13this is not always a reliable landmark. The pos-
terior semicircular canal is directed toward the
posterior fossa and can be violated when drill-
ing the posterior wall of the internal auditory
canal (Fig. 1.1d). The lateral semicircular canal
is oriented toward the middle ear. Below its
anterior portion, the facial nerve turns inferiorly
from its tympanic segment to the mastoid seg-
ment (Fig. 1.8d). The mastoid air cells posterior
to the labyrinth are drilled to reach the lateral
semicircular canal (Fig. 1.8b–d). At this point in
the exposure, car must be taken not to injure the
facial nerve.
Jugular Foramen
The jugular foramen is formed by an opening
between the petrous and occipital bones immedi-
ately inferior to the petroclival fissure (Fig. 1.1b).
It begins less than a centimeter inferior to the
internal auditory meatus and is superior to the
hypoglossal canal. The petrous bone forms a
dome over this foramen as it turns downward
(Fig. 1.2c). As with the sigmoid sinus, the jugular
foramen is often larger on the right side. It is
bounded by the jugular process of the occipital
bone and rectus capitis lateralis posteriorly, the
occipital condyle medially, the petrous carotid
canal anteriorly, and the styloid process and
extracranial facial nerve laterally (Fig. 1.2d). The
jugular bulb has a variable superior extent in the
jugular fossa that may reach as high as the laby-
rinth. Hence, a high-riding jugular bulb may hin-
der infralabyrinthine and presigmoid approaches.
At the posterior end of the petroclival fissure, the
jugular bulb receives drainage from the inferior
petrosal sinus, which becomes the petrosal part
of the foramen. The posterior sigmoid part drains
the sigmoid sinus. These two sinuses converge at
the jugular bulb medial to the exiting lower cra-
nial nerves, which then drain into the jugular vein
(Fig. 1.2a–c). Cranial nerves IX, X, and XI travel
through the medial portion of the jugular fora-
men. Branches of the ascending pharyngeal and
occipital arteries may enter the jugular foramen
and supply tumors in this location. The jugular
tubercle is located medial and inferior to the jug-
ular foramen and is often removed to provide
increased anterior exposure during far-lateral
approaches (Figs. 1.1a and 1.2a).Surgical Corridors and Related
Anatomy
The significant number of anatomical barriers
within the posterior fossa and its enclosure dic-
tate a wide variety of approaches to avoid injuring
important neurovascular structures. Approaches
have been developed for nearly every angle into
the posterior fossa, though some entail planned
morbidity. We will review these approaches start-
ing from the posterior perspective and moving
stepwise anteriorly.PosteriorOccipital Transtentorial
The occipital transtentorial approach is used to
access pineal and third ventricle lesions, precen-
tral cerebellar fissure, inferior colliculus, and
anterior vermis. The trajectory is usually either
through the occipital interhemispheric fissure lat-
eral to the straight sinus or below the occipital
lobe above the tentorium (Fig. 1.5a, b). The occip-
ital lobe is retracted, and dissection proceeds to
the tentorial incisura. The tentorium is then cut to
gain access to the ambient and quadrigeminal cis-
terns and the precentral cerebellar fissure.Supracerebellar Infratentorial
The natural corridor above the cerebellum and
below the tentorium leads to the pineal region,
tectal plate, and posterior third ventricle [ 5 ]
(Fig. 1.5d). The inferior and medial temporal
lobe may also be reached if the tentorium is cut to
perform operations such as amygdalohippocam-
pectomy. The patient is usually placed in the sit-
ting position. The transverse sinus is retracted
superiorly, and the cerebellum retracted inferi-
orly. Precentral cerebellar veins are often sacri-
ficed, but other draining veins in this region
should be preserved, particularly the internal
cerebral veins and the veins of Rosenthal.1 Surgical Anatomy of the Posterior Fossa