Internal Aspects of Base of Skull: Bones


The anterior cranial fossa is the shallowest of the three fossae and lodges the lower parts of the frontal lobes of the brain. The sulci and gyri of the lobes are mirrored in the irregularities of the bony surfaces. It is limited anteriorly and laterally by the frontal bone. On each side, the floor is formed by the slightly domed and ridged orbital plate of the frontal bone, which supports the orbital surface of the homolateral frontal lobe of the brain and its meninges and separates them from the orbit. Posterior extensions from the frontal air sinuses may expand the orbital plates for varying distances, and the medial parts of these plates overlie the ethmoidal labyrinths.


On each side of the midline crista galli are the grooved ethmoidal cribriform plates that help to form the roof of the nasal cavity, lodge the olfactory bulbs, and provide numerous orifices for the delicate olfactory nerves. A small pit exists between the frontal crest and the crista galli, the foramen cecum, which occasionally transmits a tiny vein from the nose to the superior sagittal sinus. The crista galli and frontal crest give attachment to the anterior end of the falx cerebri.


Posterior to the ethmoid and frontal bones, the floor of the anterior cranial fossa is formed by the anterior part of the body of the sphenoid bone, the jugum sphenoidale, and on each side, by the lesser wings of this bone. These lesser wings slightly overlap the anterior part of the middle cranial fossa and project into the stems of the lateral cerebral sulci, thus forming the upper boundaries of the superior orbital fissures.


The medial ends of the posterior borders of the lesser wings end in small, rounded projections, the anterior clinoid processes, which provide attachments for the anterior ends of the free border of the tentorium cerebelli. Each anterior process is grooved on its medial side by the internal carotid artery, and each may be joined to the inconstant middle clinoid process by a thin osseous bar, thus forming a narrow bony ring around the artery as it emerges from the cavernous sinus.


The middle cranial fossa is intermediate in depth between the anterior and posterior fossae. It is narrow and elevated medially but expands and becomes deeper at each side to lodge and protect the temporal lobes of the brain. It is bounded anteriorly by the posterior borders of the lesser wings of the sphenoid bone and the anterior margin of the prechiasmatic sulcus; posteriorly by the superior borders of the petrous parts of the temporal bones, which are grooved by the superior petrosal sinuses and by the dorsum sellae of the sphenoid; and laterally by the greater wings of the sphenoid, the frontal angles of the parietal bones, and the temporal squamae.


The floor in the median area is formed by the body of the sphenoid bone, containing the sphenoidal air sinuses. The lesser wings of the sphenoid are attached to its body by two roots, separated from each other by the optic canals that transmit the optic (II) nerves and ophthalmic arteries. Behind the prechiasmatic sulcus is a median elevation, the tuberculum sellae, and the hypophyseal fossa housing the pituitary gland. The fossa is limited behind by the dorsum sellae, an upward-projecting bony plate with a concave upper border expanding laterally into the posterior clinoid processes. Lateral to the sellae is a shallow, sinuous groove for the internal carotid artery; at its anterior end on the medial side may be a small tubercle, the middle clinoid process.


The lateral parts of the middle fossa are related in front to the orbits, on each side to the temporal fossae, and below to the infratemporal fossae. The middle fossa communicates with the orbits through the superior orbital fissures.


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Sep 2, 2016 | Posted by in NEUROLOGY | Comments Off on Internal Aspects of Base of Skull: Bones

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