Skull Base Surgery of the Posterior Fossa

7

Fig. 1.2 Jugular foramen and skull base venous sinuses. (a)
Posterior view. Venous drainage from the cavernous sinus
and basilar sinus empties in the superior and inferior petrosal
sinuses. The inferior petrosal sinus ends at the petrosal part of
the jugular foramen. The sigmoid, superior, and inferior
petrosal sinuses form a venous ring around the medial face of
the petrous bones, bridged by the basilar sinus anteriorly. (b)
Enlarged view of the intracranial jugular foramen. The sep-
tum of the jugular foramen separates the glossopharyngeal
meatus from the vagal meatus. (c) Posterior view of the left

jugular foramen which has been opened posteriorly. The cra-

nial nerves occupy the medial aspect of the jugular foramen,

while the jugular bulb is lateral. The hypoglossal nerve exits

the cranial cavity inferior and medial to the jugular foramen

before joining the other lower cranial nerves. (d) Inferior

view of the right jugular foramen region. The jugular fora-

men is bordered by the petrous carotid anteriorly, the styloid

process and facial nerve laterally, the occipital condyle medi-

ally, and the jugular process of the occipital bone and the

rectus capitis lateralis posteriorly

The pons (“bridge”) is developmentally a
part of the medulla and is only distinguishable
in mammals due to the pontine nuclei and the
fibers of corticopontocerebellar and corticospi-
nal tracts. It is a convex structure extending from
the pontomesencephalic sulcus superiorly to the
pontomedullary sulcus inferiorly. The descend-
ing corticospinal and corticobulbar fibers located
in its anterior half are interrupted by transverse
fibers coursing between the middle cerebellar
peduncles. Intertwined between these are pontine
nuclei involved in the corticopontocerebellar cir-
cuit. At the midlevel of the pons, the trigeminal

nerve emerges anterior to the middle cerebellar

peduncle (Figs. 1.3a and 1.4c). The posterior pons

contains several cranial nerve nuclei, ascending

tracts, and the floor of the fourth ventricle, where

the abducens nucleus makes an impression called

the facial colliculus (Fig. 1.5g). The pons is bor-

dered anteriorly and laterally by the prepontine

and cerebellopontine cisterns (Fig. 1.4b–d).

The medulla oblongata is the inferior seg-

ment of the brainstem. The pyramids represent

its most anterior prominences and contain the

corticospinal tract. At this level, 85% of the cor-

ticospinal fibers decussate. Lateral to the pyra-

1 Surgical Anatomy of the Posterior Fossa