its center, by a deep fissure, called the _anterior fissure_, and
behind, in a similar manner, by the posterior _fissure_. Each of these
fissures is lined with the pia mater, which also supports the
blood-vessels which supply the spinal cord with blood. Consequently, the
substance of the two halves of the cord is only connected by a narrow
isthmus, or bridge, perforated by a minute tube, which is termed the
_central canal_ of the spinal cord.

Each half of the spinal cord is divided lengthwise into three nearly
equal parts, which are termed the anterior, lateral, and posterior
columns, by the lines which join together two parallel series of bundles
of nervous filaments, which compose the roots of the spinal nerves. The
roots of those nerves, which are found along that line nearest the
posterior surface of the cord, are termed the posterior roots; those
which spring from the other line are known as the anterior roots.

Several of these anterior and posterior roots, situated at about the
same height on opposite sides of the spinal cord, converge and combine
into what are called the _anterior_ and _posterior bundles_; then two
bundles, anterior and posterior, unite and form the trunk of a spinal
nerve.

The nerve trunks make their way out of the spinal canal through
apertures between the vertebra, called the _inter-vertebral foramina_
and then divide into numerous branches, their ramifications extending
principally to the muscles and the skin. There are thirty-one pairs of
spinal nerves, eight of which are termed cervical, twelve dorsal, five
lumbar, and six sacral, with reference to that part of the cord from
which they originate.