Sphenoid bone

The sphenoid bone (os sphenoidale) is a bone situated at the base of the skull in front of the temporals and basilar part of the occipital.

It somewhat resembles a butterfly with its wings extended, and is divided into a median portion or body, two great wings and two small wings extending outward from the sides of the body, and two pterygoid processes which project from it below. Two sphenoidal conchæ are situated at the anterior and lower part of the body.

Body

The body (corpus sphenoidale), more or less cubical in shape, is hollowed out in its interior to form two large cavities, the sphenoidal air sinuses, which are separated from each other by a septum.

Surfaces

Superior surface

The superior surface of the body 1 presents in front a prominent spine, the ethmoidal spine, for articulation with the cribriform plate of the ethmoid bone; behind this is a smooth surface slightly raised in the middle line, and grooved on either side for the olfactory lobes of the brain.

This surface is bounded behind by a ridge, which forms the anterior border of a narrow, transverse groove, the chiasmatic groove (optic groove), above and behind which lies the optic chiasma; the groove ends on either side in the optic foramen, which transmits the optic nerve and ophthalmic artery into the orbital cavity.

Behind the chiasmatic groove is an elevation, the tuberculum sellæ; and still more posteriorly, a deep depression, the sella turcica, the deepest part of which lodges the hypophysis cerebri and is known as the hypophyseal fossa.

The anterior boundary of the sella turcica is completed by two small eminences, one on either side, called the middle clinoid processes, while the posterior boundary is formed by a square-shaped plate of bone, the dorsum sellae, ending at its superior angles in two tubercles, the posterior clinoid processes, the size and form of which vary considerably in different individuals.

The posterior clinoid processes deepen the sella turcica, and give attachment to the tentorium cerebelli.

On either side of the dorsum sellæ is a notch for the passage of the abducent nerve, and below the notch a sharp process, the petrosal process, which articulates with the apex of the petrous portion of the temporal bone, and forms the medial boundary of the foramen lacerum.

Behind the dorsum sellae is a shallow depression, the clivus, which slopes obliquely backward, and is continuous with the groove on the basilar portion of the occipital bone; it supports the upper part of the pons.

Lateral surfaces

The lateral surfaces of the body are united with the great wings and the medial pterygoid plates.

Above the attachment of each great wing is a broad groove, curved something like the italic letter f; it lodges the internal carotid artery and the cavernous sinus, and is named the carotid groove.

Along the posterior part of the lateral margin of this groove, in the angle between the body and great wing, is a ridge of bone, called the lingula.

Posterior surfaces

The posterior surface, quadrilateral in form 3, is joined, during infancy and adolescence, to the basilar part of the occipital bone by a plate of cartilage.

Between the eighteenth and twenty-fifth years this becomes ossified, ossification commencing above and extending downward.

Anterior surface

The anterior surface of the body 2 presents, in the middle line, a vertical crest, the sphenoidal crest, which articulates with the perpendicular plate of the ethmoid, and forms part of the septum of the nose.

On either side of the crest is an irregular opening leading into the corresponding sphenoidal air sinus.

These sinuses are two large, irregular cavities hollowed out of the interior of the body of the bone, and separated from one another by a bony septum, which is commonly bent to one or the other side.

They vary considerably in form and size, are seldom symmetrical, and are often partially subdivided by irregular bony laminæ.

Occasionally, they extend into the basilar part of the occipital nearly as far as the foramen magnum. They begin to be developed before birth, and are of a considerable size by the age of six.

They are partially closed, in front and below, by two thin, curved plates of bone, the sphenoidal conchæ, leaving in the articulated skull a round opening at the upper part of each sinus by which it communicates with the upper and back part of the nasal cavity and occasionally with the posterior ethmoidal air cells.

The lateral margin of the anterior surface is serrated, and articulates with the lamina papyracea of the ethmoid, completing the posterior ethmoidal cells; the lower margin articulates with the orbital process of the palatine bone, and the upper with the orbital plate of the frontal bone.

Inferior surface

The inferior surface presents, in the middle line, a triangular spine, the sphenoidal rostrum, which is continuous with the sphenoidal crest on the anterior surface, and is received in a deep fissure between the alæ of the vomer.

On either side of the rostrum is a projecting lamina, the vaginal process, directed medialward from the base of the medial pterygoid plate, with which it will be described.

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