its rotation round its own axis_. Sit upright, with the back and
shoulders well applied against the back of a chair. Note that the head
and neck can be turned as far as 60 degrees or 70 degrees. Now bend
forwards, so as to let the dorsal and lumbar vertebræ come into play,
and the head can be turned 30 degrees more.

Experiment 9. _To show how the spinal vertebræ make a firm but
flexible column._ Take 24 hard rubber overcoat buttons, or the same
number of two-cent pieces, and pile them on top of each other. A thin
layer of soft putty may be put between the coins to represent the pads
of cartilage between the vertebræ. The most striking features of the
spinal column may be illustrated by this simple apparatus.

38. How the Head and Spine are Joined together. The head rests upon
the spinal column in a manner worthy of special notice. This consists in
the peculiar structure of the first two cervical vertebræ, known as the
axis and atlas. The atlas is named after the fabled giant who
supported the earth on his shoulders. This vertebra consists of a ring of
bone, having two cup-like sockets into which fit two bony projections
arising on either side of the great opening (_foramen magnum_) in the
occipital bone. The hinge joint thus formed allows the head to nod
forward, while ligaments prevent it from moving too far.

On the upper surface of the axis, the second vertebra, is a peg or
process, called the _odontoid process_ from its resemblance to a tooth.
This peg forms a pivot upon which the head with the atlas turns. It is
held in its place against the front inner surface of the atlas by a band
of strong ligaments, which also prevents it from pressing on the delicate
spinal cord. Thus, when we turn the head to the right or left, the skull
and the atlas move together, both rotating on the odontoid process of the