INTRODUCTORY : THE ANIMAL ORGANISM 3 



formed, after several changes which do not concern us, into 

 the energy of the mechanical work of the engine. Regarded 

 as a whole, the process is a destruction or disintegration of 

 certain substances, with the liberation of a large part of the 

 energy which they contain. Now it may be shown that 

 the body receives by forced draught in breathing constant 

 supplies of oxygen, burns the complex, unstable molecules 

 of its substance, and obtains thus the energy for its actions, 1 

 discharging as waste in the breath, urine, and sweat the 

 simpler substances that it has formed. In short, the sub- 

 stance of the body is constantly undergoing disintegration 

 with evolution of energy. 



It is not difficult to prove that this process is taking 

 place. Since the molecules of the body-substance contain, 

 like those of the fuel of the engine, large quantities of 

 carbon and hydrogen, together with some oxygen and 

 nitrogen, the products of their disintegration include carbon 

 dioxide, water, and certain relatively simple compounds of 

 nitrogen, such as urea, CO(NH 2 ) 2 . (1) The intake of oxygen 

 and loss of carbon dioxide during life are easily demon- 

 strated. Men or animals enclosed in a vessel to which air 

 has not access are unable to live for more than a short time. 

 The animals are stifled, just as a fire or the flame of a 

 candle may be stifled by want of air, and subsequent 

 examination of the gases in the vessel will show that the 

 oxygen has been depleted and replaced by carbon dioxide 

 just as it would be if a candle had been burnt in it. This 

 exchange with their surroundings of carbon dioxide for 

 oxygen is characteristic of living animals and is known as 

 respiration. In man and animals like him it takes place 

 through the lungs, in breathing. If the breath be tested, it 



1 The analogy between the living body and a steam-engine must 

 not be pushed too far. It is not at all certain either (a) that the body 

 is a heat engine like the steam-engine, or (d) that the oxidation takes 

 place in the same way in the two cases. It is possible that the oxygen 

 which is taken in is first built into complex molecules of the body- 

 substance, by whose subsequent breakdown the energy of the life 

 processes is obtained. Or, again, it may be that the energy which it 

 provides in the burning of some substance is used in molecular re- 

 arrangements by which there is built up some more complex substance 

 or other system that in due course disintegrates to give the energy of 

 the visible life processes. 



