Borders and Margins of the Orbit
The base of the orbital pyramid forms the orbital opening (Aditus orbitalis). It is clearly defined by four borders that are easily palpable on the face:
- Supraorbital margin (Margo supraorbitalis);
- Infraorbital margin (Margo infraorbitalis);
- Lateral margin (Margo lateralis);
- Medial margin (Margo medialis).
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The apex of the pyramid is directed posteriorly and medially, transitioning into the optic canal (Canalis opticus). This canal provides direct communication between the orbital cavity and the middle cranial fossa.
Structure of the Orbital Walls
The orbital cavity is bounded by four walls, each formed by different bony structures:
- Superior wall (roof) (Paries superior): Formed by the orbital part of the frontal bone (Pars orbitalis) and the lesser wing of the sphenoid bone.
- Inferior wall (floor) (Paries inferior): Formed by the orbital surfaces of the maxilla and the zygomatic bone.
- Lateral wall (Paries lateralis): The thickest wall. Constructed from the zygomatic process of the frontal bone, the orbital surface of the greater wing of the sphenoid bone, and the orbital surface of the zygomatic bone.
- Medial wall (Paries medialis): Formed by the frontal process of the maxilla, the lacrimal bone, the orbital plate of the ethmoid bone, and the lateral surface of the body of the sphenoid bone. This wall also features the fossa for the lacrimal sac (Fossa sacci lacrimalis), which continues inferiorly into the lacrimal groove (Sulcus lacrimalis) and further into the nasolacrimal canal (Canalis nasolacrimalis).
Key Openings, Fissures, and Their Contents
The orbit contains numerous foramina and fissures that transmit vital neurovascular structures. Let us examine the main communication pathways.
Connections with the cranial cavity:
- Optic canal (Canalis opticus): Transmits the optic nerve (N. opticus, CN II) and the ophthalmic artery (A. ophthalmica), which gives rise to the central retinal artery.
- Superior orbital fissure (Fissura orbitalis superior): Transmits the oculomotor nerve (N. oculomotorius, CN III), trochlear nerve (N. trochlearis, CN IV), abducens nerve (N. abducens, CN VI), and the ophthalmic nerve (N. ophthalmicus, V1 branch of CN V). Vascular structures include the superior ophthalmic vein (V. ophthalmica superior) and occasionally the recurrent meningeal branches of the ophthalmic artery.
Connections with the face and other regions:
- Inferior orbital fissure (Fissura orbitalis inferior): Transmits the infraorbital nerve (N. infraorbitalis), zygomatic nerve (N. zygomaticus), infraorbital vessels, and connects the orbit to the inferior ophthalmic vein.
- Infraorbital canal (Canalis infraorbitalis): Leads to the face, maxillary sinus, and dental alveoli. It transmits the infraorbital nerve, artery, and veins, emerging onto the face via the infraorbital foramen (Foramen infraorbitale), where the nerve branches into the minor goose's foot (Pes anserinus minor).
- Supraorbital and frontal notches/foramina: Connect the superior wall to the face. The supraorbital notch/foramen transmits the frontal nerve and supraorbital vessels, while the frontal notch transmits the supratrochlear vessels and nerve (N., A., V. supratrochleares).
- Zygomaticoorbital foramen (Foramen zygomaticoorbitale): Located on the lateral wall; transmits the zygomatic nerve, which subsequently divides to exit onto the face and into the temporal fossa via the zygomaticofacial and zygomaticotemporal foramina.
- Ethmoidal foramina (anterior and posterior): Located on the medial wall, connecting the orbit to the ethmoidal air cells, nasal cavity, and anterior cranial fossa. They transmit the corresponding anterior and posterior ethmoidal arteries, veins, and nerves.