forms a
water-bed for the cord, continuous with that at the base of the brain.
The dura mater constitutes the enveloping sheath of the cord. It hangs
from the edge of the foramen magnum as a tubular sac, and is connected
to the bones only opposite the intervertebral foramina, where it is
prolonged on to each spinal nerve as part of its sheath. Between the
dura and the bony wall of the canal is a space filled with loose
areolar tissue and traversed by large venous sinuses. The dura extends
as far as the upper edge of the sacrum.

The _spinal cord_ extends from the foramen magnum to the level of the
disc between the first and second lumbar vertebrae. The cervical
enlargement, which includes the lower four cervical and the upper two
thoracic segments, ends opposite the seventh cervical spine. The
lumbar enlargement lies opposite the last three thoracic spines.

One pair of spinal nerves leaves each "segment" of the cord. On
leaving the cord the nerves incline slightly downwards towards the
foramina by which they make their exit from the canal. The obliquity
of the nerves gradually increases, till in the lower part of the
canal--from the second lumbar vertebra onward--they run parallel with
the filum terminale and together constitute the cauda equina.

It is to be borne in mind that owing to the fact that the cord is
relatively shorter than the canal, the tips of the spinous processes
lie a considerable distance lower than the segments of the cord with
which they correspond numerically. To estimate the level of the
segment of the cord which is injured: in the cervical region add one
to the number of the vertebra counted by the spines; in the upper
thoracic region add two, in the lower thoracic region add three, and
this will give the corresponding segment. The lower part of the
eleventh thoracic spinous process and the space below it are opposite
the lower three lumbar segments. The twelfth thoracic spinous process
and the space below it are opposite the sacral segments (Chipault).

_Functions._--The essential function of the spinal cord is to transmit
motor and sensory impulses between the brain and the rest of the body.
The general course of the fibres by which these impulses travel has
already been described (p. 331).

In the grey matter there are groups of nerve-cells--"centres"--which
govern certain reflex movements. The most important of these--the
centres for the rectal, the vesical, and the patellar reflexes--