The sphenoid bone is the most complex bone of the human body. Because of its shape, it is also known as the "wasp bone". It makes up most of the middle part of the base of the skull and contributes to the floor of the middle cranial fossa.
There are four main parts to the sphenoid bone: the body (corpus), the lesser and greater wings (alae minores et majores) and the pterygoid processes.
The body is the most centrally positioned portion. Anteriorly it contributes to the nasal cavity, laterally it builds the medial wall of the optic canal. Superiorly the body forms the sella turcica, the hypophyseal fossa and the dorsum sellae. They contain the anterior and posterior clinoid processes respectively. The clivus slopes posterior to the body. The sphenoidal sinuses are located in the sphenoid body behind the nasal cavity and divided by a septum.
The lesser wings arise superolaterally from the sphenoid body where they form the optic canal (→ optic nerve, ophthalmic artery). The inferior surface participates in the lateral margin of the orbit while the superior surface forms part of the cranial cavity.
The greater wings arise posterolaterally from the body. Their lateral surfaces form the infratemporal surfaces, their anterior surfaces make up part of the posterior aspect of the lateral wall of the orbit. They contain two important openings near their roots: the foramen rotundum (→ maxillary nerve) and the foramen ovale (→ mandibular nerve, accessory meningeal artery). The foramen spinosum (→ middle meningeal vessels, spinous nerve [branch of mandibular nerve]) lies at the posterior margin of the greater wings.
Between the body, lesser and greater wings is a large opening known as the superior orbital fissure where numerous nerves and vessels pass through (→ superior ophthalmic vein, ophthalmic nerve and its branches, abducent nerve, oculomotor nerve, trochlear nerve).
The pterygoid processes are extensions of the basal surface of the sphenoid body. The processes contain two canals known as the pterygoid canal (→ major petrosal nerve, deep petrosal nerve, vessels of pterygoid canal) and the palatovaginal (or pharyngeal) canal (→pharyngeal nerve). A hamulus extends bilaterally from each medial pterygoid plate.
The sphenoid bone has a common border with the frontal bone (via the sphenofrontal suture), the parietal bone (via the the sphenoparietal suture), the squamous part of the temporal bone (via the sphenosquamosal suture) and the occipital bone (via the spheno-occipital suture). As the sphenoid and occipital bone fuse during puberty ("tribasilar bone") the spheno-occipital suture disappears by the age of 25.
The body and lesser wings of the sphenoid bone mature through classic endochondral ossification, whereas the pterygoid processes undergo intramembranous ossification. The development of the greater wings of the sphenoid bone are exceptional since they are the only bony structures of the skull which go through both endochondral and intramembranous ossification.
For more information about the bones of the skull, including articles, illustrations, e-lectures and interactive quizzes, log on to kenHub.com or sign up for free!