| axis of symmetry | An axis through a particle (e.g., a virus) on such a plane that, if the particle is rotated on the axis, there are two or more positions at which the particle appears identical. (05 Mar 2000) |
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| axis, optic | <microscopy> The direction, or directions in an anisotropic crystal along which light is not doubly refracted. (05 Aug 1998) |
| axis, optical | <microscopy> Usually refers to the axis on which several principal lens axes may lie. It also refers to the axis of the eye which extends through the centre of the eye lens. The line formed by the coinciding principal axes of a series of optical elements comprising an optical system. It is the line passing through the centres of curvature of the optical surfaces. (05 Aug 1998) |
| axis opticus | 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) |
| axis pelvis | A hypothetical curved line joining the centre point of each of the four planes of the pelvis, marking the centre of the pelvic cavity at every level. Synonym: axis pelvis, plane of pelvic canal. (05 Mar 2000) |
| axis, principal | <microscopy> A line conceived as passing through the centre of a lens to connect the centres of curvature of the lens surfaces. The focal points of a lens lie on the principal axis. (05 Aug 1998) |
| axis shift | Deflection of the electrical axis of the heart to the right or left of the normal. See: left axis deviation, right axis deviation, axis. Synonym: axis shift. (05 Mar 2000) |
| axis traction | Rarely used procedure to apply traction upon the foetal head in the line of the birth canal by means of axis traction forceps. (05 Mar 2000) |
| axis-traction forceps | Obstetrical forceps provided with a second handle so attached that traction can be made in the line in which the head must move in the axis of the pelvis. (05 Mar 2000) |
| basibregmatic axis | A line extending from the basion to the bregma. (05 Mar 2000) |
| basicranial axis | A line drawn from the basion to the midpoint of the sphenoethmoidal suture. (05 Mar 2000) |
| basifacial axis | A line drawn from the subnasal point to the midpoint of the sphenoethmoidal suture. Synonym: facial axis. (05 Mar 2000) |
| biauricular axis | A straight line joining the two auricles. Compare: auriculare. (05 Mar 2000) |
| magnetic axis | <physics> This typically refers to the location of the innermost flux surface in a toroidal device, the one which encloses no volume and has therefore degenerated from a flux surface into a single field line. Roughly, the circle through the middle of the dough of the donut. Additionally, in systems with magnetic islands (see entry below), each island has a local magnetic axis, distinct from the overall magnetic axis of the torus. (09 Oct 1997) |
| mandibular axis | An imaginary line around which the mandible may rotate through the horizontal plane. Synonym: hinge axis, mandibular axis. (05 Mar 2000) |