ganglion in the upper portion of the pterygopalatine fossa. Within the carotid canal, the horizontal portion of the ICA is surrounded by a venous plexus (Bouthillier et al. 1996; Osawa et al. 2008), in direct continuity with the CS.

2.1.1Gross Anatomy

The tympanic cavity lies posterolaterally to the posterior vertical segment/posterior genu. Usually, a few millimetres of bone separate the carotid canal and the middle ear. The posterior genu sits directly posterior to the entrance of the ET into the middle ear (Osawa et al. 2008). The cochlea, placed within the petrous bone, is a little bit superior, posterior, and lateral to the posterior genu. The ET and the tensor tympani are located anteriorly to the ICAh. Proceeding medially, the vessel remains within its own canal, while the ET slopes gently downward and the cartilaginous portion of the tube is located within the sulcus tubae. This is located on the outer surface of the cranial base and placed anteriorly to the ICAh. The tensor tympani in most cases (>90 %) is located superiorly or anteriorly to the ET, and it is separated from the carotid canal by a thin bone.

2.1.2Neural Structures

2.1.2.1Greater Petrosal Nerve

The GPN originates at the level of the geniculate ganglion, exits through the hiatus Fallopii, and courses under the dura toward the gasserian ganglion. The nerve usually runs anteromedially above the horizontal portion.

2.1.2.2Geniculate Ganglion

The geniculate ganglion is placed above and usually posterolateral to the posterior genu (Osawa et al. 2008).

2.1.2.3Lesser Petrosal Nerve

The lesser petrosal nerve crosses the middle cranial fossa floor anterior to the GPN and exits the cranium through the small innominate foramen (Osawa et al. 2008), the foramen spinosum, or

the sphenopetrosal fissure (Kakizawa et al. 2007).

2.1.2.4Gasserian Ganglion

The gasserian ganglion is placed within a depression between the trigeminal impression and the foramen ovale. The stem of the trigeminal nerve is placed within the “trigeminal impression.” The inferolateral portion of the sphenopetrosal fissure (SPF) is placed above the medial third of the horizontal portion of the ICA, while the superomedial aspect rests against the ascending limb of the intracavernous segment.

2.1.3Vascular Branches

In most cases, the skull base segment of the ICA presents branches (Tubbs et al. 2007; Osawa et al. 2008).

2.1.3.1Caroticotympanic Artery

The caroticotympanic artery seems to arise from the proximal vertical part of the petrous ICA and supplies the tympanic cavity. At the level of promontory, it anastomoses with the tympanic branches of the middle meningeal artery (MMA) and stylomastoid and maxillary arteries. Not all authors agree with this position; Osawa et al. thought that this vessel comes from the ascending pharyngeal artery and not from the ICA (Osawa et al. 2008). Also, previously its presence has been doubted (Paullus et al. 1977).

2.1.3.2Arteries of the Pterygoid Canal

(Vidian Arteries)

The artery of the pterygoid canal (vidian artery) is not always present. Anatomically speaking we can consider an anterior (for ECA system) and a posterior (from ICA system) Vidian artery. The posterior one may arise as a single branch or from a common trunk with a periosteal artery (Osawa et al. 2008). This vessel may arise from the horizontal segment of the petrous part of the ICA and course into the pterygoid canal to anastomose with the same-named branch of the maxillary artery (Jinkins 2000). Furthermore, it is also possible that there is no counterpart from the

maxillary artery. In this case, the artery (about 0.5 mm) has been described to transverse the cartilage-filled FL to enter the pterygoid canal.

2.1.3.3Stapedial Artery

Rarely present in adults, the stapedial artery (SA) is a branch of the second branchial arch. The SA pierces the floor of the middle ear cavity and ascends along the promontory enclosed within a bony canal to pass between the crura of the stapes to enter the facial canal. At the level of the hiatus of the GPN, the vessel enters the middle cranial fossa to become the MMA. If the SA persists, the foramen spinosum is absent (Lasjaunias 1981).

2.1.3.4Periosteal Branch(es)

The periosteal branches are not always present. When present, most of these originate from the anterior genu and the vertical segment. When it originates as a common trunk with the vidian artery, it arises from the horizontal segment. Most periosteal arteries arising from the inferior surface of the ICA send a branch through the FL (Osawa et al. 2008).

2.1.3.5Persistent Acoustic Artery

The persistent acoustic artery is really a rare branch connecting the carotid (petrous portion) and basilar arteries (Lang 1995).