Anatomy of the skull base, Foramen of the skull base, lateral skull base, anterior skull base, internal carotid artery, cranial nerves, cavernous sinus

1. ANATOMY OF SKULL BASE
 Dr. Shilpa M J

2.  The skull base represents a central and complex bone structure of the
skull that forms the floor of the cranial cavity on which the brain lies.
 Anatomical knowledge of this particular region is important for
understanding several pathologic conditions as well as for planning
surgical procedures.
Introduction

3. Skull base boundaries:
Anterior Upper incisor teeth
Posterior  Sup. nuchal line of occipital bone,
Lateral Remaining upper teeth,the zygomatic
arch & its posterior root, the mastoid process
Composed of fivebones:
1)Ethmoid 2) Sphenoid 3)Occipital
4) Paired temporal 5) Paired frontal bones
Fossa: 1)Anteriorcranial fossa
2) Middle cranial fossa
3)Posterior cranialfossa

4. The Frontal Bone

5.  Orbital part – contains two
openings
- Anterior and Posterior ethmoidal
foramina
- At the junction of suture lines
between the lamina papyracea and
the frontal bone.

6. Frontal sinus
 Develops from pneumatic extension of anterior
ethmoidal cells into frontal bone.
 Relations :
 Posterior : Anterior cranial fossa
 Medial : Anterior ethmoidal cells
 Lateral portion of the floor : over orbital roof

7.  Connection between the Frontal
sinus and Nose – HOURGLASS
SHAPED.
 FRONTAL RECESS
BOUNDARIES :
 Superior – Skull base
 Lateral – Lamina papyracea
 Medial – Anterior portion of the
middle turbinate
 Posterior – Ethmoid bulla
 Anterior – Agger nasi cell

8. The Ethmoid Bone
Consists :
• horizontal plate(cribriform plate)
• vertical plate in the midline
- the perpendicular plate.

9. • On closer examination, the cribriform plate
Horizontal medial lamella Vertical lateral lamella
This lateral lamella articulates with
the frontal bone.

10. The skull base in this region - the ethmoid fovea -
is formed
• medially - lateral lamella of the cribriform plate
- very thin bone (0.2mm)
• laterally - frontal bone - thicker bone (0.5mm)
• The region where the anterior ethmoidal artery
pierces the dura medially is the thinnest area in
the skull base and is only 0.05 mm in thickness.

11.  Ethmoid roof configuration differentiated
based on the length of the lateral lamella of
cribriform plate – KEROS CLASSIFICATION
 Type 1 – 1-3mm
 Type 2 – 4-7mm
 Type 3 – 8-17mm

12. Temporal
bone Its made up of squamosal, mastoid, petrous & tympanic parts which ossify
separately and later fuse creating squamotympanic & petrosquamous
fissure.

13. The squamous temporal bone contains the hollow of
the glenoid fossa joined laterally to the zygomatic
process.

17. • Tympanic part has
styloid process →
behind its base,
stylomastoid foramen
• Stylomastoid
foramen transmits
facial nerve.

18. The Sphenoid Bone
• It separates the anterior and middle cranial fossa.
• looks like a bat with outstretched wings.

19. • It consist of → a central body; two sets of wings– the greater
andlesser,which course laterally ; & two pterygoid
processes, directedinferiorly.

20. Posteriorly, the
chiasmatic sulcus forms
a slight depression &
leads laterally to the
optic canal.
The tuberculum sellae,
abony elevation, just
posterior to thissulcus

21. Followed by,
posteriorly bysella
turcica & dorsum
sellae.
The dorsumsellae
terminates laterally
into the posterior
clinoid processes

23. The anterior surface ofthe body of sphenoid forms the roof &
posteriorwall of nasopharynx
Thebody housesthe sphenoidsinus.
Lesserwings→Forms
medial portionof orbital
apex.
Greaterwings → Course
upward & laterally from both sides
of the sphenoid body-forms floor of
MiddleCranial Fossa.

