The sphenoid bone is a single, unpaired component of the neurocranium, the part of the skull that encases the brain. It is structurally intricate and complex due to its unique shape and central connections. Its name originates from the Greek word sphenoeides, which translates to “wedge-shaped,” hinting at its deeply embedded location at the skull’s base. The bone is a defining feature of the cranial floor, establishing boundaries and providing protective housing for numerous delicate structures.
Central Position in the Cranial Base
The sphenoid bone is strategically positioned at the center of the skull base, acting as a structural linchpin that stabilizes the cranium. It lies in the midline, immediately behind the frontal and ethmoid bones, and anterior to the occipital bone. This deep, central placement means the bone is largely hidden from external view.
It forms a significant portion of the floor of the middle cranial fossa, the depression that cradles the temporal lobes of the brain. The bone also contributes to the posterior wall of the eye orbits and parts of the walls of the nasal cavity. Although deeply internal, it extends laterally to connect the major facial bones with those of the cranium.
The sphenoid acts as a bony bridge connecting the neurocranium to the facial skeleton. Its superior surface is visible inside the skull cavity, but only small portions of its “wings” are seen externally near the temples.
Distinctive Shape and Structural Components
The complex form of the sphenoid bone is often likened to a butterfly or a bat with its wings spread open. This single bone is functionally divided into four main parts: the central body, a pair of lesser wings, a pair of greater wings, and the downward-extending pterygoid processes. The cuboidal central body is the core structure, containing the air-filled sphenoidal sinuses, which are separated by a thin bony septum.
The lesser wings project laterally and superiorly from the front upper part of the body, forming a shelf that separates the anterior and middle cranial fossae. These wings contain the optic canals, which are small openings that allow the optic nerve and the ophthalmic artery to pass through toward the eye. The greater wings are larger, curving posterolaterally from the sides of the body, and they form part of the side walls of the skull and the posterior portion of the eye orbit.
Within the body of the sphenoid is a unique, saddle-shaped depression known as the sella turcica, which is Latin for “Turkish saddle.” This concavity provides a protective bony enclosure for the pituitary gland. The deepest part of the sella turcica is the hypophyseal fossa, where the gland rests securely. The greater wings are also pierced by several openings, such as the foramen rotundum and foramen ovale, which serve as pathways for nerves to leave the cranial cavity.
Finally, the pterygoid processes descend vertically from the junction of the body and the greater wings, resembling legs hanging down. These processes are composed of two bony plates, a medial and a lateral plate. They provide surfaces for the attachment of several muscles involved in chewing and jaw movement.
Primary Roles in Skull Function
The sphenoid bone’s central location allows it to establish the structural integrity of the skull. It articulates with nearly every other bone of the cranium, including the frontal, parietal, temporal, and occipital bones, as well as facial bones like the zygomatic and palatine bones. This extensive network of connections allows the sphenoid to function as the “keystone” of the skull, locking the surrounding bones into a stable configuration.
The numerous foramina and canals piercing the sphenoid bone are conduits for the passage of neurovascular structures. For example, the optic canal protects the optic nerve, which transmits visual information from the eye to the brain. The foramen rotundum and foramen ovale transmit branches of the trigeminal nerve, carrying sensory and motor information for the face and jaw.
The sphenoid bone also houses the paired sphenoid sinuses, which are one of the four groups of paranasal sinuses. These air-filled cavities connect to the nasal cavity, helping to lighten the mass of the skull. The sinuses also contribute to the conditioning of inhaled air, aiding in humidification and warming.