WASTE.] PHYSIOLOGY. 123 



Thus the blood feeds on the food we eat, 

 and the body feeds on the blood. 



HOW THE BLOOD GETS RID OF WASTE 



MATTERS. § VIII. 



53. But if the blood is thus continually being made 

 rich by things, it must also as continually be getting rid 

 of things. The things with which it parts are not, 

 however, the same as those which it takes. The 

 blood, as we have said, is fuel for the muscles, for the 

 brain, and for other parts of the body. These burn 

 the blood, bum it with heat but without light. But, 

 as you have learnt from your Chemistry Primer, Art. 

 4, burning is only change, not destruction ; in burn- 

 ing nothing is lost. If the muscle burns blood, it 

 bums it into something ; that something, being already 

 bumt, cannot be butnt again, and must be got rid of 



Into what things does the body bum itself while it 

 is alive ? 



I have already said that if you were to take a piece of 

 meat or some blood, and dry it and burn it, you would 

 find that it was turned into four things — water, carbonic 

 acid, ammonia, and ashes. The body is made up of 

 nitrogen, carbon, hydrogen, and oxygen, with sulphur, 

 phosphoms, and some other elements. The nitrogen 

 and hydrogen go to form ammonia; the hydrogen, 

 with the oxygen of combustion, forms water; the 

 carbon, carbonic acid ; the phosphorus, sulphur, and 

 other elements go to form phosphates, sulphates, and 

 other salts. 



In whatever way the body be oxidized, 

 whether it be rapidly burnt in a furnace, 

 whether it be slowly oxidized after death, as 



