^46 The Unity of the Organism 



We can now state briefly as much more of the bio-chemical 

 aspect of the problem as seems indispensable to our present 

 argument. A few remarks on what the physiology of our day 

 often calls tissue respiration will compass what is in mind. 

 The key fact in this is of two-fold character: (1) The tissues 

 of the organism, not its blood or any other fluids, contain the 

 substance which is in the strictest sense living. (2) This 

 substance is called living because chemical changes of a very 

 distinctive sort are going on in it. These changes are of a 

 fundamentally double nature as regards atmospheric or 

 molecular oxygen; namely, combinative and incorporative 

 change, and separative and expulsive change. The last-men- 

 tioned, the separative and expulsive change, is known as oxi- 

 dation and manifests itself to ordinary experience in the dis- 

 charge of oxygen combined with carbon as carbon dioxide, 

 and in the setting free of energy in the form of muscular and 

 other work, and of heat. The first-mentioned, or incorpora- 

 tive change, consists in taking in and storing up oxygen, 

 "somehow," as the more carefully worded physiologies put 

 it. This statement may be taken as a very brief natural 

 history description of the most fundamental steps in what 

 formal physiology calls metabolism with its two aspects, the 

 constructive, or anabolic, and the destructive, or katabolic. 

 Probably no one will question that this conception of the 

 foundations of the life process for nearly, if not quite, all 

 animal life is that held by the best physiologists since the 

 time of C. Bernard at least. No physiologist whom I have 

 consulted has stated the nature of the process more definitely 

 than has Sir Michael Foster. "The Respiration," he writes, 

 "of the muscle then does not consist in throwing into the 

 blood oxidizable substances, there to be oxidized into car- 

 bonic acid and other matters ; but it does consist in the as- 

 sumption and storing up of oxygen somehow or other in its 

 substance, in the building up by help of that oxygen of 

 explosive decomposable substances, and in the carrying out 



