The fact that the gray cells of the spinal cord can originate a countless number of reflex and automatic activities is not only of great importance in protecting the body from injury, but increases vastly the range of the activities of our daily life.

Even walking, riding the bicycle, playing on a piano, and numberless other such acts may be reflex movements.  To learn how, requires, of course, the action of the brain, but with frequent repetition the muscles become so accustomed to certain successive movements, that they are continued by the cord without the control of the brain.  Thus we may acquire a sort of artificial reflex action, which in time becomes in a way a part of our organization, and is carried on without will power or even consciousness.

So, while the hands are busily doing one thing, the brain can be intently thinking of another.  In fact, any attempt to control reflex action is more apt to hinder than to help.  In coming rapidly down stairs, the descent will be made with ease and safety if the spinal cord is allowed entire charge of the act, but the chances of stumbling or of tripping are very much increased if each step be taken as the result of the will power.  The reflex action of the cord may be diminished, or inhibited as it is called, but this power is limited.  Thus, we can by an effort of the will stop breathing for a certain time, but beyond that the reflex mechanism overcomes our will and we could not, if we would, commit suicide by holding our breath.  When we are asleep, if the palm of the hand be tickled, it closes; when we are awake we can prevent it.

[Illustration:  Fig. 120.—­Dr. Waller’s Diagrammatic Illustration of the Reflex Process.

From the sentient surface (1) an afferent impulse passes along (2) to the posterior root of the spinal cord, the nerve fibers of the posterior root ending in minute filaments among the small cells of this part of the cord (3).  In some unknown way this impulse passes across the gray part of the cord to the large cells of the anterior root (5), the cells of this part being connected by their axis-cylinder with the efferent fibers (6).  These convey the stimulus to the fibers of the muscle (7), which accordingly contract.  Where the brain is concerned in the action the circuit is longer through S and M.]

Experiment 131. To illustrate reflex action by what is called knee-jerk. Sit on a chair, and cross the right leg over the left one.  With the tips of the fingers or the back of a book, strike the right ligamentum patellae.  The right leg will be raised and thrown forward with a jerk, owing to the contraction of the quadriceps muscles.  An appreciable time elapses between the striking of the tendon and the jerk.  The presence or absence of the knee-jerk may be a most significant symptom to the physician.

282.  The Sympathetic System.  Running along each side of the spine, from the base of the skull to the coccyx, is a chain of nerve knots, or ganglia.  These ganglia, twenty-four on each side, and their branches form the sympathetic system, as distinguished from the cerebro-spinal system consisting of the brain and spinal cord and the nerves springing from them.  The ganglia of the sympathetic system are connected with each other and with the sensory roots of the spinal nerves by a network of gray nerve fibers.