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Chapter XI. 



RESPIRATION. 



<§ 1 . Respif'atioii in General. 



The action of atmospheric air is equally neces- 

 sary for the maintenance of animal and vegetable 

 life. As the ascending sap of plants cannot 

 be perfected unless exposed to the chemical 

 agency of air in the leaves ; in like manner the 

 blood of animals requires the perpetual reno- 

 vation of its vital properties by the purifying in- 

 fluence of respiration. The great importance of 

 this function is evinced by the constant provision 

 which has been made by Nature, in every class 

 of animals, for bringing each portion of their 

 nutritive juices, in its turn, into contact with air. 

 Even the circulation of these juices is an object 

 of inferior importance, compared with their 

 aeration ; for we find that insects, which have 

 but an imperfect and partial circulation of their 

 blood, still require the free introduction of air 

 into every part of their system. The necessity 

 for air is more urgent than the demand for food ; 

 many animals being capable of subsisting for a 



