When we look at the atlas (Fig. 4), we see
that it is merely a ring made up of three of the parts--the right and
left arches and the fourth element,--but the body is missing. A glance
at Fig. 4, B, will show what has become of the body of the atlas. It
has been joined to the central block of the second vertebra--the
axis--and projects upwards within the front part of the ring of the
atlas, and thus forms a pivot round which rotatory movements of the head
can take place. Here we have in the atlas an approach to the formation
of a wheel--a wheel which has its axle or pivot placed at some distance
from its centre, and therefore a complete revolution of the atlas is
impossible. A battery of small muscles is attached to the lateral levers
of the atlas and can swing it freely, and the head which it carries, a
certain number of degrees to both right and left. The extent of the
movements is limited by stout check ligaments. Thus, by the simple
expedient of allowing the body of the atlas to be stolen by the axis, a
pivot was obtained round which the head could be turned on a horizontal
plane.
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