1 66 FUNCTION OF THE LUNGS. 



acid, there is not enough oxygen to supply the needs of 

 the body and maintain life. Death in such case results 

 from suffocation, which may be more plainly named 

 oxygen-starvation. The "foul air" which is sometimes 

 present at the bottom of deep wells or pits, and kills 

 people who incautiously go down them, does so be- 

 cause it contains much carbonic acid. Carbonic acid is 

 not itself very poisonous, but air containing much of it 

 is fatal, because carbonic acid has taken the place of 

 the necessary oxygen ; and air without plenty of oxy- 

 gen will not support life. 



7. Excretion. The process of removing its wastes 

 from the body, or getting rid of things which have 

 done their work in it and are no longer wanted, is 

 named excretion. The waste substances themselves are 

 also called excretions. Organs which remove them are 

 excretory organs. 



8. The Lungs Perform a Double Duty. So far as oxygen 

 is concerned, they are organs for taking something use- 

 ful into the body, and are receptive organs. So far as 

 carbonic acid is concerned, they are excretory organs. 



9. How the Air is Purified. Every living human being 

 and every one of the lower animals is all the time taking 

 oxygen from the air and giving carbonic acid to it. So 

 is every fire and every burning candle. We may natu- 

 tallyask, How is the air kept fit to breathe? 



it ? Give another name for suffocation ? Why may carbonic acid be 

 called a " negative poison"? 



7. What is the process of excretion ? What substances are named 

 excretions ? What are excretory organs ? 



8. What are the two main duties of the lungs ? 



9. How do living animals and fires alter the air ? What living 

 things purify the air ? Name a chief food of green plants. What do 

 they do with the carbonic acid they take from the air? How do 

 animals and plants help one another ? 



