CHAPTER X 

 BESPIEATION 



1. The fundamental act of respiration. We have found in 

 studying the chemical changes which underlie cellular activity 

 (Chap. IV) that muscle fibers and gland cells and, we may 

 now add, nerve cells take in oxygen and give out carbon 

 dioxide. This cell breathing is the essential act of respiration, 

 for respiration is only another name for the oxidative proc- 

 esses of the living body. Respiration of this kind (and of 

 this kind only) is universal among living things. The one- 

 celled animal, for example, takes its oxygen directly from 

 the free oxygen of the water in which it lives, and discharges 

 its carbon dioxide into the same surrounding medium. Every 

 one of the thousands of cells of which the human body is 

 composed repeats this same process, taking its oxygen from 

 and discharging its carbon dioxide into its surrounding 

 medium in this case the lymph. The breathing move- 

 ments, which renew the air in the lungs, and the circulation 

 of blood, which affords the channel of communication between 

 the Inngs and the tissues, are merely accessory mechanisms 

 rendered necessary by the distance of the cells and the lymph 

 from the surface of the body. Their principal function is to 

 keep the lymph supplied with oxygen and to remove from it 

 the carbon dioxide. In other words, breathing, though minis- 

 tering to respiration, is not respiration itself. 



2. The quantity of oxygen and of carbon dioxide in the 

 lymph surrounding the cells of the body. The cell is the true 

 seat of oxidation. Within its imperfectly understood mecha- 

 nism are found the conditions which lead to the union of 



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