THE SKIN AND THE KIDNEYS 203 



THE KIDNEYS 



274. Getting rid of Waste Matters. Our bodies are 

 never the same for a single moment. With every breath 

 and with every beat of the heart they are ever changing. 

 Wear and waste vie with growth and repair. 



We eat food to supply the bodily engine with fuel, and 

 breathe in oxygen to feed the furnace fire. With a steady 

 burning, but without light, this engine produces not only 

 motion and heat, but uses a part of its own energy to make 

 its own repairs. Not this alone, but it even gets rid of 

 its own soot and ashes, which would otherwise clog the 

 machinery, and finally stop it. 



We have already learned that the ceaseless blood current 

 which carries fresh fuel to the tissues, in the shape of the 

 nutrient part of food, is also a kind of sewer stream that 

 rids them of waste matters. 



In other words, the blood is ever being made rich by 

 some things, and is ever getting rid of other things. The 

 blood carries fuel to the tissues of the body. In the tis- 

 sues a slow burning, or oxidation, takes place : the waste 

 or ashes must be got rid of. 



275. Principal Waste Matters of the Body. What are 

 these waste matters ? If we take a piece of beef, dry it, 

 and burn it, we shall find that it is changed into four things, 

 water, carbon dioxide, ammonia, and ashes. Now, this 

 slow burning of our tissues is really the same thing. 



Hence, in whatever way our bodies are burned, or oxi- 

 dized, whether consumed in* a furnace or buried in the 

 ground, or oxidized while they are living, the end is 

 always the same : water, carbon dioxide, a kind of ammonia 

 called urea, and a small quantity of salts, or ashes. 



