CONE

in neurology, light-sensitive neuron with a conical projection in the retina of the vertebrate eye, associated with colour vision and perception of fine detail. Shorter and far fewer than the eye's rods (light-sensitive cells), cones are less sensitive to low illumination levels and are mediators of photopic rather than scotopic (Greek skotos, dark) vision. Cones are mostly concentrated at the central yellow body containing the fovea (depression in the retina), where no rods are present, while at the outer edges of the retina, cones are sparse. Chemical changes that occur when light strikes the cones are relayed as impulses to optic-nerve fibres leading to the occipital lobe of the brain. See also colour vision. in mathematics, the surface traced by a moving straight line (the generatrix) that always passes through a fixed point (the vertex). The path, to be definite, is directed by some closed plane curve (the directrix), along which the line always glides. In a right circular cone, the directrix is a circle, and the cone is a surface of revolution. The axis of this cone is a line through the vertex and the centre of the circle, the line being perpendicular to the plane of the circle. In an oblique circular cone, the angle that the axis makes with the circle is other than 90. The directrix of a cone need not be a circle; and if the cone is right, planes parallel to the plane of the directrix produce intersections with the cone that take the shape, but not the size, of the directrix. For such a plane, if the directrix is an ellipse, the intersection is an ellipse. The generatrix of a cone is assumed to be infinite in length, extending in both directions from the vertex. The cone so generated, therefore, has two parts, called nappes or sheets, that extend infinitely. A finite cone has a finite, but not necessarily fixed, base, the surface enclosed by the directrix, and a finite, but not necessarily fixed, length of generatrix, called an element. See also conic section. also called strobilus in botany, mass of scales or bracts, usually ovate in shape, containing the reproductive organs of certain non-flowering plants. The cone, a distinguishing feature of pines and other conifers, is roughly analogous to the flower of other plants. Cones (strobili) are also found on club mosses and horsetails.