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. 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.