The skull orbit, often called the eye socket, is a bony cavity within the skull that serves as a protective housing for the eye and its associated structures. Shaped like a four-sided pyramid or cone, with its base opening towards the face and its tip pointing backward, this structure acts as a robust barrier, shielding the eyeball from external impacts and injuries, much like a protective shell around a fragile object.
Anatomical Composition
The bony framework of each orbit is formed by seven distinct bones of the skull. The roof, or superior wall, is composed of the orbital plate of the frontal bone and the lesser wing of the sphenoid bone. This superior wall separates the orbital contents from the brain’s anterior cranial fossa.
The floor, or inferior wall, is formed by the maxillary bone, which also separates the orbit from the maxillary sinus. The zygomatic bone and a portion of the palatine bone also contribute to the orbital floor. The medial wall, closest to the nose, is the thinnest and is formed by parts of the ethmoid, lacrimal, maxillary, and sphenoid bones, with the ethmoid bone making up a significant portion.
The lateral wall, the thickest and most exposed, is constructed by the zygomatic bone and the greater wing of the sphenoid bone. These bones meet at sutures, forming a strong structure around the eye. The anterior opening, known as the orbital rim, is robust.
Structures Within the Orbit
Beyond its bony walls, the orbital cavity contains several soft tissue structures integral to vision and eye movement. The eyeball, also known as the globe, is the most prominent structure. Six extraocular muscles surround the eyeball, which enable its precise movements in various directions.
The optic nerve, a bundle of nerve fibers, exits the back of the eyeball and passes through the orbit to transmit visual information directly to the brain. Other cranial nerves, specifically the oculomotor, trochlear, and abducens nerves, also reside within the orbit, controlling the extraocular muscles and other eye functions. A network of blood vessels, including the ophthalmic artery and superior and inferior ophthalmic veins, supplies and drains blood from the eye and orbital contents.
Orbital fat fills the space, acting as a protective cushion for the eyeball and the delicate structures within, allowing the eye to move smoothly. The lacrimal gland, responsible for tear production, is also situated within the orbit, typically in the upper outer region.
Protective Function and Openings
The primary role of the bony orbit is to provide physical protection for the eyeball and its associated structures from external forces and trauma. Its thick walls help to deflect impacts and absorb shock. The orbital rim, formed by the frontal, zygomatic, and maxillary bones, offers a strong anterior barrier.
Despite its protective nature, the orbit is not a completely sealed cavity; it features several openings that allow communication with other parts of the skull and face. The optic canal, located at the apex of the orbit, allows passage for the optic nerve and the ophthalmic artery to enter the orbit from the middle cranial fossa. The superior orbital fissure, situated between the greater and lesser wings of the sphenoid bone, transmits several cranial nerves, including those that control eye movement, and sensory nerves to the forehead.
The inferior orbital fissure lies between the lateral wall and the floor, allowing passage for the infraorbital nerve and the inferior ophthalmic vein. Smaller ethmoidal foramina transmit nerves and blood vessels to and from the ethmoid sinuses. These openings allow nerves and blood vessels to connect the eye to the brain and other facial regions, while maintaining the orbit’s protective integrity.
Common Orbital Conditions
Despite its protective design, the orbit can be susceptible to conditions, with orbital fractures being common. A “blowout fracture” is a specific orbital fracture that typically occurs when blunt force trauma to the eye increases pressure within the orbit. This increased pressure causes the thinnest parts of the orbital walls, often the floor or medial wall, to fracture and buckle outward, sometimes displacing orbital contents into adjacent sinuses. Symptoms often include bruising, swelling, double vision due to muscle entrapment, or numbness in the cheek if the infraorbital nerve is affected.
Orbital cellulitis is a serious bacterial infection within the soft tissues of the orbital cavity, behind the orbital septum. This infection can arise from sources including sinus infections, dental infections, or trauma like an orbital fracture. Individuals with orbital cellulitis may experience painful, swollen eyelids, eye pain with movement, bulging of the eye (proptosis), and sometimes a fever. Prompt medical attention and treatment with antibiotics are important to prevent complications such as vision loss or the spread of infection to the brain.