,cerebellar;Cer.Mes.,cerebellomesencephalic;Chor.,choroid;CN,cranialnerve;Coll.,colliculus;Dent.,
dentate;Fiss.,fissure;Flocc.,flocculus;Inf.,inferior;Lat.,lateral;Mid.,middle;Med.,median,medullary;Nucl.,nucleus;
Ped.,peduncle;Plex.,plexus;Post.,posterior;Quad.,quadrangular;Sulc.,sulcus;Sup.,superior;Tent.,tentorial;Vel.,velum;
Vent.,ventricle.
Neurosurgery,Vol.47,No.3,September2000Supplement
S12 Rhoton
FIGURE 1.4. Tentorialsurfaceandcerebellomesence-
phalicfissure.A,thetentorialcerebellarsurfacefaces
thetentoriumandslopesdownwardfromitsapex
locatedbelowthetentorialapex.Thecerebellomesence-
phalicfissureextendsforwardbetweenthecerebellum
andmidbrain.Thissurface,inwhichthevermisisthe
highestpart,differsfromthesuboccipitalsurfacein
whichthevermisisfoldedintoadeepcleft,theincisura,
betweenthecerebellarhemispheres.Thestraightsinus
andtentorialedgehavebeenpreserved.TheSCAexits
thecerebellomesencephalicfissureandsuppliestheten-
torialsurface.B,therighthalfoftheposteriorlipofthe
cerebellomesencephalicfissurehasbeenremoved.The
anteriorwallofthefissureisformedinthemidlineby
thecollicularplateandlingula,andlaterallybythesupe-
riorcerebellarpeduncles.Themiddlecerebellarpedun-
clewrapsaroundthelateralsurfaceofthesuperior
peduncle.Thetrochlearnervearisesbelowtheinferior
colliculi.C,therighthalfofthelingulaandsuperior
medullaryvelumhavebeenremovedtoexposethefourthventricle.Additionalwhitematterhasbeenremovedbelowtherightsuperior
peduncletoexposethedentatenucleusinwhichthesuperiorpeduncularfibersarise.D,enlargedview.Thedentatenucleusappearsto
wraparoundtherostralpoleofthetonsil.E,obliqueviewintothefourthventricle.Additionalcerebellumhasbeenremovedtoexpose
thenoduleandrostralpoleofthetonsil.Thedentatenucleuswrapsaroundtherostralpoleofthetonsil.Theupperhalfoftheroofis
formedbythesuperiormedullaryvelum,whichhasthelingulalayeredonitsoutersurface.Theupperpartofthelowerhalfoftheroofis
formedbythenoduleinthemidlineandbytheinferiormedullaryvelumlaterally.Theinferiormedullaryvelum,analmosttransparent
membrane,stretcheslaterallyacrosstheupperpoleofthetonsil.F,thelefthalfoftheupperpartoftheroofhasbeenremoved.The
velumarisesonthenoduleandsweepslaterallyabovebothtonsils.TheSCAcourseswithinthecerebellomesencephalicfissure.A.I.C.A.,
anteroinferiorcerebellarartery;Cer.Mes.,cerebellomesencephalic;Chor.,choroidal;CN,cranialnerve;Coll.,colliculus;Dent.,dentate;
Fiss.,fissure;Inf.,inferior;Lat.,lateral;Med.,medullary;Mid.,middle;Nucl.,nucleus;Ped.,peduncle;S.C.A.,superiorcerebellarartery;
Str.,straight;Sup.,superior;Tent.,tentorial;Vel.,velum;Vent.,ventricle.
Neurosurgery,Vol.47,No.3,September2000Supplement
