RESPIRATION 13 



the constructive the organism loses in some way — it ceases 

 to grow, or decreases in weight, or moves less, or fails to 

 reproduce. The destructive iniiuences result in the libera- 

 tion of energy, the constructive in the storing of energy. 

 The liberation of energy implies previous construction, for 

 the generation of energy from nothing is inconceivable. 

 The chief source of energy in the organism is combustion, 

 the destruction by oxidation of not merely already exist- 

 ing, but really of previously formed substances. The direct 

 result of oxidation is the evolution of heat. 



To the oxidation which goes on within the living organ- 

 ism is given the name Respiration. The interchange of 

 gases between the blood of higher animals and the air, 

 which takes place at the lungs or gills, is Taut a part of the 

 process of respiration. The oxygen taken up by the blood 

 at those surfaces exposed to the air is transferred by the 

 circulation of the blood to the tissues and cells by which it 

 is used. The oxygen is used by combining it with other 

 substances, by oxidizing these substances. Respiration is 

 physiological oxidation, or combustion, as distinguished 

 from such oxidations as take place under ordinary condi- 

 tions spontaneously. The element sodium, when exposed to 

 the air, will unite with the oxygen without heat, light, or 

 any other known form of energy being applied to encourage 

 the union. Animal and vegetable substances will not so 

 unite, the oxidation must be encouraged, and it will take 

 place outside the living body only at a temperature con- 

 siderably higher than that developed within the body of any 

 living organism. In respiration we have, then, a process 

 which differs from ordinary oxidation in the conditions 

 under which it takes place. The results, however, are the 

 same. In respiration we have to do not only with the 

 affinity of oxygen for certain elements and compounds, but 

 with the need of the living organism for energy. The^ or- 

 ganism must be actively living in order that the oxygen of 

 the air may be made to unite with those substances in the 

 body from which energy is to be liberated. Respiration may 

 be artificially continued, with the liberation of energy, only 

 for a short time after the death of the organism as a 



