The sphenoid bone (also known simply as the sphenoid) is a butterfly-shaped single bone that lies at the middle part of the base of the skull between the frontal, temporal and occipital bones.

The sphenoid bone consists of the following parts:

Body of the sphenoid - the central portion;
Greater wings (2) and lesser wings (2) - extend laterally from the body;
Pterygoid processes (2) - descend from the junction of the body and greater wings.

The body, greater wings and pterygoid processes can be identified when looking at the inferior aspect of the skull. The lesser wings of the sphenoid cannot be seen unless the skull has been opened.

Body

The body of the sphenoid is located in the midline. It is a hollow structure, containing two large air sinuses - sphenoidal sinuses - that are divided into two parts by a septum. The body of the sphenoid articulates with several other bones:

Anteriorly - with the vomer and ethmoid bones;
Posterolaterally - with the temporal bones;
Posteriorly - with the occipital bone.

The body of the sphenoid is cuboid-shaped, and it contains six surfaces - superior, two lateral, posterior, anterior and inferior surfaces.

The superior surface contains several landmarks, such as:

Prechiasmatic sulcus - also known as the chiasmatic groove or optic groove; it is a narrow, transverse groove on the anterior part of the surface, and it ends on either side of the superior surface in the optic foramen, so it extends in between the optic canals; posteriorly it is bounded by tuberculum sellae;
Sella turcica - a concave depression named the "Turkish saddle" located posterior to the prechiasmatic sulcus; it is saddle-shaped and that is why it has such a name; it contains the pituitary gland (hypophysis).

The sella turcica is formed by several anatomical structures - tuberculum sellae, dorsum sellae and hypophyseal fossa.

The tuberculum sellae is the elongated ridged anterior process that forms the anterior border of the sella turcica and posterior aspect of the prechiasmatic sulcus, while the dorsum sellae is the process forming the posterior border. In the middle between both borders is located the central depression called the hypophyseal fossa in which lies the hypophysis. Each end of the dorsum sellae contains a small superolateral projection or tubercle called the posterior clinoid process, it gives an attachment to the tentorium cerebelli. The anterior clinoid processes (structure of the lesser wing) and posterior clinoid processes surround the sella turcica.

The lateral surface contains the carotid groove - an oblique and broad groove above the connection between the greater wing, medial pterygoid plates, and body of the sphenoid, lateral to the sella turcica. The groove contains the internal carotid artery and the cavernous sinus.

The posterior surface of the sphenoid bone connects with the basilar part of the occipital bone, forming the cartilaginous junction called the sphenooccipital synchondrosis.

The anterior surface forms the upper part of the posterior wall of the nasal cavity. It contains openings of sphenoidal sinuses leading to mentioned sinuses, air-filled cavities within the body of the sphenoid, but sometimes they extend also in the greater or lesser wings. Sphenoidal sinuses are the two most posterior located sinuses in the human body separated by a bony septum. Usually, the septum is not located in the midline, but it is bent to one or the other side.

The inferior surface contains rostrum of sphenoid or sphenoidal rostrum - a triangular spine in the midline, continuous with the sphenoidal crest and septum on the anterior surface. It also articulates with the vomer.

Greater wings

Lateral to each lateral plate of each pterygoid process is the greater wing of the sphenoid, which not only contributes to the base of the skull but also continues laterally to form part of the lateral wall of the skull. Greater wings also form the posterior aspect of the lateral wall of the orbit, as well as the floor part of the middle cranial fossa.

The greater wings are bony processes extending from the side of the body of the sphenoid. They curve upward, laterally, and backward and articulate laterally and posteriorly with parts of the temporal bone.

From both greater wings inferiorly projects a triangular process called the spine of the sphenoid bone.

The greater wings have five surfaces:

The lateral surface is formed by two parts - the temporal and infratemporal surfaces. They are divided by a transverse ridge called the infratemporal crest.

There are several important openings visible on the cerebral surface of the greater wing.

Those structures include:

Lesser wings

The lesser wings of the sphenoid are two thin and sharp triangular-shaped plates, which arise from the anterior part of the superior surface of the body of the sphenoid. They project laterally and end in sharp points. The lesser wings separate the anterior cranial fossa from the middle cranial fossa.

The lesser wings have two surfaces - super and inferior.

The superior surface faces the cranial cavity. It is flat and in contact with the frontal lobe of the brain, while the inferior surface faces the orbit and forms the upper border of the superior orbital fissure.

The lesser wings also contain several important landmarks, such as:

Pterygoid processes

The two pterygoid processes extend downward from the inferior surface of the body and the junction between the greater wings and the body of the sphenoid.

Each pterygoid process has a medial and a lateral plate. The lateral plate is more prominent. Both plates connect anteriorly and participate in forming the pterygopalatine fossa. In the posterior direction, both plates move away, and they are separated by the pterygoid fossa.

Each medial plate of the pterygoid process ends inferiorly with a hook-like projection - the pterygoid hamulus and divides superiorly to form a small and shallow scaphoid fossa.

Just superior to the scaphoid fossa, at the root of the medial plate of the pterygoid process, is the opening of the pterygoid canal, which passes forward from near the anterior margin of the foramen lacerum. The pterygoid canal transmits the Vidian nerve (nerve of the pterygoid canal), which is formed by merging of the greater petrosal nerve (arising from the facial nerve (CN VII)) and deep petrosal nerve (a branch of the internal carotid plexus).

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Sphenoid bone