In
her inventive moods Nature always hits on the simplest plan possible. In
this case she adopted a ball-and-socket joint--the kind by which older
astronomers mounted their telescopes. By such a joint the telescope
becomes, just as the head is, a lever of the first order. The eyeglass
is placed at one end of the lever, while the object-glass, which can be
swept across the face of the heavens, is placed at the other or more
distant end. In the human body the first vertebra of the backbone--the
atlas--is trimmed to form a socket, while an adjacent part of the base
of the skull is shaped to play the part of ball. The kind of joint to be
used having been hit upon, the next point was to secure a safe passage
for the brain stem. That, too, was worked out in the simplest fashion.
The central parts of both ball and socket were cut away, or, to state
the matter more exactly, were never formed. Thus a passage was obtained
right through the centre of the fulcral joint of the head. The centre of
the joint was selected because when a lever is set in motion the part at
the fulcrum moves least, and the medulla, being placed at that point, is
least exposed to disturbance when we bend our heads backwards, forwards,
or from side to side.
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