The first two cervical vertebræ are especially and peculiarly modified, in order to support the skull and give to it the necessary degree of mobility upon the neck. The first vertebra, or _atlas_, is hardly more than a ring of bone with a pair of oval, cuplike depressions (_anterior cotyles_) upon the anterior face (superior in Man) into which are fitted the occipital condyles of the skull. By the rolling of the condyles upon the atlas is effected the nodding movement of the head, upward and downward, but not from side to side; this latter movement is accomplished by the partial rotation of skull and atlas together upon the second vertebra in a manner presently to be explained. On the hinder aspect are two articular surfaces (_posterior cotyles_) in shape like the anterior pair, but very much less concave, which are in contact with corresponding surfaces on the second vertebra. The neural arch of the atlas is broad and low and the neural canal is apparently much too large for the spinal cord, but, in fact, only a part of the circular opening belongs to the neural canal. In life, the opening is divided by a transverse ligament into an upper portion, the true neural canal, and a lower portion, which lodges a projection from the second vertebra. The atlas usually has no neural spine and never a prominent one; the transverse processes are broad, wing-like plates and each is perforated by a small canal, which transmits the vertebral artery. [Illustration: FIG. 12.—Axis of Wolf, left side. _od.p._ odontoid process. _cot._, anterior cotyles. _n.a._, neural arch. _n.sp._, neural spine. _pt.z._, posterior zygapophyses. _tr._, transverse process. _v.a′._, anterior opening of canal for the vertebral artery. _v.a″._, posterior opening of the same.] The second vertebra, or _axis_, is a little more like the ordinary vertebra, having a definite and usually elongate centrum, on the anterior end of which are the two articular surfaces for the atlas. Between these is a prominent projection, the _odontoid process_, which fits into the ring of the atlas and has a special articulation with the lower bar of that ring. In most mammals the odontoid process is a bluntly conical peg, varying merely in length and thickness, but in many long-necked forms the peg is converted into a semicylindrical spout, convex on the lower side and concave above. The neural spine of the axis is almost always a relatively large, hatchet-shaped plate, which is most developed in the carnivorous forms, and the transverse processes are commonly slender rods.