cord or brain, and which is, therefore, termed _afferent_;
and that a motor nerve is one which transmits an impulse from the nerve
centre, or is _efferent_. So difference in structure, or in chemical or
physical composition, can be discerned between the afferent and the
_efferent_ nerves. A certain period of time is required for the
transmission of all impulses. The speed with which an impulse travels
has been found to be comparatively slow, being even less than that of
sound, which is 1,120 feet per second.

The experiments heretofore related have been confined solely to the
nerves. We may now proceed to the consideration of what takes place when
the spinal cord is operated upon in a similar way. If the cord be
divided with a knife or other instrument, all parts of the body supplied
with nerves given off below the division will become paralyzed and
insensible, while all parts of the body supplied with nerves from the
spinal cord _above_ the division will retain their sensibility and power
of motion. If, however, only the posterior half of the spinal cord is
divided, or destroyed, there is loss of sensation alone; and, if the
anterior portion is cut in two, and the continuity of the posterior part
is left undisturbed, there is loss of voluntary motion of the lower
limbs, but sensation remains.

REFLEX ACTION OF THE SPINAL CORD. In relation to the brain, the spinal
cord is a great mixed motor and sensory nerve, but, in addition to this,
it is also a distinct nervous centre, in which originate and terminate
all those involuntary impulses which exert so potent an influence in the
preservation and economy of the body. That peculiar power of the cord by
which it is enabled to convert sensory into motor impulse is that which
distinguishes it, as a central organ, from a nerve, and is called
_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 an