BONES, MUSCLES, EXERCISE, AND REST. 285 



All exercise of the muscles directly aids the circulation in 

 the veins, and indirectly hastens it in the heart and arteries. 



657. When we walk, we breathe more rapidly than when 

 sitting still ( 342, p. 150.) If we run, or labor with great 

 violence, we breathe very rapidly. During the process of 

 exercise, more fresh blood is carried in the arteries to the 

 organs, especially the muscles, and the more impure blood 

 is brought away in the veins. The changes of living and 

 dead particles are more rapid. (252, p. 115.) There is 

 then a greater quantity of old and dead particles more 

 carbon and hydrogen to be carried out of the system, and 

 consequently a greater demand for oxygen to convert these 

 into carbonic acid gas and water. For this purpose the 

 chest expands, and we breathe more frequently, and give the 

 lungs the increased quantity of air that is needed. Hence 

 we see that exercise aids respiration, and is most advan- 

 tageously taken in the fresh air abroad. 



658. When a boy is cold in the winter, he runs to get 

 himself warm ; when a passenger is insufficiently warm in 

 his vehicle, he gets out and walks or runs by the side of his 

 horse to warm himself. When a farmer sits in his house in 

 a cold day, he has a large fire, sufficient to heat the room 

 to near 70, in order to keep himself comfortable. If he 

 ride at the same season, he wears a great coat, and wraps a 

 buffalo robe about him. But, when he takes his axe and 

 cuts his wood in the open air, he wants neithet fire nor 

 great coat, perhaps not even his close coat. His exercise 

 keeps him warm, and, if he labors violently in loading his 

 sled or his wagon with heavy logs, he becomes uncomfortably 

 hot, and may get into a free perspiration. The muscular 

 action increases the amount of wasted atoms to be con- 

 sumed in the body, and the more rapid respiration and 

 circulation increase the fuel and oxygen for the internal 

 fire, which burns more and more in proportion to the ac- 

 tivity and violence of the labor. ( 422, p. 181.) 



