526 APPLIED BIOLOGY 



meat, we should reject this absurd rule at once, because biology 

 teaches that such food is not digested by saliva. We should 

 keep a sharp lookout for such applications of biology to 

 the unscientific rules of personal hygiene which so many 

 people accept without question. 



Public hygiene or sanitation means principles of biology 

 applied to increasing the health of a community of people. 

 Problems relating to clean streets, pure food-supply, in- 

 fectious diseases, sewerage, water-supply, and others under 

 health officers, belong to public hygiene. 



RESPIRATORY ORGANS 



449. Habits of Breathing. The effect of tight clothing 

 is referred to in the preceding section. All authorities in medi- 

 cine and hygiene unite in condemning the fashion of wearing 

 any clothing which interferes with breathing movements. 



Exercise in deep breathing is important, for it trains the 

 respiratory muscles so that fuller expansion of the lungs 

 occurs regularly. This means that fresh air goes deeper into 

 the air-tubes (see 428, 429). Consult your physical-train- 

 ing teacher as. to the advantage of training in breathing for 

 athletics. 



Breathing through the nose is the natural way, because the 

 air is properly warmed and much dust is stopped in the 

 nasal passages. Mouth-breathing is abnormal, but very 

 common. Children who breathe habitually through the 

 mouth probably have enlarged adenoids in the upper part of 

 the pharynx back of the soft palate. If not removed at once 

 by a competent surgeon, they may seriously interfere with 

 the health or cause deafness, or even deformation of the upper 

 They commonly disappear after childhood, but then 



3 damage has been done. Hence any special difficulty 

 with natural breathing through the nose should be referred 

 at once to a doctor for advice. 



