CHAPTER II 

 RESPIRATION" 



A Universal Process. — All living things breathe. A 

 fish in the water and a horse on land breathe ; an ameba, 

 the simplest animal, and a human being, the most 

 complex, breathe ; a tree and a violet, a jelly fish and a 

 mushroom, breathe. They do not take in air and give 

 it off by means of a movement of the chest cavity, for 

 that is a characteristic of human beings and certain 

 other vertebrates, and is only an accident connected with 

 the process, but nevertheless they breathe. 



The Process Defined. — What do we mean by this 

 word breathe? If we study these diverse cases and 

 disregard accidental peculiarities and complications that 

 result from structure, or from habit of life, we find that 

 when an organism breathes two very simple things in- 

 variably occur ; oxygen passes into its body, and carbon 

 dioxide passes out of it. This double process is called 

 breathing or respiration. 



Necessary Conditions. — Oxygen and carbon dioxide 

 are gases. Whenever an animal breathes then a gas passes 

 into the body and a gas passes out. In order that this 

 may occur two obvious conditions must be fulfilled ; first, 

 the gases must be present ; second, some means of passage 

 must exist. As the simplest animals have the entire 



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