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 bring back its orders, it is also an independent center for what is called reflex action.  By means of its sensory nerves it receives impressions from certain parts of the body, and on its own authority sends back instructions to the muscles by its motor nerves, without consulting the brain.  This constitutes reflex action, so called because the impulse sent to the spinal cord by certain sensory nerves is at once reflected or sent back as a motor impulse to the muscles.

This reflex action is a most important function of the spinal cord.  This power is possessed only by the gray matter of the cord, the white substance being simply a conductor.

The cells of gray matter are found all along the cord, but are grouped together in certain parts, notably in the cervical and lumbar regions.  The cells of the anterior horns are in relation with the muscles by means of nerve fibers, and are also brought into connection with the skin and other sensory surfaces, by means of nerve fibers running in the posterior part of the cord.  Thus there is established in the spinal cord, without reference to the brain at all, a reflex mechanism.

279.  Reflex Centers.  For the purpose of illustration, we might consider the body as made up of so many segments piled one on another, each segment presided over by a similar segment of spinal cord.  Each bodily segment would have sensory and motor nerves corresponding to its connection with the spinal cord.  The group of cells in each spinal segment is intimately connected with the cells of the segments above and below.  Thus an impression reaching the cells of one spinal segment might be so strong as to overflow into the cells of other segments, and thus cause other parts of the body to be affected.