Sphenoid bone

Seven bones articulate to form the orbit. The sphenoid bone is in red (directly in the middle of the orbital cavity)

Wedge-shaped sphenoid bone seen laterally

It is divided into the following parts:

• a median portion, known as the body of sphenoid bone, containing the sella turcica, which houses the pituitary gland as well as the paired paranasal sinuses, the sphenoidal sinuses[5]
• two greater wings on the lateral side of the body and two lesser wings from the anterior side.
• Pterygoid processes of the sphenoides, directed downwards from the junction of the body and the greater wings.[4]

Two sphenoidal conchae are situated at the anterior and inferior part of the body.

Sphenoidal sinuses are asymmetrical air sinuses in the body of the sphenoid, closed by sphenoidal conchae.

These are two triangular wings projecting laterally from anterosuperior part of the body. Each consists of:

• A base forming medial end of the wing.
• Tip forming the lateral end of the wing.
• Superior surface forming floor of anterior cranial fossa.
• Inferior surface forming upper boundary of superior orbital fissure.
• Posterior surface projects into the Sylvian point.
• Medially, terminates in the anterior clinoid process.[4]

Development

Figure 4 : Sphenoid bone at birth, posterior aspect.

Until the seventh or eighth month of fetal development, the body of the sphenoid consists of two parts: one in front of the tuberculum sellæ, the presphenoid, with which the small wings are continuous; the other, consisting of the sella turcica and dorsum sellae, the postsphenoid, with which are associated the great wings, and pterygoid processes.

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.

This bone assists with the formation of the base and the sides of the skull, and the floors and walls of the orbits. It is the site of attachment for most of the muscles of mastication. Many foramina and fissures are located in the sphenoid that carry nerves and blood vessels of the head and neck, such as the superior orbital fissure (with ophthalmic nerve), foramen rotundum (with maxillary nerve) and foramen ovale (with mandibular nerve).[6]

The sphenoid bone of humans is homologous with a number of bones that are often separate in other animals, and have a somewhat complex arrangement.

In the early lobe-finned fishes and tetrapods, 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 olfactory nerves. Finally, the basisphenoid bone formed part of the floor of the braincase and lay immediately above the parasphenoid.[7]

Aside from the loss of the flexible joint at the rear of the palate, this primitive pattern is broadly retained in reptiles, albeit with some individual modifications. In birds, the epipterygoids are absent and the pterygoids considerably reduced. Living amphibians 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.[7]

In mammals, 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 alisphenoids 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 orbitosphenoids and presphenoid, respectively, although there is often some degree of fusion. Only the parasphenoid appears to be entirely absent in mammals.[7]

In the dog the sphenoid is represented by 8 bones: basisphenoid, alisphenoids, presphenoid, orbitosphenoids, pterygoids. These bones remain separate and are the:

• 2 Alisphenoids: each greater wing
• 2 Orbitosphenoids: each lesser wing
• Basisphenoid: back part of body
• Presphenoid: front part of body
• 2 Pterygoids: medial pterygoid plate

• 
  Position of sphenoid bone (shown in green). Animation.
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  Seen from below (mandible is removed)
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  Seen from above (parietal bones are removed)
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  Shape of sphenoid bone.
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  Facial bones.
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  Lateral wall of nasal cavity, showing ethmoid bone in position.
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  Base of skull. Inferior surface.
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  Lateral view of the skull.
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  Horizontal section of nasal and orbital cavities.
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  Floor of the skull.
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  Roof, floor, and lateral wall of left nasal cavity.
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  The skull from the front. The sphenoid is labeled with yellow to the left of the picture, both in the orbit and behind the zygomatic process
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  Sphenoid bone
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  Sphenoid bone superior view
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  Sphenoid bone and temporal bones

Wikimedia Commons has media related to Sphenoid bones.

• Sphenoidal sinus
• Pterygospinal ligament
• Basilar skull fracture

This article incorporates text in the public domain from page 147 of the 20th edition of Gray's Anatomy (1918)

• Anatomy figure: 22:01-06 at Human Anatomy Online, SUNY Downstate Medical Center - "Lateral view of skull."