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The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form the orbit. Its shape somewhat resembles that of a butterfly, bat or wasp with its wings extended. The name presumably originates from this shape, since sphekodes (σφηκώδης) means in Ancient Greek.
Two sphenoidal conchae are situated at the anterior and inferior part of the body.
These ligaments occasionally
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Foramen pierce it:
The greater part of the bone is ossified in cartilage. There are fourteen centers in all, six for the presphenoid and eight for the postsphenoid.
The sphenoidal conchae are each developed from a center that makes its appearance about the fifth month; at birth they consist of small triangular laminae, and it is not until the third year that they become hollowed out and coneshaped; about the fourth year they fuse with the labyrinths of the ethmoid bone, and between the ninth and twelfth years they unite with the sphenoid bone.
Soon after, the centers for the basisphenoid appear, one on either side of the sella turcica, and become blended together about the middle of fetal life.
Each pterygoid bone (except its hamulus) is ossified in membrane, and its center probably appears about the ninth or tenth week; the hamulus becomes chondrified during the third month, and almost at once ossifies (Fawcett).
The medial joins the lateral pterygoid plate about the sixth month.
About the fourth month, a center appears for each lingula and speedily joins the rest of the bone.
The presphenoid is united to the postsphenoid about the eighth month, and at birth the sphenoid is in three pieces Fig.: a central, consisting of the body and small wings, and two lateral, each comprising a great wing and pterygoid process.
In the first year after birth the great wings and body unite, and the small wings extend inward above the anterior part of the body, and, meeting with each other in the middle line, form an elevated smooth surface, termed the jugum sphenoidale.
By the twenty-fifth year the sphenoid and occipital are completely fused.
Between the pre- and postsphenoid there are occasionally seen the remains of a canal, the canalis cranio-pharyngeus, through which, in early fetal life, the hypophyseal diverticulum of the buccal ectoderm is transmitted.
The sphenoidal sinuses are present as minute cavities at the time of birth (Onodi), but do not attain their full size until after puberty.
In the early and , the pterygoid bones were flat, wing-like bones forming the major part of the roof of the mouth. Above the pterygoids were the epipterygoid bones, which formed part of a flexible joint between the braincase and the palatal region, as well as extending a vertical bar of bone towards the roof of the skull. Between the pterygoids lay an elongated, narrow parasphenoid bone, which also spread over some of the lower surface of the braincase, and connected, at its forward end, with a sphenethmoid bone helping to protect the . Finally, the basisphenoid bone formed part of the floor of the braincase and lay immediately above the parasphenoid.Romer, Alfred Sherwood (1977). 003910284X, Holt-Saunders International. 003910284X
Aside from the loss of the flexible joint at the rear of the palate, this primitive pattern is broadly retained in , albeit with some individual modifications. In , the epipterygoids are absent and the pterygoids considerably reduced. Living have a relatively simplified skull in this region; a broad parasphenoid forms the floor of the braincase, the pterygoids are relatively small, and all other related bones except the sphenethmoid are absent.
In , these various bones are often (though not always) fused into a single structure; the sphenoid. The basisphenoid forms the posterior part of the base, while the pterygoid processes represent the pterygoid bones. The epipterygoids have extended into the wall of the cranium; they are referred to as when separate in mammals, and form the greater wings of the sphenoid when fused into a larger structure. The sphenethmoid bone forms as three bones: the lesser wings and the anterior part of the base. These two parts of the sphenethmoid may be distinguished as and presphenoid, respectively, although there is often some degree of fusion. Only the parasphenoid appears to be entirely absent in mammals.
In the dog the sphenoid is represented by eight bones: basisphenoid, alisphenoids, presphenoid, orbitosphenoids, pterygoids. These bones remain separate and are the:
==Additional images==
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See also
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