CHAPTER XIV 

 THE EXCRETORY SYSTEM 



No machine has ever yet been invented so perfect in con- 

 struction that it will not wear out. Often it is impossible to 

 see with the naked eye the material which wears away from 

 a machine, but we know that it does disappear. The axles of 

 wagon wheels grow smaller, the tires wear away, bolts get 

 loose; all these parts have to be renewed occasionally. 



Furnaces do not show the same kind of friction as axles and 

 wheel tires but they require, at times, new piping, grates and 

 valves. In furnaces, too, there is another kind of material 

 which must constantly be removed: the ashes, which are the 

 waste from the burned fuel, must be raked out, or the fire- 

 box will become clogged so that the fire will not burn. From 

 the burning fire, too, a quantity of waste gas goes off up the 

 chimney. 



In similar ways, wastes are produced in the human body. 

 The tissues are constantly wearing out, and the parts worn 

 away are useless and must be removed.. In the body, too, 

 the oxidation of food leaves waste material corresponding 

 in a way to ashes, and in this oxidation gases are produced, 

 and considerable water. All of this waste must be eliminated, 

 and the general process of getting rid of it is called ex- 

 cretion. 



EXCRETORY ORGANS 



These wastes are conveyed to the exterior by four main 

 paths: the lungs, intestine, kidneys and the skin. It is difficult 

 to say which is the most important, for interference with the 

 functions of any one produces serious consequences. Deaths 



