XXI. RESPIRATION. 



The activity of a muscle cell was seen to be dependent 

 upon a constant supply of food and oxygen. What is 

 true of a muscle cell is true of every cell in the body. All 

 require food and oxygen, and all give off carbon dioxide 

 and wastes. In our study of the digestive and circulatory 



Fig. 144 — Position of tlie lungs in the thorax ; A, lung ; £, heart ; Z>, pulmonary 

 artery ; £, trachea. 



systems we saw the manner in ^^•hich food and oxygen 

 reached the cells of the bodj' and how the wastes were un- 



312 



