Internal Surface of Cranial Base

The internal surface of the cranial base (Latin basis cranii interna) has three large depressions that lie at different levels: the anterior, middle, and posterior cranial fossae, which form the bowl-shaped floor of the cranial cavity. The anterior cranial fossa is at the highest level, and the posterior cranial fossa is at the lowest level.

Anterior Cranial Fossa

The inferior and anterior parts of the frontal lobes of the brain occupy the anterior cranial fossa, the shallowest of the three cranial fossae. The fossa is formed by the frontal bone anteriorly, the ethmoid bone in the middle, and the body and lesser wings of the sphenoid posteriorly. The greater part of the fossa is formed by the orbital parts of the frontal bone, which support the frontal lobes of the brain and form the roofs of the orbits. This surface shows sinuous impressions (brain markings) of the orbital gyri (ridges) of the frontal lobes.

The frontal crest is a median bony extension of the frontal bone. At its base is the foramen cecum of the frontal bone, which gives passage to vessels during fetal development, but is insignificant postnatally. The crista galli (Latin cock's comb) is a thick, median ridge of bone posterior to the foramen cecum, which projects superiorly from the ethmoid. On each side of this ridge is the sieve-like cribriform plate of ethmoid bone. Its numerous tiny foramina transmit the olfactory nerves (CN I) from the olfactory areas of the nasal cavities to the olfactory bulbs of the brain, which lie on this plate.

Middle Cranial Fossa

The butterfly-shaped middle cranial fossa has a central part composed of the sella turcica on the body of the sphenoid and large, depressed lateral parts on each side. The middle cranial fossa is postero-inferior to the anterior cranial fossa, separated from it by the sharp sphenoidal crests laterally and the sphenoidal limbus centrally. The sphenoidal crests are formed mostly by the sharp posterior borders of the lesser wings of the sphenoid bones, which overhang the lateral parts of the fossae anteriorly. The sphenoidal crests end medially in two sharp bony projections, the anterior clinoid processes.

A variably prominent ridge, the limbus of the sphenoid forms the anterior boundary of the transversely oriented prechiasmatic sulcus extending between the right and the left optic canals. The bones forming the lateral parts of the fossa are the greater wings of the sphenoid, and squamous parts of the temporal bones laterally, and the petrous parts of the temporal bones posteriorly. The lateral parts of the middle cranial fossa support the temporal lobes of the brain. The boundary between the middle and the posterior cranial fossae is the superior border of the petrous part of the temporal bone laterally, and a flat plate of bone, the dorsum sellae of the sphenoid, medially.

The sella turcica (Latin Turkish saddle) is the saddle-like bony formation on the upper surface of the body of the sphenoid, which is surrounded by the anterior and posterior clinoid processes. Clinoid means "bedpost," and the four processes (two anterior and two posterior) surround the hypophysial fossa, the "bed" of the pituitary gland, like the posts of a four-poster bed. The sella turcica is composed of three parts:

1. The tuberculum sellae (horn of saddle): a variable slight to prominent median elevation forming the posterior boundary of the prechiasmatic sulcus and the anterior boundary of the hypophysial fossa.

2. The hypophysial fossa (pituitary fossa): a median depression (seat of saddle) in the body of the sphenoid that accommodates the pituitary gland (Latin hypophysis).

3. The dorsum sellae (back of saddle): a square plate of bone projecting superiorly from the body of the sphenoid. It forms the posterior boundary of the sella turcica, and its prominent superolateral angles make up the posterior clinoid processes.

On each side of the body of the sphenoid, a crescent of four foramina perforate the roots of the cerebral surfaces of the greater wings of the sphenoids:

1. Superior orbital fissure: Located between the greater and the lesser wings, it opens anteriorly into the orbit.

2. Foramen rotundum (round foramen): Located posterior to the medial end of the superior orbital fissure, it runs a horizontal course to an opening on the anterior aspect of the root of the greater wing of the sphenoid into a bony formation between the sphenoid, the maxilla, and the palatine bones, the pterygopalatine fossa.

3. Foramen ovale (oval foramen): A large foramen posterolateral to the foramen rotundum; it opens inferiorly into the infratemporal fossa.

4. Foramen spinosum (spinous foramen): Located posterolateral to the foramen ovale and opens into the infratemporal fossa in relationship to the spine of the sphenoid.

The foramen lacerum (lacerated or torn foramen) is not part of the crescent of foramina. This ragged foramen lies posterolateral to the hypophysial fossa, and is an artifact of a dried cranium. In life, it is closed by a cartilage plate. Only some meningeal arterial branches and small veins are transmitted vertically through the cartilage, completely traversing this foramen. The internal carotid artery and its accompanying sympathetic and venous plexuses pass across the superior aspect of the cartilage (i.e., pass over the foramen), and some nerves traverse it horizontally, passing to a foramen in its anterior boundary.

Extending posteriorly and laterally from the foramen lacerum is a narrow groove for the greater petrosal nerve on the anterosuperior surface of the petrous part of the temporal bone. There is also a small groove for the lesser petrosal nerve.

Posterior Cranial Fossa

The posterior cranial fossa, the largest and deepest of the three cranial fossae, lodges the cerebellum, pons, and medulla oblongata. The posterior cranial fossa is formed mostly by the occipital bone, but the dorsum sellae of the sphenoid marks its anterior boundary centrally, and the petrous and mastoid parts of the temporal bones contribute its anterolateral "walls."

From the dorsum sellae there is a marked incline, the clivus, in the center of the anterior part of the fossa leading to the foramen magnum. Posterior to this large opening, the posterior cranial fossa is partly divided by the internal occipital crest into bilateral large concave impressions, the cerebellar fossae. The internal occipital crest ends in the internal occipital protuberance formed in relationship to the confluence of the sinuses, a merging of dural venous sinuses.

Broad grooves show the horizontal course of the transverse sinus and the S-shaped sigmoid sinus. At the base of the petrous ridge of the temporal bone is the jugular foramen, which transmits several cranial nerves in addition to the sigmoid sinus that exits the cranium as the internal jugular vein (IJV). Anterosuperior to the jugular foramen is the internal acoustic meatus for the facial (CN VII) and vestibulocochlear nerves (CN VIII), and the labyrinthine artery. The hypoglossal canal for the hypoglossal nerve (CN XII) is superior to the anterolateral margin of the foramen magnum.