40 THE STORY OF THE LIVING MACHINE. 



up that fuel into simple compounds which may be 

 chiefly considered as three carbonic dioxide(CO 2 ), 

 water (H 2 O), and ash. The energy contained in 

 the original compound can not be held by these 

 simpler bodies, and it therefore escapes as heat. 

 Just the same process, with of course difference 

 in details, is found in the living machine. The 

 food, after reaching the living cell, is united with 

 the oxygen, and, so far as chemical results are con- 

 cerned, the process is much the same as if it oc- 

 curred outside the body. The food is broken into 

 simpler compounds and the contained energy is 

 liberated. The energy is, by the mechanism of the 

 machine, changed into motion or nervous impulse, 

 etc. The food is broken into simple compounds, 

 which are chiefly carbonic dioxide, water, and ash; 

 the ash being, however, quite different from the 

 ash obtained from burning coal. Now the engine 

 must have its chimney to remove the gases and 

 vapours (the CO 2 and H 3 O) and its ashpit for the 

 ashes. In the same way the living machine has 

 its excretory system for removing wastes. In 

 the removal of the carbonic acid and water we 

 have to do once more with the respiratory sys- 

 tem, and the process is simply a repetition of the 

 story of gas diffusion, chemical union, and osmo- 

 sis. It is sufficient here to say that the process is 

 just as simple and as easily explained as those al- 

 ready described. The elimination of these wastes 

 is simply a problem of chemistry and mechanics. 



In the removal of the ash, however, we have 

 something more, for here again we are brought 

 up against the vital action of the cell. This ash 

 takes chiefly the form of a compound known as 

 urea, which finds its way into the general circula- 

 tory system. From the blood it is finally removed 



