Anatomy Made Easy : Base of the Skull

Base of the Skull:

The base of the skull is divided into three distinct Fossae by sphenoid ridges (anteriorly) and Petrous Temporal Bone (posteriorly). These are the Anterior, Middle and Posterior Cranial Fossae.

Anterior Cranial Fossa:

Anterior cranial fossa lies above the nasal cavity and the orbits and houses the frontal lobe of the two cerebral hemispheres. Bones that form the anterior cranial fossa are:

· The Ethmoid bone (in the midline)

· The orbital part of frontal bone (anterolateral to Ethmoid bone)

· The Sphenoid Bone (posteriorly)

The Ethmoid bone found in the middle of the anterior cranial fossa has a small prominent bony ridge called the Cristal Galli. This along with the Frontal Crest on the anteriorly provide attachment site for Falx Cerebri –vertical extension of Dura matter, partially separating the two cerebral hemispheres-. On either side of the Crista Galli are cribriform plates –for the olfactory bundle- that form the narrow roof of the nasal cavity. Anteriorly between the Crista Galli and frontal crest is Foramen Cecum.

Lateral to the Ethmoid bone are the orbital part of the frontal bone, forming the floor of the anterior cranial fossa and roof of the orbits. Posterior to these two bones is the Ethmoid bone, the body of which extends posteriorly into the middle cranial fossa. Laterally projecting from the body are the lesser wings of the sphenoid bone that extends as far as the junction between the temporal and greater wing of sphenoid bone on the lateral wall of the skull. The lesser wing of sphenoid bone also forms boundary between the anterior and middle cranial fossa. Medially the lesser wing of sphenoid fossa form the Anterior Clinoid Process that provide attachment site for the Tentorium Cerebri –sheet of dura matter separating parts of cerebral hemisphere from cerebellum. Anterior to it is an opening –optic canal- for the ophthalmic artery and optic nerve.

Middle Cranial Fossa:

Middle cranial fossa is bounded anteriorly by the chiasmatic sulcus and the lesser wing of the sphenoid bone. While the posterior boundary is formed by the anterior surface of the superior part, of the petrous part of the temporal bone. The bones that form the floor of this fossa are parts of sphenoid and temporal bone.

The floor at the midline of the middle cranial fossa is formed by the body of the sphenoid bone. On either side of it are large depressions formed by the greater wing of the sphenoid and squamous part of temporal bone. These depressions contain the temporal lobe.

Sella Tursica is a complex bony structure formed by the sphenoid bone. The anterior wall of the Sella Tursica is formed called the Tuberculum Sellae. The posterior wall – the Dorsum Sellae- of Sella Tursica have prominent bony projection on the lateral edges which form the Posterior Clinoid Process, which like its anterior counterpart, provide attachment site for the Tentorium Cerebri. Between the two walls is the Hypophysial Fossa containing the pituitary gland.

On the petrous part of temporal bone, medially there is a slight depression called the trigeminal impression for the sensory ganglia of trigeminal nerve. Lateral to that is a round bony protuberance – the Arcuate eminence –produced by the underlying semicircular canal (internal ear). Anterolateral to that is Tegmen Tempani which is a slight depression that marks the roof of the middle ear cavity.

Foramen and fissures that are found in the middle cranial fossa are:

· The Superior Orbital Fissure – Occulomotor, Trochlear, Opthalmic (nerve and vein) and Abducent Nerve

· Foramen Rotundum – maxillary branch of trigeminal nerve

· Foramen Ovale – Mandibular nerve, Petrosal Nerve and the accessory middle meningeal artery

· Foramen Spinosum – The Middle meningeal artery and vein.

Posterior Cranial Fossa:

It is formed mainly by the occipital bone and temporal bone. It is the largest and deepest cranial fossa and houses the brainstem (medulla, pons, and midbrain) and cerebellum. The anterior boundary of the posterior cranial fossa is the Dorsum Sellae and Clivus. The Clivus slopes from the foramen magnum and is partly formed by the sphenoid and partly by the occipital bone. It also forms the base of the occipital bone. Laterally the anterior boundary of the posterior cranial fossa is formed by part of the petrous part of temporal bone. Posteriorly it is bounded by the internal surface of the squamous part of the occipital bone.

There is a large foramen in the deepest part of the posterior cranial fossa, the Foramen magnum, allows the spinal cord to pass into the skull and connect to the brainstem.