30 Animal Life and Intelligence. 



products of the combustion that takes place in the 

 muscular, nervous, and other tissues. 



The animal organism has sometimes been likened to 

 a steam-engine, in which the food 'is the fuel which enters 

 into combustion with the oxygen taken in through the 

 lungs. It may be worth while to modify and modernize 

 this analogy always remembering, however, that it is an 

 analogy, and that it must not be pushed too far. 



In the ordinary steam-engine the fuel is placed in the 

 fire-box, to which the oxygen of the air gains access ; the 

 heat produced by the combustion converts the water in 

 the boiler into steam, which is made to act upon the 

 piston, and thus set the machinery in motion. But there 

 is another kind of engine, now extensively used, which 

 works on a different principle. In the gas-engine the fuel 

 is gaseous, and it can thus be introduced in a state of 

 intimate mixture with the oxygen with which it is to unite 

 in combustion. This is a great advantage. The two can 

 unite rapidly and explosively. In gunpowder the same 

 end is effected by mixing the carbon and sulphur with 

 nitre, which contains the oxygen necessary for their explo- 

 sive combustion. And this is carried still further in dyna- 

 mite and gun-cotton, where the elements necessary for 

 explosive combustion are not merely mechanically mixed, 

 but are chemically combined in a highly unstable com- 

 pound. 



But in the gas-engine, not only is the fuel and the 

 oxygen thus intimately mixed, but the controlled explo- 

 sions and the resulting condensation are caused to act 

 directly on the piston, and not through the intervention 

 of water in a boiler. Whereas, therefore, in the steam- 

 engine the combustion is to some extent external to the 

 working of the machine, in the gas-engine it is to a large 

 extent internal and direct. 



Now, instead of likening the organism as a whole to 

 a steam-engine, it is more satisfactory to liken each cell 

 to a gas-engine. We have seen that the cell-substance 

 around the nucleus is composed of a network of proto- 



