Dr. B.D. Chaurasia was a renowned Indian anatomist whose textbooks inspired the author to pursue a career in surgery. His line diagrams helped simplify anatomy. He produced line diagrams of skull base anatomy, including the 360° view of the skull base in Part 2. The document discusses cranial nerve anatomy in the skull base and relationships between nerves and vessels.

1. Dr.B.D.CHAURASIA - Great anatomist of
india whose books inspired me to take
up surgical field . Simplified the
anatomy with line diagrams .
Line diagrams - Skull
base 360°- Part 2
24-05-2017
2.38 pm

2. 1. 3rd nerve supplies to the muscles from medially – so when you are doing principle of
divergence [ to separate the ocular muscles ] to remove the intraconal tumors , don’t stretch
too much , chances of nerve avulsion from the muscle is there sothat muscle palsy
2. SO4 LR6 – all muscles by 3rd nerve , super oblique by 4th nerve , lateral rectus by 6th nerve
3. MG = medial group of nerves – 3rd, 6rth , nasociliary ; LG = lateral group of nerves – LFT [
mneumonic = Lacrimal , frontal , trochlear ]
4. 6th nerve is lateral to nasociliary nerve in between two divisions of 3rd nerve because it has
to supply lateral rectus .

3. Red rings = thicker lower division , thinner upper division ; Green ring = 6th nerve ; Yellow ring =
Nasociliary nerve
Mneumonic = My left hand middle finger is thicker than left hand index finger , so Lower
division of 3rd nerve is thicker than upper division of 3rd nerve .
Nasociliary nerve present between two divisions of 3rd nerve & 6th nerve present lateral to
nasociliary nerve

4. 1. In SOF [ superior orbital fissure ] & Orbital apex in nasal endosopic view - 3rd nerve
devides into thicker lower division & thinner upper division – in between these two
divisions 6th nerve & nasociliary nerve is seen .
2. 6th nerve is lateral to nasociliary nerve in between two divisions of 3rd nerve
because it has to supply lateral rectus .

5. Levator palpebrae superioris originates
from lesser wing of sphenoid

6. 1. Lateral part- LFT [ Liver functional tests ] Menumonic – Lacrimal N., Frontal
N.,TrochlearN. & Superior Opthalmic Vein.
2. The frontal and trochlear nerves ascend above the Levator muscle & superior
rectus muscle.
Frontal N. devides into Sup.Troch.N.
& Supraorb.N. – NOTE Fal.Lig

7. A segment of the orbital portion of the optic
nerve has been removed. This exposes the
branch of the inferior division of the
oculomotor nerve, which passes below the
optic nerve and enters the medial rectus
muscle.

8. 1. In the posterior part of the CS the trochlear nerve is below the oculomotor nerve, while
anteriorly it turns upward and becomes the most superior structure of the CS (at the level of
the optic strut) (Iaconetta et al. 2012 ) .
2. Trochlear nerve is always superior to V1.

9. 1. 6th nerve crosses para-clival & para-sellar carotid junction in the dorellos canal
2. in SOF 4TH nerve is above the 3rd nerve where as in cavernous sinus 3rd nerve above 4th nerve
3. supratrochlear & infratrochlear triangle above & below the 4th nerve .

10. 1. Thicker inferior division of 3rd N. & thinner superior division of 3rd N .
2. Nasociliary N passes between divisions of 3rd N.
3. In the annulus, the nasociliary nerve passes medially, and it is located
between the two divisions of the oculomotor nerve; the abducens nerve runs
superiorly and laterally to reach the lateral rectus muscle.

11. M-OCR

12. Two potential iatrogenic carotid injury areas
We have to very careful at m-OCR in
transtubercular & transplanum drilling
because praclinoidal & supraclinoidal
junction is exactly m-OCR
Posterior genu is the most common
area of iatrogenic injury of carotid

13. The optic chiasm is referred to as
prefixed when it is located above the
tuberculum sellae and as a
postfixed chiasm when it is situated
superior to the dorsum sellae

14. Parasagittal cadaveric specimen with a postfixed chiasm. Note the more
inclined pituitary stalk (stalk). For reference, note the left and right optic
nerves

15. Parasagittal cadaveric specimen with a normally positioned optic
chiasm. For reference, note the right optic nerve (II)

16. Subfrontal cadaveric dissection in a specimen found to have a
prefixed chiasm. Note the anterior location of the pituitary stalk (arrow).
For reference, note the optic nerves (II) and left internal carotid artery
(ICA)

18. Fig. 2.21 Sagittal sections and superior views of the sellar region showing the optic nerve and chiasm and
the carotid artery. The prefixed chiasm is located above the tuberculum. The normal chiasm is located
above the diaphragma. The postfixed chiasm is situated above the dorsum.

