the medulla--that part of the great nerve stem which joins the brain to
the spinal cord. The medulla is part of the brain stem.

This was only one of the difficulties which had to be overcome. The eyes
are set on the pre-fulcral lever of the head. For our safety we must be
able to look in all directions--over this shoulder or that. We must also
be able to turn our heads so that our ears may discover in which
direction a sound is reaching us. In fashioning a fulcral 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