In time, however, acts which were first done only with effort and by a conscious will, become automatic. The will ceases to concern itself. By what is called reflex action, memory is developed in the spinal cord and the muscular centers (sec. 273). There is thus a great saving of actual brain work, and one important cause of fatigue is removed. 83. Effect of Exercise on Important Organs. The importance of regular exercise is best understood by noting its effects upon the principal organs of the body. As the action of the heart is increased both in force and frequency during exercise, the flow of blood throughout the body is augmented. This results from the force of the muscular contractions which play their part in pressing the blood in the veins onward towards the heart. Exercise also induces a more vigorous respiration, and under increased breathing efforts the lung capacity is increased and the size of the chest is enlarged. The amount of air inspired and expired in a given time is much larger than if the body were at rest. The blood is thus supplied with a much larger amount of oxygen from the air inhaled, and gives off to the air a corresponding excess of carbon dioxid and water. Again, exercise stimulates and strengthens the organs of digestion. The appetite is improved, as is especially noted after exercise in the open air. The digestion is more complete, absorption becomes more rapid, the peristaltic movements of the bowels are promoted, and the circulation through the liver is more vigorous. More food is taken to supply the force necessary for the maintenance of the mechanical movements. Ample exercise also checks the tendency towards a torpid circulation in the larger digestive organs, as the stomach and the liver, so common with those who eat heartily, but lead sedentary lives. In short, exercise may be regarded as a great regulator of nutrition. Exercise increases the flow of blood through the small vessels of the skin, and thus increases the radiation of heat from the surface. If the exercise be vigorous and the weather hot, a profuse sweat ensues, the rapid evaporation of which cools the body. The skin is thus a most important regulator of the bodily temperature, and prevents any rise above the normal which would otherwise result from vigorous exercise. (See secs. 226 and 241).