64 ELEMENTARY PHYSIOLOGY. 



lungs are filled with air, the oxygen of the air, in a 

 wonderful way, passes through the wall of the cell 

 and -the wall of the capillary and unites with the 

 blood. At the same time, impurities from the blood 

 pass through capillary wall and cell wall into the 

 cells and out through the air-passages with the 

 escaping breath. In this way the blood is renewed, 

 purified and brightened by the life-giving oxygen, 

 and starts vigorously off on another round through the 

 body. 



Breathing is the act of the body by 



Breathing. ^.^ ^ ^^ ^ fiUed ^.^ ^ ^ 



emptied again at proper intervals. This operation 

 is so important and essential to life, that it has been 

 entrusted to the performance of muscles of the invol- 

 untary kind, that is, such as are not dependent on 

 the direction of the mind. It is true, the will may 

 interfere with the work of these muscles, so that we 

 may suspend breathing to some extent. But it is 

 Nature's plan that this work should be committed to 

 faithful nerve-centers and muscles appointed for the 

 purpose. It is well that this is so, for, otherwise* 

 during sleep or other unconscious moments, when 

 the mind gives no direction to the body, breathing 

 would stop and life would end. Even when awake 

 and in health, in this very busy age, we might for- 

 get to breathe. 



The scientific name for breathing is 

 respiration. That act of respiration 

 which brings air into the lungs is called 



