nsion, lateral
flexion, and rotation around the long axis of the column. Flexion is
accompanied by compression of the intervertebral discs, and by a
slight forward movement of each vertebra on the one below it. This
forward movement is checked by the tension of the ligamenta flava
which stretch between the laminae.

In the infant, the spine is either straight or presents one long
antero-posterior curve with its convexity backwards. With the
assumption of the erect posture the normal S-shaped curve is
developed, the cervical and lumbar segments arching forward, while the
thoracic and sacral segments arch backward.

Through the skin it is often difficult to identify with certainty the
individual spinous processes. The spine of the seventh cervical
vertebra,--vertebra prominens--and that of the first thoracic, are
those most readily felt. While the arm hangs by the side, the root of
the spine of the scapula is opposite the third thoracic spine, and the
lower angle of the scapula is on the same level as the seventh. The
twelfth thoracic vertebra may be recognised by tracing back to it the
last rib. A line joining the highest points of the iliac crests
crosses the fourth lumbar spine; and the second sacral spine is on the
same level as the posterior superior iliac spine. The bodies of the
upper cervical vertebrae may be felt through the posterior wall of the
pharynx. The cricoid cartilage corresponds in level to that of the
lower border of the sixth cervical vertebrae and its transverse
process.

It is important for surgical purposes to bear in mind that most of the
spinous processes do not lie on the same level as their corresponding
bodies. The tips of the spines of the cervical and first two or three
thoracic vertebrae lie, roughly speaking, opposite the lower edge of
their respective bodies; those of the remaining thoracic vertebrae lie
opposite the body of the vertebrae below; while the spines of the
lumbar vertebrae lie opposite the middle of their corresponding bodies.

The _vertebral canal_ contains the spinal cord so suspended within its
membranes that it does not touch the bones, and is not disturbed by
the movements of the vertebral column.

The _membranes_ of the cord are continuous with those of the brain.
The arachno-pia invests the cord and furnishes a sheath to each of the
spinal nerves as it passes out through the intervertebral foramen. The
arachno-pial space is filled with cerebro-spinal fluid, which