24. Its the skull base part situated between the foramen magnum and the dorsum
sellae.
Formed from sphenoid and occipital bones.
CLIVU
S

25. Posterior: lesser wing of the sphenoid & anteior clinoid processes.
Floor : roof of the nasal cavity & ethmoidsinuses medially.
Lateral wall : thick and strong orbital plates of the frontal bone

26. MIDDLE CRANIAL FOSSA
• The middle cranial base
forms the floor of the
middle cranial fossa.
• Boundaries
 Anterior : Tuberculum
sellae, anterior clinoid
process, post margin of lesser
sphenoid wing.
 Posterior : superior border
of petrous part of temporal
bone and dorsum sellae of
sphenoid.

27. Posterior Cranial fossa
•Anterior margin :- The posterior surfaceof the
clivus.
• Laterally:- posterior surfaceof the petrous part of
temporal bone
• Posteriorly :- mastoid
portion of temporal bone
& the squamous part of
occipitalbone.

28. Frontal crest :- Midline bony ridge that projects upwards & provide attachment to
the falx cerebri.
Foramen caecum:-Transmits emissary vein from nose to superior sagittal sinus
Crista galli :- Provides site for ant. most attachment of the falx cerebri.
Cribriform plate :- Sheet of bone contaning many small Olfactory foramina 
Transmit olfactory nerve fibres into the nasal cavity.
Ant & post ethmoidal foramen:-TransmitsAnt & post.ethmoidal artery,nerve,vein
Thecontents & foramina's ofACF

29. RULE OF SIX

30. Triangular shaped fissure bounded med. body of sphenoid, sup. lesser wing,
& inf. greater wing and is completed lat frontal bone as greater & lesser
wings converge.
Optic strut separates optic canal from superior orbital fissure.
Optic canal & superior orbital fissure together form the orbital apex.
Superior orbitalfissure
Abducent nerve is most likely to damage 1st in
sup. orbital fissuresyndrome

31. Extends from pterygopalatine fossa along orbital floor.
„Separates greater wings of the sphenoid from the maxilla.
Content –1) Maxillary branch of trigeminal nerve 2)Infra orbital vessels.
3)Emissary veins connecting inf ophthalmic vein to pterygoid venous plexus.
4)Zygomatic nerve.
Inferior orbitalfissure

32. Subdivisions of the lateral skull base
1. Pharyngeal;
2. Tubal;
3. Neurovascular;
4. Auditory;
5. Articular;
6. Infratemporal fossa.

33. Situated centrally in the skull base forming roof of the nasopharynx,
Boundaries Formed by the line of attachment of the pharyngeal wall.
The pharyngobasilar fascia is attached to the skull base and medial pterygoid
plates thickened post into apharyngeal ligament that continues inferiorly as
the pharyngeal raphe.
Separated from the prevertebral
muscles post by prevertebralfascia.
 Tubal area lies just lateral to
pharyngeal area, comprises the
region occupied by eustachiantube
Pharyngeal &Tubal area

34. The pharyngobasilar fascia is attached to undersurface of the tube, & two
'paratubal' muscles arise one on each side of it.
The levator palati arises medially (within the pharynx) & the tensor palati arises
laterally (outside thepharynx).
Both muscles are partly attached to the tube, and open it during swallowing

35. NEUROVASCULARAREA
• Posterior to the tubal area lies the neurovascular area:-
• Carotid sheath;
• Styloid apparatus;
• Facial nerve

36. Carotid sheath
• It is attached to the skull base around the carotid
foramen and continues downwards as far as the aortic
arch.
• Content :
Internal carotid artery Vagus
nerve.
Internal jugular vein

37. 3muscles : Stylopharyngeus, Stylohyoid,Styloglossus.
Stylopharyngeus:- Passlateral toICA.
Origin Deep aspect of base of styloid process.
InsertionThyroid cartilage & side wall of pharynx.
Nerve supply Ninth nerve.
Function:- Elevates larynx &pharynx.
Stylohyoid:- Passlateral toECA.
Origin Back of the base of styloid process
Insertion2 slips over base of greater cornu ofhyoid
Nerve supply Seventhnerve.
Function:- Elevates & retracts the hyoid.
Styloid apparatus

38. Styloglossus:-
Origin front of the styloid process &upper part of stylohyoid ligament
Insertion Side of thetongue
Nerve supply Hypoglossalnerve,
Function:- Retract thetongue .

