CHAPTER XII 



RESPIRATION, AERATION, AND FERMEN- 

 TATION 



THE importance of air in the maintenance of animal 

 efficiency has been recognized as long as mankind has 

 existed. Shortly after the discovery of oxygen by La- 

 voisier and Priestly (1774), Scheele showed that air exhaled 

 by animals contains a smaller proportion of oxygen and 

 an increased content of carbon dioxid. That marks the 

 beginning of our knowledge of one of the products of 

 respiration and of the extensive use of oxygen (O 2 ) by 

 active living things. It was not, however, until later that 

 the relation of plants to oxygen was understood ; and at 

 that time, of course, the fundamental nature of the cata- 

 bolic or destructive changes taking place with or without 

 free oxygen in both animal and plant cells could not be 

 suspected. 



160. The term " respiration." -Long before there was 

 any accurate knowledge respecting the nature of the chem- 

 ical changes (whether anabolic or catabolic) which may 

 proceed in the cell, the term ' ' respiration ' ' was in use to 

 denote in animals " breathing," -this latter term never 

 aptly applying to any part of the process in plants. 

 With the progress of physiological study upon plants and 



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