The orbit, commonly called the eye socket, is the bony cavity that houses and protects the eyeball and its associated structures. It is not a single bone but a complex, cone-shaped space formed by the articulation of multiple skull bones. This structure provides protection and structural integrity, allowing the delicate eye to function safely within a secure environment.
Where the Orbital Bone is Located
The orbit is a symmetrical, paired structure situated bilaterally on the face, resembling a pyramid with four walls. The base opens outward, creating the orbital rim that can be felt around the eye. The apex points backward and slightly inward toward the center of the head, terminating near the middle cranial fossa.
The two orbits are separated by the nasal cavity and the paranasal sinuses, particularly the ethmoid air cells. The roof lies just below the frontal lobe of the brain, while the floor sits on top of the maxillary sinus. This positioning means the bony walls form a boundary with several air-filled spaces, which affects both protection and the potential spread of infection.
The Seven Bones That Form the Eye Socket
The structure commonly referred to as the orbital bone is a mosaic of seven distinct bones that come together to form the four walls of the socket. These bones are the frontal, zygomatic, maxilla, ethmoid, lacrimal, palatine, and sphenoid bones. Their complex articulation provides the necessary contours and strength for the eye’s housing.
The roof (superior wall) is primarily formed by the orbital plate of the frontal bone, with a small contribution from the lesser wing of the sphenoid bone at the back. The floor (inferior wall) is composed of the maxilla, the zygomatic bone, and a small part of the palatine bone. The floor is structurally weaker than the roof and is separated from the underlying maxillary sinus by a thin layer of bone.
The lateral wall is the strongest of the four walls, formed by the zygomatic bone and the greater wing of the sphenoid bone. This strength is important as the lateral wall is often subjected to high impact forces. The medial wall is the thinnest, composed of four bones: the lacrimal bone, the ethmoid bone’s orbital plate, a small part of the sphenoid bone, and the frontal process of the maxilla. The majority of this wall is the paper-thin ethmoid bone, known as the lamina papyracea, which separates the orbit from the ethmoid air sinuses.
Protecting the Eye and Soft Tissues
The primary function of the bony orbit is to act as a physical barrier, shielding the delicate visual apparatus from external trauma. Within this protective cavity, the eyeball (globe) is housed along with its associated soft tissues. These contents include the six extraocular muscles responsible for eye movement, the optic nerve, and the lacrimal gland.
Any space not taken up by these structures is filled with orbital fat, which acts as a cushion to stabilize the globe and facilitate the smooth movement of the extraocular muscles. The walls of the orbit also contain various openings, such as the optic canal and orbital fissures, which allow nerves and blood vessels to pass through, connecting the eye to the nervous and circulatory systems.
Despite the protective strength of the orbit, certain areas are structurally less resilient and prone to injury, most notably the floor and the medial wall. Blowout fractures, which typically result from blunt force trauma to the eye area, most commonly affect the orbital floor, as the thin bone collapses into the maxillary sinus below. The medial wall is the second most common site for a blowout fracture, where the thin lamina papyracea can buckle into the ethmoid sinuses.
The lateral wall, formed by the thicker zygomatic and sphenoid bones, requires a much greater force to fracture, making injuries to this area far less common. When a fracture occurs, the orbital contents, such as the fat or the inferior rectus muscle, can herniate or become trapped in the broken bone fragments. This can lead to double vision or limited eye movement.