39. STYLOMASTOID FORAMEN
It transmits the facial nerve and
the stylomastoid artery.
As soon as it emerges from the
foramen, VII gives off the
posterior auricular nerve
(supplying the occipital belly of
occipitofrontalis)
and a muscular branch (supplying
the posterior belly of digastric and
stylohyoid).
It then swings forward into
the parotid gland, dividing
zygomaticofacial &
cerviciofacial division

40. AUDITORY
AREA
This small area
anterolateral to the
neurovascular area
forming the floor
and anterior wall of
the external
auditory canal and
middle ear.

41. Situated on each side of the body of sphenoid bone & extend
from sup. orbital fissure ant to petrous apex post.
Receives :- Sup.& inf. ophthalmic vein, Central vein of retina
Sphenoparietal sinus.
Drains into:- Petrosal sinus, Pterygoid plexus, Basilar plexus.
Contents:- 1) CN III, IV,V1,V2 & VI 2) ICA
Thecontents & foramina's of MCF
CavernousSinus Only anatomic
location in the
body in which an
artery travels
completely
through a
venousstructure

43. Dural invagination at posterior aspect of cavernous sinus. Contains gasserian
ganglion (trigeminal).
Dural layers shows thinperipheral enhancement.
Optic canal
 Formed by the lesser wing of sphenoid.
 The contents are :- Optic nerve .
OphthalmicArtery.
Sympathetic fibers from carotid plexus
MECKEL’S CAVE

44. ForamenRotundum Foramenovale
 situatedpost-lat to F.rotundum
Contents :-1) Mandibular Nerve (CN
V3) 2) Accessory meningealartery
3) Lesser petrosal nerve4)Emissary
vein
Canal in the base of the greater
sphenoid wing, situated - inf &
lateral toSOF.
It extends obliquely forward & slightly
inferiorly, connecting the MCFto
pterygopalatine fossa.
Transmits the maxillary nerve (V2),
artery of the foramen Rotundum &
emissary veins.

45. Foramenspinosum ForamenLacerum
Its located at the base of medial
pterygoid plate, ant to the petrous
apex.
Structures passing wholelength:
1)Meningeal branch of Ascending
pharyngeal artery 2) Emissaryvein
Other structures partiallytraversing:
3)Internal carotid artery
4)Greater petrosal nerve.
Its an aperture in the greater wing
of the sphenoid posterolateral to
foramen ovale.
Contents :- 1) Middle meningeal
artery & vein. 2) Emissary vein.
3) Nervous spinosus (Meningeal
branch of mandibularnerve)

46. VidianCanal
Pterygoid canal.
Located in the floor of sphenoid sinus at the junction of the pterygoid process &
the sphenoid body connecting the pterygopalatine fossa ant & the foramen
lacerum posteriorly.
Contents:- 1) VidianArtery ( Br.Of MaxillaryArtery). 2)Vidian Nerve (greater
superficial petrosal nerve & deep petrosal nerve )

47. Its a passage within petrous temporal bone & transmits the ICA& sympathetic
plexus enters the MCFfrom the neck.
Its initially directed superiorly, then turns anteromedially to reach up to
the petrous apex.
CAROTID SHEATH
 Fibrous connective tissue thatsurrounds
vascular compartment ofthe neck.
 Cervical part of the sympathetic trunk is embedded in prevertebral fascia
immediately post. tosheath.
Carotid canal

48. ForamenMagnum Internal acousticcanal
Thecontents & foramina's of PCF
 from pontomedullary junctionto inner ear
Divided by a bony lamina (falciform crest) into :- 1)
Smaller superior part:Superior vestibular N. &Facial
N
2) Larger Inferior part:- Inferior vestibular N.
& Cochlearnerve
The foramen magnum is entirely formed TransmitsVII & VIII cranial nerves
within the occipital bone.
Contents :- 1. Medullaoblongata.
2.Vertebral arteries andveins.
3.Anterior & posterior spinal arteries.
4. spinal component ofCNXI.
5.Tectorial membrane &alar ligaments.

