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Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents. In the adult human, the volume of the orbit is 30 mL, of which the eye occupies 6.5 mL.〔Duane's Ophthalmology, Chapter 32 ''Embryology and Anatomy of the Orbit and Lacrimal System''. (eds Tasman W, Jaeger EA) Lippincott/Williams & Wilkins, 2007〕 The orbital contents comprise the eye, the orbital and retrobulbar fascia, extraocular muscles, cranial nerves II, III, IV, V, and VI, blood vessels, fat, the lacrimal gland with its sac and nasolacrimal duct, the eyelids, medial and lateral palpebral ligaments, check ligaments, the suspensory ligament, septum, ciliary ganglion and short ciliary nerves.
==Foramina and fissures==
The orbits are conical or four-sided pyramidal cavities, which open into the midline of the face and point back into the head. Each consists of a base, an apex and four walls.〔"eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009〕 There are two important foramina, or windows, two important fissures, or grooves, and one canal surrounding the globe in the orbit. There is a supraorbital foramen, an infraorbital foramen, a superior orbital fissure, an inferior orbital fissure and the optic canal, each of which contains structures that are crucial to normal eye functioning. The supraorbital foramen contains the supraorbital nerve, the first division of the trigeminal nerve or V1 and lies just lateral to the frontal sinus. The infraorbital foramen contains the second division of the trigeminal nerve, the infraorbital nerve or V2, and sits on the anterior wall of the maxillary sinus. Both foramina are crucial as potential pathways for cancer and infections of the orbit to spread into the brain or other deep facial structures.
The optic canal contains the optic nerve (cranial nerve II) and the ophthalmic artery, and sits at the junction of the sphenoid sinus with the ethmoid air cells, superomedial and posterior to structures at the orbital apex. It provides a pathway between the orbital contents and the middle cranial fossa. The superior orbital fissure lies just lateral and inferior to the optic canal, and is formed at the junction of the lesser and greater wing of the sphenoid bone. It is a major pathway for intracranial communication, containing cranial nerves III, IV, VI which control eye movement via the extraocular muscles, and the ophthalmic branches of cranial nerve V, or V1. The third division of the trigeminal nerve enters the skull base at the foramen rotundum, or V3. The inferior orbital fissure lies inferior and lateral to the ocular globe at the lateral wall of the maxillary sinus. It is not as important in function, though it does contain a few branches of the maxillary nerve and the infraorbital artery and vein. Other minor structures in the orbit include the anterior and posterior ethmoidal foramen and zygomatic orbital foramen.
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