190 Neuroanatomy: Draw It to Know It
Skull Foramina
Here, we will draw the skull foramina. First, draw the
anterior half of the skull base in axial view; include the
following topographic landmarks: the frontal bone, eth-
moid bone, lesser wing of the sphenoid bone (include its
anterior clinoid process), and the ridge of the petrous
portion of the temporal bone. Next, we will label the
major neurovascular structures and cranial nerves that
pass through the foramina; we will denote the cranial
nerves in brackets. Show that the foramina of the cribri-
form plate of the ethmoid bone contain the olfactory
nerve bundles (cranial nerve 1). Next, medial to the
anterior clinoid process, label the optic canal and lateral
to it label the superior orbital fi ssure. Show that the optic
nerve (cranial nerve 2) traverses the optic canal and
that the oculomotor, trochlear, and abducens nerves
(cranial nerves 3, 4, and 6, respectively) and the fi rst divi-
sion of the trigeminal nerve (the ophthalmic nerve
[5(1)]) pass through the superior orbital fi ssure. Next,
show that the ophthalmic artery traverses the optic canal
and that the superior ophthalmic vein passes through
the superior orbital fi ssure. Th e ophthalmic artery is a
direct branch of the internal carotid artery, which tra-
verses the carotid canal; indicate that the carotid canal
lies along the petrous ridge. Note that adjacent to the
carotid canal is the foramen lacerum, which the internal
carotid artery runs above in its lacerum segment (see
Drawing 11-8 ).^15
Next, posterior to the superior orbital fi ssure, within
the greater wing of the sphenoid bone, from anterior to
posterior, label foramen rotundum, foramen ovale, and
foramen spinosum. Indicate that the second division of
the trigeminal nerve (the maxillary nerve [5(2)]) tra-
verses foramen rotundum and that the third division of
the trigeminal nerve (the mandibular nerve [5(3)]) tra-
verses foramen ovale. To remember that foramen rotun-
dum houses the second division of the trigeminal nerve,
think of the “Star Wars” character R2D2 , whose round
head and name ( R2D2 ) should serve as a helpful mne-
monic. Note that within the mandible is foramen
mentum, which houses the distal mandibular nerve
branch — the mental nerve, which provides sensory cov-
erage to the mentum (the chin). Lastly, indicate that
foramen spinosum contains the meningeal branch of the
mandibular nerve (aka nervus spinosus) and also the
middle meningeal artery — injury to this vessel can lead
to intracranial epidural hematoma.^9 , 16
Now, draw the posterior surface of the skull base in
axial view; include the posterior extension of the petrous
ridge and the foramen magnum as topographic land-
marks.^17 First, along the petrous apex, draw the internal
acoustic meatus. Indicate that both the facial nerve (cra-
nial nerve 7) and vestibulocochlear nerve (cranial nerve
8) and the internal auditory artery (aka labyrinthine
artery) pass through the internal acoustic meatus. Next,
below the internal acoustic meatus, draw the jugular
foramen, which lies at the border of the temporal and
occipital bones. Show that the glossopharyngeal nerve
(cranial nerve 9), vagus nerve (cranial nerve 10), and
spinal accessory nerve (cranial nerve 11) pass through
the jugular foramen; and also show that the internal jug-
ular vein passes through the jugular foramen — it receives
the sigmoid sinus and inferior petrosal sinus.^18 N o w ,
along the foramen magnum, label the hypoglossal canal
and show that both the hypoglossal nerve (cranial nerve
12) and also a venous plexus pass through the hypoglos-
sal canal. Th en, indicate that in addition to traversing the
jugular foramen, the spinal accessory nerve also runs
through foramen magnum; it originates in the cervical
spinal cord, passes up through foramen magnum, and
then passes out of the cranium through the jugular fora-
men. Finally, indicate that the vertebral arteries and
spinal vessels traverse the foramen magnum as well.^3 , 4 , 6 – 11