OUR BODIES AND HOW WE LIVE 



229. The Air we breathe. The air that we breathe is 

 chiefly a mixture of two gases, oxygen and nitrogen, in the 

 proportion of one part of the former to four of the latter. 



Oxygen is the active gas, the life-giving principle of 

 nature. It has been well named " the great supporter of 

 animal life." 



Nitrogen is mixed with it, otherwise the oxygen would 

 be too strong for us, and would burn us up too fast. In 



short, nature kindly 

 tempers with nitro- 

 gen the air which we 

 breathe. 



230. How the Air 

 is changed in Breath- 

 ing. If we examine 

 the air before it en- 

 ters the lungs, and 

 again after it has 

 passed through them, 

 we shall find that, 



FIG. 107. A Portion of a Child's Lung. while the bulk is al- 

 Lobules of various sizes are well shown. most exactly the 



same, the composi- 

 tion has been changed. It has left behind about one 

 quarter of its oxygen, and has taken in exchange for it 

 nearly the same quantity of carbon dioxide, a gas which is 

 destructive to life when present in large amounts. Carbon 

 dioxide is not poisonous in itself, but its presence excludes 

 the life-giving oxygen. 



About thirty cubic inches of air pass in and out of the 

 lungs with every breath, and more than three hundred 

 cubic feet every twenty-four hours. 



