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. When we examine the base of the skull, all that we
see of the ball of the joint are two knuckles of bone (Fig. 3, A),
covered by smooth slippery cartilage or gristle, to which anatomists
give the name of occipital condyles. If we were to try to complete the
ball, of which they form a part, we should close up the great
opening--the _foramen magnum_--which provides a passageway for the brain
stem on its way to the spinal canal. All that is to be seen of the
socket or cup is two hollows on the upper surface of the atlas into
which the occipital condyles fit (Fig. 3, B). Merely two parts of the
brim of the cup have been preserved to provide a socket for the
condyles or ball.

[Illustration: Fig. 3.--A, The opening in the base of the skull, by
which the brain stem passes to the spinal canal. The two occipital
condyles represent part of the ball which fits into the cup formed by
the atlas. B, The parts of the socket on the ring of the atlas.]

As we bend our heads, the occipital condyles revolve or glide on the
sockets of the atlas. 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,