A Book of Exposition | Page 4

joint for the
head, then, two different objects had to be secured: free mobility for the
head, and a safe transit for the medullary part of the brain stem. How
well these objects have been attained is known to all of us, for we can
move our heads in the freest manner and suffer no damage whatsoever.
Indeed, so strong and perfect is the joint that damage to it is one of the
most uncommon accidents of life.
Let us see, then, how this triumph in engineering has been secured. 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. 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, 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. 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