nsists of an outer portion, mainly of nervous fibres,
the white matter, and of inner, ganglionated, and more highly
vascular grey matter. (In the cerebrum the grey matter is external, and
the white internal.) The cord, like the brain, is surrounded by a
vascular fibrous investment, and protected from concussion by a
serous fluid. The nerves which emerge from the vertebral column
between the vertebrae, arise, unlike the cranial nerves, by two roots.
The dorsal of these, the sensory root (d.n.), has a swelling upon it, the
dorsal ganglion, and-- by experiments upon living animals-- has been
shown to contain only afferent fibres; the ventral, the motor
root, is without a ganglion, and entirely or mainly motor. The two unite
outside the cord, and thereafter the spinal nerves are both sensory
and motor.

Section 132. Besides the great mass of brain and spinal cord
(cerebro-spinal axis), there is, on either side of the dorsal wall of the
body cavity, a sympathetic nervous chain. The nerve fibres of this
system, like the nerve fibres of invertebrates, are non-medullated. It
may be seen as a greyish thread running close by the common
carotid in the neck (sym., Figure 1); it then runs over the heads of
the ribs in the thorax and close beside the dorsal aorta in the
abdominal region. In the anterior region of the neck it dilates to form
a superior cervical ganglion, and opposite the first rib it forms an
inferior cervical ganglion. Thence, backwards, there is a ganglion on
each sympathetic chain opposite each spinal nerve, and the two
exchange fibres through a thread, the ramus communicans. To the
sympathetic chain is delegated much of the routine work of reflex
control of the bloodvessels and other viscera, which would otherwise
fall upon the spinal cord.

Section 133. There are eight cervical (spinal) nerves, one in front of
the atlas, and one behind each of the cervical vertebrae. The last four
and the first thoracic (spinal) contribute to a leash of nerves running
out to the fore limb, the brachial plexus (plexus, literally network, but
here meaning a plaited cord). The fourth cervical also sends down a
phrenic nerve (p.n., Figure 1), along by the external jugular vein and
the superior caval vein to the diaphragm. The last three lumbar and
the sacral nerves form a sacral plexus, supplying the hind limb.

Section 134. From the sympathetic in the hinder region of the thorax
a nerve, the great splanchnic nerve, arises,