- The axis is the second cervical vertebra (symbol: C2). It is called the "axis" because the uppermost cervical vertebra (called the atlas) rotates about the odontoid process of C2. The joint between the axis and atlas is a pivot type of joint. It allows the head turn. The Latin word "axis" means axle or pole. The axis bone serves as the axle about which the atlas (and the head) turn.
* * *1. [TA] A straight line joining two opposing poles of a spherical body, about which the body may revolve. 2. [TA] The central line of the body or any of its parts. 3. The vertebral column. 4. The central nervous system. 5. [TA] The second cervical vertebra. SYN: vertebra C2, epistropheus, odontoid vertebra, second cervical vertebra, toothed vertebra, vertebra dentata. 6. An artery that divides, immediately upon its origin, into a number of branches, e.g., celiac a.. See trunk. [L. axle, a.]- basifacial a. a line drawn from the subnasal point to the midpoint of the sphenoethmoidal suture. SYN: facial a..- cerebrospinal a. the central nervous system; the brain and spinal cord. SYN: encephalomyelonic a., neural a..- condylar a. a line through the two mandibular condyles around which the mandible may rotate during a part of the opening movement. SYN: condyle cord.- craniofacial a. a straight line passing through the mesethmoid, presphenoid, basisphenoid, and basioccipital bones.- electrical a. the net direction of the electromotive forces developed in the heart during its activation, usually represented in the frontal plane. See triaxial reference system.- encephalomyelonic a. SYN: cerebrospinal a..- external a. of eye [TA] that part of the optic a. from the midpoint of anterior surface of the cornea to the posterior surface of the posterior pole of the external surface of the sclera. SYN: a. externus bulbi oculi [TA].- axes of Fick three axes that pass through the center of the eye vertically (Z), horizontally in the coronal plane (X), and horizontally in the sagittal plane (Y). All ocular rotations can be described by rotation along one of these axes.- hinge a. SYN: transverse horizontal a..- instantaneous electrical a. the resultant a. of the electromotive forces developing in the heart at any given moment.- internal a. of eye [TA] that part of the optic a. from the midpoint of the posterior surface of the cornea to the anterior surface of the retina opposite the posterior pole. SYN: a. internus bulbi oculi [TA].- a. of lens a line connecting the anterior and posterior poles of the lens of the eye. SYN: a. lentis.- long a. a line extending through the center of an object lengthwise; in dentistry, the line extending inciso- (occluso-) cervically parallel to axial surfaces of a tooth.- long a. of body the imaginary straight line in the median plane which nearly intersects the center of all transverse planes through the body, running from the apex of the skull through the center of the perineum and continuing between the lower limbs, parallel to and equidistant from the long axes of the limbs; in theory, this is the line about which the body's mass is equally distributed. SEE ALSO: embryonic a.. SYN: cephalocaudal a..- mean electrical a. the average magnitude and direction of all the electromotive forces developed during the cardiac event under consideration; e.g., atrial or ventricular depolarization, or ventricular repolarization. SEE ALSO: a. deviation.- neutral a. of straight beam the a. perpendicular to the plane of loading of a beam at stresses within the proportional limit; it lies at the gravity a. of the cross-section of the beam.- normal electrical a. a mean electrical a. of the heart situated between minus 30° and +90°. See hexaxial reference system.- opening a. an imaginary line around which the mandibular condyles may rotate during opening and closing movements. Cf.:fulcrum line.- optic a. [TA] the a. of the eye connecting the anterior and posterior poles; it usually diverges from the visual a. by five degrees or more. SYN: a. opticus [TA].- a. opticus [TA] SYN: optic a..- orbital a. the line from the center of the optic foramen (apex of orbit) extending anteriorly, laterally, and inferiorly to the middle of the orbital opening.- a. of pelvis [TA] a hypothetical curved line joining the center point of each of the four planes of the pelvis, marking the center of the pelvic cavity at every level. SYN: a. pelvis [TA], pelvic a., plane of pelvic canal.- principal optic a. a line passing through the center of the lens of a refracting system at right angles to its surface.- pupillary a. a line perpendicular to the surface of the cornea, passing through the center of the pupil; the “direction of gaze.”- rotational a. SYN: fulcrum line.- sagittal a. in dentistry, the line in the frontal plane around which the working side condyle rotates during mandibular movement.- a. of symmetry an a. through a particle ( e.g., a virus) on such a plane that, if the particle is rotated on the a., there are two or more positions at which the particle appears identical.- transporionic a. an imaginary line connecting the upper central points of the external auditory meatuses; used in radiographic cephalometry. See porion.- transverse horizontal a. an imaginary line around which the mandible may rotate through the horizontal plane. SYN: hinge a., mandibular a..- vertical a. in dentistry, the line around which the working side condyle rotates in the horizontal plane during mandibular movement.- visual a. the straight line extending from the object seen, through the center of the pupil, to the macula lutea of the retina. SYN: line of vision.- Y-a. 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.
* * *1 a) a straight line about which a body or a geometric figure rotates or may be thought of as rotatingb) a straight line with respect to which a body, organ, or figure is symmetrical2 a) the second vertebra of the neck of the higher vertebrates that is prolonged anteriorly within the foramen of the first vertebra and united with the dens which serves as a pivot for the atlas and head to turn upon called also epistropheusb) any of various central, fundamental, or axial parts <the cerebrospinal \axis> <the skeletal \axis>c) AXILLA
* * *n.1. a real or imaginary line through the centre of the body or one of its parts or a line about which the body or a part rotates.2. the second cervical vertebra, which articulates with the atlas vertebra above and allows rotational movement of the head.
* * *ax·is (akґsis) pl. axґes [L., from Gr. axōn axle] 1. a line about which a revolving body turns or about which a structure would turn if it did revolve. 2. a line around which specified parts of the body are arranged. 3. [TA] the second cervical vertebra; called also epistropheus. 4. one of the reference lines in a coordinate system. In a two-dimensional coordinate system there are two axes, one horizontal (designated the x-axis), and the other intersecting it (designated the y-axis). Cf. abscissa and ordinate.
Axis. (A), (Top) transverse aspect; (bottom) superior aspect. (B), Position.
Medical dictionary. 2011.