412 THE HUMAN BODY. 



quantities of carbon dioxide although from the conditions 

 of the experiment it can receive from outside no uncombined 

 oxygen, and other experiments show that it contains none. 

 Hence the living muscular fibre must contain a substance 

 which is decomposed during activity and yields carbon 

 dioxide as one product of decomposition ; and this quite in- 

 dependent of any simultaneous direct oxidation. 



2. What is true of muscle is probably true of most of the 

 tissues. During rest they take up oxygen and fix it in the 

 form of complex compounds, bodies which, like nitro-glycer- 

 ine, are readily decomposed into simpler, and in such decom- 

 positions liberate energy which is used by the working tissue. 

 One product of the decomposition is the highly oxidized 

 carbon dioxide, and this is eliminated; other products are 

 less oxidized, and possibly are not eliminated but built up 

 again, with fresh oxygen taken from the blood and fresh 

 carbon from the food, into the decomposable substance. 



3. During the day a man gives off from his lungs more 

 oxygen in carbon dioxide, than he takes up by the same 

 organs from the air. During the night the reverse is the 

 case. This, however, has nothing to do with the alternating 

 periods of light and darkness, as it has in the case, of a green 

 plant, which in the light evolves more oxygen than it con- 

 sumes, and in the dark the contrary. It depends, rather, on 

 the fact that during the day more muscular effort is exerted 

 than at night, and the meals are then taken and digested. 

 The activity of the muscles and the digestive glands is de- 

 pendent on processes which give rise to a large production of 

 carbon dioxide and, during the night, when both are at rest, 

 more oxygen is taken up than is contained in the carbon 

 dioxide eliminated. If a man works and takes his meals at 

 night, and sleeps in the day, the usual ratios of his gaseous 

 exchanges with the exterior are entirely reversed. 



4. The amount of work that a man's organs do, is not 

 dependent on the amount of oxygen supplied to them, but 

 the amount of oxygen used by him depends on how much he 

 uses his organs. The quantity of oxygen supplied must of 

 course always be, at least, that required to prevent suffoca- 

 tion; but an excess above this limit will not make the tissues 

 work. Just as a man must have a certain amount of food 

 to keep him alive, so he must have a certain amount of 

 oxygen; but as extra food will not make his tissues or Jiim 



