ring back its orders, it is also an independent center for what
is called reflex action. By means of its sensory nerves it receives
impressions from certain parts of the body, and on its own authority sends
back instructions to the muscles by its motor nerves, without consulting
the brain. This constitutes reflex action, so called because the
impulse sent to the spinal cord by certain sensory nerves is at once
reflected or sent back as a motor impulse to the muscles.

This reflex action is a most important function of the spinal cord. This
power is possessed only by the gray matter of the cord, the white
substance being simply a conductor.

The cells of gray matter are found all along the cord, but are grouped
together in certain parts, notably in the cervical and lumbar regions. The
cells of the anterior horns are in relation with the muscles by means of
nerve fibers, and are also brought into connection with the skin and other
sensory surfaces, by means of nerve fibers running in the posterior part
of the cord. Thus there is established in the spinal cord, without
reference to the brain at all, a reflex mechanism.

279. Reflex Centers. For the purpose of illustration, we might
consider the body as made up of so many segments piled one on another,
each segment presided over by a similar segment of spinal cord. Each
bodily segment would have sensory and motor nerves corresponding to its
connection with the spinal cord. The group of cells in each spinal segment
is intimately connected with the cells of the segments above and below.
Thus an impression reaching the cells of one spinal segment might be so
strong as to overflow into the cells of other segments, and thus cause
other parts of the body to be affected.

Take as an example the case of a child who has eaten improper food, which
irritates its bowels. Sensory nerves of the bowels are disturbed, and
powerful impressions are carried up to a center in the spinal cord. These
impressions may now overflow into other centers, from which spasmodic
discharges of nerve energy may be liberated, which passing to the muscles,
throw them into violent and spasmodic contraction. In other words, the
child has a fit, or convulsion. All this disturbance being the result of
reflex action (the spasmodic motions being quite involuntary, as the brain
takes no part in them), the child meanwhile is, of course, entirely
unconscious and, however it may seem to be distressed, really suffers