Take as an example the case of a child who has eaten improper food, which irritates its bowels.  Sensory nerves of the bowels are disturbed, and powerful impressions are carried up to a center in the spinal cord.  These impressions may now overflow into other centers, from which spasmodic discharges of nerve energy may be liberated, which passing to the muscles, throw them into violent and spasmodic contraction.  In other words, the child has a fit, or convulsion.  All this disturbance being the result of reflex action (the spasmodic motions being quite involuntary, as the brain takes no part in them), the child meanwhile is, of course, entirely unconscious and, however it may seem to be distressed, really suffers no pain.

Scattered along the entire length of the spinal cord, especially in the upper part, are groups of nerve cells which preside over certain specific functions of animal life; that is, definite collections of cells which control definite functions.  Thus there are certain centers for maintaining the action of the heart, and the movements of breathing; and low down in the cord, in the lumbar regions, are centers for the control of the various abdominal organs.

Numerous other reflex centers are described by physiologists, but enough has been said to emphasize the great importance of the spinal cord as an independent nerve center, besides its function as a conductor of nervous impulses to and from the brain.

280.  The Brain as a Reflex Center.  The brain, as we have just stated, is the seat of consciousness and intelligence.  It is also the seat of many reflex, automatic, and coordinating centers.  These give rise to certain reflex actions which are as entirely independent of consciousness as are those of the spinal cord.  These acts take place independently of the will, and often without the consciousness of the individual.  Thus, a sudden flash of light causes the eyes to blink, as the result of reflex action.  The optic nerves serve as the sensory, and the facial nerves as the motor, conductors.  The sudden start of the whole body at some loud noise, the instinctive dodging a threatened blow, and the springing back from sudden danger, are the results of reflex action.  The result ensues in these and in many other instances, without the consciousness of the individual, and indeed beyond his power of control.

281.  The Importance of Reflex Action.  Reflex action is thus a marvelous provision of nature for our comfort, health, and safety.  Its vast influence is not realized, as its numberless acts are so continually going on without our knowledge.  In fact, the greater part of nerve power is expended to produce reflex action.  The brain is thus relieved of a vast amount of work.  It would be impossible for the brain to serve as a “thinking center” to control every act of our daily life.  If we had to plan and to will every heart-beat or every respiration, the struggle for life would soon be given up.