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.
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