68 Life and Health 



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, 

 habit is developed in the nerve centers of the spinal cord. 

 There is thus a great saving of actual brain work, and one 

 important cause of fatigue is removed. 



95. Effect of Exercise on the Circulation and Respiration. 

 As the action of the heart is increased both in force and 

 in 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 press- 

 ing 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 dioxide and water. 



96. Effect of Exercise on Digestion. Exercise stimulates 

 and strengthens the organs of digestion. The appetite is 

 improved, especially 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. 



Ample exercise also checks the tendency so common 

 with those who eat heartily, but lead sedentary lives, 

 towards a torpid circulation in the larger digestive organs, 

 especially in the stomach and the liver. 



97. Exercise and the Bodily Temperature. Exercise 

 increases the flow of blood through the small vessels of 

 the skin, and thus increases the radiation of heat' from 



