| opening axis | An imaginary line around which the mandibular condyles may rotate during opening and closing movements. Compare: fulcrum line. (05 Mar 2000) |
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| optical axis | <physics> The line passing through both the centres of curvature of the optical surfaces of a lens, the optical centreline for all the centres of a lens system. (09 Oct 1997) |
| optic axis | The axis of the eye connecting the anterior and posterior poles; it usually diverges from the visual axis by five degrees or more. Synonym: axis opticus. (05 Mar 2000) |
| orbital axis | The line from the centre of the optic foramen (apex of orbit) extending anteriorly, laterally, and inferiorly to the middle of the orbital opening. (05 Mar 2000) |
| thoracic axis | <anatomy, artery> Origin, axillary; distribution, muscles and skin of shoulder and upper chest; anastomoses, branches of superior thoracic, internal thoracic, lateral thoracic, posterior and anterior circumflex humeral, and suprascapular. Synonym: arteria thoracoacromialis, acromiothoracic artery, thoracic axis, thoracoacromial trunk. (05 Mar 2000) |
| thyroid axis | A short arterial trunk arising from the subclavian artery, giving rise to the suprascapular (which may instead arise directly from the subclavian artery) and terminating by dividing into the ascending cervical and inferior thyroid arteries. Synonym: truncus thyrocervicalis, thyroid axis. (05 Mar 2000) |
| electrical axis | The net direction of the electromotive forces developed in the heart during its activation, usually represented in the frontal plane. See: triaxial reference system. (05 Mar 2000) |
| electron optical axis | <physics> The path of an electron through an electron optical system along which it suffers no deflection due to lens fields. This axis does not necessarily coincide with the mechanical axis of the system. (05 Aug 1998) |
| embryonic axis | The cephalocaudal axis established in the embryo by the primitive streak. (05 Mar 2000) |
| encephalomyelonic axis | The central nervous system; the brain and spinal cord. Synonym: encephalomyelonic axis, neural axis. (05 Mar 2000) |
| transporionic axis | An imaginary line connecting the upper central points of the external auditory meatuses; used in radiographic cephalometry. See: porion. (05 Mar 2000) |
| transverse horizontal axis | An imaginary line around which the mandible may rotate through the horizontal plane. Synonym: hinge axis, mandibular axis. (05 Mar 2000) |
| external axis of eye | That part of the optic axis from the midpoint of anterior surface of the cornea to the posterior surface of the posterior pole of the external surface of the sclera. Synonym: axis bulbi externus. (05 Mar 2000) |
| facial axis | A line drawn from the subnasal point to the midpoint of the sphenoethmoidal suture. Synonym: facial axis. (05 Mar 2000) |
| Y-axis | A cephalometric indicator of the vertical and horizontal coordinates of mandibular growth expressed in degrees of the inferior facial angle formed by the intersection of the sella-gnathion plane with the Frankfort horizontal plane. (05 Mar 2000) |