The sphenoid bone is a single, unpaired bone located at the center of the human skull. This unique bone is often described as having a distinctive butterfly, bat, or wasp-like shape due to its complex structure with outstretched “wings” and processes. Its central placement within the skull means it connects with many other cranial and facial bones.
Location and Distinctive Shape
The sphenoid bone occupies a central position within the human skull, situated in the middle and towards the front. It rests along the midline, forming a substantial part of the base of the skull. This bone is one of seven that contribute to the formation of the eye sockets, or orbits, and helps to create the floor of the middle cranial fossa, a depression that houses parts of the brain.
The sphenoid bone’s highly distinctive and complex shape includes a central body, two greater wings, two lesser wings, and two pterygoid processes. The body, located centrally, contains the sphenoidal sinuses and an indentation known as the sella turcica. Extending laterally from the body are the greater wings, which contribute to the sides of the skull and the posterior wall of the orbits. The lesser wings project forward and upward from the body, forming part of the anterior cranial fossa. Descending from the junction of the body and the greater wings are the pterygoid processes, which provide attachment points for muscles.
Major Functions
The sphenoid bone plays several significant roles in the structural integrity and functionality of the skull. It assists in forming the base and the sides of the skull, providing rigidity to protect the brain and associated nerve structures. Portions of this bone also contribute to the facial skeleton and the orbital floors. Its central location allows it to articulate with numerous other bones, acting as a “keystone” that locks many cranial and facial bones together.
The sphenoid bone provides important attachment sites for various muscles. Specifically, its rear parts and pterygoid processes serve as origins for muscles involved in mastication. The sphenoid bone also contains sphenoidal sinuses, which connect to the nasal cavity. These sinuses contribute to reducing the overall weight of the skull.
Important Structures It Contains or Protects
The sphenoid bone contains numerous passageways for nerves and blood vessels. The sella turcica is a saddle-shaped depression on the upper surface of its body. This specialized concavity is designed to house and protect the pituitary gland. The sella turcica comprises three parts: the tuberculum sellae anteriorly, the hypophyseal fossa where the pituitary gland rests, and the dorsum sellae posteriorly.
The sphenoid bone is also permeated by several foramina (openings) and canals, which are essential for the passage of neurovascular structures to and from the cranium. The optic canals allow the optic nerves and ophthalmic arteries to pass through, connecting the eyes to the brain. The superior orbital fissure, located between the greater and lesser wings, provides a pathway for several cranial nerves, including the oculomotor, trochlear, abducent, and ophthalmic nerves, along with the superior ophthalmic vein. Other significant openings include the foramen rotundum for the maxillary nerve, the foramen ovale for the mandibular nerve, and the foramen spinosum for the middle meningeal vessels.