The eye orbit is a bony cavity within the skull. It protects the eyeball and its associated structures. This socket positions the eye for proper vision and movement. Its design ensures the eye remains secure and enables complex visual functions.
The Bony Housing
The eye orbit is not a single bone but a complex structure formed by seven distinct bones working together: the frontal, zygomatic, maxillary, ethmoid, lacrimal, palatine, and sphenoid. These bones contribute to the overall shape and integrity of the roughly cone-shaped orbital cavity.
The roof of the orbit is formed by the orbital plate of the frontal bone and a smaller contribution from the lesser wing of the sphenoid. This superior wall separates the orbital contents from the anterior cranial fossa of the brain. The lateral wall, considered the strongest of the orbital walls, is composed of the zygomatic bone and the greater wing of the sphenoid.
The medial wall is the thinnest, formed by four bones: the maxillary, lacrimal, ethmoid, and part of the sphenoid. This wall is susceptible to fractures. The floor includes parts of the maxillary, palatine, and zygomatic bones.
Openings, known as foramina, fissures, and canals, perforate these walls, serving as conduits for various structures. The optic canal, at the apex, allows passage for the optic nerve and the ophthalmic artery. The superior orbital fissure, lateral and inferior to the optic canal, transmits cranial nerves III, IV, and VI, which control eye movement, along with branches of the trigeminal nerve. The infraorbital foramen, found on the orbital floor, allows the infraorbital nerve to exit.
Inside the Orbit
Beyond the eyeball, the orbit contains structures supporting vision and eye movement. Six extraocular muscles: the superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, and inferior oblique, attach to the eyeball and orbital bones. These muscles coordinate precise eye movements, including elevation, depression, adduction, abduction, and rotation.
The optic nerve (cranial nerve II) transmits visual information from the retina to the brain. Other cranial nerves, the oculomotor (III), trochlear (IV), and abducens (VI), innervate the extraocular muscles. The trigeminal nerve (V) provides sensory innervation to the orbital region.
Blood vessels, including branches of the ophthalmic artery, supply oxygen and nutrients to the orbital contents. The central retinal artery, a branch of the ophthalmic artery, supplies the retina.
Orbital fat fills much of the space, surrounding the eyeball and extraocular muscles. This fat cushions the eye from external forces and facilitates smooth eye movement by providing a flexible medium.
The lacrimal gland, in the upper outer orbit, produces tears to lubricate and protect the eye surface. Tears flow across the eye and drain into the lacrimal sac, housed in a groove on the medial orbital wall.
Protecting and Supporting Vision
The orbit’s bony architecture and its contents work together to protect the eyeball and facilitate its function. Its strong walls, particularly the lateral wall, act as a physical barrier, shielding the eye from blunt trauma. The conical shape helps distribute impact forces, reducing direct injury to the globe.
The combined presence of orbital fat, precisely arranged extraocular muscles, and vital nerves and blood vessels within this stable environment ensures the eye is cushioned, capable of coordinated movement, and receives constant communication and supply for complex visual processes.
Despite its protective design, the orbit’s integrity can be compromised. Fractures of the orbital bones, often from trauma, can affect eye position or the function of orbital muscles and nerves. Additionally, direct communication pathways between the orbit and other parts of the skull, through its various openings, allow infections or conditions from elsewhere to spread into the orbital cavity.