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.  These 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