188 Neuroanatomy: Draw It to Know It
Skull Base
Here, we will draw the skull base in axial view. Draw an
outline of one half of an axial view of the skull base and
label its anterior one third as the frontal bone. Next, in
midline, demarcate the ethmoid bone, which comprises
the steeply peaked crista galli and the surrounding cribri-
form plate. Th en, posterior to the frontal bone, draw the
sphenoid bone, which subdivides into a body and lesser
and greater wings. First, label the midline portion of the
sphenoid bone as the sphenoid body. Show that it subdi-
vides into the jugum sphenoidale, anteriorly, and the
sella turcica, posteriorly. Th e posterior end of the mid-
line sphenoid bone and the neighboring anterior occipi-
tal bone (drawn later), together, form the clivus. Now,
draw the sphenoid wings. Label the lesser wing , anteri-
orly, and the greater wing , posteriorly. Topographically,
the lesser sphenoid wing angles up over the greater sphe-
noid wing , which rolls downward. Label the protuber-
ance along the posteromedial ridge of the lesser wing as
the anterior clinoid process, an important anatomic
landmark.
Next, draw the temporal bone posterior to the greater
wing of the sphenoid bone. Label the squamous part,
laterally, and the petrous part, medially. Th e squamous
part makes up the bulk of the external surface of the tem-
poral bone, whereas the petrous part makes up the bulk
of the internal surface. Next, posteromedial to the tem-
poral bone, draw the occipital bone; it extends back to
the occiput. In the anterior one third of the occipital
bone, draw the foramen magnum, which is the entry
zone of the brainstem. Now, label the combined anterior
occipital bone and posterior sphenoid bone as the clivus,
which is steeply sloped. Next, along the lateral edge of
the skull base, label the parietal bone. Th e parietal bones
make up much of the lateral and superior surfaces of the
skull.
Now, let’s draw a sagittal section of the skull to illus-
trate how the peaks and valleys within the skull base form
diff erent fossae. Draw a downward-sloping line with two
peaks. Label the anterior depression as the anterior cra-
nial fossa, the middle depression as the middle cranial
fossa, and the posterior depression as the posterior cra-
nial fossa.^1
Next, let’s defi ne the borders of these fossae in our
axial diagram. First, let’s include the major peaks within
the skull base (the fossae comprise the valleys between
these peaks). Dot a line along the lesser wing of the sphe-
noid bone. Anterior to it, label the anterior cranial fossa,
which comprises the frontal bone, ethmoid bone, jugum
sphenoidale, and lesser wing of the sphenoid bone — the
basal portions of the frontal lobes lie within this fossa.
Next, dot a diagonal line through the petrous temporal
bone. Indicate that the middle cranial fossa lies between
the lesser wing of the sphenoid bone and the petrous
ridge of the temporal bone. Th e middle cranial fossa
comprises the greater wing of the sphenoid bone, a por-
tion of the squamous temporal bone, a portion of the
petrous temporal bone, and the sella turcica of the sphe-
noid bone — the basal portions of the temporal lobes lie
within this fossa. Lastly, posterior to the petrous ridge,
label the posterior cranial fossa, which comprises the
posterior portion of the petrous bone, the occipital bone,
and the clivus — the cerebellum and brainstem lie within
this fossa. Note that the occipital and parietal lobes lie
superior to the plane of the skull base.^13
Next, we will draw the foramina of the skull base.
Skull base injuries and diseases present with unique pat-
terns of neurologic defi cit, so we need a good under-
standing of the skull foramina and their neurovascular
contents to diagnose the various presentations of skull
base injury.^3 , 4 , 6 , 13 , 14