PART III. HOW THE BODY IS 

 NOURISHED 



CHAPTER X 



THE RESPIRATION 



246. Why Breathing Organs are needed. Every cell in 

 the body requires oxygen to enable it to do its appointed 

 work. When the supply of oxygen stops, the activity of 

 the cell ceases at once. If it is a muscle cell, motion can 

 be generated in the muscle only by the union of oxygen 

 with the contents of the cell. If it is a gland cell, it can- 

 not do its work of secreting useful fluids without the help 

 of oxygen, for the substances which the gland cell takes 

 from the blood must be changed to form the secretion. If 

 the cell is a brain cell, although it may not use as much as 

 a muscle cell uses, oxygen is still indispensable. The oxi- 

 dation that takes place in the various cells results in the 

 formation of carbon dioxid and other waste products which 

 would destroy the life of the cell if allowed to remain ; 

 these are removed from the body by the same organs that 

 supply the oxygen. 



247. What must be the Nature and Structure of a Breath- 

 ing Organ? It is obvious that in animals of large size 

 with many tissues, the great majority of the living cells of 

 the tissues must be buried deep away from the external 

 surface. But even if deep-seated and away from the air, 

 the living cells have the same need of oxygen as though 



