But what will happen if we roll our heads
backwards to such an extent that the bony edge of the opening in the
base of the skull is made to press hard against the brain stem and crush
it? That, of course, would mean instant death. Such an accident has been
made impossible (1) by making the opening in the base of the skull so
much larger than the brain stem that in extreme movements there can be
no scissors-like action; (2) the muscles which move the head on the
atlas arrest all movements long before the danger-point is reached; (3)
even if the muscles are caught off their guard, as they sometimes are,
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.
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