This article is about a bone named the axis. Second cervical vertebra, or epistropheus, atlas of human anatomy netter pdf free download above.
The body is deeper in front than behind, and prolonged downward anteriorly so as to overlap the upper and front part of the third vertebra. Its under surface is concave from before backward and convex from side to side. The internal structure of the odontoid process is more compact than that of the body. The odontoid peg is the ascension of the atlas fused to the ascension of the axis.
The peg has an articular facet at its front and forms part of a joint with the anterior arch of the atlas. The alar ligaments, together with the apical ligaments, are attached from the sloping upper edge of the odontoid peg to the 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.
They are covered above by the superior articular surfaces. The axis is ossified from five primary and two secondary centers. 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 base of the process is separated from the body by a cartilaginous disk, which gradually becomes ossified at its circumference, but remains cartilaginous in its center until advanced age.
In this cartilage, rudiments of the lower epiphysial lamella of the atlas and the upper epiphysial lamella of the axis may sometimes be found. Type I Fracture – Extends through the tip of the dens. This type is usually stable. Type II Fracture – Extends through the base of the dens. It is the most commonly encountered fracture for this region of the axis. This type is unstable and has a high rate of non-union.