The Body-Machine and Its Work 337 



to bring away from the organs the waste products of their activ- 

 ity, the carbonic acid (carbon dioxide), which is got rid of in 

 the lungs, and the soluble nitrogenous waste, which is got rid of 

 by the kidneys. The work that is done in the various organs- 

 such as the muscles, nerves, and glands may be roughly com- 

 pared to the work done in a simple steam-engine. Fuel must be 

 supplied; then oxygen (the essential element of air) must be 

 supplied to unite chemically with the fuel and convert the energy 

 which is locked up in it into heat and active work. The stomach 

 supplies the fuel. The lungs, like the blacksmith's bellows, 

 supply the draught of oxygen. If we do not forget that in the 

 animal body the chemical action is far more subtle and indirect 

 than in the furnace, this may be taken as a useful simple view 

 of what goes on. 



Let us follow the draught of air into the lungs and the 

 blood. We saw that, for the food to become available to the 

 tissues, the blood-vessels have to become finer and finer until at 

 last their walls are so thin that the nutritive material can pass 

 through them. It is the same with the air-passages through which 

 we breathe. The air enters by the nostrils; we will suppose, at 

 least, that the reader is sensible enough to breathe always through 

 the nostrils, and teach his children to do the same. In the nose 

 there is a warming chamber, richly supplied with blood (and the 

 supply automatically increases in cold weather), and there is a 

 sort of sieve or filter (the hairs in the nostrils) for "screening" 

 foreign bodies from the air. Dry air is also moistened in the 

 nostrils. There is a mucous membrane in it which is most useful 

 if you treat it reasonably; but if you treat it unreasonably if 

 you pack yourself with others into a moist, stuffy, unventilated 

 railway-carriage or small room it will get gored with blood and 

 "boggy," and offer a good field for certain microbes, and you 

 will have a "cold in the head." 



Behind the root of the tongue the air-way crosses the food- 

 way and enters the windpipe or trachea. At this point there is 