19. Opthalmic artery

20. classification of the ophthalmic artery types
http://www.springerimages.com/Images/MedicineAndPublicHealth/1-10.1007_s10143-006-
0028-6-1
a = intradural type,
b = extradural supra-optic strut type [ Optic strut = L-OCR ]
c = extradural trans-optic strut type

21. Type b = extradural supra-optic strut
type [ Optic strut = L-OCR

22. In Type c = extradural trans-optic strut type , the Opthalmic
foramen in Optic strut

23. classification of the ophthalmic artery types
http://www.springerimages.com/Images/MedicineAndPublicHealth/1-
10.1007_s10143-006-0028-6-1
a = intradural type,
b = extradural supra-optic strut type [ Optic strut = L-OCR ]
c = extradural trans-optic strut type
on optic nerve, pr proximal ring, cdr carotid dural
ring= upper dural ring , ica internal carotid artery
I think this variation is type c

24. In both type a = intradural type,
b = extradural supra-optic strut types Opthalmic
foramen is in Optic canal

25. In Type c = extradural trans-optic strut type , the Opthalmic
foramen in Optic strut

26. http://www.nature.com/eye/journal/v20/n10/fig_tab/6702377f3.html#figure
-title
The upper diagram is Type a or b Opthalmic artery , the lower diagram is Type
c Opthalmic artery
Dup OC = Duplicate Opthalmic
canal

27. Anterior limit of Transplanum approach is PEA – when
we are removing a triangular piece of bone in
Transplanum approach , the base of traingle is PEA

28. PEA roughtly corresponds with Optic tubercle
continuation above

29. Petrous apex

30. Petrous bone has three surfaces with three boarders &
divided into three 1/3rds

31. The petrous portion of the temporal
bone or pyramid is pyramidal

32. Petrous apex =Anterior Triangular ( T ) area +
Posterior Quadrangular ( Q ) area

34. Approaches to petrous apex
LATERAL SKULL BASE
1. From above the labyrinth
a. Middle cranial fossa transpetrous
[ = Trans-apical ] approach
2. From posterior to the labyrinth
a. Retrolabyrinthine transpetrous
[ = Trans-apical ] approach /
endoscopic retrolabyrinthine
approach
3. From through the labyrinth
a. Translabyrinthine transpetrous
[ = Trans-apical ] approach
b. Transcochlear transpetrous
[ = Trans-apical ] approach
4. From below the labyrinth
a. Infralabyrinthine/Infra-otic =
Infratemporal fossa type A
transpetrous [ = Trans-apical ]
approach
b. POTS
c. Infralabyrinthine/Infra-otic =
Infratemporal fossa type B & C
transpetrous [ = Trans-apical ]
approach
ANTERIOR SKULL BASE
1. From anterior to the labyrinth
a. Suprapetrous approach
b. Infrapetrous approach

35. Lateral skull base view

36. Middle cranial fossa transpetrous
approach

37. the anterior petrosectomy with preoperative embolization of the inferior
petrosal sinus is a time-conserving approach giving one of the best routes to
reach the ventral brainstem while working in front of the cranial nerves and
preserving hearing.
http://www.worldneurosurgery.org/article/S0090-3019(00)00271-8/fulltext

38. Retrolabyrinthine transpetrous
approach
– dotted red line means trajectory medial to labyrinth & cochlea

39. Tranaslabyrinthine Transpetrous [=
Transapical ] approach
After drilling the LSC , PSC & SSC
– IAC is exposed
Transpetrous path is above & below
the IAC & medial to cochlea

40. The Enlarged Translabyrinthine Approach with Transpetrous ( =
Transapical ) Extension – intradurally above the IAC you will get 5th nerve
where below the IAC you will get 6th nerve & lower cranial nerves .
Schematic drawings showing the amount of bone removal
around the internal auditory canal in the different variants of the
translabyrinthine approach. Note that in the transapical modification the
exposure is 320° and about 360° in types I and II, respectively. Abbreviations
as in Fig. 5.1. cn, cranial nerve; CN, cochlear nerve; FN, facial nerve;
IV, inferior vestibular nerve; SV, superior vestibular nerve.

