152 ELEMENTARY BIOLOGY 



inches of air. If he takes a very deep breath, his lungs 

 receive about 130 cubic inches. By forcing out of the lungs 

 as much air as possible and then taking a deep breath, he can 

 take in about 230 cubic inches. When the lungs have been 

 emptied as completely as possible by a forced expiration, they 

 still contain some 100 cubic inches of air. The lungs do not 

 work to full capacity in regular breathing ; that is, they are 

 never perfectly empty, and they are rarely perfectly full. 



The 30 cubic inches of ordinary tidal air will partially fill 

 some of the air sacs of the lungs. The 100 cubic inches of 

 additional air will fill more of the air sacs and distend them 

 more. But some of the air sacs farthest away from the main 

 air pipes will be filled only by forced breathing. This is true 

 especially of the air sacs in the extreme upper corners of the 

 lungs. These air sacs are never reached in the breathing of 

 some people, and it is in these very corners that tuberculosis 

 of the lungs most frequently begins. 



For the sake of the health of the lungs it is desirable that 

 they be thoroughly ventilated at least three or four times a day. 

 Vigorous exercise of the large muscles of the legs, shoulders, 

 or abdomen will automatically increase the depth of the breath- 

 ing, so that the athlete or the shovel-man will ventilate his 

 lungs without needing to think about the matter. But the book- 

 keeper or the seamstress does need to think of the matter. 

 A person whose occupation does not regularly compel deep 

 breathing should acquire the habit of ventilating the lungs by 

 means of ten to fifteen very deep breaths, taken three or four 

 times a day, outdoors if possible, but at least at the open 

 window. A very good habit that does not cut into one's time 

 is to take a dozen very deep breaths on first going out in the 

 morning, and again at noon and in the evening, or in fact 

 whenever one passes from a house to the outdoor air. 



184. Posture and clothing. You cannot give the fresh air 

 a chance to reach all the corners of your lungs unless the 

 shoulders are back far enough to let the chest expand freely. 



