bones, placed one above the other, called the _vertebrae_, and two
bones found below the vertebrae, known as the sacrum and the coccyx (Fig.
98). These twenty-six bones supply the central axis of the body, support
the head and upper extremities, and inclose and protect the spinal cord.

[Fig. 98]

Fig. 98--The spinal column.

The upper seven vertebrae form the neck and are called the _cervical_
vertebrae. They are smaller and have greater freedom of motion than the
others. The first and second cervical vertebrae, known as the _atlas_ and
the _axis_, are specially modified to form a support for the head and
provide for its movements. The head rests upon the atlas, forming with it
a hinge joint (used in nodding to indicate "yes"); and the atlas turns
upon an upward projection of the axis forming a pivot joint (used in
shaking the head to indicate "no").

The next twelve vertebrae, in order below the cervical, are known as the
_thoracic_ vertebrae. They form the back part of the framework of the
thorax and have little freedom of motion. The five vertebrae below the
thoracic are known as the _lumbar_ vertebrae. These bones are large and
strong and admit of considerable motion. Below the last lumbar vertebra is
a wedge-shaped bone which has the appearance of five vertebrae fused
together. This bone, known as the _sacrum_, connects with the large bones
which form the pelvic girdle. Attached to the lower end of the sacrum is a
group of from two to four small vertebrae, more or less fused, called the
_coccyx_.

[Fig. 99]

Fig. 99--*Two views of a lumbar vertebra.* _A._ From above. _B._ From the
side. 1. Body. 2, 3, 4, 5. Projections from the neural arch.

*The Joining of the Vertebrae.*--A typical vertebra consists of a heavy,
disk-shaped portion in front, called the _body_, which is connected with a
ring-like portion behind, called the _neural arch_. The body and the
neural arch together encircle a round opening which is a part of the canal
that contains the spinal cord (Fig. 99). From the neural arch are seven
bony projections, or processes, three of which serve for the attachment of
muscles and ligaments, while the other four, two above and two below, are
for the interlocking of the vertebrae with each other. The separate
vertebrae are joined together in the spinal column, as follows:

_a._ Between the bodies of ad