41. Transcochlear transpetrous approach

42. Infratemporal fossa approach –A & B
Transpetrous approach

43. POTS = Petro-occipital trans-sigmoid
approach – sigmoid sinus is opened
– dotted red line means trajectory medial to labyrinth & cochlea

44. Anterior skull base – suprapetrous &
infrapetrous approach

45. CP angle

47. Various Transpetrous approaches to get lateral
view of CP angle ( = to reach Lateral part of
Posterior cranial fossa dura )
predominently to reach Level 1 =
Trigeminal nerve area & Level 2 =
AFB area
1. Retrolabyrinthine Transpetrous
( = Transapical )
2. Translabyrinthine Transpetrous (
= Transapical )
3. Transcochlear Transpetrous ( =
Transapical )
predominently to reach Level 3 =
Lower cranial nerve area
4. POTS = Petro-Occipital Trans-
Sigmoid approach
predominently to reach
Level 4 = Foramen magnum area
5. Exrtreme lateral or Far lateral or
Transcondylar approach

48. identify the superior
ampullary nerve first and then find the facial nerve
(F) after eliminating this nerve and the superior
vestibular nerve (Vs).

49. The Enlarged Translabyrinthine Approach with Transpetrous ( =
Transapical ) Extension – intradurally above the IAC you will get 5th nerve
where below the IAC you will get 6th nerve & lower cranial nerves .
Schematic drawings showing the amount of bone removal
around the internal auditory canal in the different variants of the
translabyrinthine approach. Note that in the transapical modification the
exposure is 320° and about 360° in types I and II, respectively. Abbreviations
as in Fig. 5.1. cn, cranial nerve; CN, cochlear nerve; FN, facial nerve;
IV, inferior vestibular nerve; SV, superior vestibular nerve.

51. Drill from inside to outside in middle
cranial fossa approach of IAC

52. (Right ear) Diagram of the internal auditory canal near the fundus.
The superior ampullary nerve (AN) is shown overlapping the superior vestibular
nerve (SVN). CN, cochlear nerve; FN, facial nerve; HC, horizontal crest; IVN,
inferior vestibular nerve; VC, vertical crest (Bill’s bar).

53. Cranial nerves

56. ICA Vertebro-basillar

57. 1. 3th nerve between PCA & SCA
2. 4th nerve coming from dorsal
brain stem passes above SCA [ some
times SCA has two branches]
3. 6th nerve originates at VBJ [
Vertebro-basillar junction ] . 6th
nerve may have two rootlets of
origin , one above & one below the
AICA
4. In 30 % of cases AICA passes in
between 7th & 8th nerves
5. PICA passes between two bundles
of 12th nerve & between two roots
of 11th nerve [ 11c = 11th cervical ,
11s = 11th spinal root ]
6. The exit zones of the 6th and 12th
nerves are at the same level [ same
vertical line when view from
Transclival approah ( through lower
clivus ) ]
7. 11th nerve behind left vertebral
artery at cervico-medullary junction
. 11th is closely related to the
vertebral artery (VA) at the point of
dural entrance

58. 1. 3th nerve between PCA & SCA
2. 4th nerve coming from dorsal brain
stem passes above SCA [ some times SCA
has two branches]
3. 6th nerve originates at VBJ [ Vertebro-
basillar junction ] . 6th nerve may have
two rootlets of origin , one above & one
below the AICA
4. In 30 % of cases AICA passes in
between 7th & 8th nerves
5. PICA passes between two bundles of
12th nerve & between two roots of 11th
nerve [ 11c = 11th cervical , 11s = 11th
spinal root ]
6. The exit zones of the 6th and 12th nerves
are at the same level [ same vertical line
when view from Transclival approah (
through lower clivus ) ]
7. 11th nerve behind left vertebral artery
at cervico-medullary junction . 11th is
closely related to the vertebral artery (VA)
at the point of dural entrance

60. 1. 3th nerve between PCA & SCA
2. 4th nerve coming from dorsal brain stem passes above SCA [ some times SCA has two branches]
3. 6th nerve originates at VBJ [ Vertebro-basillar junction ] . 6th nerve may have two rootlets of origin , one above & one below the AICA
4. In 30 % of cases AICA passes in between 7th & 8th nerves
5. PICA passes between two bundles of 12th nerve & between two roots of 11th nerve [ 11c = 11th cervical , 11s = 11th spinal root ]
6. The exit zones of the 6th and 12th nerves are at the same level [ same vertical line when view from Transclival approah ( through lower
clivus ) ]
7. 11th nerve behind left vertebral artery at cervico-medullary junction . 11th is closely related to the vertebral artery (VA) at the point of dural
entrance

61. 1. Laceral carotid & jugular
tubercle & lower cranial
nerves 9th ,10th ,11th are in
the same line .
2. hypoglossal canal present
between occipital
condyle/foramen magnum &
jugular tubercle

62. 1. Laceral carotid & jugular tubercle & lower cranial nerves 9th ,10th ,11th are in the same
saggital line .
2. hypoglossal canal present between occipital condyle/foramen magnum & jugular tubercle
3. PICA passes between two bundles of 12th nerve & between two roots of 11th nerve [ 11c =
11th cervical , 11s = 11th spinal root ] & encirlces lower cranial nerves 9th ,10th ,11th

