certain strong ligaments--fastenings of tough fibres--are so set as
automatically to jam the joint before the edge of the foramen can come
in contact with the brain stem.

These are only some of the devices which Nature had to contrive in order
to secure a safe passageway for the brain stem. But in obtaining safety
for the brain stem, the movements of the head on the atlas had to be
limited to mere nodding or side-to-side bending. The movements which are
so necessary to us, that of turning our heads so that we can sweep our
eyes along the whole stretch of the skyline from right to left, and from
left to right, were rendered impossible. This defect was also overcome
in a simple manner. The joints between the first and second
vertebrae--the atlas and axis--were so modified that a turning movement
could take place between them instead of between the atlas and skull.
When we turn or rotate our heads, the atlas, carrying the skull upon it,
swings or turns on the axis. When we search for the manner in which this
has been accomplished, we see again that Nature has made use of the
simplest means at her disposal. When we examine a vertebra in the course
of construction within an unborn animal, we see that it is really made
up by the union of four parts (see Fig. 4): a central block which
becomes the "body" or supporting part; a right and a left arch which
enclose a passage for the spinal cord; and, lastly, a fourth part in
front of the central block which becomes big and strong only in the
first vertebra--the atlas. 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