The orbit (or orbital compartment, latin: orbita) is a paired skeletal cavity that is located in the skull on either side of the root of the nose. The orbit accommodates and protects the eye and its accessory structures.
The shape of the orbit resembles a pyramidal structure with an apex directed posteriorly and a base situated anteriorly. Each orbit opens on the anterior aspect of the skull with an approximately quadrangular shaped orbital opening. The orbital opening is formed by two margins:
- supraorbital margin (formed by the orbital part of the frontal bone),
- infraorbital margin (formed by the orbital processes of the maxilla and zygomatic bone).
Seven bones contribute to formation of the orbit, and these are:
- frontal bone (its orbital part),
- lacrimal bone (its orbital surface),
- ethmoid bone (its orbital plate),
- zygomatic bone (its orbital process),
- maxilla (its orbital surface),
- palatine bone (its orbital process),
- sphenoid bone (greater and lesser wings, body of sphenoid).
The bony orbit is bounded by four walls formed by several bones of the skull:
- superior wall (roof),
- medial wall,
- lateral wall,
- inferior wall (floor).
The roof of the orbit is formed anteriorly by the orbital part of the frontal bone, and posteriorly by the orbital surface of the lesser wing of the sphenoid.
Anteromedially on the supraorbital margin lies a supraorbital foramen (or sometimes a notch), which transmits the supraorbital vessels and nerve. Anterolaterally, the lacrimal fossa is situated, which houses the lacrimal gland.
The medial wall of the orbit is formed anteriorly by the lacrimal bone, medially - the orbital plate of the ethmoid bone, posteriorly - the body of the sphenoid, inferiorly - the frontal process of the maxilla.
The orbital surface of the lacrimal bone presents a fossa for the nasolacrimal sac.
The lateral wall of the orbit is formed anteriorly by the orbital surface of the zygomatic bone, posteriorly - the orbital surface of the greater wing of the sphenoid.
The lateral wall of the orbit contains the zygomaticofacial and zygomaticotemporal foramina that transmit the corresponding nerves.
Within the midpoint of the lateral orbital wall the orbital tubercle is situated, where the lateral palpebral ligament, the check ligament of the lateral rectus and the aponeurosis of the levator palpebrae all attach.
The floor of the orbit is formed by the orbital surface of the maxilla.
The orbital surface of the maxilla presents the infraorbital notch, which opens on the face at the infraorbital foramen or notch and carries the infraorbital nerve and vessels.
The orbits contain the eyeballs and their associated structures such as nerves, blood vessels and skeletal extraocular muscles. Any space within the orbit that is not occupied is filled with orbital fat that cushions the eyes and stabilises the extraorbital muscles.
The following structures are situated within the orbit along with the eye:
- nerves and ganglia: motor nerves - oculomotor, trochlear, abducens nerves; parasympathetic fibers form the oculomotor nerve and from the facial nerve; sympathetic fibers; sensory nerves - optic, ophthalmic nerves; and a parasympathetic ganglion - ciliary ganglion;
- blood vessels: ophthalmic artery, infraorbital branch of the maxillary artery, superior and inferior ophthalmic veins, infraorbital vein;
- extraocular muscles: levator palpebrae superioris, superior and inferior oblique, and four recti - superior, inferior, lateral, medial;
- structures of lacrimal apparatus: lacrimal gland, lacrimal canaliculi, lacrimal sac.
There are several openings that connect the orbit with other cavities of the skull and transmit nerves and blood vessels. These are the following:
- optic canal,
- superior orbital fissure,
- Inferior orbital fissure,
- anterior ethmoidal foramen,
- posterior ethmoidal foramen,
- nasolacrimal canal,
- infraorbital canal.
The optic canal connects the orbit with the middle cranial fossa to conduct the optic nerve and the ophthalmic artery.
The superior orbital fissure also connects the orbit with the middle cranial fossa and it transmits the oculomotor, trochlear and abducens nerves, branches of the ophthalmic nerve and the ophthalmic veins
The inferior orbital fissure connects the orbit with both pterygopalatine fossa and the infratemporal fossa. It transmits the infraorbital and zygomatic branches of the maxillary nerve and their accompanying vessels, orbital branches arising from the pterygopalatine ganglion, and a connection between the inferior ophthalmic vein and the pterygoid venous plexus.
The anterior ethmoidal foramen is an opening between the orbit and the anterior cranial fossa and it transmits the anterior ethmoidal nerve and its accompanying blood vessels.
The posterior ethmoidal foramen connects the orbit with the nasal cavity and conducts the posterior ethmoidal nerve and blood vessels.
The nasolacrimal canal leads from the orbit to the inferior nasal meatus in the nasal cavity and it carries the nasolacrimal duct.
The infraorbital canal runs within the floor of the orbit and opens with the infraorbital foramen on the anterior surface of the maxilla and it transmits the infraorbital nerve, artery and veins.