l fissure;
E, lateral column;
F, anterior column;
G, posterior column;
H, posterior median column;
K, anterior root;
L, posterior root;
M, ganglion of
N, a spinal nerve.
]

275. The Functions of the Spinal Nerves. The messages which pass
along the spinal nerves to and from the brain are transmitted mostly
through the gray matter of the cord, but some pass along the white matter
on the outer part. As in the brain, however, all the active powers of the
cord are confined to the gray matter. The spinal nerves themselves have
nothing to do with sensation or will. They are merely conductors to carry
messages to and fro. They neither issue commands nor feel a sensation.
Hence, they consist entirely of white matter.

276. Functions of the Spinal Cord. The spinal cord is the principal
channel through which all impulses from the trunk and extremities pass to
the brain, and all impulses to the trunk and extremities pass from the
brain. That is, the spinal cord receives from various parts of the body
by means of its sensory nerves certain impressions, and conveys them to
the brain, where they are interpreted.

The cord also transmits by means of its motor nerves the commands of the
brain to the voluntary muscles, and so causes movement. Thus, when the
cord is divided at any point, compressed, as by a tumor or broken bone, or
disorganized by disease, the result is a complete loss of sensation and
voluntary movement below the point of injury. If by accident a man has his
spinal cord injured at some point, he finds he has lost all sensation and
power of motion below that spot. The impulse to movement started in his
brain by the will does not reach the muscles he wishes to move, because
traveling _down_ the spinal cord, it cannot pass the seat of injury.

So the impression produced by pricking the leg with a pin, which, before
pain can be felt, must travel up the spinal cord to the brain, cannot
reach the brain because the injury obstructs the path. The telegraph wire
has been cut, and the current can no longer pass.

277. The Spinal Cord as a Conductor of Impulses. The identity in
structure of the spinal nerves, whether motor or sensory, and the vast
number of nerves in the cord make it impossible to trace for any distance
with the eye, even aided by the microscope and the most skillful
dissection, the course of nerve fibers. The paths by which the motor
impulses travel down the cord are fairly well known. Th