can take place. Here we have in the atlas an approach to the formation
of a wheel--a wheel which has its axle or pivot placed at some distance
from its centre, and therefore a complete revolution of the atlas is
impossible. A battery of small muscles is attached to the lateral levers
of the atlas and can swing it freely, and the head which it carries, a
certain number of degrees to both right and left. The extent of the
movements is limited by stout check ligaments. Thus, by the simple
expedient of allowing the body of the atlas to be stolen by the axis, a
pivot was obtained round which the head could be turned on a horizontal
plane.

[Illustration: Fig. 4.--A, The original parts of the first or atlas
vertebra. B, Showing the "body" of the first vertebra fixed to the
second, thus forming the pivot on which the head turns.]

Nature thus set up a double joint for the movements of the head, one
between the atlas and axis for rotatory movements, another between the
atlas and skull for nodding and side-to-side movements. And all these
she increased by giving flexibility to the whole length of the neck.
Makers of modern telescopes have imitated the method Nature invented
when fixing the human head to the spine. Their instruments are mounted
with a double joint--one for movements in a horizontal plane, the other
for movements in a vertical plane. We thus see that the young engineer,
as well as the student of medicine, can learn something from the
construction of the human body.

In low forms of vertebrate animals like the fish and frog, the head is
joined directly to the body, there being no neck.

No matter what part of the human body we examine, we shall find that its