Sphenoid Bone
Irregular Bone, part of the axial skeleton (the core of the skull)
Also known as: Butterfly Bone or Wasp Bone
Classification
Irregular Bone, part of the axial skeleton (the core of the skull)
Dimensions
In humans, this bone spans the entire width of the mid-skull, measuring approximately 4 to 5 inches across. It is defined by its complex, winged symmetry, featuring a central body with two sets of flared extensions.
Key Features
Often described as looking like a butterfly or a moth in flight, the sphenoid is recognized by its central 'saddle' (the sella turcica, where the blue arrows in the image point) and two pairs of wings. The larger, outer 'greater wings' form part of the temples, while the 'lesser wings' sit higher up and help form the eye sockets. It is riddled with small holes and passages for nerves and vessels.
Similar Bones
The sphenoid can be confused with the ethmoid bone, which is also a complex core skull bone. However, the ethmoid is much smaller, sits further forward, and lacks the wide, sweeping 'wings' characteristic of the sphenoid. The temporal bone also shares similar complex ridges but forms the outer side of the skull rather than the central floor.
Across Species
The sphenoid acts as the cornerstone of the skull across vertebrate species. In birds, the bone is highly fused and lighter to accommodate flight, while in whales, it is broadened significantly to support massive sensory structures. In most mammals, the 'wings' of the sphenoid provide essential anchoring points for powerful jaw muscles, showing great variation between predators with strong bites and grazers with grinding teeth.
Evolutionary History
Tracing back to early jawed fish, the sphenoid originated from several distinct cartilaginous elements that eventually fused into a single complex bone. This consolidation allowed for a sturdier skull base in land-dwelling tetrapods. Over millions of years, the bone shifted its orientation to create a protective cradle for the central nervous system as brains expanded in complexity.
Photography Tips
To identify this bone, photograph it from directly above or below to capture the butterfly-like symmetry. Use a soft, angled light to highlight the deep central cradle and the thin channels near the inner side of the wings. Placing a small coin or ruler next to it helps viewers understand its scale relative to other skull components.
Notes
What are the arrows pointing to