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The orbits are two bony sockets at the front of the face that primarily house and protect the eyes and associated structures.
Ocular or optic refers specifically to the globe (eyeball). Orbital refers to all the contents of the bony orbit, encompassing both the intra and extraocular structures.
Orbits are roughly pyramidal in shape, broad based anteriorly and tapering to an apex, posteriorly. The normal volume in an adult is approximately 30 mL, of which the globe occupies 6.5 mL.
The orbit 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.
- extraocular muscles
- optic nerve (CN II)
- branches of the oculomotor nerve (CN III)
- superior division
- inferior division
- trochlear nerve (CN IV)
- branches of the ophthalmic division of the trigeminal nerve (CN Va)
- branches of the maxillary division of the trigeminal nerve (CN Vb)
- abducens nerve (CN VI)
- autonomic nerves and ganglia
- lacrimal gland
- fascia bulbi (Tenon's capsule)
The bony margins of the orbits, known as the bony orbit, are composed of the parts of seven craniofacial 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.
Other than the globe of the eye, and the optic nerve the orbit can be thought of containing two compartments with reference to the musculofascial cone:
Three major communications of the orbit are:
1. The superior orbital fissure, which opens laterally, lies between the greater and lesser wings of sphenoid bone, and houses
- superior and inferior ophthalmic veins
- cranial nerves lll, lV, V1 and VI
2. The optic canal that opens posteriorly into the cranial cavity and transmits the optic nerve and ophthalmic artery.
Communications of the pterygopalatine fossa (PPF):
- medially: PPF opens into the nasal cavity via the sphenopalatine foramen
- laterally: PPF communicates with the inferior temporal fossa via the pterygomaxillary fissure
- posterosuperiorly: PPF opens into the middle cranial fossa via foramen rotundum
- posteroinferiorly: PPF opens into the vidian canal
The inferior orbital fissure 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).
Nerves of the orbit
There are many nerves within the orbit: see orbital nerve supply.
Stability of the globe
The fascial sheath of the eye (Tenon's fascia), orbital fat, obliques and bony attachment of the recti provide stability to the eyeball within the orbit and prevent it from sinking or retracting. See article titled 'stability of the eye'.
The fascial sheath of the eye (Tenon's fascia) prevents the eye from sinking. The fascia is applied like a bursa to the back of the eye, from the corneoscleral junction to the attachment of the optic nerve and is thickened over the extraocular muscles.
Over the lateral rectus, the tubular prolongation is thickened to form the lateral check ligament, which attaches to the marginal tubercle of Whitnall (bony elevation at the orbital surface of the zygoma). Over the medial rectus, this prolongation is thickened to form the medial check ligament, which attaches to the posterior lacrimal crest (of the lacrimal bone). Between these ligaments, the inferior part of the sheath is thickened to form the suspensory ligament of Lockwood, which supports the eye within the orbit.
The eye does not rest on the orbital floor but is held up (in fact, closer to the roof) by the suspensory ligament. As a result, the whole maxilla can be removed with the medial orbital wall up to the marginal tubercle without descent of the eye. Above this level removal of bone destroys the attachment of the suspensory ligament, hence the eye sinks down, and diplopia results.
The eyes always rotate about a fixed center, which is its own geometrical center. The contracting recti muscles do not displace the eye posteriorly due to:
- bony attachment of the recti muscles
- presence of orbital fat
- forward pull of the superior and inferior oblique muscles
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- 4. Mcminn. Last's Anatomy. (2003) ISBN: 9780729537520