External Aspect of Skull Base

The incisive foramen transmits the terminal branches of the nasopalatine nerves and greater palatine vessels. The major and minor palatine foramina are traversed by the corresponding arteries and nerves. The choanae are the posterior nasal apertures.

The foramen ovale pierces the greater sphenoidal wing near the lateral pterygoid plate and the sulcus for the auditory tube; the mandibular nerve, the accessory meningeal artery, and communications between the cavernous sinuses and pterygoid venous plexus pass through it. The foramen spinosum, anteromedial to the sphenoidal spine, transmits the middle meningeal artery and the meningeal branch of the mandibular nerve.

The foramen lacerum is an irregular canal between the sphenoidal body, the apex of the petrous part of the temporal bone and the basilar part of the occipital bone. The upper end of the carotid canal opens into it, and the internal carotid artery with its nerves and veins, on emerging from the canal, turn upward to enter the cavernous sinus. Meningeal branches of the ascending pharyngeal artery and emissary veins from the cavernous sinus pass through the foramen lacerum, and the deep and greater petrosal nerves unite within it to form the nerve of the pterygoid canal.

The anterior part of the mandibular fossa articulates with the mandibular head and belongs to the temporal squama, but the posterior nonarticular part is derived from the tympanic plate. The tympanosquamous fissure between them is continued medially as the petrotympanic fissure, through which the chorda tympani nerve emerges. The stylomastoid foramen behind the root of the styloid process transmits the facial (VII) nerve and the stylomastoid branch of the posterior auricular artery.

The lower opening of the carotid canal is anterior to the jugular fossa, which lodges the superior bulb of the internal jugular vein. The canal bends at right angles within the petrous part of the temporal bone, and its upper end opens into the foramen lacerum. The tympanic canaliculus pierces the ridge between the carotid canal and the jugular fossa and conveys the tympanic branch of the glossopharyngeal (IX) nerve to the tympanic plexus. The mastoid canaliculus opens on the lateral wall of the fossa and transmits the auricular branch of the vagus (X) nerve. The jugular foramen in the depth of the fossa may be partly or completely divided into three parts by bony spicules. The anteromedial compartment transmits the inferior petrosal sinus and a meningeal branch of the ascending pharyngeal artery; the intermediate part transmits the glossopharyngeal, vagus, and accessory (XI) nerves; and the posterolateral part conveys the sigmoid sinus to the superior bulb of the internal jugular vein. Often seen near the posterior border of the mastoid process is a mastoid foramen, which is traversed by an emissary vein from the sigmoid sinus and a meningeal twig from the occipital artery. The anterior end of the hypoglossal canal (for the hypoglossal [XII] nerve and some small meningeal vessels) is above the anterior end of the occipital condyle. Behind the condyle is a shallow condylar fossa, usually pierced by a condylar foramen conveying an emissary vein between the sigmoid sinus and cervical veins.

The posterior part of the base of the skull is formed predominantly by the occipital squama; these are marked by nuchal lines, occipital crest, and so forth, which serve mainly for muscular and ligamentous attachments. However, the most notable feature is the foramen magnum, through which the medulla oblongata and spinal cord become continuous. The vertebral arteries, spinal roots of the accessory nerves, and recurrent meningeal branches from the upper cervical nerves ascend through the foramen magnum, while down through it pass the anterior and posterior spinal arteries.

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