RESPIRATION AND THE VOICE. 123 



Fires 1 



Decaying matters L add to the carbonic-odd gas. 



Living animals 



Living plants add to the oxygen. 



6, Oxygen is the most remarkable of the elements. 

 Three-fourths of the material of which our bodies are 

 made is oxygen: eight-ninths of the material of which 

 water is made is oxygen. It is more than half the weight 

 of many other substances. We can not live five minutes 

 without a fresh supply of it. It is infinitely more valu- 

 able than gold, but is free to all. We do not have to 

 work for it, as we do for food: we take it from the air we 

 breathe. 



THE LUNGS. 



SECTION II. 1. The whole of the air that we take in 

 does not enter the blood. Just as the digestible rtarts of 

 our food are absorbed, while the indigestible parts are 

 cast out, so that part of the air that we need, which is 

 oxygen, is absorbed in the lungs, and the rest is breathed 

 out. 



2. The lungs are masses of little cells, with very thin 

 walls, placed in the chest. On the outside of the walls of 

 these sacs are the capillaries, millions of them. They 

 cover the sacs as the netting covers a balloon, only they 

 are much closer than such a net. The blood from the 

 pulmonary artery pours into these capillaries, and so 

 spreads out all over the lung-cells. The oxygen, then, has 

 only to pass through the thin lung-cell and the thin capil- 

 lary wall to enter the blood. 



3. Frogs get a part of their oxygen through their skins. 

 These are delicate and - moist, and just beneath them the 

 net-work of capillaries is spread out. Oxygen easily 



