16 Neuroanatomy: Draw It to Know It
Meninges
Here, we will draw the meninges, which comprises
the dura mater, arachnoid mater, and pia mater. First
draw an outline of the skull and upper spinal canal.
Intracranially, draw the outermost meningeal layer: the
dura mater, directly underlying the skull. Th en, within
the spinal canal, show that space exists between the dura
mater and the vertebral column, which creates a true epi-
dural space. Epidural hematoma more commonly occurs
in the spinal canal, because of its true epidural space,
than in the cranial vault, where the dura mater directly
adheres to the skull. Th e classic example of intracranial
epidural hematoma is that which occurs from trauma to
the temporal bone — the fractured bone severs the under-
lying middle meningeal artery and the high pressure of
this arterial vessel rips the dura away from the skull so
that blood collects between the dura and the cranium.
Clinically, we commonly discuss the dura mater as a
single layer, but within the cranium, it actually comprises
two separate anatomic sublayers. Label the layer we just
drew as the periosteal dural sublayer, which tightly
adheres to the skull. Next, label the underlying menin-
geal sublayer. Th e periosteal layer ends within the cra-
nium; within the spinal canal, only the meningeal dural
sublayer exists. Indicate that for much of the cranial dura
mater, the periosteal and meningeal sublayers closely
adhere; however, show that meningeal dural sublayer
refl ections also exist. Th ese refl ections form the falx cere-
bri, which separates the cerebral hemispheres; the tento-
rium cerebelli, which separates the cerebellum from the
overlying occipital lobes; and the falx cerebelli (not
shown here), which separates the cerebellar hemispheres.
Th en, show that the superior sagittal dural venous sinus
forms within the falx cerebri and that the transverse
sinuses form within the tentorium cerebelli. Th ese dural
venous sinuses function in cerebrospinal fl uid absorp-
tion and blood-fl ow return; we discuss them in detail
elsewhere.^1 Next, indicate that the tentorium cerebelli
divides the cranial vault into a supratentorial compart-
ment, which contains the cerebral hemispheres (note
that we grossly distort the cerebral hemispheric propor-
tions for diagrammatic purposes, here), and an infraten-
torial compartment, which contains the cerebellum and
brainstem. Intracranial herniation syndromes involve
pathologic displacement of central nervous system struc-
tures. Th ree forms of supratentorial herniation exist.
Indicate that in subfalcine herniation, one hemisphere
herniates underneath the falx cerebri (this shift is also
called cingulate herniation because it is the cingulate
g yrus that fi rst herniates under the falx); next, show that
in uncal herniation, the medial temporal lobe (the uncus)
herniates over the tentorium cerebelli; and then, show
that in central herniation (aka transtentorial herniation),
the diencephalon herniates directly down through the
tentorium cerebelli. Next, let’s show the two forms of
infratentorial herniation that exist. Indicate that in
upward cerebellar herniation, the cerebellum herniates
upward into the supratentorial cavity, and then, show
that in tonsillar herniation, the cerebellar tonsils undergo
downward herniation through the foramen magnum.^2 , 3
Now, label the next innermost meningeal layer as the
arachnoid mater and then show that a potential space
exists between the dura and arachnoid mater layers: the
subdural space. Show that this space is actually fi lled
with the loosely arranged dural border cell layer. Th e clas-
sic cause of subdural hematoma is from rupture of low-
pressure bridging veins as they run within this space.
Next, show that the pia mater directly contacts the
central nervous system parenchyma; it is the delicate,
innermost layer of the meninges. Th en, label the space
between the pia and arachnoid mater layers as the suba-
rachnoid space. Unlike the subdural space, this is a true
(actual) space, which bathes the nervous system. Th e
subarachnoid space contains cisternal fl uid collections,
which we draw in Drawing 2-4.^4 – 8