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.
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