ese impulses
originate in the brain, and passing down keep to the same side of the
cord, and go out by nerves to the same side of the body.

The sensory impulses, however, soon after they enter the cord by the
nerve of one side, cross in the cord to the opposite side, up which they
travel to the brain. Thus the destruction of one lateral half of the cord
causes paralysis of motion on the _same side_ as the injury, but loss of
sensation on the _opposite side_, because the posterior portion destroyed
consists of fibers which have crossed from the opposite side.

Experiment proves that if both roots of a spinal nerve be cut, all those
parts of the body to which they send branches become paralyzed, and have
neither sense of pain nor power of voluntary movement. The parts might
even be cut or burned without pain. It is precisely like cutting a
telegraph wire and stopping the current.

[Illustration: Fig. 119.--The Base of the Brain.

A, anterior lobe of the cerebrum;
B, olfactory nerve;
C, sphenoid portion of the posterior lobe;
D, optic chiasm;
E, optic tract;
F, abducens;
H, M, hemispheres of the cerebellum;
K, occipital portion of the occipital lobe;
L, fissure separating the hemispheres;
N, medulla oblongata;
O, olivary body;
P, antenor pyramids;
R, pons Valoru;
S, section of olfactory nerve, with the trunk removed to show sulcus in
which it is lodged;
T, anterior extremity of median fissure
]

Experiment also proves that if only the posterior root of a spinal nerve
be cut, all sensation is lost in the parts to which the nerve passes, but
the power of moving these parts is retained. But if the anterior root
alone be divided, all power of motion in the parts supplied by that nerve
is lost, but sensation remains. From these and many other experiments, it
is evident that those fibers of a nerve which are derived from the
anterior root are motor, and those from the posterior root sensory,
fibers. Impulses sent _from_ the brain and spinal cord to muscles will,
therefore, pass along the anterior roots through those fibers of the
nerves which are derived from these (motor) roots. On the other hand,
impressions or sensations passing _to_ the brain will enter the spinal
cord and reach the brain through the posterior or sensory roots.

278. The Spinal Cord as a Reflex Center. Besides this function of the
spinal cord as a great nerve conductor to carry sensations to the brain,
and b