The first vertebra or Atlas (fig. 1, Plate XXXVIII), so described because in the human family it supports the head, differs in a striking manner from the typical vertebra, being a mere ring of bone, having two broad wings or transverse processes jutting out from the sides. In front it presents two deep concave surfaces, which articulate with corresponding convexities or condyles (occipital condyles) at the back of the head. It is by the peculiar construction of this joint that the free up-and-down movement of the head upon the neck is rendered possible.