Orbit

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The orbit is a feature of the face and contains the globe.

Gross anatomy

In the adult, the orbit has a volume of approximately 30 mL, of which the globe occupies 6.5 mL. It has a roof, floor, medial and lateral wall. The orbit is open anteriorly where it is bound by the orbital septum, which forms part of the eyelids. Posteriorly, the orbit angles inward such that their apices communicate with the intracranial compartment via the optic canal and superior orbital fissure.

globe
extraocular muscles
cranial nerves
- optic nerve (CN II)
- branches of the oculomotor nerve (CN III)
- trochlear nerve (CN IV)
- ophthalmic division of the trigeminal nerve (CN Va)
- abducens nerve (CN VI)
autonomic nerves and ganglia
- ciliary ganglion
- sympathetic root to the ciliary ganglion (parasympathetic root travels in the oculomotor nerve)
arteries
- ophthalmic artery
veins
- superior ophthalmic vein
- inferior ophthalmic vein
fat
lacrimal gland
fascia bulbi (Tenon's capsule)

Bony margins

The bony margins of the orbits are composed of seven bones:

pars orbitalis of the frontal bone
lacrimal bone
lamina papyracea of the ethmoid bone
orbital process of the zygomatic bone
orbital surface of the maxillary bone
orbital process of the palatine bone
greater and lesser wings of the sphenoid bone

The four bones of the medial wall are remembered with this mnemonic.

Communications

The orbit communicates posteriorly with the intracranial cavity via the optic canal, through which the optic nerve and ophthalmic artery is transmitted. Immediately inferolateral to the optic canal is the superior orbital fissure, through which most neurovascular structures pass. The infratemporal fossa is accessed via the inferior orbital fissure, which is in direct continuation with the infraorbital foramen, through which the infraorbital nerve exits to supply the skin below the eye (and where it is often damaged by a blow-out fracture).

Medially, small communications with the paranasal sinuses are via the anterior ethmoidal foramen and posterior ethmoidal foramen.

Anteriorly, the supraorbital notch is closed inferiorly by the orbital septum forming a fibrous supraorbital foramen. The nasolacrimal duct drains the nasolacrimal sac via the nasolacrimal foramen.

Image 7 Diagram ( update )

Caption was added:

Figure 7: bones of the orbit

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