49. Located behind the carotid canal & formed in front
by petrous portion of the temporal, & behindby
occipital.
It is divided into 3 compartments by two fibrous
septa.
Anterior : Glossopharyngeal , Inferior petrosal
sinus
Middle : vagus and spinal accessory nerves
Posterior : Emerging internal jugular vein
Jugular foramen

50. When the roof of the jugular bulb isseen above the level of floor of IAC, it
is called a high riding jugular bulb, which is more common on the right
side.
This is a dangerous variant & exposing during translabyrinthine surgery.

51. Ant condylar canal. Located within occipital bone.
Transmits hypoglossal nerve.
Hypoglossal canal.

52. Afat filled space between the pterygoid plates and the posterior wall of maxillary
sinus. Shaped like an invertedpyramid.
Borders :- Med - Perpendicular plate palatine bone, Lat - Narrowing to
pterygomaxillary fissure,Ant - Post wall of maxillary sinus, Post - Med & Lat
pterygoid plates; inferior aspect of greater wing of sphenoid bone.
It contains:-1) Pterygopalatine ganglion 2)Terminal third of the maxillary artery,
3)CNV2 4)Greater & deep petrosal nerve.
Pterygopalatine fossa

53. The PPFis an important pathway for the spread of neoplastic and infectious
processes:
Med - with nasal cavity via sphenopalatine F.
Lat - infratemporal fossa via the
pterygomaxillaryfissure.
Ant - with orbit via the inferior orbital fissure.
Post & sup - with Meckel cave & cavernous
sinus (of MCF) via the F.rotundum.
Post & inf- with MCFvia the vidian
canal, which transmits the Vidian nerve.
Inferiorly - with palate via the greater
and lesser palatine canals
Pterygopalatine fossaCommunications

55. It is the space between the skull base,
lateral pharyngeal wall & the ramus of
mandible.
Boundaries :- 1) Lat.- Ascending ramus of
the mandible and zygomatic arch.
2) Med.- Lateral pterygoid plate.
3)Ant. –Posterolateral wall ofmaxilla and
IOF.
4) Post. –mastoid and tympanic portions of
the temporal bone.
5)Sup. - Greaterwing of the sphenoid bone.
6) Inf.–Superior limit ofposterior belly ofdigastric
andangle ofmandible.
Infratemporal fossa

56. Medial Pterygoid muscle
 Origin:- Superficial head Maxillary tuberosity & pyramidal process of palatine
bone
 Deep head Medial surface of lateral pterygoid plate of the sphenoid bone
 Action: Elevates & Protrusion of the mandible ,Side to side movement
 Nerve supply:- Medial pterygoid nerve branch of mandibular nerve
 Blood Supply:- Branch of maxillary artery
ContentsOf Infratemporal fossa

57. Origin:- Upper head infra-temporal surface & crest of greater wing of sphenoid
 Lower head Lateral surface of the lateral pterygoid plate
Action: Depression & Protrusion of the mandible, Side to side movement
Nerve supply:- branches from the masseteric or buccal nerve, branch of the ant.
trunk of the mandibular nerve
Blood Supply:- Pterygoid vessels from Maxillary artery
Lateral Pterygoidmuscles

58. Its the larger of the 2 terminal branches of ECAArises behind neck of the
mandible Passforward bwt ramus of mandible & sphenomandibular
ligament Then runs sup to the lower head of lateral pterygoid b/w two
heads of lateral pterygoid - Pterygomaxillary fissure Pterygopalatine fossa.
MAXILLARYARTERY

59. Described in 3parts: before, on & beyond the lat. pterygoid muscle.
From 1ST& 3RD parts all branches enter bony foramina; from 2nd part noneof
the branches go through bony foramina
BRANCHESOFMAXILLARYARTERY

60. PLEXUS:- Lies within & on latsurface of lat pterygoid muscle, &
tributaries correspond to branches of maxillaryartery.
Drains into maxillary vein to join superficial temporal vein & form
retromandibularvein.
Communicating veins: 1) Inferior orbital fissure  Inferior opthalmic vein
2) aconnecting vein
passesvertically
down from the
cavernous sinus
3)deep facial veinto
join ant facialvein.
Thepterygoid plexus & maxillary veins

