The human skull is composed of 22 bones that interlock to form a protective helmet for the brain. Within this complex structure, a single bone occupies a central position, contributing significantly to the stability and functionality of the entire head. This bone serves as a central anchor point for a vast network of nerves and blood vessels.
Defining the Sphenoid Bone
The sphenoid bone is one of the eight bones that make up the cranium, the bony case that encloses the brain. Its name is derived from the Greek word “sphenoeides,” meaning “wedge-shaped,” although its appearance is often compared to a butterfly or a bat with its wings extended. It is located at the base of the skull, situated between the frontal bone anteriorly and the occipital bone posteriorly.
It holds a special designation as the skull’s “keystone” bone because of its extensive articulations. The sphenoid connects with almost every other bone in the skull, including the frontal, parietal, temporal, occipital, ethmoid, and vomer bones. This central position allows it to connect the neurocranium, which protects the brain, to the facial skeleton, providing structural continuity.
The Butterfly Structure: Component Parts
The sphenoid bone’s complex anatomy is described as having four main components: a central body, two greater wings, two lesser wings, and two pterygoid processes. The body is the most central, cuboidal part of the bone, housing the air-filled sphenoidal sinuses, which help lighten the skull’s weight.
Projecting upward from the superior surface of the body is a landmark known as the sella turcica, Latin for “Turkish saddle.” This depression has a central part called the hypophyseal fossa, which provides a protected bony seat for the pituitary gland. The lesser wings extend outward from the anterior part of the body, forming a ridge that separates the skull’s anterior and middle cranial compartments.
The greater wings flare out laterally from the body, resembling the main wings of a butterfly or bat. These wings contribute to the floor of the middle cranial compartment and form part of the side walls of the skull. Extending downward from the junction of the body and the greater wings are the paired pterygoid processes, which are thin, vertically oriented bony plates. These processes serve as attachment sites for muscles involved in chewing, such as the lateral pterygoid muscle.
Role in Cranial Stability and Nerve Pathways
The sphenoid bone acts as a central hub for communication between the brain and the face. Its numerous articulations ensure the skull maintains its structural integrity and shape. The bone also contributes to the formation of the orbital walls, providing a stable structure for the eye sockets.
A primary functional characteristic of the sphenoid bone is the presence of multiple foramina and fissures, which serve as protected passageways for vital structures. One significant opening is the optic canal, which transmits the optic nerve and the ophthalmic artery. The superior orbital fissure, a gap between the greater and lesser wings, allows passage for several cranial nerves that control eye movement.
Other foramina facilitate the passage of branches of the trigeminal nerve (the main sensory nerve of the face) and blood vessels. These include the foramen rotundum, which transmits the maxillary nerve, and the foramen ovale, which transmits the mandibular nerve. These tunnels ensure a protected route for nerve and vascular communication between the cranial cavity and the face.
The Sphenoid Bone and Medical Procedures
The sphenoid bone’s unique anatomy and central location make it a common target and pathway in various medical contexts. Its large, air-filled cavity, the sphenoidal sinus, is positioned directly beneath the brain and behind the nasal cavity. This proximity means that an infection, known as sphenoid sinusitis, can be serious due to the risk of the infection spreading into the cranial cavity.
The bone’s structure is utilized in a procedure called transsphenoidal surgery. This is a minimally invasive technique primarily used to remove tumors growing on the pituitary gland, which is housed within the sella turcica. Surgeons access the pituitary gland by navigating through the nose and the sphenoidal sinus, avoiding the need to open the skull (craniotomy).
This approach offers benefits like a shorter recovery time and less postoperative discomfort. The procedure relies on the bone’s central position to provide a direct route to the pituitary gland at the base of the brain.