65. 3rd nerve is sandwiched between posterior
cerebral artery [ PCA ] & superior
cerebellar artery [ SCA ]

66. 1. 3rd nerve from interpeduncular fossa
2. Only cranial nerve which comes from dorasal surface is 4th nerve
3. 6th nerve originates from ponto-medullary junction = vertebro-
basillar junction

67. Both sides 6th nerves in dorello’s canals present medial to
para-clival carotids in mid-clivus & crosses Para-clival & Para-
sellar carotids juction in cavernous sinus

71. 6th nerve (the snake nerve)
6th nerve originates above the VBJ [
vertebro-basillar junction ] – Prof.
Amin Kassam

72. 3rd ventricle

73. Anterior choraoid arteries goes
towards optic tract

76. Optic chiasma – infundibulum – Mamillary
bodies

77. Circle of willis

79. In anterior skull base approach

80. Middle cranial fossa approach

81. In anterior skull base approach - Type C Modified
Transcochlear Approach – after cutting the tentorium
With mild retraction of the temporal lobe, the bifurcation of the internal carotid artery (ICA) into
the anterior (ACA) and middle cerebral (MCA) arteries is seen. The ipsilateral (ON) and
contralateral (ONc) optic nerves are seen. The oculomotor nerve (III) is embraced by the
posterior cerebral artery (PCA) superiorly and the superior cerebellar artery (SCA) inferiorly

82. In the descriptive analysis of the 20 specimens, the PCoAs
distance was 9 to 18.9 mm, mean of 12.5 mm, median of 12.2
mm, standard deviation of 2.3 mm.

83. http://www.ajnr.org/content/27/8/1770/F2.expansion.html
Classification of the anatomic variations in the circle of Willis. In the “textbook” type, both the
precommunicating segment of the anterior cerebral artery (A1) and that of the posterior cerebral
artery (P1) were normal in size. The next group included both right and left A1 hypoplasia.
Because no significant difference between cerebral arteries on the right and left sides has been
established,5,18 we combined right and left A1 hypoplasia into A1 hypoplasia. The next group
included right and left P1 hypoplasia, which again were treated as a single category, P1
hypoplasia. “Other” type included a combination of A1 hypoplasia and P1 hypoplasia, bilateral P1
hypoplasia, as well as other unclassified variations. ACA indicates anterior cerebral artery; ACo,
anterior communicating artery; MCA, middle cerebral artery; ICA, internal cerebral artery; PCo,
posterior communicating artery; PCA, posterior cerebral artery; BA, basilar artery

84. http://www.ajnr.org/content/27/8/1770/F3.expansion.html
Relative contribution of proximal arteries to total volume flow in variations in the circle of Willis.
Values signify mean percentage ± SD. The upper left value corresponds to the relative
contribution of the right internal carotid artery in the “textbook” type or of the internal carotid
artery ipsilateral to hypoplastic A1 or P1 in the other variations. The upper right value
corresponds to the relative contribution of the left internal carotid artery in the “textbook” type,
or of the internal carotid artery contralateral to hypoplastic A1 or P1 in the other variations. The
value at the bottom corresponds to the relative contribution of the basilar artery.
* The value for A1 hypoplasia variation was significantly smaller than those for “textbook” type
and P1 hypoplasia variation. The value for P1 hypoplasia variation was significantly larger than
that for “textbook” type.
** The value for A1 hypoplasia variation was significantly larger than that for “textbook” type.
*** The value for P1 hypoplasia variation was significantly smaller than that for “textbook” type.

85. http://stroke.ahajournals.org/content/30/12/2671/F2.expansion.html
Scheme of anatomic variations of the posterior part of the CW: variant types a
through c are complete, whereas the remainder are incomplete. a, Bilateral PCoA
present. b, PCA originating predominantly from the ICA. This type is known as a
unilateral fetal-type PCA (FTP), indicated by the arrows; the PCoA on the other side is
present. c, Bilateral FTP, with both P1 segments patent. d, Unilateral PCoA present. e,
Hypoplasia or absence of both PCoAs, with isolation of the anterior and posterior
circle parts at this level. f, Unilateral FTP, with hypoplasia or absence of the P1
segment. g, Unilateral FTP, with hypoplasia or absence of the contralateral PCoA. h,
Unilateral FTP, with hypoplasia or absence of the P1 and PCoA. i, Bilateral FTP, with
hypoplasia or absence of both P1 segments. j, Bilateral FTP, with hypoplasia or
absence of one P1 segment.

86. Open skull base approaches

92. Infra-temporal
fossa type B
approach

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