The orbit is the bony, cone-shaped socket in the skull that houses and protects the eye and its associated components. It shields these sensitive parts from injury and provides a stable base for the complex network of muscles, nerves, and vessels necessary for sight.
The Bony Framework
The framework of the orbit is a mosaic formed by seven different bones that create its four walls. The roof, or superior wall, is primarily formed by the frontal bone, with a smaller contribution from the lesser wing of the sphenoid bone. This formation separates the orbital cavity from the anterior cranial fossa, which houses the frontal lobes of the brain.
The floor of the orbit is composed of the maxillary, palatine, and zygomatic bones. This inferior wall is particularly thin, making it susceptible to fractures from direct impact. The maxilla also separates the orbit from the maxillary sinus located directly beneath it, meaning infections or trauma affecting the sinus can impact the orbital contents.
The medial wall, situated towards the nose, is formed by the ethmoid, maxilla, lacrimal, and sphenoid bones. The ethmoid bone is the primary contributor and is characterized by its paper-thin structure, separating the orbit from the ethmoid sinuses.
The lateral wall, which faces the temple, is the thickest and strongest part of the orbit. It is formed by the zygomatic bone and the greater wing of the sphenoid, providing significant protection from side impacts.
Contents of the Orbit
The Eyeball (Globe)
The most prominent occupant of the orbital cavity is the eyeball, also known as the globe. This complex sensory organ is responsible for detecting light and converting it into electrochemical signals. The eyeball itself occupies only a fraction of the orbit’s total volume, which is approximately 30 milliliters in an adult human.
Extraocular Muscles
Movement of the eyeball is controlled by a group of six extraocular muscles. These muscles originate from the back of the orbit and attach to the outer surface of the eyeball, enabling a wide range of precise movements. The four rectus muscles—superior, inferior, medial, and lateral—are responsible for the primary directional movements, while the two oblique muscles—superior and inferior—handle rotational adjustments.
Nerves and Blood Vessels
A complex network of nerves and blood vessels traverses the orbit to support its contents. The optic nerve is the primary neural pathway, transmitting visual information from the retina to the brain. Other cranial nerves, including the oculomotor, trochlear, and abducens nerves, control the movements of the extraocular muscles. The ophthalmic artery is the main blood supplier to the eye and its structures, while venous drainage is managed by the superior and inferior ophthalmic veins.
Fat and Glands
A significant portion of the orbit is filled with orbital fat, which acts as a cushion for the eyeball and other structures. This adipose tissue allows the eye to move smoothly within its socket and absorbs shock from minor impacts. Also housed within the orbit is the lacrimal gland, located in the upper, outer portion of the socket. This gland produces the watery component of tears, which keep the surface of the eye moist and clean.
Pathways Into and Out of the Orbit
The bony orbit is not a completely enclosed structure; it features several openings that allow for the passage of nerves and blood vessels. These pathways connect the orbital contents to the cranial cavity and other regions of the face and skull. The primary openings are the optic canal, the superior orbital fissure, and the inferior orbital fissure.
The optic canal is a cylindrical opening located at the apex of the orbit. This passageway allows the optic nerve to travel from the back of the eyeball into the middle cranial fossa to connect with the brain. The ophthalmic artery also passes through the optic canal to enter the orbit.
The superior orbital fissure is a larger, slit-like opening situated between the lesser and greater wings of the sphenoid bone. A multitude of structures pass through this fissure, including the cranial nerves for eye movement (the oculomotor, trochlear, and abducens nerves) and the ophthalmic division of the trigeminal nerve. The superior ophthalmic vein also exits the orbit through this opening.
The inferior orbital fissure, located along the floor of the orbit, provides passage for the zygomatic nerve and the inferior ophthalmic vein.
Common Medical Conditions
A common injury is a “blowout fracture,” caused by blunt force trauma to the eye. This impact can cause a break in the thin floor of the orbit, potentially trapping the inferior rectus muscle and limiting upward eye movement.
Orbital cellulitis is a serious infection of the fat and muscles contained within the orbit. This condition can arise from a sinus infection that spreads through the thin ethmoid bone of the medial wall or from a skin infection around the eye. The resulting inflammation and swelling can cause pain, vision problems, and bulging of the eye. Prompt treatment is necessary to prevent the infection from spreading to the brain.
Thyroid eye disease, an autoimmune condition often associated with an overactive thyroid gland, can also affect the orbit. In this condition, the body’s immune system attacks the tissues within the orbit, causing the extraocular muscles and orbital fat to swell. This swelling can push the eyeball forward, a condition known as proptosis, and may lead to double vision or damage to the optic nerve.