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. The structures transmitted are ophthalmic veins; ophthalmic nerve (CN V₁); CN III, IV, and VI; and sympathetic fibers.

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.