| C3 | third cervical nerve; third cervical vertebra; third component of complement |
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| CIII | third cranial nerve |
| C4 | fourth cervical nerve; fourth cervical vertebra; fourth component of complement |
| CIV | fourth cranial nerve |
| C5 | fifth cervical nerve; fifth cervical vertebra; fifth component of complement |
| orbital muscle | <anatomy> A rudimentary nonstriated muscle, crossing the infraorbital groove and sphenomaxillary fissure, intimately united with the periosteum of the orbit. Synonym: musculus orbitalis, Muller's muscle, orbital muscle. (05 Mar 2000) |
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| orbital neoplasms | Neoplasms of the bony orbit and contents except the eyeball. (12 Dec 1998) |
| orbital opening | The somewhat quadrangular anterior entrance to the orbit which forms the base of the pyramid-shaped orbital cavity. It is bounded by the sharp supra-, infra-, and lateral orbital margins and a less obvious medial margin on each side of the upper nose. Synonym: aditus orbitae, aperture of orbit. (05 Mar 2000) |
| orbital ophthalmoplegia | Ophthalmoplegia due to a lesion within the orbit. (05 Mar 2000) |
| orbital part of frontal bone | The portion of the frontal bone that contributes to the formation of the orbits; the most rostral of three cortical convolutions that togetther form the inferior frontal gyrus. Synonym: pars orbitalis ossis frontalis. (05 Mar 2000) |
| orbital part of lacrimal gland | See: lacrimal gland. (05 Mar 2000) |
| orbital part of orbicularis oculi muscle | orbital part of orbicularis oculi muscle |
| orbital plane | The orbital surface of the maxilla, lying perpendicular to the orbitomeatal plane at the orbitale. Synonym: planum orbitale. (05 Mar 2000) |
| orbital plate | A thin plate of ethmoid bone forming part of the medial wall of the orbit and the lateral wall for the ethmoidal labyrinth. Synonym: lamina orbitalis ossis ethmoidalis, lamina papyracea, orbital lamina of ethmoid bone, orbital layer of ethmoid bone, orbital plate, paper plate, papyraceous plate. (05 Mar 2000) |
| orbital plate of ethmoid bone | A thin plate of ethmoid bone forming part of the medial wall of the orbit and the lateral wall for the ethmoidal labyrinth. Synonym: lamina orbitalis ossis ethmoidalis, lamina papyracea, orbital lamina of ethmoid bone, orbital layer of ethmoid bone, orbital plate, paper plate, papyraceous plate. (05 Mar 2000) |
| orbital process | The anterior and larger of the two processes at the upper extremity of the vertical plate of the palatine bone, articulating with the maxilla, ethmoid, and sphenoid bones. Synonym: processus orbitalis. (05 Mar 2000) |
| orbital pseudotumour | <radiology> Non-specific inflammation of orbit, sudden, painful proptosis, Treatment: steroids, middle age, M=F, unilateral (90%), EOM enlargement: one muscle, inferior rectus most common, lacrimal gland enlargement, optic nerve enlargement and irregularity, scleral enhancement (50%), streaky intraconal fat, orbital mass (intra- or extraconal, discrete or irregular) (12 Dec 1998) |
| orbital region | The region about the orbit. Synonym: regio orbitalis. (05 Mar 2000) |
| orbital ridge | The bony ridge beneath the eyebrow. (12 Dec 1998) |
| orbital rim | The mostly sharp edge of the orbital opening which is the peripheral border of the base of the pyramid-shaped orbit. The superior half of the orbital rim is the supraorbital margin; the inferior half is the infraorbital margin. The frontal, maxillary, and zygomatic bones contribute to the orbital rim, which is generally strong to protect the orbital contents. Weak, potential fracture sites of the rim coincide with the sutures between the participating bones. Synonym: margin of orbit. (05 Mar 2000) |