plane. 1

A level surface; a surface such that if any two of its points are joined of by a straight line that line will lie wholly within the surface. 2. An ideal p. as above defined, used as a basis or standard of reference by means of which the positions of the parts of a body or organism are indicated, alveolocondylar p. The p. tangent to the alveolar point and most prominent points on the lower aspects of the condyles of the occipital bone. coccygeal p. The fourth parallel p. of the pelvis, diagonal p. Of a flower, any vertical p. which is not anteroposterior or lateral. double inclined p. Two boards united by hinges and resting upon a third; used to support the lower limb flexed in the treatment of fractures. first focal p. See focal p's. first principal p. See focal p's. focal p's. Two p's drawn respectively through the anterior and posterior principal foci of a dioptric system (e. g., the eye) and perpendicular to the line (axis of the system) connecting the two. The p. pa ing through the anterior focus is called the anterior (or first) focal p.; that pa ing through the posterior focus the posterior (or second) focal p. glabellooccipital p. The vertical p. of the maximum anteroposterior diameter of the skull, inclined p. A surface situated obliquely to the p. of the horizon; one of the elementary mechanical powers. inclined p's of the pelvis. "The sciatic spines," says Lusk, "divide the pelvic cavity into two unequal sections. In the larger, anterior section, the lateral walls slope toward the symphysis and arch of the pubes, while posteriorly the walls slope in the direction of the sacrum and coccyx. The declivities in front of the spines are termed the anterior inclined p's of the pelvis, over which rotation of the occiput takes place in the mechanism of normal labor. Behind the spines the lateral slopes are known as the posterior inclined p's." medial p., median p., mesial p. A p., usually anteroposterior, which divides a body or organ into two equal and symmetrical parts. The median p. of the body is known as the meson, orbital p. See planum orbitale. parallel p's of the pelvis. P's intersecting the axis of the pelvic canal at right angles, and therefore in reality not parallel. Four such p's are distinguished: the first parallel p. of the pelvis is the p. of the superior strait; the second is the p. extending from the middle of the sacral vertebra to the level of the subpubic ligament; the third is the p. at the level of the spines of the ischia; and the fourth is the p. at the outlet, p. of refraction. A p. pa ing through a refracted ray of light and drawn perpendicular to the surface at which refraction takes place, p. of regard. A p. pa ing through the fovea of the eye and the fixation point, p's of the pelvis. Imaginary p's touching the same parts of the pelvic canal on the two sides, principal p's. See focal p's. sagittal p. The median anteroposterior p. of the body, single inclined p. A support for the lower limb in the treatment of certain fractures, consisting of a board laid under the limb which is raised at the distal end and inclines downward to the buttocks. [Lat., plan us, flat.]