The sphenoid bone is a single, unpaired, and highly complex bony structure situated at the base of the skull. It acts as a central scaffolding element, connecting the cranial vault, which protects the brain, with the facial skeleton. This intricate bone is often described by anatomists as the “keystone” of the cranium because it articulates with almost all other bones of the skull. Its complexity is derived from its multiple articulations and the numerous openings it contains for the head and neck’s major nerves and blood vessels.
Location and Overall Structure
The sphenoid bone is located centrally, spanning the width of the skull base just in front of the basilar part of the occipital bone and behind the frontal bone. Its position is deep within the skull, making it largely inaccessible without dissection, but its influence is widespread throughout the cranial architecture. The bone articulates with twelve other bones, including the frontal, parietal, temporal, occipital, vomer, and ethmoid bones of the cranium, as well as several facial bones.
The bone’s overall structure is often likened to a butterfly or a bat with its wings spread wide, highlighting its four main components. These parts radiate outward from a central body. The central body contains hollow air-filled spaces and forms the core connection point for the two pairs of lateral extensions.
Major Anatomical Features
The sphenoid bone is systematically divided into a central cuboidal body, two greater wings, two lesser wings, and two downward-projecting pterygoid processes. The body of the sphenoid is the innermost section, containing the sphenoidal sinuses, which are a pair of air-filled cavities separated by a bony septum. The superior surface of this body features a distinctive saddle-shaped depression known as the sella turcica.
The sella turcica is a crucial landmark as it forms the protective bony seat for the pituitary gland. The deepest part of this depression is called the hypophyseal fossa, where the gland itself is securely nestled. Surrounding this fossa are raised bony edges: the tuberculum sellae at the front and the dorsum sellae at the back. This arrangement shields the pituitary gland from external pressures and movements.
The greater wings of the sphenoid extend laterally and upward from the body, contributing significantly to the floor of the middle cranial fossa and the side walls of the skull. These expansive surfaces also help form the posterior part of the eye sockets, or orbits. The lesser wings are triangular plates that project horizontally forward from the body, forming part of the floor of the anterior cranial fossa and the roof of the orbit.
The pterygoid processes extend downward from the junction of the body and the greater wings, providing attachment points for several muscles used in chewing, known as the muscles of mastication. Each process divides into two plates, the lateral and medial pterygoid plates, which contribute to the walls of the nasal cavity and the roof of the mouth.
Essential Roles in Cranial Function
The sphenoid bone performs several integrated functions. It provides the stable foundation for the cranium and contributes to the lateral walls of the skull, maintaining the rigidity and shape required to protect the brain. The bone’s extensions form a major part of the boundaries for several facial structures, including the posterior walls of the orbits and the lateral walls of the nasal cavity.
The sphenoid bone is perforated by numerous openings, or foramina. These openings allow major cranial nerves and blood vessels to pass through the thick bony barrier without compression. For example, the optic nerve passes through the optic canal, while other openings, such as the foramen rotundum and foramen ovale, transmit branches of the trigeminal nerve.
The sphenoidal sinuses, the hollow spaces within the bone’s body, serve a dual purpose by reducing the overall weight of the skull. These air-filled cavities also contribute to the resonance of the voice. Furthermore, the bone serves as an attachment site for muscles that allow the lower jaw to move.
Clinical Relevance
Fractures of the sphenoid bone are uncommon but are typically associated with high-energy blunt trauma to the head or face. Because of the close proximity of the bone to the internal carotid artery, optic nerve, and multiple other cranial nerves, a fracture can lead to serious complications, including vision loss, nerve damage, or severe bleeding.
The sphenoid bone and its sinuses are often used as a direct pathway for specialized surgical procedures. The transsphenoidal approach is a common surgical technique where surgeons access the pituitary gland by passing instruments through the nasal cavity and the sphenoid sinus. This technique allows for the removal of pituitary tumors with minimal external incision, using the natural anatomical corridor to reach the sella turcica.
Inflammation of the sphenoidal sinuses, known as sphenoid sinusitis, can be particularly problematic due to the bone’s proximity to the brain and optic nerves. An infection here can cause deep-seated headaches and, if left untreated, carries a risk of spreading to the surrounding neurological structures.