18 Neuroanatomy: Draw It to Know It
Cerebrospinal Fluid Flow
Here, we will draw the fl ow of cerebrospinal fl uid
through the nervous system in coronal view. First, estab-
lish the relevant meningeal layers: draw the outermost
dural sublayer — the periosteal sublayer, and then the
innermost layer — the meningeal sublayer. Show that
together they form the dura mater, which contains dural
venous sinuses within the dural refl ections. Next, move
inward and draw the arachnoid mater. Between the
arachnoid mater and the overlying meningeal sublayer,
label the subdural space, which is constituted by the
dural border cell layer. Now, draw the pia mater as the
layer directly adhering to the nervous system paren-
chyma. Between the pia mater and the arachnoid mater,
label the subarachnoid space.
Next, let’s draw the cerebral ventricles. Show a
T-shaped coronal view of the paired lateral ventricles,
third ventricle, and fourth ventricle, and then, show the
central canal of the spinal cord. Also, show some repre-
sentative choroid plexus, the secretory epithelial tissue
that produces cerebrospinal fl uid, in the lateral ventri-
cles; we show the ventricular locations of the choroid
plexus in Drawing 2-3. Th e choroid plexus is formed
where invaginations of vascularized meninges, called tela
choroidea, merge with ventricular ependyma. Th e tela
choroidea are variably defi ned histologically as either
combinations of pia and ependyma or double pial layers.
Th e tight junctions within the choroid plexus cuboidal
epithelium form an important blood–cerebrospinal
fl uid barrier.^9 – 12
Now, show that cerebrospinal fl uid empties through
the paired foramina of Monro into the third ventricle,
then into the fourth ventricle, and then down the central
canal, which is mostly obliterated by middle adulthood.
Next, show that cerebrospinal fl uid empties from the
fourth ventricle through the foramen of Magendie, in
midline, and the foramina of Luschka, laterally, to enter
the subarachnoid space. Th en, show that fl uid descends
into the spinal canal to bathe the spinal cord, and show
that it also ascends into the cranial vault to bathe the rest
of the brain.
Next, show a representative arachnoid villus extend
from the subarachnoid space through the subdural
space into a dural venous sinus. Finally, indicate that
cerebrospinal fl uid passes into the arachnoid villus to be
reabsorbed within the dural venous sinus. Neoplastic
arachnoid villi cells form meningiomas, a common brain
tumor type; the greatest concentration of meningiomas
is found where there is the greatest concentration of
arachnoid villi: at the cerebral convexity, falx cerebri,
and base of the skull.
Note that although we have highlighted the cerebro-
spinal fl uid resorption into the dural venous sinuses,
here, the majority of fl uid within these dural venous
channels is blood. Th is is because the rate of cerebrospi-
nal fl uid production and resorption is far slower than the
rate of blood entrance and reabsorption into and out of
the cranial vault. Cerebrospinal fl uid is produced and
reabsorbed at a rate of roughly 0.35 milliliters per minute,
which equals about 20 milliliters per hour. In a typical
lumbar puncture, anywhere from 10 to 20 milliliters
of fl uid are withdrawn; thus, this fl uid is replaced within
a half-hour to one hour aft er the procedure. Th ere is
roughly 150 milliliters of cerebrospinal fl uid in the ner-
vous system at any given time.^4 – 8