A WONDERFUL WORLD 



it releases again, through the life processes, as 

 carbonic-acid gas, to be again drawn into the cycle 

 of vegetable life. 



The act of breathing well illustrates our mysteri- 

 ous relations to Nature — the cunning way in which 

 she plays the principal part in our lives without our 

 knowledge. How certain we are that we draw the 

 air into our lungs — that we seize hold of it in some 

 way as if it were a continuous substance, and pull 

 it into our bodies ! Are we not also certain that the 

 pump sucks the water up through the pipe, and 

 that we suck our iced drinks through a straw? We 

 are quite unconscious of the fact that the weight of 

 the superincumbent air does it all, that breathing is 

 only to a very limited extent a voluntary act. It is 

 controlled by muscular machinery, but that machin- 

 ery would not act in a vacuum. We contract the 

 diaphragm, or the diaphragm contracts under 

 stimuli received through the medulla oblongata from 

 those parts of the body which constantly demand 

 oxygen, and a vacuum tends to form in the chest, 

 which is constantly prevented by the air rushing 

 in to fill it. The expansive force of the air under its 

 own weight causes the lungs to fill, just as it causes 

 the bellows of the blacksmith to fill when he works 

 the lever, and the water to rise in the pump when 

 we force out the air by working the handle. Another 

 unconscious muscular effort under the influence of 

 nerve stimulus, and the air is forced out of the lungs, 



53 



