Objectives:
Describe the features (and
reflections/foldings) of cranial dura mater.
Describe the intracranial dural venous
sinuses.
The brain is covered from outside
inwards by three meningeal layers namely
i)
Dura mater
ii) Arachnoid
mater and
iii) Pia
mater.
Dura
Mater:
It is tough and consists of
outer
endosteal/periosteal layer which serves as inner periosteum
(covering inside of the bones that form the calvaria of skull) and
inner
meningeal layer which is a strong fibrous membrane that is continuous at
the foramen magnum with the spinal dura mater covering the spinal cord.
The
meninegeal layer is fused with the endosteal layer all over except where it
projects inwards in the form of four folds – falx cerebri, tentorium cerebella,
falx cerebella and diaphragm sellae.
Reflections
of (cranial) dura mater
The internal meningeal layer of dura mater is
a supporting layer that reflects away from the external endosteal/periosteal
layer of dura to form dural infoldings (reflections). The dural infoldings
include:
Cerebral
falx (Latin –falx cerebri)
Cerebellar
tentorium (L. tentorium cerebelli)
Cerebellar
falx (L. falx cerebelli)
Sellar
diaphragm (L. diaphragmasellae)
The
cerebral falx (L. falx, a sickle-shaped structure), the largest dural
infolding, lies in the longitudinal cerebral fissure that separates the right
and left cerebral hemispheres.
Attaches
in the median plane to the internal surface of the calvaria as
Anteriorly
- the frontal crest of the frontal bone and crista galli of the ethmoid bone
Posteriorly
- the internal occipital protuberance
It
ends by becoming continuous with the cerebellar tentorium.
The
cerebellar tentorium, is a wide crescent shaped septum that separates
the occipital lobes of the cerebral hemispheres from the cerebellum.
Attachments
–
Rostrally to the clinoid
processes of the sphenoid,
Rostrolaterally to the
petrous part of the temporal bone, and
Posterolaterally to the
internal surface of the occipital and part of the parietal bone
The
cerebral falx attaches to the cerebellar tentorium and holds it up, giving it a
tent-like appearance (L. tentorium, tent). The cerebellar tentorium divides the
cranial cavity into supratentorial and infratentorial compartments.
The
cerebellar falx is a vertical dural infolding that lies inferior to the
cerebellar tentorium in the posterior part of the posterior cranial fossa.
It
is attached to the internal occipital crest and partially separates the
cerebellar hemispheres.
The
sellar diaphragm, the smallest dural infolding, is a circular sheet of
dura that is suspended between the clinoid processes forming a partial roof
over the hypophysial fossa in the sphenoid bone. The sellar diaphragm covers
the pituitary gland in this fossa and has an aperture for passage of the
infundibulum and hypophysial veins.
Intracranial
dural venous sinuses
The
dural venous sinuses are endothelium-lined spaces between the endosteal and the
meningeal layers of the dura except the inferior and straight sinuses which
lodge only within the fold of meningeal layer.
Large
veins from the surface of the brain empty into these sinuses and most of the
blood from the brain ultimately drains through them into the internal jugular
veins. The dural venous sinuses are:
Unpaired Paired
Superior
sagittal Transverve
Inferior
sagittal Sigmoid
Straight Cavernous
Occipital
superior
petrosal
Anterior
intercavernous Inferior
petrosal
Posterior
intercavernous spheno-parietal
Basilar
venous plexus petro-squamous
Middle meningeal
1) The
superior sagittal sinus lies in the convex attached border of the
cerebral falx. It begins at the crista galli and ends near the internal
occipital protuberance at the confluence of sinuses, a meeting place of
the superior sagittal, straight, occipital, and transverse sinuses.
The
superior sagittal sinus receives the superior cerebral veins and communicates
on each side through slit-like openings with the lateral venous lacunae.
2) The
inferior sagittal sinus runs in the inferior concave free border of the
cerebral falx and ends in the straight sinus.
3) The
straight sinus (L. sinus rectus) is formed by the union of the inferior
sagittal sinus with the great cerebral vein. It runs inferoposteriorly along
the line of attachment of the cerebral falx to the cerebellar tentorium, where
it joins the confluence of sinuses.
4) The
transverse sinuses pass laterally from the confluence of sinuses,
coursing along the posterolateral attached margins of the cerebellar tentorium
and then become the sigmoid sinuses. Blood received by the confluence of
sinuses is drained by the transverse sinuses, but rarely equally. Usually the
left sinus is larger.
5) The
sigmoid sinuses follow S-shaped courses in the posterior cranial fossa,
forming deep grooves in the temporal and occipital bones. Each sigmoid sinus
turns anteriorly and then continues inferiorly as the Internal Jugular Vein.
6) The
occipital sinus lies in the attached border of the cerebellar falx and
ends superiorly in the confluence of sinuses. It communicates inferiorly with
the internal vertebral venous plexus.
7) The
cavernous sinus is located on each side on the upper surface of the body
of the sphenoid. The cavernous sinus consists of a venous plexus of extremely
thin-walled veins that extends from the superior orbital fissure anteriorly to
the apex of the petrous part of the temporal bone posteriorly. It receives
blood from the superior and inferior ophthalmic veins, superficial middle
cerebral vein, and sphenoparietal sinus. The venous channels in these sinuses
communicate with each other through venous channels anterior and posterior to
the stalk of the pituitary glands - the intercavernous sinuses and sometimes
through veins inferior to the pituitary gland. The cavernous sinuses drain
posteroinferiorly through the superior and inferior petrosal sinuses and
emissary veins to the pterygoid plexuses.
The
structures passing through the cavernous sinus:
Inside
each cavernous sinus is the internal carotid artery with its small branches,
surrounded by the carotid plexus of sympathetic nerve(s), and the abducent
nerve (CN VI) . The oculomotor (CN III) and trochlear (CN IV) nerves, plus two
of the three divisions (ophthalmic and maxillay) of the trigeminal nerve (CN V)
are embedded in the lateral wall of the sinus.
8) The
superior petrosal sinuses run from the posterior ends of cavernous sinus
to the transverse sinuses. Each superior petrosal sinus lies in the
anterolateral attached margin of the cerebellar tentorium, which attaches to
the superior border (crest) of the petrous part of the temporal bone.
9) The
inferior petrosal sinuses also commence at the posterior end of
the cavernous sinus inferiorly. Each inferior petrosal sinus runs in a groove
between the petrous part of the temporal bone and the basilar part of the
occipital bone. The inferior petrosal sinuses drain the veins of the lateral
cavernous sinus directly into the origin of the IJVs.
10) The
basilar plexus connects the inferior petrosal sinuses and communicates
inferiorly with the internal vertebral venous plexus.
REFERENCES:
Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):
Gray's Anatomy
K. L. Moore's Clinically Oriented Anatomy
R. Snell's Clinical Anatomy