body parts of animal arranged around a central axis
Radial symmetry
title: Axis (anatomy) text: foramen large, but smaller than that of the atlas. The transverse processes are very small, and each ends in a single tubercle; each is perforated by the transverse foramen, which is directed obliquely upward and laterally. The superior articular surfaces are round, slightly convex, directed upward and laterally, and are supported on the body, pedicles, and transverse processes. The inferior articular surfaces have the same direction as those of the other cervical vertebrae. The superior vertebral notches are very shallow, and lie behind the articular processes; the inferior lie in front of the articular processes, as in the other cervical title: Axis (anatomy) text: vertebrae The spinous process is large, very strong, deeply channelled on its under surface, and presents a bifurcated extremity. Contact sports are contraindicated for individuals with an anomalous dens, as any violent impact may result in a catastrophic injury. This is because a malformed odontoid process may lead to instability between the atlas and axis (the C1 and C2 cervical vertebrae). The axis is ossified from five primary and two secondary centers. The body and vertebral arch are ossified in the same manner as the corresponding parts in the other vertebrae, viz., one center for the body, and two for title: Axis (anatomy) text: the vertebral arch. The centers for the arch appear about the seventh or eighth week of fetal life, while the centers for the body appear in about the fourth or fifth month. The dens or odontoid process consists originally of a continuation upward of the cartilaginous mass, in which the lower part of the body is formed. About the sixth month of fetal life, two centers make their appearance in the base of this process: they are placed laterally, and join before birth to form a conical bilobed mass deeply cleft above; the interval between the sides of the cleft title: Body plan text: Body plan A body plan, Bauplan (German plural Baupläne), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrate body plan is one of many: invertebrates consist of many phyla. This term, usually applied to animals, envisages a "blueprint" encompassing aspects such as symmetry, segmentation and limb disposition. Evolutionary developmental biology seeks to explain the origins of diverse body plans. Body plans have historically been considered to have evolved in a flash in the Cambrian explosion, but a more nuanced understanding of animal evolution suggests gradual development of body plans title: Axis (anatomy) text: margins of the foramen magnum. The inner ligaments limit rotation of the head and are very strong. The weak apical ligament lies in front of the upper longitudinal bone of the cruciform ligament, and joins the apex of the deltoid peg to the anterior margin of the foramen magnum. It is the fibrous remnant of the notochord. The pedicles are broad and strong, especially in front, where they coalesce with the sides of the body and the root of the odontoid process. They are covered above by the superior articular surfaces. The laminae are thick and strong, and the vertebral