Orbit
The orbit, in the adult human, has a volume of approximately 30ml, of which the globe occupies 6.5ml. It has a roof, floor, medial and lateral wall. The orbit is open anteriorly where it is bound by the orbital septum, which also contributes to the eyelids. Posteriorly the orbit angles inward, such that their apicies communicate with the intracranial compartment via the optic canal and superior orbital fissure (see below). Its content includes:
- 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
- veins
- fat
- lacrimal gland
- fascia bulbi aka Tenon's capsule
The orbit's bony margins are made up 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 orbit communicates posteriorly with the intracranial cavity via the optic foramen through which is transmitted the optic nerve. Immediately infrolateral to it is the superior orbital fissure through which most traffic occurs. The infratemporal fossa is accessed via the inferior orbital fissure, which is in direct continuation with the infraorbital foramen, though 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 ethmoid foramen and posterior ethmoidal foramen.
Anteriorly the supraorbital notch is closed inferiorly by the orbital setpum forming a 'supraorbital foramen' of sorts. Finally, the nasolacrimal duct drains the nasolacrimal sac via the nasolacrimal foramen.