61. The mandibularnerve
Course:- Middle cranial fossa 
Passing through foramen ovale 
inf r at em por al fossa – Main trunk
divides into anterior and posterior
trunk.
Branches :
• Main trunk – Meningeal branch and
nerve to medial pterygoid
• Ant trunk – Buccal nerve,masseteric,
deep temporal and nerve to lateral
pterygoid
• Post trunk – Auriculotemporal,
Lingual, Inferior alveolar nerve

62. Internal carotid artery: Carotid foramen→curves upwards
into F.lacerumin MCF → apex of petrous bone→
enters the cavernoussinus
It lies in front of cochlea & middle ear cavity, separated by
thin plate of bone (may be dehiscent)
→ gives off small intrapetrous branches, including carotico-
tympanic artery → feeding vessels for a glomus tumour.
Structureswithin the skullbase

63.  C1 – Cervical
 C2 – Petrous - Ascending / Genu /
Horizontal
 C3 – Lacerum ‘Extradural’
 C4 – Cavernous –
 Ascends – Posterior clinoid
process
 Forwards – side of the body of
sphenoid
 Upwards – medial side of
anterior clinoid process
 C5 – Clinoid segment ‘intradural’
 C6 – Ophthalmic segment
 C7 – Communicating segment

64. Branches of the internal carotid artery

65. Jugularbulb
It isthe point at which sigmoid
sinus feeds the upper end ofIJV.
Liesbelow posterior part of the floor
of the middleear.
Inferior petrosal sinusjoints
jugular bulb at the skull base

66. DURAL VENOUS
SINUSES

67. Parapharyngeal , masticator, carotid & retropharyngeal spaces seen in close
contact with the skull base along their cephalad aspect.
Parapharyngeal space extends caudally to the submandibular space & cranially
abuts the base skull Contains fat, which acts asamedium for infection.
Relation of skull baseto the deep facial spaces

68. Also c/a Lateral pharyngeal space, Pharyngomaxillaryspace.
Boundaries:- Shape like an invertedpyramid
SupSkull base, sphenoid & temporal bones.
Inferior Greater cornu of hyoid bone
Anterior Pterygomandibular raphe.
Posterior Carotid sheath post-lat.
& retropharyngeal space post-med.
Med Sup.Constrictor,
Buccopharyngeal fascia
Lat Ramus ofmandible, deep lobe
of parotid gland, medial pterygoid
muscle
Parapharyngeal
space

69. Has two compartments Prestyloid compartment
Contains 2 musclesTensor palati & Levator palati muscles
2 artery Ascending palatine & ascending pharyngeal artery & int. jugular vein.
Retrostyloid compartment
It isneurovascular space,
& contains the carotid sheath.

70.  There are 4 muscles asfollow
Masseter muscle
 Origin:- From zygomaticarch
 Insertion:-Lateral aspect of mandible
from the angle forwards along the lower
border, & upwards over the lower
part ofthe ascending ramus.
 Nerve supply:- massetericbranch
from ant. division of the mandibular N.
 Action:-Elevation &protrusion
of mandible
Musclessuperficial to the lateral skull base

71. Largest muscle of mastication & fan shape.
Origin: From inf. temporal line , floor of temporal fossa & from overlying
temporal fascia of the side of the skull.
Insertion: Superior border & medial tip of the coronoid process.
Action: Elevation (anteriorfibers)
& Retraction (posteriorfibers)
Nerve supply:Ant div. of mandibular N.
Blood Supply:- middle temporal artery,
branch ofsup. temporal artery
deep temporal arteries,branches
of the maxillaryartery
Temporalismuscle

72. Origin:- from 2 heads: manubrium & clavicle.
Inserted:- Curved line extending from tip of the mastoid process to superior
nuchal line of theocciput.
Nerve supply:- eleventhCN
Action:-Toprotract the head (moving it forwards while keeping it vertical with a
horizontal gaze).
Sternocleidomastoid
muscle

73. Two bellies united bytendon
Origin –Anterior belly from diagastric fossa of mandible. Posterior belly from
mastoid notch of temporal bone.
Insertion –Both meet at the intermediate tendon and held by the fibrous pulley.
Nerve supply:- Post. bellyis supplied
by seventh nerve (nerve todigastric)
& the ant. belly by the fifth nerve
(mylohyoid nerve).
Action:-To depress & retract the chin
Digastric muscle