_reflex action_.

The gray matter, and not the white, is the part of the cord which
possesses this power. This reflex action is a special function of the
spinal cord, and serves as a monitor to, and regulator of the organs of
nutrition and circulation, by placing them, ordinarily, beyond the
control of conscious volition.

[Illustration: Fig. 57.]

If the foot of a decapitated frog is irritated, there is an instant
contraction of the corresponding limb; if the irritation is intense the
other limb also contracts. These motions indicate the existence, in some
part of the spinal cord, of a distinct nerve-centre, capable of
converting and reflecting impulses. It has been found by experiment,
that the same movements will take place if the irritation be applied to
any portion of the body to which the spinal nerves are distributed, thus
giving undoubted evidence that the spinal cord in its entirety is
capable of causing these reflections. Fig. 57 represents the course of
the nervous impulses. The sensory impulse passes upward along the
posterior root, _a_, until it reaches the imbedded gray matter, _b_, of
the cord, by which it is reflected, as a motor impulse, downward along
the anterior root, _c_, to the muscles whence the sensation was
received. This is the reflex action of the spinal cord. There is no
consciousness or sensation connected with this action, and the removal
of the brain and the sympathetic system does not diminish its activity.
Even after death it continues for some time, longer in cold-blooded than
in warm-blooded animals, on account of the difference in temperature,
thus showing this property of the spinal cord. By disease, or the use of
certain poisons, this activity may be greatly augmented, as is