62 GENERAL SCIENCE 



of oxygen into the blood and the flow of carbon dioxide 

 out of the blood. It also makes the lungs less able to 

 resist disease, (c) Mouth breathing causes a narrowing 

 of the upper jawbone and the teeth may protrude forward 

 and not have enough space for healthy growth. This in 

 later life will cause the person to have continually an 

 open mouth, (d) Mouth breathing causes improper 

 enunciation of words and a harsh tone due to injured 

 vocal cords. A mouth breather can never have a good 

 voice for singing. 



45. Nose Breathing. (a) By inhaling through the 

 nose the air is warmed and irritation of the throat and 

 lungs is prevented, (b) Most of the dust and germs are 

 taken out of the air before it reaches a place where injury 

 might be done, (c) The air is moistened and so the throat 

 is not irritated by dryness. (d) Colds are not so easily 

 caught as by mouth breathing, (e) It permits the proper 

 development of the face and avoids other evils of mouth 

 breathing. 



Deep breathing exercises should be practiced daily to 

 cause the air to go to every part of the lungs and keep 

 them open and also to develop the breathing muscles. 

 Boys should never smoke, because tobacco smoke con- 

 tains two dangerous poisons, namely, nicotine and carbon 

 monoxide, which affect the whole air passage by irritating 

 the mucous membrane, besides getting these poisons into 

 the blood. 



46. Ventilation. Ventilation consists of keeping in 

 our homes and buildings a supply of pure air for breath- 

 ing. When it goes into our lungs, the air if pure contains 

 about 20 per cent of oxygen and about .03 per cent of car- 

 bon dioxide. When it comes out of the lungs it is impure 

 air and contains about 16 per cent of oxygen and 4.03 per 



